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Materials for energy and environment


Phonons and fluctuations in low dimensional structures

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The subject of the symposium is the transfer of energy across atomic interfaces and the propagation of energy in the nm-scale. The objective is to bring together concepts from solid state and statistical physics to converge towards a comprehensive understanding of coherence, energy-temperature in the nm-scale, chaos and noise phenomena.



Steering heat by light or vice versa, examining nano-scale energy conversion, as in thermoelectricity and harvesting in biological systems, are two aspects that share the underlying science of energy processes across atomic interfaces and energy propagation in the nanoscale and or in confined systems. The nanometer scale defies several of the bulk relationships as confinement of electrons and phonons, locality and non-equilibrium become increasingly important.

A deep understanding is sought using advanced experimental methods to reach the nm scale for electronic and thermal properties, as well as the possibility to perform theoretical work with many more atoms than in the past by means of advanced molecular dynamics and ab-initio methods among others. State of the art knowledge in statistical mechanics, quantum physics and quantum biology, noise, low frequency fluctuations, phonon engineering, combined phononic-photonic crystals, acoustic metamaterials, among others are topics that converge in the underpinning science, which we wish to explore in this symposium.

The propagation of phonons as energy carriers impacts not only heat transfer (thermoelectricity, thermal interface materials), but also the very concept and handling of temperature in non-equilibrium and highly localised conditions. Much of the needed progress depends on the materials studies and this symposium will target the interface material aspects as well as the emerging concepts to advance in this field. There are multiple applications areas, which will be impacted such as phase change materials, coherence in quantum information, energy conversion in thermoelectric materials, thermal management in nanoelectronics, etc...


Hot topics to be covered by the symposium:

  • Radiative heat transfer
  • Energy Conversion in the Nano scale
  • Nano scale thermal conductivity
  • Micro to Nanoscale thermal management
  • Phononic crystals
  • Photon-phonon interactions
  • Electron-phonon interactions in low dimensions
  • Phonons in Metrology and in Biology
  • Coherent acoustic phonons and Phonon sources
  • Thermal rectifiers, memories and computation


The confirmed invited speakers are:

  • Giuliano Benenti, University Insubria, Italy
  • Gang Chen, MIT, USA
  • Davide Donadio, Max Planck Institute for Polymer Research, Mainz, Germany
  • George Fytas, University of Crete, Greece
  • Tobias Kippenberg, EPFL, Switzerland
  • Bernard Perrin, INSP, Paris, France
  • Gyaneshwar P Srivastava, University of Exeter, UK


Scientific Committee:

  • Bahram Djafari-Rouhani (University Lille 1 France)
  • Anthony Kent (University of Nottingham, UK)
  • Fabio Marchesoni (University of Camerino, Italy)
  • Natalio Mingo (CEA, France)
  • Cesar A. Rodriguez-Rosario (University of Bremen, Germany)
  • Javier Rodríguez-Viejo (Autonomous University of Barcelona, Bellaterra, Spain) 
  • Pascal Ruello (University of Le Mans, France)
  • Thomas Dekorsy (University of Konstanz, Germany)


Conference Proceedings:

The proceedings of Symposium D: Phonons and Fluctuation in Low Dimensional Structures will be published in IOP Conference Series.
The manuscripts must follow the IOP Conf. Ser. guidelines and template. Deadline for the submission of manuscripts: 19th May 2014.
Contact: Prof. Dr Clivia M. Sotomayor Torres, mail to:


Symposium organizers:


Clivia M. Sotomayor Torres
Catalan Institute of Nanotechnology
Phononic and Photonic Nanostructures Group
Campus UAB, Edifici CM3
08193 Bellaterra (Barcelona)
Phone: +34 93 586 8304
Fax: +34 93 586 8313

Sebastian Volz
Ecole Centrale Paris
Thermal Nanosciences Group
Laboratoire d'Energétique Moléculaire et Macroscopique, Combustion, UPR CNRS 288
92295 Châtenay Malabry
Phone: +33 14113 1070
Fax: +33 14702 8035

