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2018 Fall Meeting



Recent progress in superconductivity of two-dimensional layered systems

Superconductivity in two-dimensional (2D) layered materials is currently one of the most attractive research subjects in solid state physics. One of keys for emergence of superconductivity is a carrier doping. In this symposium, we exchange the information of the superconductivity of carrier- accumulated 2D layered materials and discuss the future perspective.


This symposium will provides the forum of discussion on superconductivity in two-dimensional (2D) layered materials. The superconductivity is currently one of the most attractive and exciting research subjects in solid state physics because many superconductors with high superconducting transition temperatures (T ’s) have been successfully synthesized using the 2D layered materials. c One of keys for emergence of superconductivity is a carrier doping of the 2D layered materials. The carrier doping has been performed not only by chemical methods such as metal-doping and element substitution of precursor material, but also by electrostatic / electrochemical methods. Furthermore, superconductivity which emerges from the interface between two 2D layered materials is also very attractive. Thus, the 2D layered materials have provided a fruitful research stage for realization of superconductivity. In addition to this, the superconducting properties of 2D layered material may provide a very exciting physics from viewpoint of dimensionality and pairing mechanism, in particular the superconductivity of 2D layered materials, which are classified as ‘topological insulator and Weyl semimetal’, is very exciting; the superconductivity often emerges under high pressure.

Special emphasis will be put on the following subjects:

  • Superconductivity produced by metal doping of 2D layered materials 
  • Superconductivity produced by electrostatic / electrochemical carrier doping of 2D layered materials
  • Interface superconductivity in 2D layered materials 
  • Emergence of superconductivity from topological insulator and Weyl semimetal
  • Superconductivity of 2D layered materials under extreme condition
  • Synthesis of new 2D layered materials for superconductivity

Hot topics to be covered by the symposium

  • High T superconductivity of metal-doped Fe-based materials c
  • Emergence of superconductivity in transition metal dichalcogenides by electrostatic carrier doping 
  • Interface superconductivity
  • Topological superconductor 
  • Emergence of high-T superconductivity under high pressure c
  • Dimensionality and paring mechanism of superconductivity
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Session 1 : Roman Puzniak
Authors : Jun Akimitsu
Affiliations : Research Institute for Interdisciplinary Science, Okayama University

Resume : To realize the superconductivity in the Iridate system with the strong spin-orbit coupling will open the new world in the condensed matter physics. We call it ?J-Physics?. In particular, Sr2IrO4/Ba2IrO4 has been predicted to be a high-temperature superconductor upon electron doping since it highly resembles the cuprate in crystal structure and magnetic coupling constant. Particularly, the remarkable resemblance between Sr2IrO4/Ba2IrO4 and La2CuO4 makes a good candidate to expect the unconventional HTSC in the Sr2IrO4/Ba2IrO4. Indeed, 1) A low temperature STM study [1] on the K-doping (effectively electron doping) in the clean surface of Sr2IrO4 demonstrates the clear spin gap state. 2) Moreover, Y. K. Kim et al. observed the low temperature nodal Fermi surface and high-temperature Fermi arcs[2]. These experimental results suggest the clear evidence of the d-wave pairing correlation. However, the direct evidence of the superconductivity such as zero resistivity (E=0) and Meisser effect (B=0) can not be observed in the bulk system. We have tried to observe the evidence of superconductivity in the carrier doped bulk material. Recently, we published our present experimental data for this problem to the following paper [3] [4]. In particular, Terashima et. al. demonstrated that the d-wave gapped state approaches the Fermi energy as the doped carrier increases, which show the striking similarity with those observed for underdoped cuprate, suggesting that superconductivity can be realized with increasing the carrier concentration. In this talk, we present the present situation of ?how to dope the carrier? and discuss its expremental results. References : [1] Y. J.Yan et al., Phys. Rev. X 5, 041018 (2015). [2] Y. K. Kim et al., Nature Physics 12, 37 (2016). [3] K. Terashima et al. Phys. Rev. B 96, 041106 (R) (2017) [4] K. Horigane et al. Phys. Rev. B 97, 064425. (2018).

Authors : Simon J. Clarke, Simon J. Cassidy, Jack N. Blandy, Daniel N. Woodruff
Affiliations : Department of Chemistry, University of Oxford, Inorganic Chemistry Laboratory, South Parks Road, Oxford, OX1 3QR, UK

Resume : Soft (low temperature) chemical control of solid state systems will be described in which intercalation or deintercalation chemistry are used to control structures and properties of transition metal compounds. In particular the factors that control superconductivity in layered iron selenides will be demonstrated in the context of intercalates of FeSe containing ammonia and electropositive metals, and the lithiation of layered iron hydroxide selenides [1,2], with comparison made to thin film FeSe samples. The extension of this chemistry to control a range of other transition metal systems, notably layered oxide chalcogenides of mid-1st row transition metals [3] in order to access unusual oxidation states and coordination environments in metastable compounds will be described. References : [1] The Parent Li(OH)FeSe Phase of Lithium Iron Hydroxide Selenide Superconductors. Woodruff, D. N.; Schild, F.; Topping, C. V.; Cassidy, S. J.; Blandy, J. N.; Blundell, S. J.; Thompson, A. L.; Clarke, S. J. Inorg. Chem., 2016, 55, 9886?9891. [2] Soft chemical control of superconductivity in lithium iron selenide hydroxides Li1?xFex(OH)Fe1?ySe. Sun, H.; Woodruff, D. N.; Cassidy, S. J.; Allcroft, G. M.; Sedlmaier, S. J.; Thompson, A. L.; Bingham, P. A.; Forder, S. D.; Cartenet, S.; Mary, N.; Ramos, S.; Foronda, F. R.; Williams, B. H.; Li, X.; Blundell, S. J.; Clarke, S. J. Inorg. Chem., 2015, 54, 1958?1964. [3] Soft chemical control of the crystal and magnetic structure of a layered mixed valent manganite oxide sulfide, Blandy, J. N.; Abakumov, A. M.; Christensen, K. E.; Hadermann, J.; Adamson, P.; Cassidy, S. J.; Ramos, S.; Free, D. G.; Cohen, H.; Woodruff, D. N.; Thompson, A. L.; Clarke S. J. APL Mat. 2015, 3, 041520.

Authors : Kosmas Prassides
Affiliations : WPI-AIMR, Tohoku University, Japan

Resume : Superconductors and magnets in which the electronically-active components are molecules rather than atoms often consist of π-electron open-shell molecular units as the main component of the electronic conduction and spin network, respectively. Current best-in-class materials are the all-carbon π-electron fullerides – these show the highest known superconducting critical temperature, Tc at 38 K coupled with a record upper critical magnetic field, Hc2 in excess of 90 T. The dominance of strong electron correlations in defining their behaviour poses significant challenges for understanding the highly robust superconducting response to both temperature and magnetic field in these highly correlated molecular systems. If the fullerenes were the sole molecular system displaying such electronic functionalities, the specialized chemistry of the spherical electronically-degenerate building units might be thought to be the cause. But we have recently unveiled other extended molecular carbon-based π-systems such as polyaromatic hydrocarbons (PAHs) as new electronic materials platforms beyond C60 – here alkali metal phenacenes were shown to provide the first example of a 3D quantum spin-liquid state to 50 mK arising purely from π-electrons, while at the same time harbouring orbitally entangled states, prerequisites of the emergence of quantum magnetism and exotic superconductivity.

Session 2 : Hugo Keller
Authors : Sergey Medvedev
Affiliations : Max-Planck Institute for Chemical Physics of Solids

Resume : The exploration of the chemical, structural and electronic parameters of a solid, which lead to the appearance or the enhancement of superconductivity, is an ongoing topic of intensive research. Application of the high pressure is a powerful method to change drastically but in controllable manner the interatomic distances in solids and investigate consequent properties of the system. The impact of high pressure on the crystalline solid results in structural, electronic, magnetic and other phase transitions and leads to unusual and sometimes even unexpected properties of matter. The family of transition-metal dichalcogenides with their well-defined structure and wide range of electronic properties provides a platform to study the regularities of pressure-induced structural phase transitions and the interplay between structure and properties of matter. Transition-metal dichalcogenides (TMDs) TX2 (T is transition metal cation and X is chalcogen anion) have attracted the interest in mineralogy, chemistry, solid-state physics and materials science over more than five decades due to their interesting structural chemistry, unusual electronic properties, rich intercalation chemistry, and wide spectrum of potential applications. Originating from the competition between cationic (metal) d-orbitals and anionic (chalcogen) sp-levels, TMDs form either two- (2D) or three-dimensional (3D) structures. In this talk, the pressure tuning of crystal and electronic structure and pressure-induced superconductivity in layered 2D β-MoTe2 and 3D PdSe2 will be discussed.

Authors : Yoshiya Uwatoko
Affiliations : Institute for Solid State Physics, University of Tokyo, Kashiwanoha 5-1-5, Kashiwa, Chiba 277-8581, Japan

Resume : β-FeSe has the simplest crystal structure among the iron-based superconductors. At ambient pressure, it is a bulk superconductor with Tc = 8.5 K within the nonmagnetic nematic phase below Ts = 90 K. High-temperature superconductivity with Tc above 30-40 K (SC-I) has been successfully achieved by electron doping via co-intercalation some alkali-metal ions with ammonia, or organic molecules in between the adjacent FeSe layers. We recently performed detailed magneto-transport measurements on two heavily electron-doped FeSe materials, i.e. (Li1-xFex)OHFe1-ySe and Li0.36(NH3)yFe2Se2, under hydrostatic pressures up to 12.5 GPa by using the palm cubic anvil cell. We found that the SC-I phase is suppressed gradually by increasing pressure to Pc, above which a second high-Tc superconducting phase (SC-II) with Tc higher than 50 K emerges gradually. More importantly, our high-precision resistivity data enabled us to uncover a sharp transition of the normal state from a Fermi liquid for SC-I to a non-Fermi liquid for the SC-II phase. References : [1] J. P. Sun, et al., Nat. Commun. 9 (2018) 380(1-7). [2] P. Shahi, et al., Phys. Rev. B 97 (2018) 020508(1-6).

Authors : K. Shimizu, H. Nakao, M. Kitagaki, M. Einaga, M. Sakata, N. Hirao, S. Kawaguchi, Y. Ohishi
Affiliations : KYOKUGEN, Grad. Sch. Eng. Sci., Osaka University, 1-3 Machikaneyama, Toyonaka, Osaka 560-8531, Japan; SPring-8, 1-1-1, Sayo-cho, Sayo-gun, Hyogo 679-5198, Japan

Resume : A high-temperature superconductor, H3S was recently reported with the critical temperature (Tc) exceeding 200 K [1]. Our previous works with the powder x-ray diffraction and electrical resistance measurements revealed the superconducting H3S was synthesized by low-temperature compression of H2S under high pressure [2]. Here we report the observation of the superconducting H3S by another synthesis pathway, directly from elemental hydrogen H2 and sulfur under high pressure and high temperature. The crystallinity was improved compared to the previous pathway (low-temperature compression of H2S) and the Tc was 200 K at 146 GPa. This work was supported by JSPS KAKENHI Grant Number 26000006. References : [1] A. Drozdov et al., Nature 525, 73 (2015). [2] M. Einaga et al., Nature Physics 12, 835 (2016).

