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2014 Fall Meeting



Superconductivity in low dimensional systems

Symposium focuses on the understanding of various novel aspects of superconductivity observed in low-dimensional systems, which differ from superconductivity in bulk materials. The issues such as design of low-dimensional structures, physical characterization, theories and the potential applications in novel devices will be discussed.


In recent years the continuous advancement in the fabrication methods has opened new areas of research of novel phenomena which appear in low-dimensional systems, such as ultrathin films, superlattices and nano-engineered mesoscopic structures. These systems are of enormous importance for applications in the modern nanoelectronic devices. The electronic states in these systems are greatly affected by the presence of surfaces or interfaces, resulting in many microscopic processes which differ greatly from these observed in the bulk materials. In particular, the phenomenon of superconductivity is expected to show novel features, and indeed experiments confirm many of these expectations. The effects such as interface superconductivity in oxides, superconductivity of Dirac fermions in topological insulators, or proximity-induced superconductivity in graphene are among these novel effects. In addition, new technologies allow for designing artificial hybrid systems containing constituent films with very different ground states, such as, for example, superconductor/ferromagnet hybrids. The close proximity between different ground states results in the interactions between these subsystems, leading to a wealth of unusual effects, including long-range proximity effect, modifications of the phase diagram, and magnetic-domain induced pinning of vortices. The symposium will provide a forum for discussion of these new phenomena, and their possible applications.

Hot topics to be covered by the symposium

  • Interfacial superconductivity in oxides
  • Topological insulators
  • Graphene
  • Superconductor-insulator transition in 2D systems
  • Ferromagnet/superconductor heterostructures
  • Mesoscopic superconductors, nano-engineered structures and devices
  • Nano-characterization techniques

List of invited speakers

  • C. Africh, Laboratoro TASC Trieste (Italy), Growth mechanisms and electronic structure of CVD graphene on Ni
  • E. Y. Andrei, Rutgers University (USA), Scanning tunneling microscopy and spectroscopy of graphene
  • N.P. Armitage, The Johns Hopkins University (USA), Microwave studies of superconductor-insulator transition in InO
  • H. Beidenkopf, Weizmann Institute (Israel) (tentative)
  • M.G. Blamire, University of Cambridge (UK), Ferromagnetic insulator Josephson junctions
  • H. Boschker, Max Planck Institute (Germany), Interface superconductor with gap behaviour like a high-temperature superconductor
  • V. Bouchiat, Institut Neel (France), Gate controlled superconductor-insulator transition and exotic metallic states in Graphene decorated with tin nano dots
  • D. Di Castro, Universita di Roma Tor Veragata (Italy), The interface between "infinite layer" and perovskite structure oxides for the occurrence of high Tc superconductivity
  • K. Dybko, Institute of Physics PAN (Poland), Proximity effect in manganite/cuprate heterostuctures
  • M. Ferrier, Laboratoire des Physiques des Solids (France), Dissipation and Supercurrent Fluctuations in a Diffusive Normal-Metal–Superconductor Ring
  • S. Gariglio, University of Geneva (Switzerland), Superconductivity at the LaAlO3/SrTiO3 interface
  • I. Guillamon, Universidad Autonoma de Madrid (Spain), STM of the 2D vortex lattice at very low temperatures
  • T. Hanaguri, RIKEN Center for Emergent Matter Science (Japan), STM/STS studies on topological insulators
  • P. Ioselevich, Landau Inst. (Russia), Theory of Majorana states in superconductor-topological insulator hybrids
  • A. Kapitulnik, Stanford University (USA) (tentative)
  • L. Krusin-Elbaum, The City College of New York (USA), Superconductivity of disordered Dirac fermions
  • J. Lesueur, Paris Tech (France), Superconductivity and quantum phase transition at oxides interface
  • V.V. Ryazanov, ISSP RAS Chernogolovka (Russia), Hybrid Josephson superconductor-ferromagnet-superconductor structures for superconducting electronics and spintronics
  • K. S. Tikhonov, Landau Institute (Russia) (tentative)
  • B. Sacepe, Institute Neel, Grenoble (France), Localization of preformed Cooper pairs
  • C. Strunk, University of Regensburg (Germany), Size-dependent conduction near the superconductor-insulator transition in TiN thin films
  • E. Zeldov, Weizmann Institute (Israel), NanoSQUID-on-tip: Towards scanning magnetic microscopy with single spin sensitivity

Scientific committee:

  • Helen Bouchiat, Laboratoire de Physiques des Solides, Orsay (France)
  • Mikhail V. Feigel’man - Landau Institute for Theoretical Physics, Moscow (Russia)
  • Jochen Mannhart, Max Planck Institute, Stuttgart (Germany)
  • Dmitry Roditchev, Institut des Nanosciences de Paris, Université Pierre et Marie Curie, Paris 6 (France)

The symposium will be co-organized by the EU 7th Framework Programme under the project REGPOT-CT-2013-316014 (EAgLE). 