Jouni Ahopelto
Technical Research Centre of Finland VTT
Microsystems and Nanoelectronics
P.O. Box 1000
FI-02044 VTT
Phone: +358 20 722 6644
Fax: +358 20 722 7012

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Nanoscale Thermal Transport I : Clivia M Sotomayor Torres (ICN2)
Authors : Gang Chen
Affiliations : Mechanical Engineering Department Massachusetts Institute of Technology Cambridge, MA 02139 USA

Resume : Understanding phonon transport is important for many technological developments, examples are thermoelectric energy conversion, for which phonon heat conduction should be minimized, and thermal management of microelectronics, photonic devices, and batteries, for which heat conduction should be maximized. In this talk, I will start with a discussion of first-principles simulation on phonon heat conduction in bulk crystals, which reveals details on phonon scattering and mean free path distributions. I will explain the importance of resonant bonding on the low thermal conductivity of III-V semiconductors. I will then present a recently developed thermal conductivity spectroscopy technique to measure phonon mean free distribution and discuss experimental evidence on coherent contribution of phonons to heat conduction in superlattices, supported by detailed simulations. This material is based upon work supported as part of the “Solid State Solar-Thermal Energy Conversion Center (S3TEC), an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under Award Number: DE-SC0001299/DE-FG02-09ER46577.

Authors : Oleg V. Kolosov, Manuel Pumarol, Benjamin J. Robinson
Affiliations : Physics Department, Lancaster University, Lancaster, LA1 3BE, UK

Resume : We experimentally explore one of the least defined areas in scanning thermal microscopy (SThM) – the thermal contact between the SThM probe tip and the studied sample by monitoring normal and shear forces between the nanoscale tip and the studied surface while measuring the heat transport from the tip. By using the measurements in both ambient and high vacuum (1x 10-7 Torr) environment we directly differentiate contributions of various channels of heat conduction in SThM. In order to measure in real-time the shear forces an SThM X-piezo was dithered creates a lateral sliding forces acting on the cantilever. The torsional and normal forces are both measured through and recorded simultaneously with SThM signal. We found that the shear forces and thermal response are inversely correlated to the heat transport to the sample. Once the mechanical contact is established, our data both in air and vacuum, suggest that it is the solid-solid contact that is the dominant heat transfer channel of the thermal contact. If liquid bridge would be dominant, we should be able to observed significant decrease of the shear force, with the constant heat conduction that was not the case in our experiments suggesting that liquid bridge may be much less essential for nanoscale heat transport in SThM than generally accepted.

Authors : S. Merabia, J. Lombard, F. Detcheverry
Affiliations : CNRS and Université Lyon 1

Resume : Despite decades of research, our understanding of interfacial thermal transport is still quite poor. Traditional models such as AMM and DMM fail to give a good description of the thermal boundary resistance between a metal and a dielectrics. In this contribution, I will discuss the effect of electron-phonon coupling in the value of the thermal boundary resistance between a metal and silicon or silica, using a combination of analytic approach and numerical simulations. I will analyze two applications: the thermal response of thin metallic films deposited on silicon or silica, and hyperthermia driven by core-shell nanoparticles.

Authors : M. Prunnila, D. Gunnarsson, J. Richardson-Bullock, M. J. Prest, T. E. Whall, E. H. C. Parker, L. Donetti, F. Gamiz
Affiliations : VTT Technical Research Centre of Finland; University of Warwick, UK; Universidad de Granada, Spain;