Session 3 : Leonardo Degiorgi
Authors : A. Gauzzi, Y. Klein, D. Santos Cottin, H. Yang, L. Paulatto, M. Casula, N. Nilforoushan, M. Marsi, L. de? Medici, R. Lobo, Ph. Werner, T. Miyake
Affiliations : IMPMC-Sorbonne University, CNRS, IRD, MNHN, Paris, France; Laboratoire de Physique des Solides-CNRS, Université Paris-Sud, Université Paris-Saclay, Orsay, France; LPEM-ESPCI, PSL, CNRS, Sorbonne University, Paris, France; Department of Physics, University of Fribourg, Fribourg, Switzerland; Research Center for Computational Design of Advanced Functional Materials, AIST, Tsukuba, Japan

Resume : The 2D system BaCo1-xNixS2 (BCNS) has attracted interest for a Mott metal-insulator transition (MIT) controlled by electronic doping, x, similar to that observed in high-Tc superconductors, e.g. cuprate and Fe-based pnictides. However, no superconductivity has been observed yet in BCNS. The possibility that this system may host unconventional electronic states has been put into question by recent studies on the metallic (x=1) end-member BaNiS2 [1,2], which have unveiled a peculiar Fermi surface characterized by small pockets and Dirac-like cones, explained by first-principles calculations that properly include the screened exchange and electronic correlations. In this talk, we discuss the implications of the above features on the topological properties of the electronic states and on the transport properties of BCNS. We focus on the stability of the Fermi liquid with respect to the Co/Ni substitution. An optical conductivity study supported by DMFT calculations [3] suggest that the MIT observed at x=0.2 is accompanied by a band reshuffling and leads to a borderline behavior between a conventional Mott insulator and an incoherent gapless metal. We find that the transport properties arising from this peculiar ground state consist in a remarkably linear behavior of the optical conductivity in a wide energy range. References : [1] D. Santos Cottin et al., Nature Communications 7, 11258 (2016). [2] Y. Klein et al., Phys. Rev. B 97, 075140 (2018). 3. D. Santos-Cottin et al., arXiv:1712.01539 (2017).

Authors : Hidenori Goto, Ritsuko Eguchi, Yoshihiro Kubozono
Affiliations : Research Institute for Interdisciplinary Science, Okayama University, Okayama 700-8530, Japan

Resume : Carrier doping is a key issue to induce superconductivity in two-dimensional layered materials (2DMs). For this purpose, two methods have been widely used. One utilizes field-effect transistor structure to apply gate voltage to 2DMs (gating method). An electric-double-layer transistor with ionic-liquid gate enables accumulation of high density carriers. The other uses electron transfer from/to 2DMs (doping method). Carrier density in 2DMs is modified when molecules are adsorbed on the surface to donate electrons or holes. Using these methods, we explored novel electronic properties in highly-doped graphene. Graphene is expected to show unprecedented phenomena such as chiral superconductivity, when the Fermi level reaches van Hove singularity. This doping level corresponds to 1/4 carriers per carbon atom, whereas such a high carrier density was not achieved with the two methods. The ineffective doping is ascribed to the low density of states of graphene. Mechanism of carrier doping with ionic liquid and electron-transfer molecules is quantitatively discussed. In addition, electron acceptor and donor molecules were adsorbed on both sides of bilayer graphene to produce perpendicular electric field and open a band gap. From the results obtained, we suggest that uniform electric field is not produced when dopant molecules are randomly distributed. The potential fluctuation is more prominent in the doping method, and may affect the occurrence of superconductivity in doped 2DMs.

Authors : Tong He, Kaya Kobayashi, Jun Akimitsu, Hidenori Goto and Yoshihiro Kubozono
Affiliations : Research Institute for Interdisciplinary Science, Okayama University, Okayama 700-8530, Japan

Resume : Topological insulators have been extensively studied during the past decade, because of their exciting electronic properties and possible application in electronics. The topological insulator possesses a band gap in bulk and gapless surface state. These electronic states are completely protected by time-reversal symmetry. A wide variety of materials have currently been suggested as a candidate of topological insulator. The most popular topological insulator, Bi2Se3, has a simple surface state with a single Dirac cone [1], but the electronic state of real Bi2Se3 single crystal is not simple because of naturally accumulated electrons due to the deficiency of Se. The Fermi level traverses the conduction band of the bulk crystal, leading to difficulty in detecting surface states. We prepared Ag-doped Bi2Se3, AgxBi2-xSe3, in which the electron-depletion was successfully achieved for Bi2Se3 to down-shift the Fermi level [2]. The application of pressure was made for Ag0.05Bi1.95Se3, and the electric transport was measured at 0 ? 27 GPa. The crystal structure was also investigated at 0 ? 30 GPa by synchrotron powder X-ray diffraction (XRD) measurement. Ag0.05Bi1.95Se3 showed no superconductivity down to 2.0 K at 0 GPa. The superconductivity was observed above 11GPa, with a maximum Tc value of ~7.06 K. The presence of three different structural phases, rhombohedral (phase I), monoclinic (phase II), and tetragonal structures (phase III), was confirmed from the XRD patterns at 0 ? 30 GPa. The emergence of superconductivity seems to correlate with the structural transition from phase I to phase II. To know the origin of emergence of superconductivity above 11 GPa, the electronic structures of the above three phases is now studied by Hall-effect measurement under pressure. References : [1] Y. Xia et al. Nat. Phys. 5, 398 (2009). [2] E. Uesugi et al. submitted.

Session 4 : Yoshihiro Kubozono
Authors : Samuele Sanna
Affiliations : Department of Physics and Astronomy, University of Bologna

Resume : Poisoning of superconductivity occurs in optimally electron doped LaFeAsO0.89F0.11 by substituting Fe by Mn at a remarkably low critical content xc = 0.2% [1,2]. Here we show that as soon as superconductivity is destroyed the parent undoped LaFeAsO ground state is recovered: both the same stripe ordered magnetic structure and tetragonal to orthorhombic transition are detected in optimally electron doped LaFeAsO0.89F0.11 with 0.5% of Mn [3]. Interestingly, we show that the Mn critical threshold xc for the suppression of superconductivity is rapidly increased to few percent of Mn by squeezing the volume cell by substituting La by Y or Sm (xc = 8%) [4,5,6]. Recent theoretical calculations [3,7] display that electronic correlations enhance the RKKY exchange interactions between Mn ions driving the compound with La at the verge of an electronic instability, i.e. xc tends to 0. On the other hand, the squeezing of the volume cell weakens the electronic correlations stabilizing the superconducting state, i.e. xc increases. References : [1] F. Hammerath et al., Phys. Rev. B 89, 134503 (2014). [2] F. Hammerath et al., Phys. Rev. B 92, 020505(R) (2015). [3] M. Moroni et al., Phys. Rev. B 95, 180501(R) (2017). [4] M. Moroni et al. Phys. Rev. B 94, 054508 (2016). [5] G. Lamura et al., Phys Rev. B 94, 214517 (2016). [6] R. Kappenberger et al., Phys. Rev. B 97, 054522 (2018). [7] M. N. Gastiasoro et al., Phys. Rev. Lett. 117, 257002 (2016).

Authors : Kaya Kobayashi, T. Ueno, A. Ide, T. Takahashi, S. Sasakura, H. Fujiwara, T. Yokoya, J. Akimitsu
Affiliations : Research Institute for Interdisciplinary Science, Okayama University, Department of Physics, Okayama University; Department of Physics, Okayama University; Department of Physics, Okayama University; Department of Physics, Okayama University; Department of Chemistry, Okayama University; Department of Physics, Okayama University; Research Institute for Interdisciplinary Science, Okayama University, Department of Physics, Okayama University; Research Institute for Interdisciplinary Science, Okayama University

Resume : Layered chalcogenides have been intensively studied recently due to their characteristic electronic structure and the accessibility toward device fabrication. One of the main topics in the studies of the materials is to modulate the electronic structure by cleavage and the device fabrication. This technique allowed us to modify the indirect semiconductor to superconductivity in MoS2. The intercalation of metals is another widely-used technique to modify the electronic state of layered chalcogenides. Often times, the intercalation induces electron-doping resulting the superconductivity in the system. Recent studies revealed that we could also intercalate transition metals in some of the layered chalcogenides. This technique can be expanded to other crystal structure component leading to homologues structures. We have found some of the new structures using the technique and superconductivity in the materials. Some of the materials show the characteristic sign of topological superconductors.

Authors : L. Degiorgi
Affiliations : Laboratorium für Festkörperphysik, ETH - Zürich, 8093 Zürich, Switzerland

Resume : The temperature-doping phase diagram of almost all iron-based superconductors is characterized by an antiferromagnetic dome centred at the parent compound. The onset of the magnetically ordered state is coincident with or follows a tetragonal-to-orthorhombic structural phase transition, that is driven by electronic nematic order. The latter breaks the tetragonal symmetry of the underlying lattice, without altering the translational symmetry. Superconductivity emerges in proximity of such a broken symmetry state, at the bottom of the magnetic dome in both electron- and hole-doped systems. It is currently debated to what extent nematic fluctuations contribute to the pairing-mechanism. Here, we offer a comprehensive optical investigation of the optimally hole-K-doped 122 material over a broad spectral range, as a function of temperature and of tunable applied stress, which acts as an external symmetry breaking field. We show that the stress-induced optical anisotropy in the infrared spectral range, which is reversible upon sweeping the applied stress, occurs only below the superconducting transition temperature. These findings demonstrate that there is a large nematic susceptibility at optimal doping which extends right under the superconducting dome.

Authors : *L. Piperno (1,2), A. Angrisani Armenio (2), A. Vannozzi (2), V. Pinto (2), F. Rizzo (2), A. Augieri (2), A. Mancini (2), A. Rufoloni (2), G. Celentano (2), R. B. Mos (3), L. Ciontea (3), T. Petrisor (3), G. Sotgiu (1)
Affiliations : (1) Engineering Department, Roma Tre University, Via Vito Volterra 62, 00146, Rome, Italy; (2) ENEA, Frascati Research Centre, Via E. Fermi, 45 ? 00044 Frascati, Italy; (3) Centre for Superconductivity, Spintronics and Surface Science, Technical University of Cluj-Napoca, Str. Memorandumului, Nr. 28, 400114 Cluj-Napoca, Romania

Resume : YBa2Cu3O7-x (YBCO) films grown by metal organic decomposition (MOD) are promising candidates for the production of high performance HTS wires. Moreover, artificial pinning centers (APC) inclusions improve the current transport performance of the films both in high magnetic fields and temperatures. Among the numerous possible pinning landscapes, chemically decorated surfaces have attracted attention as powerful tools for the improvement of transport properties in YBa2Cu3O7-x films. This approach makes use of low-cost, easily-tunable chemical methods to obtain self-assembled oxide nanostructures on a substrate that will serve, in a second step, for the deposition of the superconducting film itself. The structures are supposed to produce in the superconducting matrix a specific amount of strain which is generally held responsible for the increased transport capacity of variously doped samples. In this work we analyze two YBCO/oxide combinations to investigate the potential of the surface decoration technique. For the surface decoration a method called Polymer Assisted Deposition (PAD) was used. Patented in 2008, it makes use of polymer and complexing agents to obtain a homogeneous distribution of metallic ions in the to-be-deposited solutions. The oxides chosen for the deposition are those most commonly used for artificial pinning in YBa2Cu3O7-x, namely BaZrO3, and ZrO2. The density and dimensions of these nanostructures can be easily controlled by varying simple parameters in the precursor solutions preparation. The nanostructures were grown on SrTiO3 crystalline substrates. The thus decorated samples are then analyzed via AFM and XRD for structural and morphological characterization. The subsequent deposition of YBa2Cu3O7-x is carried out via standard low-fluorine MOD route and pulsed laser deposition (PLD). The obtained films are analyzed via SEM, XRD, VSM, DC resistivity and critical current measurements. The results show that the decorated surfaces are highly effective in straining the YBCO matrix leading to a significant improvement in transport properties.