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Authors : I. Zaytseva1, O. Abal’oshev1, P. Dłużewski1, W. Paszkowicz1, L.Y. Zhu2, C. L.Chien2 and Marta Z.Cieplak1
Affiliations : 1Institute of Physics, Polish Academy of Sciences, 02 668 Warsaw, Poland 2Department of Physics and Astronomy, The Johns Hopkins University, Baltimore, Md 21218, USA

Resume : It is well established that upon the reduction of the thickness of superconducting film the film resistance increases and the superconductivity (S) is suppressed. In addition, in a strongly-disordered thin films the external magnetic field induces superconductor-insulator transition. However, there exists also large class of weakly disordered materials, in which a saturation of resistance is observed in the limit of very low temperatures, suggesting that the transition occurs to a metallic, nonsuperconducting state. The nature of these transitions is still under debate. In this work we study the structural and magnetotransport properties in a series of Si/Nb/Si trilayers grown by magnetron sputtering at room temperature. The thickness of Nb, d, is varied from 50 nm down to 1.1 nm with a fixed Si thickness of 10 nm. With decreasing d, the structure of the Nb layer changes from polycrystalline to amorphous at d near 3.3 nm, while the superconducting temperature monotonically decreases, reaching zero for d < 1.2 nm. The Hall coefficient, which is positive in thick films, starts decreasing for d < 6 nm. Eventually, it changes sign into negative in ultrathin films with d < 1.6 nm, indicating that two types of carriers contribute to the conduction, most likely due to the influence of boundry scattering on the relaxation rate of carriers. We also show that the magnetic-field induced transition occurs from superconducting to the metallic, nonsuperconducting state. The possible origins of this metallic state will be discussed.

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Authors : Hans Boschker, Christoph Richter, Jochen Mannhart
Affiliations : Max Planck Institute for Solid State Research, Stuttgart, Germany

Resume : The electron liquid at the LaAlO3/SrTiO3 interface is a model system for the study of superconductivity as it provides a two-dimensional superconductor whose properties can be tuned with an electrical gate field. We developed planar tunnel junctions to study the superconductivity spectroscopically. Our tunnel junctions give access to two important physical parameters: the size of the superconducting gap and the electron-phonon spectral function. The gap increases with decreasing charge density in the entire phase diagram and persists up to temperatures far above Tc in the underdoped regime. These results are analogous to the pseudogap behavior of the high-transition-temperature copper oxide superconductors. The electron-phonon spectral function is independent of the carrier density and the likelihood of the conventional electron-phonon coupling mechanism for superconducting pairing will be discussed.

Authors : F. Baiutti1, G. Logvenov1, G. Gregori1, G. Christiani1, Y. Wang2, W. Sigle2, P. A. van Aken2, J. Maier1
Affiliations : 1Max Planck Institute for Solid State Research, Heisenbergstr. 1, 70569 Stuttgart, Germany; 2Max Planck Institute for Intelligent Systems, Heisenbergstr. 3, 70569, Stuttgart, Germany

Resume : The role of the local charge carrier concentration profiles is paramount for the occurrence of superconductivity in low-dimensional systems. Atomic layer-by-layer oxide MBE technique together with state-of-the-art characterization methods have disclosed new ways for the synthesis of novel superconducting epitaxial heterostructures. In this contribution, we present the exciting findings of the study of Sr delta-doped La2CuO4. Delta-doping, which represents a novel approach in the field of layered complex oxides, here relies on the substitution of a full La-O atomic layer by a Sr-O layer. Artificial superlattices having different spacing between two subsequent Sr-O layers were grown on LaSrAlO4 substrates; by appropriately tuning the superlattice structure, high-Tc superconductivity up to about 40 K was obtained. In order to rationalize this effect, detailed studies were performed with a spherical aberration-corrected STEM, which allows imaging and spectroscopic analyses (EDX as well as EELS) of each atomic column. The results indicate that high-Tc superconductivity, confined in few unit cells across the delta-doped atomic layer, can be attributed to holes enrichment on both sides of the interface, while Sr cations undergo an asymmetric spatial redistribution from the nominal Sr-O plane, confined within ~3 unit cells in the growth direction. These findings, suggesting a rather complex mechanism of charge rearrangement, are discussed.