Resume : Operation of electron refrigerators and thermal detectors, such as bolometers, depends on the strength of the coupling of the electrons to the thermal bath. The coupling is, in the end, deter-mined by the thermal electron-phonon coupling (TEPC), i.e., the thermal conductance between the electron and phonon systems. TEPC depends strongly on the microscopic details of the system at hand. In semiconductors and also in graphene the coupling can be tuned, for example, by strain and by adjusting the carrier concentration. Strain-tuning of TEPC has been investigated theoretically and experimentally in many-valley systems [1,2] and applied in enhancing the performance of silicon based electron refrigerators [3]. However, the effect of strain on TEPC is orders of magnitude smaller than suggested by the theory. In this contribution, we discuss this discrepancy and other heat dissipation channels that can become dominant at very low temperatures. One possible channel is provided by the near-field electron-electron interaction between closely spaced conductors [4], which suggests that thermal management of low-temperature devices could possibly utilize near-field heat transfer even in the case of solid-solid contact. References: [1] M. Prunnila, Phys. Rev. B 75, 165322 (2007). [2] J.T. Muhonen et al. Appl. Phys. Lett. 98, 182103 (2011). [3] M.J. Prest et al., Appl. Phys. Lett. 99, 251908 (2011). [4] M. Prunnila and S. Laakso, New J. Phys, 15 033043 (2013).

Authors : Konstantinos TERMENTZIDIS and David LACROIX
Affiliations : LEMTA, CNRS UMR-7563, University of Lorraine, Vandoeuvre les Nancy, France

Resume : The reduction of the thermal conductivity of nanostructured materials is of great interest for almost all devices and crucial for the thermoelectric efficiency. The development of new fabrication methods allows tailoring nanostructured materials and their transport properties. Bulk Bismuth Telluride is the most efficient thermoelectric material near room temperature. Bi2Te3 nanowires are of particular interest as they are inexpensive to be fabricated (electrochemical deposition) and their reduced thermal conductivity due to the lower dimension can be lowered further with structural modulation. Important parameters that can change the transport properties are the stoichiometry, the roughness of the external surfaces, the state of the interfaces in the case of core/shell type nanowires, as well as the doping. With means of molecular dynamics and using realistic potential, we predict the thermal conductivity of a series of modulated Bi2Te3 nanowires. We will prove that the roughness provides efficient scattering across a broad spectra of phonons, and can reduce the thermal conductivity close to the amorphous limit.

Authors : Pierre-Olivier CHAPUIS, Yunxin WANG, Nabil DJATI
Affiliations : Centre for Thermal Sciences, Lyon (CETHIL) CNRS - INSA Lyon - UCBL Campus La Doua - LyonTech 69621 Villeurbanne (Lyon), France

Resume : The Boltzmann transport equation (BTE) allows simulating heat transfer when the characteristic dimensions of the medium start to be comparable to the heat carrier mean free paths. However, the equation is not straightforward to solve and it can be useful to consider approximated equations. The ballistic-diffusive heat conduction equations (BDE) have been proposed as a tool to simulate heat transfer in subdiffusive configurations. Our aim is to investigate their advantages and drawbacks. We analyze the results of phonon simulations with the BDE in monodimensional and bidimensional geometries for grey media, where heat carriers possess a single mean free path, and non-grey media. The goal is to split the effect of confinement and dispersion. The 1D case is simple and it is verified that the results can be approximated with a simple analytical equation. The 2D case is analyzed based on the information got from the previous analyses. It is shown that care should be taken close to the boundaries. We discuss the results obtained with a heat source with size comparable with the mean free path, useful for many experiments involving thermal constrictions.

Authors : Xiangjun Liu, Gang Zhang, Yong-Wei Zhang
Affiliations : Institute of High Performance Computing, A-STAR, Singapore; Institute of High Performance Computing, A-STAR, Singapore; Institute of High Performance Computing, A-STAR, Singapore

Resume : Tailoring interfacial thermal transport in graphene-based nanoarchitectures is important for many applications including nanoelectronics, solid-state lighting, energy generation and nanocomposites. We demonstrate that interfacial thermal conductance G can be fivefold enhanced by introducing covalent bonds at the interfaces using molecular dynamics simulations. The simulations captured the trend of thermal transport enhancement with the increment of interfacial covalent bond density. The results confirm that the observed G enhancements at the interfaces are due to strong interfacial covalent bonds and resultant coupling in the atomic vibrational spectra near the interface. The spectral analysis indicates that the coupling between graphene out-of-plane motion and bonded linkage group motion at low frequencies serves as the most important channels for thermal transport across the interface. Thus, covalently bonding functionalization is an attractive approach to tune the heterointerfacial thermal transport in a variety of material systems.