Poster Session : Dirk Johrendt
Authors : P. Gierlowski, B. C. Camargo, M. Jaworski, D. Gawryluk, K. Kosyl, W. Paszkowicz
Affiliations : Institute of Physics, Polish Academy of Sciences, Al. Lotnikow 32/46, PL-02668 Warszawa, Poland

Resume : Microwave properties of ?11? layered Co-doped iron chalcogenide Fe_{1-y}Co_yTe_{0.65}Se_{0.35} crystals, with a maximum doping y= 0.036, were measured in the temperature range between 5.5 K and 15 K, by means of a superconducting cylindrical microwave resonator working at 26.2 GHz in the TE_{011} mode. The samples were grown by the Bridgman technique and were characterized by X-ray diffraction. Our crystals were fast-cooled samples, having good superconducting properties but extrinsic structural imperfections. They displayed up to 90 % of the tetragonal (superconducting) phase by weight. Based on variations of the resonant frequency width and its position, the surface resistance R_s as well as the London penetration depth change \delta\lambda_L vs temperature was calculated for small samples with lateral sizes of approx. 1 mm. \delta\lambda_L showed a different temperature dependence for different dopant concentrations and can be approximated by a T^x power function with 2.714< x< 3.072 for samples with the biggest amount of the tetragonal phase, typically also observed for 122-pnictides in the s-wave superconducting state. This work was supported by the Polish National Science Centre Grant No. 2014/15/B/ST3/03889.

Authors : V. V. Chabanenko (1), V. F. Rusakov (2), A. Chumak (1,3), A. Nabia?ek (3), R. Pu?niak (3)
Affiliations : (1) Galkin Donetsk Institute of Physics and Technology NAS Ukraine, 46 Nauki av., Kiev 03028, Ukraine; (2) Vasyl' Stus Donetsk National University, 21, 600-Richchya st., Vinnytsia 21021, Ukraine; (3) Institute of Physics, Polish Academy of Sciences, Al. Lotników 32/46, Warsaw 02-668, Poland

Resume : Superconductors of type II in mixed state contain quantized nanosize whirlpools of electrons, called Abrikosov vortices. Dynamical properties of single vortex line in YBa2Cu3O7-x (hard superconductors) were investigated experimentally with the aid of magnetic force microscopy (Auslaender O. M. et al, Nature Physics, 2009). Here the image, motion, and deformation of individual vortices are presented. The simple theory of this experiment considering the dragging force, the pinning force, and an elastic force were built in (Brandt E. H., et al, Physica C, 2010). In the present report, we consider single vortex line (H ? Hc1) dynamics under the action of external force, which varies in the plane and decays into the interior of the sample taking into account additionally inertial properties of the vortex (Kopnin N. B., Rep. Prog. Phys., 2002) and Lorentz force. Different scenarios of vortex line trajectories in two-dimensional model were considered. The data obtained make it possible to qualitatively explain the experimental results of manipulations of a single vortex. The influence of each of the forces on the shape of the vortex trajectory is analyzed, and it is shown that the Lorentz force plays the most important role.

Authors : I. Zaytseva (1), K. Kosyl (1), D. J. Gawryluk (1,2), and Marta Z Cieplak (1)
Affiliations : (1) Institute of Physics, Polish Academy of Sciences, 02-668 Warsaw, Poland; (2) Laboratory for Scientific Developments and Novel Materials, Paul Scherrer Institut, CH-5232 Villigen PSI, Switzerland

Resume : FeTe(0.65)Se(0.35) is an iron-based superconductor. It is a multiband material, in which substitution of Ni into Fe-site induces electron doping of the system [1]. However, the properties of single crystals are frequently complicated by inhomogeneity of the structure [2]. In the present work we study the structure, the Hall effect, and the angle-dependent magnetoresistance (AMR) of the Fe(1?y)Ni(y)Te(0.65)Se(0.35) crystals, with y in the range from 0 to 0.08, grown by Bridgman?s method with different cooling rates, slow (S) and fast (F). Single, tetragonal phase is observed in S crystals, while F crystals show admixture of monoclinic phase. While the Hall effect data confirm the electron doping of both types of crystals, the in-plane AMR shows strong, y-dependent anisotropy at low temperatures (T < 8K), which disappears on warming. The possible origins of these findings will be discussed. References : [1] V. L. Bezusyy, et al., Phys. Rev. B 91, 100502(R) (2015). [2] A. Wittlin, et al., Supercond. Sci. Technol. 25, 065019 (2012). Acknowledgments: This work was supported by Polish National Science Centre Grant No. 2014/15/B/ST3/03889.

Authors : E. P??ko and A. Krzto?-Maziopa
Affiliations : Warsaw University of Technology, Faculty of Chemistry, Noakowskiego St.3, 00-664 Warsaw, Poland

Resume : Superconductivity in layered FeSe1-xChx (Ch = S, Te) intercalated with various metal-organic donors is currently a subject of hot scientific debate, mostly due to high critical temperatures (Tc ~ 46K) and possible coexistence of magnetic and superconducting phases. Metal-organic donors are effectively inserted into the van der Waals gaps of host structure by low temperature solvothermal reactions from highly reductive media (solutions of alkali metals Lewis bases). These reactions are usually performed in an excess of liquid medium and the amount of the intercalant in the structure is adjusted by controlling the concentration of dissolved metal, temperature and time of intercalation. Solvothermal approach enables preparation of polycrystalline materials characterized by high superconducting volume fractions although for more spatial donors the whole process runs even few weeks due to kinetic limitations. In addition, due to prolonged reaction time in the presence of strong reductant, the product is usually contaminated by unwanted magnetic impurities. Another approach, that provides better control of the process is an electrochemical intercalation. Herein, we report the results of work on electrochemical intercalation of alkali metals into FeSe1-x host. Cyclic voltammetry of iron selenide electrodes immersed in nonaqueous electrolyte was employed to characterize the potential dependence of an alkali metal ion intercalation. The progress of intercalation process and diffusion of intercalant into the host structure was monitored by an electrochemical impedance spectroscopy measurements. The obtained results were correlated with structure, morphology and chemical composition of the prepared materials.

Authors : Artem Lynnyk (1), Anna Krzton-Maziopa (2), Edyta Pesko (2), and Roman Puzniak (1)
Affiliations : (1) Institute of Physics, Polish Academy of Sciences, Aleja Lotników 32/46, PL-02668 Warsaw, Poland; (2) Faculty of Chemistry, Warsaw University of Technology, Noakowskiego 3, PL-00664 Warsaw, Poland

Resume : Layered iron selenides intercalated with organic molecules have attracted a lot of attention in the last few years. The magnetic and nonmagnetic mesoscopic phase separation in these materials opened discussion on the nature of superconducting phase and the role of organic intercalants in tuning of superconducting properties. Superconductors with general formula (Li-O)x(FeSezCh1-z)y (O – Py, EDA; Ch – S, Te) were obtained by solvothermal intercalation of polycrystalline FeSezCh1-z using Schlenk technique. Magnetic measurements of their superconducting state properties have been performed by means of SQUID. Samples have been characterized by superconducting state transition temperature in the range 9 – 43 K, depending on their chemical composition. Relatively high transition temperature Tc around 43 K was shown by (Li EDA)y(FeSe0.88S0.1)x. Upper critical field, Hc2, was determined with ac magnetic susceptibility in external magnetic field. It was found that Hc2, recorded in the temperature range up to 30 K, decreases with increasing temperature, with a coefficient dHc2/dT typical for superconducting layered AxFe2-ySe2 (A = K, Rb, Cs). Importantly, the magnetic field range, where the superconductivity at the temperatures above 30 K is observed, corresponds to irreversibility field in the hysteresis loop. This may suggest that appearance of superconductivity at the temperatures higher than Tc of AxFe2-ySe2 is most likely related to magnetic inhomogeneity in the studied system.

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Session 5 : Simon J. Clarke
Authors : Gennady Logvenov
Affiliations : Max Planck Institute for Solid State Research, Stuttgart, Germany

Resume : Two dimensional (2D) phenomena in complex oxides heterointerfaces attract attention of scientific community during the last ten years due to their multitudinous functionalities and promising applications. The progress in this field has been mostly due to the substantial improvments of the deposition techniques such as pulse laser deposition (PLD) and atomic layer-by-layer (ALL) oxide molecular beam epitaxy (MBE). Compared with PLD, the MBE method has several extraordinary advantages: (i) the lowest energy of impinging atoms - providing less intermixing at the interface of different oxides; (ii) a possibility to control the sequences of deposition species and stoichiometry with high precision - providing engineering of interfaces via changing sequence and a stoichiometry of the interface layers; and (iii) in-situ monitoring reflection high energy diffraction (RHEED) - providing a primary feedback for control of the quality of the films growth. All together, all these distinctive features allow to achieve a precise stoichiometry and deposition control to synthesis high quality oxide films and different heterostructures, as well as to understand the dynamics of the deposition process itself, what is crucial for the chemistry and physics of the oxide interfaces. In my talk I focus only on high temperature superconducting (HTSC) interfaces in different complex oxide heterostructures. First I present ALL oxide MBE method, then different heterostructures with 2D HTSC interfaces and their characterizations by using different experimental methods including mutual inductance, electrical resistance measurements, scanning transmission electron microscopy etc. In conclusions I present implications of these finding on future work. The full information on these HTSC interfaces could be found in following publications [1-7]. References : [1] Baiutti, F., et al., in Metal Oxide-Based Thin Film Structures (Ed. Ghenadii Korotcenkov) pp.53-78 (2018) Elsevier Inc. ISBN: 978-0-12-811166-6. [2] Gozar, A. et al., Nature 455 (2008) 782. [3] Bauitti, F. et al., Nature Communications 6 (2015) 8586. [4] Wang, Yi, et al., ACS Appl. Mater. Interfaces. 8 (2016) 6763. [5] Baiutti, F., et al., ACS Appl. Mater. Interfaces, 8 (2016)27368. [5] Suyolcu, Y. E. et al., Scientific Reports 7 (2017) 453. [6] Suyolcu, Y. E. et al., Advanced Materials Interfaces 4 (2017) 1700737. [7] Baiutti, F., et al., Nanoscale, 10 (2018) 8712.