Authors : S. Gariglio, A. Fête, D. Li, D. Stornaiuolo, J.-M. Triscone
Affiliations : DPMC University of Geneva, 24 quai Ernest-Ansermet, CH-1211 Geneva, Switzerland

Resume : The conducting interface between the two band insulators LaAlO3 and SrTiO3 has drawn a large share of attention, as it presents a variety of exciting electronic properties that are tunable by an electric field [1]. At low temperatures, magnetotransport analysis has revealed a strong Rashba spin-orbit interaction originating from the breaking of inversion symmetry [2] and, in field effect devices, the ground state has been tuned from an insulating to a superconducting state. In this presentation I will discuss field effect experiments to probe the superconducting state and map the phase diagram of the system and its link with doped bulk SrTiO3. I will also present recent experiments on nanostructures that reveal a remarkable tuning of the electronic properties and allow weak localization and weak anti-localization as a function of doping and temperature to be followed [3]. Finally, I will discuss an approach that should allow superconducting coupling between different gases to be studied [4]. [1] A. D. Caviglia et al., Nature 456, 624 (2008). [2] A. D. Caviglia et al., Phys. Rev. Lett. 104, 126803 (2010); A. Fête et al., Phys. Rev. B 86, 201105 (2012). [3] D. Stornaiuolo et al. Appl. Phys. Lett 101, 222601 (2012). [4] D. Li et al. Appl. Phys Lett. Mat. 2, 012102 (2014).

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Topological insulators, majoranas : Chair - Eva ANDREI, Vincent BOUCHIAT
Authors : Lia Krusin-Elbaum
Affiliations : Department of Physics, The City College of New York, CUNY, New York, NY 10031, USA

Resume : Surfaces of three-dimensional (3D) topological insulators have emerged as one of the most remarkable states of condensed quantum matter. These surfaces host robust spin-protected helical Dirac particles that emerge from topological order and can support unusual electronic phases when electron correlations are at play. One particularly exciting phase is topological superconductivity which, if found, could lead to a paradigm change in quantum information. Here we report on high-temperature surface superconductivity of hole carriers we discovered in a 3D topological material Sb2Te3 by fine-tuning the crystal growth protocol. This new superconducting state has low bulk carrier density, huge carrier mobility, and a sharp diamagnetic (Meissner) onset near 55 K, at which also the local superconducting gap is observed by STM. The unconventional nature of this state is witnessed by the persistence of quantum oscillations inside the superconducting state and two-dimensional (2D) quantum beats arising from strong spin-orbit coupling. This unusual superconductivity resides in surface Dirac puddles, with global coherence mediated by interpuddle diffusion of quasiparticles. Our findings suggest that by tuning the material's parameters (e.g., the quasiparticle mean free path and the system's Fermi velocity) of this mesoscopic structure even higher temperature surface superconductivity may be achieved. (This work was supported in part by the NSF DMR-1122594 and DOD-W911NF-13-1-0159).

Authors : Aharon Kapitulnik, Alan Fang, Carolina Adamo, Elizabeth Schemm
Affiliations : Geballe Laboratory for Advanced Materials, Stanford University

Resume : The new class of topological states that has recently emerged, called quantum spin Hall (QSH) states or topological insulators, are topologically distinct from all other known states of matter. QSH systems are insulating in the bulk-they have an energy gap separating the valence and conduction bands - but on the boundary they have gapless edge or surface states that are topologically protected and immune to impurities or geometric perturbations. A direct analogy can be drawn between superconductors and insulators by observing that the Hamiltonian for the quasiparticles of a superconductor is analogous to the Hamiltonian of a band insulator, with the superconducting gap corresponding to the band gap of the insulator (using the so-called Bogoliubov-de Gennes formalism). By analogy, some unconventional superconductors are also predicted to show topological effects, hence dubbed topological superconductors. The search for topological superconductivity that started with simple heterostructures of interfaces between a topological insulator and a superconductor expanded recently to more complex compounds. In particular, half-Heusler alloys were predicted to be topological insulators with strong band inverseion, and many of them are predicted to be topological superconductors. Of the known half Heusler alloys, the family of MPtBi (where M=Y,La,Lu) are predicted to have the largest band inversion with superconductivity below a Tc~1 K. In this talk we will survey our recent results on this family of compounds emphasizing the superconducting state through scanning tunneling microscopy and spectroscopy, transport and optical measurements.


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Symposium organizers
Marcin KonczykowskiCentre National de Recherche Scientifique

Laboratoire des Solides Irradies/UMR 7642 Ecole Polytechnique F-91128 Palaiseau France

Marta Z. CieplakInstitute of Physics

Polish Academy of Sciences Al. Lotnikow 32/46 02-668 Warsaw Poland

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