Authors : Chester Szwejkowski [1], Kai Sun [2], Costel Constantin [1], Ashutosh Giri [3], Christopher Saltonstall [3], Patrick E. Hopkins [3]
Affiliations : [1] Department of Physics and Astronomy, James Madison University, Harrisonburg, Virginia USA; [2] Department of Materials Science and Engineering, University of Michigan, Ann Arbor, Michigan USA; [3] Department of Mechanical and Aerospace Engineering, University of Virginia, Charlottesville, Virginia USA

Resume : Gallium nitride (GaN) is considered the most important semiconductor after the discovery of Silicon. Understanding the properties of GaN is important in determining the utility and applicability of this class of materials to devices. We present results of time domain thermoreflectance (TDTR) measurements as a function of surface root mean square (RMS) roughness. We used commercially available 5mm x 5mm, single-side polished GaN (3-7 µm)/Sapphire (430 µm) substrates that have a wurtzite crystal structure and are slightly n-type doped. The GaN substrates were annealed in the open atmosphere for 10 minutes (900-1000 °C). This high-temperature treatment produced RMS values from 1- 60 nm and growth of gallium oxide (GaO) as measured with an atomic force microscopy and transmission electron microscopy respectively. A gold film (~80 nm) was deposited on the GaN surface using electron beam physical vapor deposition which was verified using ellipsometry and profilometry. The TDTR measurements suggest that the thermal conductivity decays exponentially with RMS roughness and that there is a minimum value for thermal boundary conductance at a roughness of 15nm. Future experiments will explore these results further.

Authors : A. Fraile1, G. Tsironis1, N. Lazaridis1, K. Papagelis2 and D. Campbell3
Affiliations : 1) CCQCN, Department of Physics, University of Crete, Heraklion, Greece 2) Department of Materials Science, University of Patras, Greece 3) Physics Department, Boston University, MA, USA.

Resume : Discrete breathers or intrinsic localized modes have been theoretically predicted in many different materials. This is the case also of graphene [1, 2], hydrogenated graphene [3] etc. However, the results presented in [1] and [2] are not completely compatible and clearly further research is necessary. More important, experimental evidence is still lacking. In this work we present our current research using classical molecular dynamics (MD) and different interatomic potentials (Tersoff, AIREBO, LCBOP and reaxFF). Our MD simulations show the existence of breathers but, for example, the lifetime can change one order of magnitude or more depending on the force field used to describe the carbon-carbon interaction. (In fact this can be also observed comparing the lifetimes and frequencies presented in [1] and [2]). Hence, the properties of the breathers clearly depend on the interatomic potential and the differences between the potentials has to be considered. Finally we present our future experimental plans to complement our theoretical effort. References: [1] Y. Yamayose et al. Excitation of intrinsic localized modes in a graphene sheet. EPL, 80 (2007) 40008 [2] L. Z. Khadeeva et al. Discrete Breathers in Deformed Graphene. JETP Letters, 2011, Vol. 94, No. 7, pp. 539–543 [3] B. Liu et al 2013 J. Phys. D: Appl. Phys. 46 305302. Discrete breathers in hydrogenated graphene.