Authors : A. Ricci (1,2), B. Joseph (3,4), G. Campi (5), N. Poccia (6), D. Innocenti (7), C. Gutt (8), M. Tanaka (9,10), H. Takeya (10), Y. Takano (10), T. Mizokawa (11), A. Bianconi (2), M. Sprung (1), N. L. Saini (3)
Affiliations : (1) Deutsches Elektronen-Synchrotron DESY, Notkestra?e 85, D-22607 Hamburg, Germany; (2) Rome International Center for Material Science, RICMASS, Rome, Italy; (3) Dipartimento di Fisica, Università di Roma ?La Sapienza?, P. le Aldo Moro 2, 00185, Roma, Italy; (4) Elettra-Sincrotrone Trieste, Strada Statale 14, Km 163.5, Basovizza 34149, Trieste, Italy; (5) Institute of Crystallography, CNR, via Salaria Km 29.300, Monterotondo Roma, I-00015, Italy; (6) Department of Physics, Harvard University, 17 Oxford Street Cambridge, MA 02138, USA; (7) Department of Chemistry, University of Liverpool, Liverpool L69 3BX, United Kingdom; (8) Department of Physics, University of Siegen, Emmy-Noether-Campus, Walter-Flex-Str. 3, 57072 Siegen, Germany; (9) Graduate School of Engineering, Kyushu Institute of Technology, 1-1 Sensui-cho, Tobata, Kitakyushu 804-8550, Japan; (10) National Institute for Materials Science, 1-2-1 Sengen, Tsukuba 305-0047, Japan; (11) Department of Applied Physics, Waseda University, Tokyo 169-8555, Japan

Resume : Coexistence of phases, characterized by different electronic degrees of freedom, commonly occurs in layered superconductors in which the presence of inhomogeneity has been widely observed. In particular, alkaline intercalated FeSe chalcogenides are model systems showing nanoscale coexistence of paramagnetic (PAR) metal and antiferromagnetic (AFM) texture characterized by a large magnetic moment. While the spatial distribution of different phases has been studied by a variety of techniques, their temporal behavior is not yet known. The granular nanostructure suggests the existence of slow phase dynamics with important consequences for the related physical properties unlike homogeneous electronic systems in which fast dynamics prevails. Here, we report the first direct visualization of the atomic motion in the coexisting phases in KxFe2?ySe2 chalcogenide. We have used X-ray Photon Correlation Spectroscopy (XPCS), in which the temporal evolution of X-ray speckle patterns provides direct visualization of the local nano-domains configurations. We have found that, while the PAR phase shows slowing down upon cooling, the AFM texture reveals an anomalous non-equilibrium dynamics with avalanches as in the martensites. When the sample is cooled down across the superconducting Tc the AFM phase shows an anomalous slowing behavior suggesting a nanoscale reorganization of AFM domains and a critical correlation between the AFM phase and superconductivity. This observation provides compelling evidence of avalanche-like collective excitations to drive superconductivity in the layered iron-chalcogenides.

Authors : Roland Wiesendanger
Affiliations : Interdisciplinary Nanoscience Center Hamburg, University of Hamburg, Germany

Resume : In this presentation, we will focus on interfacial superconductivity in novel types of heterostructures. In particular, we will present a low-temperature SP-STS study of ultrathin FeTe1-xSex (x = 0, 0.5) films grown on prototypical Bi-based bulk topological insulators. We observe fully developed U-shaped superconducting gaps in FeTe0.5Se0.5 layers of one unit cell (UC) thickness with a transition temperature (Tc) of ~ 11 K, close to the one of the corresponding bulk system (Tc ~ 14.5 K) [1]. Surprisingly, we also find clear evidence for superconductivity up to Tc ~ 6 K for one UC thick FeTe layers grown on Bi2Te3 substrates [2], in contrast to the non-superconducting FeTe bulk compound which exhibits bicollinear antiferromagnetic (AFM) order in a wide temperature range up to 70 K. Even more surprisingly, detailed investigations of the atomic-scale spin-resolved local density of states by SP-STS reveal that superconductivity in one UC layers of FeTe grown on Bi2Te3 appears to spatially coexist with bi-collinear AFM order. By using 3D-vector-resolved SP-STM techniques [3] we find an unusual reorientation of the diagonal double-stripe spin structure at Fe1+yTe thin film surfaces [4]. Moreover, variable-temperature SP-STM studies [5] reveal an enhanced Néel temperature for AFM spin ordering of the ultrathin FeTe films grown on topological insulators [6]. These findings open novel perspectives for theoretical studies of competing orders in Fe-based superconductors as well as for experimental investigations of exotic phases in heterostructures of topological insulators and superconducting layers. References : [1] A. Kamlapure, S. Manna, L. Cornils, T. Hänke, M. Bremholm, Ph. Hofmann, J. Wiebe, and R. Wiesendanger, Phys. Rev. B 95, 104509 (2017): "Spatial variation of the two-fold anisotropic superconducting gap in a monolayer of FeSe0.5Te0.5 on a topological insulator". [2] S. Manna, A. Kamlapure, L. Cornils, T. Hänke, E. M. J. Hedegaard, M. Bremholm, B. B. Iversen, Ph. Hofmann, J. Wiebe, and R. Wiesendanger, Nature Commun. 8, 14074 (2017): "Interfacial superconductivity in a bi-collinear antiferromagnetically ordered FeTe monolayer on a topological insulator". [3] S. Meckler, M. Gyamfi, O. Pietzsch, and R. Wiesendanger, Rev. Sci. Instrum. 80, 023708 (2009): "A low-temperature spin-polarized scanning tunneling microscope operating in a fully rotatable magnetic field". [4] T. Hänke, U. R. Singh, L. Cornils, S. Manna, A. Kamlapure, M. Bremholm, E. M. J. Hedegaard, B. B. Iversen, Ph. Hofmann, J. Hu, Z. Mao, J. Wiebe, and R. Wiesendanger, Nature Commun. 8, 13939 (2017): "Reorientation of the diagonal double-stripe spin structure at Fe1+yTe bulk and thin film surfaces". [5] J. Warmuth, M. Bremholm, Ph. Hofmann, J. Wiebe, and R. Wiesendanger, NPG Quantum Materials 3, 21 (2018): "Domain imaging across the magneto-structural phase transition in Fe1+yTe". [6] U. R. Singh, J. Warmuth, A. Kamlapure, L. Cornils, M. Bremholm, Ph. Hofmann, J. Wiebe, and R. Wiesendanger, Phys. Rev. B 97, 144513 (2018): "Enhanced spin ordering temperature in ultrathin FeTe films grown on a topological insulator".

Session 6 : Katsuya Shimizu
Authors : H. Boschker (1), L. Kuerten (1), C. Richter (2), N. Mohanta (20, T. Kopp (2), A. Kampf (2), J. Mannhart (1)
Affiliations : (1) Solid State Quantum Electronics, MPI Solid State Research, Stuttgart, Germany; (2) Center for Electronic Correlations and Magnetism, University of Augsburg, Augsburg, Germany

Resume : We identied quasiparticle states at well-dened energies inside the superconducting gap of the electron system at the LaAlO3-SrTiO3 interface using tunneling spectroscopy. The features are found only in a number of samples and depend upon the thermal-cycling history of the samples. The features consist of a peak at zero energy and other peaks at finite energies, symmetrically placed around zero energy. These peaks disappear, together with the superconducting gap, with increasing temperature and magnetic field. We discuss the likelihood of various physical mechanisms that are known to cause in-gap features in superconductors and conclude that none of these mechanisms can easily explain the results. The conceivable scenarios are the formation of Majorana bound states, Andreev bound states, or the presence of an odd-frequency spin triplet component in the superconducting order parameter [1]. References : [1] L. Kuerten, et al., In-gap features in superconducting LaAlO3 - SrTiO3 - interfaces observed by tunneling spectroscopy, Phys. Rev. B 96, 014513 (2017).

Authors : S. Gariglio, D. Li, A. Fête,, M. Boselli, A. Waelchli, and J.-M. Triscone
Affiliations : DQMP, University of Geneva, 24 Quai E.Ansermet, CH-1211 Geneva, Switzerland

Resume : The two-dimensional electron liquid present at the LaAlO3/SrTiO3 interface exhibits superconductivity and hosts a large spin-orbit interaction. In field effect devices, both phenomena can be tuned by a gate voltage, revealing a dome-like phase diagram for the superconducting state with a critical temperature Tc reaching 300 mK. In a set of field-effect devices based on SrTiO3 interfaces having different amplitudes of polar discontinuity, we have mapped the evolution of the superconducting properties in magnetic fields applied parallel and perpendicular to the conducting plane, revealing that the estimation of the thickness of the superconducting layer changes across the phase diagram and scales with the strength of the polarization mismatch. Funding from Swiss NSF and ERC is acknowledged. References : [1] S. Gariglio, A. Fête and J.-M. Triscone, J. Phys. Condens Matter. 27, 283201 (2015). [2] S. Gariglio, M. Gabay, J. Mannhart and J.-M. Triscone, Physica C 514, 189 (2015). [3] S. Gariglio, M. Gabay and J.-M. Triscone, APL Materials 6, 060701 (2016). [4] D. Li et al., Advanced Science (2018).

Authors : Ritsuko Eguchi (1,2), Margherita Boselli (1), Adrien Waelchli (1), Gernot Scheerer (1), Danfeng Li (1,3), Stefano Gariglio (1), Jean-Marc Triscone (1)
Affiliations : (1) DQMP, Department of Physics, University of Geneva, Geneva 1211, Switzerland; (2) Research Institute for Interdisciplinary Science (RIIS), Okayama University, Okayama 700-8530, Japan; (3) GLAM, Stanford University, California 94305, USA

Resume : The superconducting state of carrier-doped SrTiO3 is somehow unique since it appears at the lowest carrier concentration among the superconducting materials and is close to a ferroelectric order. Recent work has investigated the influence of ferroelectric fluctuations on superconductivity [1-3]. An enhance-ment of the superconducting transition temperature (Tc) is theoretically predicted in the vicinity of the ferroelectric quantum critical point (QCP) [1]. Experimental studies on ferroelectric SrTi18O3-d [2] and Sr1-xCaxTiO3-d (0.002 < x < 0.02) [3] have indeed revealed that, for the same value of carrier concentration, Tc is higher for 18O and Ca-doped SrTiO3 samples compared with SrTiO3-d samples. In this study, we investigate superconductivity at LaAlO3 (10 u.c.)/Sr1-xCaxTiO3 interfaces (x = 0.0025, 0.005, 0.01, 0.05) to clarify how the 2D electron liquid at the interface is affected by the ferroelectric QCP. A clear metallic behavior is observed in LaAlO3/Sr1-xCaxTiO3 (x = 0.025, 0.005 and 0.01) heterostructures. On the other hand, LaAlO3/Sr1-xCaxTiO3 (x = 0.05) shows a metal-insulator transition at around 70 K. For LaAlO3/Sr1-xCaxTiO3 (x = 0.005 and 0.01) interfaces, we observe a superconducting transition, but not for LaAlO3/Sr1-xCaxTiO3 (x = 0.0025 and 0.05). We discuss the electrostatic modulation of superconductivity and magnetotransport properties in the LaAlO3/Sr1-xCaxTiO3 (x = 0.005 and 0.01) interfacial system. References : 1. J. M. Edge, Y. Kedem, U. Aschauer, N. A. Spaldin, and A. V. Balatsky, Phys. Rev. Lett., 115, 247002 (2015). 2. A. Stucky, G. W. Scheerer, Z. Ren, D. Jaccard, J.-M. Poumirol, C. Barreteau, E. Giannini and D. van der Marel, Sci. Rep. 6, 37582 (2016). 3. C. Rischau, X. Lin, Ch. P. Grams, D. Finck, S. Harms, J. Engelmayer, T. Lorenz, Y. Gallais, B. Fau-qué, J. Hemberger and K. Behnia, Nat. Phys. 13, 643?648 (2017).

Authors : Guru Prakash Neupane
Affiliations : College of Chemistry and Environmental Engineering, Shenzhen University, Shenzhen 518052, Guangdong, China Research School of Engineering, College of Engineering and Computer Science, the Australian National University, Canberra ACT 2601, Australia

Resume : Enormous research activities in graphene and hexagonal boron nitride (h-BN) directed the two-dimensional (2D) materials science research trends towards the discovery of new 2D materials. Many promising 2D materials such as transition metal dichalcogenides (TMDs), group IV monochalcogenides (MNs) and black phosphorous (BP) have been explored in this decade. These new 2D materials exhibits exciting electrical, optical, optoelectronics and mechanical properties. However, more research activities are mainly focused in TMDs materials especially in MoS2, MoSe2, WS2, WSe2, MoTe2, etc. Recent anisotropic optical properties in BP and GeS have increased the research attentions in many MNs such as SnS, SnSe, GeSe etc. too. [1,2] Since the discoveries of large family of 2D materials, opportunities to build up various 2D heterostructures, multi heterostructures or super lattices for future applications are available. Many TMDs materials are optically and electrically isotropic; and BP, MNs, ReS2, ReSe2 etc. are anisotropic in nature. Moreover, isotropy materials lead to formations of quasi 2D excitons and trions while the anisotropic 2D materials lead to the formation of quasi-one-dimensional (quasi-1D) excitons and trions in a 2D system. Hence, isotropic/anisotropic 2D heterostructures can provide unique particle interactions in between quasi-1D and 2D states. This type of system can help for the understanding of the fundamental scientific phenomenon in 2D limit. Moreover, in-plane isotropic/anisotropic 2D heterostructures may open the possibility of engineering smart devices in the nano-domain. Furthermore, this study will direct the research activities for finding of innovative applications in nano optoelectronics and various other fields such as energy storage, chemical/bio sensors, bio imaging, cancer therapy etc. This work will summaries the recent research progress in in-plane isotropic/anisotropic 2D semiconducting heterostructures, and twill open the potential applications in optoelectronics, thermoelectricity, piezoelectricity, photostriction etc. Keywords: 2D materials, isotropy/anisotropy, hetero structure, trions/excitons. References : 1. Xu, R. et al. Extra ordinarily bound quasi-one-dimensional trions in two-dimensional phosphorene atomic semiconductors. ACS Nano 10, 2046-2053 (2016). 2. Tan, D. et al. Anisotropic optical and electronic properties of two-dimensional layered germanium sulfide. Nano Res. 10, 546-555 (2017).

Session 7 : Jun Akimitsu
Authors : Arthur F. Hebard (1), Ang J. Li (1,2), Xiaochen Zhu (1), Haoming Jin (1)
Affiliations : (1) University of Florida, FL; (2) GlobalFoundries, NY

Resume : The crystalline layered high-Tc superconductor Bi-2212 can be easily cleaved into smoothly faceted flakes which, when placed into intimate physical contact with a variety of layered materials or bulk semiconductors, form heterogeneous junctions. Two such junctions are discussed in this talk: Bi-2212/1T-TaS2 where at low temperatures the 1T-TaS2 is a Mott insulator harboring charge density waves (CDWs) and Bi-2212/n-GaAs Schottky barrier junctions which manifest quantum mechanical tunneling at low bias voltages. The CDW order in the 1T-TaS2 appears to play an important role by coexisting with an unexpected and surprisingly high Tc of the induced proximity gap which, for junctions with high transparencies, is seen to have a surprisingly large value (∼ 20 meV) equal to half that of intrinsic Bi-2212 (∼ 40 meV). Proximity-induced high-Tc superconductivity in the 1T-TaS2 is driven by coupling to the metastable metallic phase coexisting within the Mott commensurate (CCDW) phase and associated with a concomitant change of the CCDW order parameter in the inter-facial region. For the Bi-2212/n-GaAs Schottky barrier junctions, modifications to the thermionic emission equation provide an excellent description of the I-V characteristics even at low temperatures where tunneling is found by differential conductance spectroscopy measurements to be important and capacitance measurements under reverse bias suggest an unexpectedly long electric field screening length in the superconductor.

Authors : Fabian O. von Rohr (1), Robert J. Cava (2), Liling Sun (3), Ronny Thomale (4)
Affiliations : (1) University of Zurich, Department of Chemistry, CH-857 Zurich, Switzerland; (2) Princeton University, Department of Chemistry, NJ-08540, USA; (3) Institute of Physics and Beijing National Laboratory for Condensed Matter Physics, Chinese Academy of Sciences, Beijing 100190, China; (4) Institut fuer Theoretische Physik und Astrophysik, Julius-Maximilians-Universitaet Wuerzburg, 97074 Wuerzburg, Germany

Resume : Layered black phosphorus (black-P) is a narrow-gap semiconductor, which crystallizes in the orthorhombic A17 structure at ambient pressure and temperature. The layers of black-P consist of six-rings in a so-called chair conformation, and they are weakly bound by van der Waals forces. Under pressure, early studies showed that black-P exhibits several phase transitions [1,2]. The structure changes from A17 (orthorhombic) to semimetallic A7 (rhombohedral) at 4.5 GPa, followed by a metallic primitive cubic phase at 10 GPa. Part of the difficulty of establishing the full electronic phase diagram of black-P has been the unavailability of large, high-quality single crystals. Recent advances in black-P sample preparations [3] and high-pressure techniques have enabled us to perform a series of very detailed experiments on the superconducting and normal state of black-P [4,5]. We have established the detailed phase diagram revealing an anomalous pressure dependence of the critical temperature and the importance of hole carriers in developing superconductivity. Our studies reveal that the intriguing Tc valley, as found in our experiment should be attributed to the Lifshitz transitions. We also find that the experimentally observed Tc plateau at high pressure cannot be fully understood with solely considering an electron-phonon mechanism. Our findings suggest that besides electron-phonon coupling, plasmonic contributions or electronic correlations may be relevant for systems with d-orbital character such as black-P. References : [1] J. C. Jamieson, Science 139, 1291 (1963). [2] H. Kawamura, I. Shirotani, K. Tachikawa, Solid State Commun. 54, 775 (1985). [3] T. Nilges, M. Kersting, T. Pfeifer, Journal of Solid State Chem. 181, 1707 (2008). [4] J. Guo, H. Wang, F.O. von Rohr, W. Yi, Y. Zhou, Z. Wang, S. Cai, S. Zhang, X. Li, Y. Li, J. Liu, K. Yang, A. Li, S. Jiang, Q. Wu, T. Xiang, R.J. Cava, L. Sun, Phys. Rev. B 96, 224513 (2017). [5] X. Wu, H.O. Jeschke, D. Di Sante, F.O. von Rohr, R.J. Cava, R. Thomale, Phys. Rev. Materials 2, 034802 (2018).

Authors : T. L. Yu (1,2,3+), Q. Song (1,2,3+), X. Lou (1,2,3), B. P. Xie (2), H. C. Xu (2), C. H. P. Wen (1,2,3), Q. Yao (1,2,3), S. Y. Zhang (4,5), X. T. Zhu (4,5), J. D. Guo (4,5,6), R. Peng (2,*), D. L. Feng (1,2,3,*)
Affiliations : (1) State Key Laboratory of Surface Physics and Department of Physics, Fudan University, Shanghai, 200433, China; (2) Laboratory of Advanced Materials, Fudan University, Shanghai 200438, China; (3) Collaborative Innovation Center of Advanced Microstructures, Nanjing, 210093, China; (4) Beijing National Laboratory for Condensed Matter Physics and Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China; (5) School of Physical Sciences, University of Chinese Academy of Sciences, Beijing 100049, China; (6) Collaborative Innovation Center of Quantum Matter, Beijing 100871, China; *Correspondence to:;; + These authors contributed equally

Resume : At the interface between monolayer FeSe films and SrTiO3 substrate the superconducting transition temperature (Tc) is unexpectedly high, triggering a surge of excitement. The mechanism for the Tc enhancement has been the central question, as it may present a new strategy for searching for higher Tc materials. To reveal this enigmatic mechanism, by combining advances in high quality interface growth, oxygen isotope substitution, and extensive data from angle resolved photoemission spectroscopy, we provide striking evidence that the high Tc in FeSe/SrTiO3 is the cooperative effect of the intrinsic pairing mechanism in the FeSe and interactions between the FeSe electron and SrTiO3 phonon. Furthermore, our results point to the fascinating prospect that similar cooperation between different Cooper pairing channels may be a general framework to understand and design high-temperature superconductors.

Authors : A. Kalaboukhov (1), P. P. Aurino (1), L. Galletti (1), T. Bauch (1), F. Lombardi (1), D. Golubev (2), D. Winkler (1), and T. Claeson (1)
Affiliations : (1) Department of Microtechnology and Nanoscience (MC2), Chalmers University of Technology, 412 96 Göteborg, Sweden; (2) Low Temperature Laboratory, Department of Applied Physics, Aalto University, FI-00076 Aalto, Finland

Resume : An interface between wide band-gap insulators, LaAlO3 and SrTiO3 (LAO/STO) possesses exciting properties such as a quasi-two-dimensional electron gas, two-dimensional superconductivity, ferromagnetic behavior and giant spin-orbit coupling. We have systematically investigated superconductivity in the LAO/STO nanostructures with dimensions of 100 ? 300 nm [1]. The nanostructures were fabricated using our patterning method based on low-energy Ar ion beam irradiation [2]. Analysis of current-voltage characteristics suggests that our nanostructures behave like clean superconducting filaments without formation of weak links. Moreover, we observed a SQUID-like periodic modulation of the critical current in nano-rings corresponding to the Little-Parks fluxoid quantization. A most remarkable observation is an enhancement of the critical current by a small perpendicular magnetic field. This effect may be explained by the suppression of spin flip scattering on magnetic domains by external magnetic field or by induced unconventional pairing. The fact that the dip in critical current at zero magnetic field is similar in structures with different geometries suggests that it is of local origin. Our results point towards the coexistence of a homogeneous interfacial superconductivity and spatially separated magnetism at the LAO/STO interface. References : [1] A. Kalaboukhov et al., Phys. Rev. B 96, 184525 (2017). [2] P.P. Aurino et al., Appl. Phys. Lett. 102, 201610 (2013).

Session 8 : Sergey Medvedev
Authors : Dirk Johrendt
Affiliations : Department Chemie Ludwig-Maximilians-Universität München Butenandtstr. 9-13 81377 München Germany

Resume : The intercalation chemistry of iron chalcogenide superconductors has developed well in the last years. Motivated by the very high critical temperatures observed with FeSe monolayers [1-3] deposited on SrTiO3 substrates, it appeared conceivable that intercalated FeSe layers may mimic this scenario. Indeed, several groups have reported superconductivity up to 45 K through intercalation of molecular species (mostly amines) in FeSe and additional electron doping [4-8]. Unfortunately, the sample qualities of these materials are often rather poor and hamper the precise structural characterization. One reason is the still insufficient knowledge about and control of phase formation under hydro- or solvothermal conditions. Even the seemingly simple hydrothermal synthesis of FeSe leads to perfectly stoichiometric, but oddly enough, non-superconducting samples [9]. On the other hand, the solvothermal synthesis in ethylenediamine/glycerol yields superconducting FeSe, where the typical tetragonal-to-orthorhombic (nematic) phase transition no longer occurs. More reliable is the hydrothermal synthesis of LiOH-intercalated FeSe and FeS. We found that substitution of selenium by sulfur in [(Li,Fe)OH]Fe(Se1-xSx) gradually suppresses superconductivity, while the ferromagnetic properties of the [(Li,Fe)OH] layer persist [10]. Substitution of iron by cobalt likewise suppresses superconductivity of [(Li,Fe)OH]FeSe, whereby cobalt replaces iron in the FeSe and (Li,Fe)OH layers. stoichiometric [LiOH]FeSe has been shown to be non-superconducting [11], while this is still unclear for FeSe intercalated with neutral spacers like ethylenediamine (en). We have synthesized non-superconducting Fe1-xSe(en)0.3 [12], while a recent report by Gao et al. shows superconductivity at 10 K in this undoped compound [13]. This is unexpected given the fact that superconductivity was mostly observed by electron doping in FeSe intercalates as well as in FeSe@STO. References : [1] Q.-Y. Wang et al., Chin. Phys. Lett., 2012, 29, 037402. [2] S. He et al., Nat Mater, 2013, 12, 605-610. [3] J.-F. Ge et al., Nat Mater, 2015, 14, 285-289. [4] M. Burrard-Lucas et al., Nat. Mater., 2013, 12, 15-19. [5] T. Hatakeda et al., J. Phys. Soc. Jpn., 2013, 82, 123705. [6] T. Noji et al, Physica C, 2014, 504, 8-11. [7] T. Hatakeda et al., Journal of Physics: Conference Series, 2014, 568, 022032. [8] S. J. Sedlmaier et al., J. Am. Chem. Soc., 2013, 136, 630-633. [9] U. Pachmayr et al., Chem. Commun., 2016, 52, 194-197 [10] U. Pachmayr et al., Chem. Commun., 2015, 51, 4689-4692. [11] H. Sun et al., Inorg. Chem. 2015, 54, 1958-1964. [12] J. Stahl et al., Dalton Trans. 2018, 47, 3264-3271. [13] Z. Gao et al., Sci. Chin. Mater. 2018,