Authors : Thomas E. Wilson(a)*, Erich Kasper(b), Michael Oehme(b), Jörg Schulze(b), Konstantin Korolev(a)
Affiliations : (a) Department of Physics, Marshall University, Huntington, WV, U.S.A., 25755 (b) IHT, University of Stuttgart, Stuttgart, Germany, 70569

Resume : We report upon a novel means of producing coherent acoustic phonons; namely, the direct excitation of high-frequency acoustic phonons in silicon doping superlattices by the resonant absorption of nanosecond-pulsed far-infrared (FIR) laser radiation of the same frequency. We have used silicon-doping superlattices, fabricated on 0.6 mm thick, float zone (100) silicon substrates, with 30 periods (34.5 nm) and 2-D doping (Sb and B) concentrations of 8 x 1012 cm-2. The corresponding conversion efficiency (FIR to acoustic power) is of order 10-9. Our cavity-dumped, optically-pumped molecular FIR gas laser can provide 10-kW of peak power in 5-ns pulses at 10 pps. The FIR laser radiation is transported to the cryostat entrance window by a corrugated waveguide terminated with Teflon lens to focus the beam onto a germanium prism in contact with the superlattice. The prism coupler, operating in a total internal reflection mode, converts the incident transverse FIR electric field into an evanescent longitudinal field over the thickness of the superlattice. A fast granular aluminum/palladium current-biased microbolometer (10 micron x 20 micron x 100 nm), fabricated upon the rear sample surface and biased near its transition temperature at 1.86 K, is used for detection. Measured time-of-flight across the thin (0.6-mm) substrate is used to verify that the phonons are longitudinal. We note we observe no larger and delayed transverse acoustic phonon pulse (expected to occur with heat pulse production and phonon focusing in the (100) direction in Si), indicating that this technique provides for single-mode coherent phonon generation.

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Thermal Rectification : Giuliano Benenti (U Insubria)
Authors : P. Ferrando, A. F.Lopeandia, X. Alvarez, G. Garcia, LL. Abad, M. I. Alonso, M. Garriga, A. R. Goni, J. Rodríguez-Viejo
Affiliations : Nanoamterials and Microsystems group. Dep. Physics. Universitat Autónoma de Barcelona, 08193 Bellaterra, Spain Institut de Microelectrònica de Barcelona- Centre Nacional de Microelectrónica, Campus UAB, 08193 Bellaterra, Spain. Institut de Ciència de Materials de Barcelona (ICMAB-CSIC), Campus UAB, 08193 Bellaterra, Spain

Resume : The experimental discovery of thermal rectification in asymmetrically mass-loaded carbon nanotubes in 2006 boosted the search for thermal analogs of electronic devices. However, the realization of efficient thermal rectifiers based on simple materials, such as Si or SiGe, remains a challenge. In this work, we provide experimental evidence of outstanding heat flow asymmetries in two remarkably different systems: trapezoidal Si nanowires and compositionally graded SiGe superlattices. The structure of the trapezoidal nanowires is asymmetrically defined between suspended heater/sensors by Focus Ion Beam. The origin of the heat flow asymmetry is related to the presence of Ga ions at the outer shell of the nanowire that produces a mass–damping of the oscillations with a strong influence in the atoms located at the narrow outer edge of the structure. On the other hand, specific Si1-xGex/Si SLs superlattices with well-defined compositional gradients across the SiGe layer show a strong reduction of the cross plane thermal conductivity and an outstanding difference of around 40% in its value depending if the heat flow is parallel or antiparallel to the concentration gradient.

Authors : R. Rurali (1), X. Cartoixe (2), L. Colombo (3)
Affiliations : (1) Institut de Ciencia de Materials de Barcelona (ICMAB-CSIC), Spain (2) Universitat Autonoma de Barcelona, Spain (3) Universita di Cagliari, Italy

Resume : Semiconducting nanowires (NWs) have attracted a growing interest in recent years and are recognized as important building blocks for emerging applications in nanoelectronics [1-3]. The understanding of thermal transport has lately acquired a great importance as well, because NWs have been proposed to be a pathway for the engineering of efficient thermoelectric materials. Here we study thermal transport in SiGe nanowires across a Si/Ge axial interface by means of non-equilibrium molecular dynamics simulations. We calculate the interface thermal resistance (ITR) of realistic models of axial SiGe heterojunctions, whose morphology depends strongly on the different experimental conditions [4-7]. We also investigate if these asymmetric junctions can yield thermal resistances that depend on the applied thermal gradient, i.e. thermal rectification. We find that diffuse interfaces result in larger ITR, while sharp junctions yield a small, but non-negligible thermal rectification, favoring heat transport from Si to Ge. [1] D. K. Ferry, Science 319, 579 (2008) [2] R. Rurali, Rev. Mod. Phys. 82, 427 (2010) [3] M. Amato, M. Palummo, R. Rurali, and S. Ossicini, Chem. Rev., doi:10.1021/cr400261y [4] Clark et al., Nano Lett. 8, 1246 (2008) [5] Wen et al., Science 326, 1247 (2009) [6] Perea et al., Nano Lett. 11, 3117 (2011) [7] Geaney et al., Nano Lett. 13, 1675 (2013)