Authors : Yu Saito
Affiliations : Department of Applied Physics, The University of Tokyo RIKEN AIP

Resume : Recent advances have developed methods to produce ideal two-dimensional (2D) electron systems, which are highly-crystalline with the minimal disorder [1]. Here, we introduce the recent developments of highly-crystalline 2D superconductors and a series of unprecedented physical properties discovered originating from inversion symmetry in these systems. First of all, we highlight the quantum phases, i.e., quantum metallic state [2] and the quantum Griffiths phase [3] in out-of-plane magnetic fields. In addition, we focus two novel phenomena owing to broken inversion symmetry originating from crystal structure in ion-gated MoS2: one is the experimental observation of enhanced in-plane upper critical field up to 52 Tesla by spin-valley locking (Ising superconductivity) [4] and the other is the nonreciprocal superconducting transport, the latter of which is later expected to be universal phenomena in noncentrosymmetric superconductors [5]. This nonreciprocal transport can be regarded as the intrinsic ratchet effect originating from the noncentrosymmetric structure. These series of unprecedented phenomena suggest that highly-crystalline 2D superconductors evidently offer tremendous opportunities to unveil the intrinsic exotic nature of superconductors, leading to a new era of 2D superconductivity. References : [1] Y. Saito, T. Nojima and Y. Iwasa Nature Reviews Materials 2, 16094 (2016). [2] Y. Saito et al. Science 350, 409 (2015). [3] Y. Saito et al. Nature Communications 9, 778 (2018). [4] Y Saito et al. Nature Physics 12, 144 (2016). [5] R. Wakatsuki and Y. Saito et al. Science Advances 3, e1602390 (2017).

Authors : B. C. Camargo, P. Gierlowski
Affiliations : Institute of Physics, Polish Academy of Sciences

Resume : When juxtaposed to superconductors, normal materials are expected to develop superconducting behavior in a region close to the interface. This effect usually survives in mesoscopic scales, hardly surpassing tenths of micrometers in reach. Here, we probe the electrical transport properties of macroscopic (millimeter-size) graphite, bismuth and antimony samples with current electrodes made out of superconducting alloys. Our results reveal the induction of a partial superconducting-like behavior in the bulk of our devices, occurring concomitantly to superconductivity in the current injection leads. Our results suggest the triggering of a network of superconducting grains in the materials studied, which persist in scales of the order of hundredths of micrometers.

Authors : Rinat F. Mamin
Affiliations : Zavoisky Physical-Technical Institute of FIC KazanSC RAS, Sibirskii tract 10/7, Kazan 420029, Russia

Resume : A high-mobility electron gas was observed in 2004 [1] at the interface of heterostructure LaAlO3 (LAO) and SrTiO3 (STO). Such heterointerfaces involving two insulating nonmagnetic oxides were comprehensively studied. In particular, it was found that the metallic phase (quasi-two-dimensional electron gas, 2DEG) is formed in the STO layers at the LAO/STO interface when the number of LAO layers is larger than three [2]. Such a system undergoes a transition to a superconducting state at temperatures below 300 mK [3]. We report the tailoring quasi-two-dimensional electron gas (q2DEG) state as well as a high-Tc quasi-two-dimensional superconductivity (HTq2DSC) in heterostructure Ba0.8Sr0.2TiO3/La2CuO4 consisting of an insulating ferroelectric film deposited by magnetron sputtering on a non-atomically-flat surface of an insulator single crystal. First, the ab-initio calculation of the structural and electronic properties of the BaTiO3/La2CuO4 heterostructure have been performed. It is shown that already with one BTO layer the band gap of the heterostructure is closed. Second, the temperature dependence of the electrical resistance for heterostructure formed by antiferromagnetic La2CuO4 (LCO) single crystals with epitaxial films of ferroelectric Ba0.8Sr0.2TiO3 (BSTO) deposited onto them has been studied. The measured electrical resistance is compared to that exhibited by LCO single crystals without the films. The interface of that heterostructure shows superconducting behavior with transition temperature Tc is about 30 K, which 100 time larger than Tc in LaAlO3/SrTiO3.The beginning of a transition to superconducting state occurs around 40 K, similar to what is observed in bulk La2-xSrxCuO4 single crystals at optimal doping. Important that, when we applied electrodes for resistance measurements on the back side of the heterostructure (from single crystal side, without contact with interface), superconducting state is not observed. Therefore, we conclude that this is not a case of surface superconductivity, and that the oxygen does not penetrate the surface layer during the sputtering of the film. Our results open a new page in creating interfaces with HTq2DSC, since it has been shown experimentally that using of a ferroelectric material in the heterostructures with parent compound of high temperature superconductor can leads to q2DEG and as well as HTq2DSC state. The heterostructure in that case is created by a relatively simple method at the non-atomically-flat boundary of the two insulating oxides with different elementary cell structures. It opens up new physics, when a polarization catastrophe is associated not with the polar oxides, but with the ferroelectric oxides. If a weak magnetic field is applied perpendicularly to the interface of the heterostructure, a resistance appears. This confirms a quasi-two-dimensional nature of the superconductive state. This highly robust phenomenon is confined within the interface area. The proposed concept promises ferroelectrically controlled interface superconductivity which offers the possibility of novel design of electronic devices. The reported study was funded by Russian Scientific Foundation, research project No. 18-12-00260. References : [1] A. Ohtomo and H. Y. Hwang, Nature 427, 423 (2004); [2] S. Thiel,, Science 313, 1942 (2006). [3] N. Reyren, S., Science 317, 1196 (2007).

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Session 9 : Arthur F. Hebard
Authors : Antonio Bianconi
Affiliations : Rome International Center for Materials Science, Superstripes Via dei Sabelli 119A, 00185 Roma, Italy

Resume : The material design to get high temperature superconductors in anisotropic quantum matter tuned at a topological Lifshitz transition in multi-valley metals was proposed on Dec 7, 1993 [1] . It was based on the coexistence of strong coupling spots in the k-space and free carriers. The chemical potential is tuned to a Lifshitz transition by control of anisotropic strain and electronic density. The predictions have been confirmed in cuprates [3-6], diborides [7], iron based superconductors [8-10], pressurized H3S [11-14]. A major step in the field has been the development of the theory of superconductivity of with a steep and a flat band by numerical solution of Bogolibov de Gennes equations in the BCS and in the BCS-BEC crossover near Lifshitz transitions [6,15]. All these materials are described by multi-band-Hubbard models with a complex phase separation occurring at the Lifshitz transitions [6,15] observed by using scanning micro XANES [2] and scanning micro x-ray diffraction [3]. Here we discuss the emerging role of Fano resonances in sulfur hydrides, in organics and related matter where high energy intrinsic localized modes and the interplay between the flat band at the topological Lifshitz transition and a steep band gives high temperature superconductivity [16-17]. We show that the physics of systems showing high temperature superconductivity coexists with fluctuations of short range Periodic Spin Charge Lattice Orbital modulations driven by the proximity to Lifshitz transitions controlled by strain, charge density or pressure. References : [1] A. Bianconi, M. Missori Sol. State Commun. 91, 287 (1994) doi:10.1016/0038-1098(94)90304-2. [2] A. Bianconi, US Patent 6,265,019 (2001). [3] G. Campi et al., Nature 525, 359 (2015). doi:10.1038/nature14987. [4] A. Shengelaya and K. A. Müller, EPL (Europhysics Letters) 109, 27001 (2015). [5] A. Bianconi, Solid State Communications 91, 1 (1994). [6] A. Perali, et al. Supercond. Sci. Technol. 25, 124002 (2012). [7] A. Bianconi, Journal of Superconductivity 18, 625 (2005) doi:10.1007/s10948-005-0047-5. [8] M. Fratini, et al., Supercond. Sci. Technol 21, 092002 (2008). [9] R. Caivano, et al., Sup. Sci. and Tech. 22, 014004 (2009).  doi:10.1088/0953-2048/22/1/014004. [10] A. Bianconi, Nature Physics 9, 536-537 (2013). doi:10.1038/nphys2738. [11] A. Bianconi, T. Jarlborg, EPL (Europhysics Letters) 112, 37001 (2015). doi:10.1209/0295-5075/112/37001. [12] A. Bianconi, T. Jarlborg, Nov. Supercond. Mater. 1, 37 (2015) doi:10.1515/nsm-2015-0006. [13] T. Jarlborg, A. Bianconi, Scientific Reports 6, 24816 (2016). doi:10.1038/srep24816. [14] M.V. Mazziotti, et al. EPL (Europhysics Letters) 118, 37003 (2017). doi:10.1209/0295-5075/118/37003. [15] G. Campi, A. Bianconi, Journal of Superconductivity and Novel Magnetism 29, 627 (2016). doi:10.1007/s10948-015-3326-9. [16] Bussmann-Holder, A., Köhler, J., Simon, A., Whangbo, M.-H., Bianconi, A., and Perali, A. Bussmann-Holder, A. et al.. Condensed Matter 2, 24 (2017). doi:10.3390/condmat2030024. [17] Bussmann-Holder, A., Köhler, J., Simon, A., Whangbo, M. & Bianconi, Journal of Superconductivity and Novel Magnetism 30, 151 (2017). doi:10.1007/s10948-016-3947-7.

Authors : Annette Bussmann-Holder (1), Jürgen Köhler (1), Arndt Simon (1), Andrea Perali (2), Antonio Bianconi (2,3), and Myung -Hwan Whangbo (4)
Affiliations : (1) Max-Planck-Institute for Solid State Research, Heisenbergstr. 1, D-70569 Stuttgart, Germany; (2) Rome International Centre for Material Science Superstripes, RICMASS, via dei Sabelli 119A, 00185 Rome, Italy; (3) Institute of Crystallography, CNR, via Salaria Km 29.300, Monterotondo Roma, I-00015, Italy; (4) Department of Chemistry, North Carolina State University, Raleigh, NC 27695-8204, USA;

Resume : The present work addresses the investigation of the optimal conditions for amplification of the superconducting critical temperature Tc in a system of multiple electronic components. We use mean field theory for multiband superconductivity focusing on the case where the first electronic component is assumed to have a vanishing Fermi velocity corresponding to a case of the intermediate polaronic regime, and the second electronic component is in the weak coupling regime with standard high Fermi velocity in agreement with the experimental results in cuprates, diborides and iron based superconductors. By keeping a constant and small exchange interaction between the two electron fluids, we search for the optimum coupling strength in the electronic polaronic component which gives the largest amplification of the superconducting critical temperature in comparison with the case of a single electronic component. References : [1] Annette Bussmann-Holder, Jürgen Köhler, Arndt Simon, Myung-Hwan Whangbo, Antonio Bianconi and Andrea Perali, Condens. Matter 2017, 2(3), 24.

Authors : Joel Hutchinson, Jorge Hirsch, Frank Marsiglio
Affiliations : University of Alberta, University of California San Diego

Resume : In two-dimensional crystals, crystalline inversion asymmetry or Fermi surface instabilities can lead to the electronic spin degeneracy being lifted by Rashba spin-orbit coupling. This has unique implications for superconductivity, where spin splitting ensures that Cooper pairs form in mixed singlet-triplet states. The result is a gap function with unconventional symmetry, and an atypical doping dependence arising from the split single-particle spectrum. In this talk we explore these consequences in a two-dimensional tight-binding model on a square lattice with Rashba spin-orbit coupling as well as a correlated hopping term arising from an off-diagonal part of the electron-electron interaction. It is known that the Rashba coupling typically decreases the critical temperature in the presence of attractive interactions. Here we show that with correlated hopping, Rashba coupling actually enhances the critical temperature as well as the tunnelling asymmetry. This may explain some recent observations in the LaAlO3/SrTiO3 interface where Rashba physics is important.

Session 10 : Annette Bussmann-Holder
Authors : Janez Bonca, Marcin Mierzejewski
Affiliations : Faculty of Mathematics and Physics, University of Ljubljana, SI-1000 Ljubljana, Slovenia and J. Stefan Institute, SI-1000 Ljubljana, Slovenia; Department of Theoretical Physics, Faculty of Fundamental Problems of Technology, Wroc?aw University of Science and Technology, 50-370 Wroc?aw, Poland

Resume : We address a long standing problem concerning the origin of bosonic excitations that strongly interact with charge carriers. We show that the time - resolved pump - probe experiments are capable to distinguish between regular bosonic degrees of freedom, e.g. phonons, and the hard-core bosons, e.g., magnons. The ability of phonon degrees of freedom to absorb essentially unlimited amount of energy renders relaxation dynamics nearly independent on the absorbed energy or the fluence. In contrast, the hard core effects pose limits on the density of energy stored in the bosonic subsystems resulting in a substantial dependence of the relaxation time on the fluence and/or excitation energy. Very similar effects can be observed also in a different setup when the system is driven by multiple pulses. J. Kogoj, M. Mierzejewski and J. Bon?a, Phys. Rev. Lett., 117, 227002 (2016).

Authors : Milorad V. Milosevic
Affiliations : Department of Physics, University of Antwerp, Belgium

Resume : The effects specific to multi-gap superconductors include novel vertical and skyrmionic states, giant-paramagnetic response, hidden criticality, and time-reversal symmetry breaking. In this talk, I will discuss the evolution of multigap superconductivity in MgB2 at its ultrathin limit. Atomically thin MgB2 is distinctly different from bulk MgB2, in that both free Mg- and B-surfaces contribute additional surface electronic state, comparable in electronic density to the bulk-like σ- and π-bands. Using the ab initio electron-phonon coupling and anisotropic Eliashberg equations, we show that these surface states strongly recast superconductivity in MgB2 at few-monolayer thickness, providing the gap that hybridizes with σ-gap for 4+ monolayer films, but shifts towards the π-gap in thinner structures. As a consequence, the observable superconducting properties and vortex matter radically change with every added monolayer in the thinnest limit. Furthermore, a single monolayer of MgB2 develops three distinct superconducting gaps, on completely separate parts of the Fermi surface. We show that this three-gap superconductivity is robust over the entire temperature range that stretches up to considerably high critical temperature of 20 K. We also reveal that Tc can be boosted to > 55 K under tensile strain of ~ 4%. Finally, I will discuss how hydrogenation hybridizes with all prominent coupling channels in monolayer MgB2, and enhances superconductivity to even higher temperatures!

Authors : Masaki Fujita, Kentaro Sato
Affiliations : Institute for Materials Research, Tohoku University

Resume : Origin of characteristic spin excitations in doped Mott insulator is a fascinating issue, since it is relevant to the mechanism of high-Tc superconductivity. To gain a deep insight into the nature of spin excitations, we studied the doping and the thermal evolutions of spin excitations in La2-xSrxCuO4 by neutron and resonant inelastic X-ray scattering measurements. From the systematic study, we found the evidence of two energy scales in the hour-glass shaped spin excitations, which is separated by the waist energy. The feature will be discussed in connection with the two spin degrees of freedom, that is, the spins of itinerant and localized electrons.

Authors : Oleksandr Zheliuk (1), Jianming Lu (1,2), Qihong Chen (1), Jie Yang (1)' Ali A. El Yumin (1) Jianting Ye (1*)
Affiliations : (1) Device Physics of Complex Materials, Zernike Institute for Advanced Materials, Nijenborgh 4, 9747 AG, Groningen, The Netherlands; (2) State key laboratory for mesoscopic physics, School of Physics, Peking University, 209 Chengfu Road, Beijing 100871, P. R. China

Resume : Ising-type superconductivity in monolayer TMD?s becomes a platform of intensive studies experimentally and theoretically. The strong Zeeman type spin-orbit coupling (SOC) aligns the spins of electrons in Cooper pairs along the out-of-plane direction, making a superconducting state robust against external magnetic field applied parallel to the crystal plane. As the SOC originates from broken inversion symmetry, restoring the symmetry is expected to suppress the Ising protection. Here, we use bilayer MoS2 - crystal which is globally centrosymmetric but possesses broken local inversion symmetry. By implementing it into electric double layer transistor (EDLT), we are able to induce superconductivity in both layers and gradually tune a coupling strength ? a parameter that can reveal hidden degrees of freedom.

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Session 11 : Alessandro Ricci
Authors : C. Q. Jin
Affiliations : Institute of Physics, Chinese Academy of Sciences; School of Physics, University of Chinese Academy of Science, Beijing 100190

Resume : High Tc cuprates are especially attractive since they exhibit so far the record high transition temperatures, i.e. up to 130 K at ambient conditions. An interesting question is whether there is space for cuprates to further increase Tc. Here we will present our work on pressure tuned cuprates with variants modulation structures related to oxygen ordering in system with simple composition consisting of copper alkaline earth oxides wherein Tc enhancements are promising upon improvement of dopant ordering.

Authors : A. Shengelaya (1), T. Gagnidze (2), F. La Mattina (2), G.-L. Bona (2), K. A. Müller (3)
Affiliations : (1) Department of Physics, Tbilisi State University, Chavchavadze 3, 0128 Tbilisi, Georgia; (2) Empa, Ueberland Str. 129, 8600 Dübendorf, Switzerland; (3) IBM Research Division, Zürich Research Laboratory, Rüschlikon 8803, Switzerland

Resume : With advances in thin-film growth, atomic-level control of interfaces has reached an unprecedented level of sophistication. At the ultrathin range, interface phenomena can induce unusual changes of some electronic properties in a way that is impossible in the bulk materials. One of the possibilities is the modification of Coulomb interaction between charge carriers by changing effective dielectric constant of the material. Such an effect which was called the Coulomb interaction engineering was demonstrated in a thin semiconducting layers sandwiched by insulators with a different dielectric constant. It would be interesting to check if similar effect can exist in thin films of functional oxide materials. Following this idea we recently proposed to create a multilayer structure in which an underdoped copper-oxide high-temperature superconductor is sandwiched between high-dielectric-constant insulator layers such as ferro- or ferri-electrics, thereby reducing the Coulomb repulsion between the intrinsically present clusters or stripes in the CuO2 layers of the pseudogap phase. This might lead to an increase in the size of such clusters, resulting in smaller distances between them and coherence at higher temperature, i.e., a higher Tc. In order to check the possibility of Coulomb interaction engineering, epitaxial heterostructures of cuprate superconductors and high dielectric constant insulator layers were grown by means of pulsed laser deposition technique. In-situ Reflection High-Energy Electron Diffraction (RHEED) system was used to verify the layer-by-layer growing condition. High quality fully epitaxial heterosturctures were produced and characterized by various experimental techniques. Obtained results will be presented and remaining challenges will be discussed.

Authors : Anna Krzton-Maziopa
Affiliations : Faculty of Chemistry, Warsaw University of Technology, Noakowskiego 3, PL-00-664 Warsaw, Poland

Resume : Molecularly intercalated layered iron monochalcogenides, {M}FeSe1-xChx ({M} = organic or metal-organic donor, Ch = S, Te) are currently the subject of intensive research due to their fascinating chemistry and tunable electric and magnetic properties related both to the possible doping of host material and the specific nature of guests residing in the structure. The rich chemistry and physics of these novel iron-based systems affect the intricate relations between crystal structure, superconductivity and magnetic properties. Controlling of the mutual interactions could be achieved by subtle adjustment of the crystal structure through substitutions either on iron or selenium sites and most favourably by intercalation of electron donating guests between the FeCh sheets, which in the parent inorganic host, FeSe1-xChx , are held together by weak van der Waals forces. In recent years, significant progress has been made in the field of molecular intercalates of layered iron selenides, revealing superconducting inorganic-organic hybrid materials with critical temperatures, Tc, up to 46 K. Many of these novel materials have been synthesized by low-temperature intercalation of molecular species, such as adducts/solvates of alkali metals and nitrogen-containing donor molecules into layered structure of inorganic hosts. It has been shown that both the chemical nature as well as orientation of organic molecules in the van der Waals gaps of the inorganic host are crucial for structural modifications and may be used for alteration of superconducting properties of these hybrid organic-inorganic superconductors. In this talk, the most important aspects of intercalation chemistry leading to the formation of novel molecularly intercalated superconducting phases with respect to the effect of the chemical nature of intercalating guest molecules on the crystal structure and physical properties will be discussed. This work was partly supported by the Department of the Navy, Office of Naval Research Global under ONR GRANT CONTRACT NO. N62909-17-1-2126

Session 12 : Janez Bonca
Authors : Vadim Ksenofontov
Affiliations : Institut für Anorganische und Analytische Chemie, Johannes Gutenberg-Universität, Mainz, Germany

Resume : The pairing mechanism in novel Fe-based superconductors is still an opened question. Conventional and pressure Mössbauer studies of simplest FeSe superconductor and its derivatives are very important for the understanding of the mechanisms of superconductivity in these systems. Application of nuclear inelastic scattering of synchrotron radiation to study of the local phonon DOS in FeSe-based SC as function of temperature and pressure could prove that electron-phonon interactions could not be the main mechanism for superconductivity in these systems. Mössbauer experiments appeared very informative in study of influence of static non-compensated magnetic moments on superconductivity in iron-based superconductors. Doping of small amounts of Cu into the FeSe suppresses superconductivity and introduces local static moments at the Fe sites. High-pressure studies using the Synchrotron Mössbauer Source (SMS) revealed that restoration of superconductivity under pressure in Cu-doped FeSe occurs because of the suppression of the static spin-glass state. We conclude that only nano-scale phase separation of insulating antiferromagnetic and metallic non-magnetic FeSe-similar domains provides conditions for coexistence of static magnetism and superconductivity. Many experimental facts provide evidence that antiferromagnetic spin fluctuations can mediate superconductivity acting as ?glue? for Cooper pairs in Fe-based superconductors. Our Mössbauer studies of FeSe intercalated with Li/NH3 spacer layers with a superconducting transition temperature of TC = 43 K support this idea. They demonstrate that simultaneously with superconducting transition in 57Fe Mössbauer spectra appears a magnetic subspectrum of dynamic nature and its intensity scales with a transition curve when passing to the superconducting state. Pressure measurements using the 57Fe-SMS confirmed important role of the antiferromagnetic spin fluctuations in pairing mechanism in FeSe-based superconductors.