Authors : M. Lejman, V. Shalagatskyi, O. Kovalenko, T. Pezeril, V. Temnov, P. Ruello
Affiliations : Institut des Molécules et Matériaux du Mans, UMR 6283 CNRS, Université du Maine

Resume : The understanding of the transport of hot electrons and their interactions with the bulk lattice and the interface is of prime importance for the improvement of advanced nanoscale devices. In this communication, we show that supersonic hot electrons are responsible for the emission of coherent acoustic phonons deeply beneath the free surface where they are optically excited consistently with previous experiments [1-3]. Moreover, these hot electrons can also travel enough far at room temperature to interact with a buried interface (Cu-Ti in our case) located at 220 nm beneath the free surface. In particular, we evidence that this interaction leads to coherent acoustic phonon emission. Such phenomenon has been reported only one time up to now in the case of aluminum [4]. In order to go deeper in the experimental investigations and to discuss in more details the underlying physics, several ultrafast optical pump-probe configurations been used [2]. In particular, we have performed different experiments where the time of flight of hot electrons and coherent acoustic phonons have been characterized. From an original probe wavelength dependence study of the optical detection process, we clearly establish the signature of superdiffusive hot transport within the copper film and the link with the acoustic phonon emission. These results and observations are important to quantify the mechanism of electron-phonon interaction at materials interfaces. [1] O. B. Wright, “Ultrafast nonequilibrium stress generation in gold and silver,” Phys. Rev. B 49, 9985–9988 (1994). [2] O. B. Wright and V. Gusev, “Ultrafast generation of acoustic waves in copper,” IEEE Trans. Ultrason. Ferroelectr. Freq. Control 42, 331–338 (1995). [3] G. Tas and H. J. Maris, “Electron diffusion in metals studied by picosecond ultrasonics,” Phys. Rev. B 49, 15046–15054 (1994). [4] M. Lejman, V. Shalagatskyi, O. Kovalenko, T. Pezeril, V. Temnov, P. Ruello “Ultrafast optical detection of coherent acoustic phonons emission driven by superdiffusive hot electrons”, J. Opt. Soc. Am. B, to be published in Feb 2014.

Authors : C L Poyser, A V Akimov, R P Campion, A J Kent
Affiliations : School of Physics and Astronomy, University of Nottingham, University Park, Nottingham NG7 2RD UK

Resume : We describe the use of an AlGaAs p-i-n diode to monitor the output of a single pass phonon amplification device. The optical resonance of quantum wells, incorporated in the intrinsic region of the diode, strongly depend on the strain associated with acoustic waves. This can be exploited to create an optically-gated detector with picosecond resolution by monitoring changes in induced photocurrent caused by acoustic waves [1]. In the current scheme, the p-i-n detector is fabricated on one side of a 150µm GaAs substrate, and two GaAs/AlAs superlattices (SLs), the lower of which can be placed under an electrical bias, are grown on the other side. The lower, SASER gain, SL was grown to specifications which have been previously shown, using an incoherent bolometric detection technique, to provide phonon amplification [2]. Femtosecond optical pumping of the top SL generates quasi-monochromatic sub-Terahertz phonons which propagate through the gain SL and the substrate to the p-i-n diode. This is gated by a time-delayed femtosecond pulse providing a high resolution coherent detection, this shows evidence of coherent amplification in the SASER device. [1] Moss, D et al. Phys. Rev. B 83, 245303 (2011) [2] Beardsley, R. P. et al. New J. Phys. 13, 073007 (2011)