Authors : Z. Guguchia, F. v. Rohr, A. Kerelsky, D. Edelberg, S. Banerjee, D. A. Rhodes, B. A. Frandsen, D. Scullion, M. Augustin, M. Scully, Z. Shermadini, H. Luetkens, A. Shengelaya, C. Baines, E. Morenzoni, A. Amato, J. C. Hone, R. Khasanov, S. J. L. Billinge, E. Santos, R.J. Cava, A. N. Pasupathy, Y. J. Uemura
Affiliations : (1) Department of Physics, Columbia University, New York, NY 10027, USA; (2) Department of Chemistry, Princeton University, Princeton, New Jersey 08544, USA; (3) Laboratory for Muon Spin Spectroscopy, Paul Scherrer Institute, CH-5232 Villigen PSI, Switzerland; (4) Department of Applied Physics and Applied Mathematics, Columbia University, New York, NY 10027, USA; (5) Department of Mechanical Engineering, Columbia University, New York, NY 10027, USA; (6) Department of Physics, University of California, Berkeley, California 94720, USA; (7) School of Mathematics and Physics, Queen?s University Belfast, UK; (8) Department of Physics, Tbilisi State University, Chavchavadze 3, GE-0128 Tbilisi, Georgia; (9) Andronikashvili Institute of Physics of I. Javakhishvili Tbilisi State University, Tamarashvili str. 6, 0177 Tbilisi, Georgia; (10) Condensed Matter Physics and Materials Science Department, Brookhaven National Laboratory, Upton, NY 11973, USA

Resume : Transition metal dichalcogenides (TMDs), a family of 2D layered materials like graphene, have been subject to tremendous experimental and theoretical studies not only due to their exciting physical properties but also as systems that may solve critical technological problems. I will describe a few TMD systems with novel electronic properties, where the application of hydrostatic pressure lead to large and unexpected effects. These include the Weyl semimetal Td-MoTe2 and metallic NbSe2 (where a strong pressure effect on superfluid density, its linear scaling with Tc in both, as well as a possible sign changing s+- gap in MoTe2 were observed), and semiconducting 2H-MoTe2 (where we provide the first evidence for involvement of magnetic ordering in the physics of TMDs). I will discuss these results from an experimental perspective using a combination of muon-spin rotation, X-ray/Neutron powder diffraction, and atomic-resolution scanning tunneling microscopy techniques. References : [1] Z. Guguchia et. al., Nature Communications 8, 1082 (2017). [2] Z. Guguchia et. al., arXiv:1711.05392v2 (2017).

Authors : Rustem Khasanov (1), William R. Meier (2.3), Yun Wu (2,3), Daixiang Mou (2,3), Sergey L. Bud'ko (2,3), Ilya Eremin (4), Hubertus Luetkens (1), Adam Kaminski (2,3), Paul C. Canfield (2,3), and Alex Amato (1)
Affiliations : (1) Laboratory for Muon Spin Spectroscopy, Paul Scherrer Institut, CH-5232 Villigen PSI, Switzerland; (2) Division of Materials Science and Engineering, Ames Laboratory, Ames, Iowa 50011, USA; (3) Department of Physics and Astronomy, Iowa State University, Ames, Iowa 50011, USA; (4) Institut fur Theoretische Physik III, Ruhr-Universitat Bochum, 44801 Bochum, Germany

Resume : The temperature dependence of the in-plane magnetic penetration depth (\lambda_ab) in an extensively characterized sample of superconducting CaKFe_4As_4 (T_c = 35K) was investigated using muon-spin rotation (\muSR). A comparison of \lambda_ab^-2(T) measured by \muSR with the one inferred from ARPES data confirms the presence of multiple gaps at the Fermi level. An agreement between \muSR and ARPES requires the presence of additional bands, which are not resolved by ARPES experiments. These bands are characterised by small supercondcting gaps with an average zero-temperature value of \Delta_0 =2.4(2)meV. Our data suggest that in CaKFe_4As_4 the s+- order parameter symmetry acquires a more sophisticated form by allowing a sign change not only between electron and hole pockets, but also within pockets of similar type.

Session 13 : Changqing Jin
Authors : Harald O. Jeschke
Affiliations : Research Institute for Interdisciplinary Science, Okayama University, Okayama 700-8530, Japan

Resume : Unconventional superconductivity in iron pnictides and chalcogenides is widely believed to be controlled by the interplay of low-energy antiferromagnetic spin fluctuations and the particular topology of the Fermi surface in these materials. Based on these ingredients one would also expect the large class of isostructural and isoelectronic iron germanide compounds to be good superconductors. In reality however, they superconduct at very low temperatures or not at all. We establish that superconductivity in iron germanides is suppressed by strong ferromagnetic tendencies [1,2], which surprisingly do not occur due to changes in bond-angles or -distances compared to iron pnictides and chalcogenides, but due to changes in the high-energy electronic structure upon exchange of elements. Our results indicate that superconductivity in iron-based materials can not be fully understood based on low-energy model Hamiltonians. This work was done in collaboration with D. Guterding, I. I. Mazin, J. K. Glasbrenner, E. Bascones, R. Valenti. References : [1] H. O. Jeschke, I. I. Mazin, R. Valenti, Phys. Rev. B 87, 241105(R) (2013). [2] D. Guterding, H. O. Jeschke, I. I. Mazin, J. K. Glasbrenner, E. Bascones, R. Valenti, Phys. Rev. Lett. 118, 017204 (2017).

Authors : E. Piatti (1), D. De Fazio (2), D. Daghero (1), S. R. Tamalampudi (2), D. Yoon (2), Q. Chen (3), J. T. Ye (3), A. C. Ferrari (2), R. S. Gonnelli (1)
Affiliations : (1) Department of Applied Science and Technology, Politecnico di Torino, 10129 Torino, Italy; (2) Cambridge Graphene Center, University of Cambridge, CB3 OFA Cambridge, UK; (3) Device Physics of Complex Materials, University of Groningen, 9747 AG Groningen, The Netherlands

Resume : Most of the results of electric-field-induced superconductivity in MoS2 have so far been interpreted in terms of population of the conduction band minima at the K point of the First Brillouin Zone (FBZ). By performing low-temperature electric transport measurements in ion-gated few-layer MoS2 devices we observed two maxima in the residual square resistance at the increase of charge doping. According to recent theoretical calculations they correspond to the Fermi level crossing of the high-energy, spin-orbit-coupling split minima (Q1 and Q2) at the Q point in the FBZ. Superconductivity appears only in proximity of the crossing of the high-energy sub-band at Q2 and maximum Tc is obtained only when this sub-band is highly populated. These results highlight for the first time the essential role of the Lifshitz transition associated to the crossing of the sub-band at Q2 in the induction and optimization of superconductivity in MoS2. In a second experiment we overcame the limits in the maximum surface carrier density achievable in the electrostatic regime by exploiting a strong electric field to drive ions in between the van-der-Waals-bonded layers. After optimization of this intercalation process we observed pronounced ?hump? structures around 200 K in the T dependence of the resistivity of ultrahighly-doped LixMoS2 thin flakes. We interpret these anomalies as a sign of the appearance of a charge-density-wave phase that was recently theoretically predicted in highly-doped MoS2.

Authors : Zbigniew Bukowski
Affiliations : Institute of Low Temperature and Structure Research, Polish Academy of Sciences, 50-422 Wroclaw, Poland

Resume : The coexistence of superconductivity and ferromagnetism in doped EuFe2As2 is not only a fascinating fundamental state of matter but also might be potentially interesting for spintronics and high magnetic field applications. In a brief overview I shall summarize the results of experimental investigations of the effects of applied magnetic field, hydrostatic pressure and chemical substitution on the crystal structure, magnetic ordering and superconductivity of EuFe2As2. I will present recent results of our investigations of peculiar transport and magnetic properties of EuFe2-xCoxAs2 single crystals as well as neutron diffraction studies of the magnetic structure evolution with Co doping or under influence of high pressure. EuFe2-xNixAs2 is exceptionally intriguing because in this system Ni doping although suppresses the SDW order, it does not lead to the appearance of superconductivity. Finding the reason of the absence of superconductivity in EuFe2-xNixAs2 could make a significant contribution to a better understanding of the superconductivity mechanism in the iron pnictides. I present the magnetic phase diagram as a function of Ni doping derived from magnetization and resistivity data obtained on single-crystalline material. The search for superconductivity in this system was carried out down to 0.55 K and also under high pressure up to 2.5 GPa. The ab initio calculations of the electronic structure properties were performed using density functional theory.

Authors : J. Pietosa (1), D. J. Gawryluk (1,2), R. Puzniak (1), A. Wisniewski (1)
Affiliations : (1) Institute of Physics, Polish Academy of Sciences, Aleja Lotników 32/46, PL 02668 Warsaw, Poland; (2) Laboratory for Multiscale Materials Experiments, Paul Scherrer Institute, CH 5232 Villigen, PSI, Switzerland

Resume : Magnetic measurements at ambient and at hydrostatic pressure were performed for single crystals of Fe0.99Te0.66Se0.34 and Fe0.994Ni0.007Te0.66Se0.34 in order to study influence of pressure on the basic parameters describing the superconducting state. An enhancement of all the investigated parameters for both Fe0.99Te0.66Se0.34 and Fe0.994Ni0.007Te0.66Se0.34 under hydrostatic pressure was noticed. The critical current density (jc) at ambient pressure is much smaller for Fe0.994Ni0.007Te0.66Se0.34 than that for both Fe0.99Te0.66Se0.34 and FeTe0.5Se0.5, but under hydrostatic pressure the Ni-substituted sample exhibits bigger improvement of jc than that for both undoped Fe Te-Se samples. Superconducting state properties of Fe0.99Te0.66Se0.34 single crystal, such as lower and upper critical fields, were found to be poorer at both ambient and hydrostatic pressure than those observed for multiphase FeTe0.5Se0.5 sample. However, the recovery of superconducting state properties was not observed in Fe0.994Ni0.007Te0.66Se0.34 crystal as a result of disorder introduced by Ni substitution, as compared with those in Fe0.99Te0.66Se0.34. It means that an inhomogeneous spatial distribution of ions with nanoscale phase separation is essential for enhancement of superconductivity but disorder introduced by simple substitution of another kind of ions alone is not sufficient.


Symposium organizers
Hugo KELLERUniversity of Zurich

Institute of Physics, Winterthurerstrasse 190, CH-8057 Zürich, Switzerland
Jianting YEUniversity of Groningen

Zernike Institute for Advanced Materials, Nijenborgh 4, 9747AG Groningen, The Netherlands
Roman PUZNIAKInstitute of Physics, Polish Academy of Sciences

Aleja Lotnikow 32/46, PL-02-668 Warsaw, Poland
Yoshihiro KUBOZONOOkayama University

Research Institute for Interdisciplinary Science, Tsushima-naka 3-1-1, Kita-ku, Okayama 700-8530, Japan