Authors : A.F. Lopeandía#; A.P. Perez-Marín#; Ll. Abad&; P. Ferrando#; G. Garcia#; J.Rodríguez-Viejo#;
Affiliations : #Grupo de Nanomateriales y Microsistemas, Dep. Física, Universitat Autónoma de Barcelona, 08193 Bellaterra, Spain. &Instituto de Microelectrónica de Barcelona- Centre Nacional de Microelectrónica, Campus UAB, 0893 Bellaterra, Spain.

Resume : We report the characterization of a thermoelectric microgenerator build up using standard CMOS technologies. As a thermoelectric active material we use ultrathin single-crystalline Si membranes, 100 nm in thickness, with embedded n and p-type doped regions. The planar design, with the n-p couples electrically connected in series and thermally in parallel, includes a central heater/sensor that permits to impose several small temperature gradients and then characterize the average thermoelectric lumped parameters. Finite element modeling is used to derive, from the experimental data, the intensive parameters for the thin film couples, and thus evaluate ZT in the full range measured from 50 to 350K. A 3-fold reduction in thermal conductivity due to the phonon boundary scattering with the thin film surfaces supposes an effective improvement in ZT respect to bulk values since resistivity and Seebeck coefficients are respected. The ZT obtained grows monotonically in the measured range. As a test of the maximum output power density produced, we imposed a 200K temperature gradient with the frame at 350K obtaining 250 µW/cm^2.

Authors : Giuliano Benenti
Affiliations : University of Insubria, Italy

Resume : The understanding of coupled particles and heat transport in complex systems is a fundamental problem, also of practical interest in connection with the challenging task of developing high-performance thermoelectric heat engines and refrigerators. Recently discovered general mechanisms of optimizing the figure of merit of thermoelectric efficiency are discussed, also in connection to momentum-conserving interacting systems [1.2], to the breaking of time-reversal symmetry by an applied magnetic field [3-6], and to multiterminal steady-state quantum thermal machines [7]. [1] G. Benenti, G. Casati and W. Jiao, Conservation laws and thermodynamic efficiencies, Phys. Rev. Lett. 110, 070604 (2013). [2] G. Benenti, G. Casati and C. Mejia-Monasterio, Thermoelectric efficiency in momentum-conserving systems, New J. Phys. 16, 015014 (2014). [3] G. Benenti, K. Saito and G. Casati, Thermodynamic bounds on efficiency for systems with broken time-reversal symmetry, Phys. Rev. Lett. 106, 230602 (2011). [4] K. Saito, G. Benenti, G. Casati and T. Prosen, Thermopower with broken time-reversal symmetry, Phys. Rev. B 84, 201306(R) (2011). [5] M. Horvat, T. Prosen, G. Benenti and G. Casati, Railway switch transport model, Phys. Rev. E 86, 052102 (2012). [6] V. Balachandran, G. Benenti and G. Casati, Efficiency of three-terminal thermoelectric transport under broken time-reversal symmetry, Phys. Rev. B 87, 165419 (2013). [7] F. Mazza, R. Bosisio, G. Benenti, V. Giovannetti, R. Fazio and F. Taddei, Thermoelectric efficiency of three-terminal quantum thermal machines, preprint.

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Optomechanics : Achim Kittel (U Oldenburg)
Authors : Tobias J. Kippenberg
Affiliations : Institute of Condensed Matter Physics, EPFL Switzerland

Resume : The mutual coupling of optical and mechanical degrees of freedom via radiation pressure has been a subject of interest in the context of quantum limited displacements measurements for Gravity Wave detection for many decades. Over the past years these radiation pressure “backaction” phenomena have been observed – starting from observations in in high Q optical microresonators(1) – in a variety of micro and nanoscale opto and electro-mechanical systems. The high Q of the microresonators, not only enhances nonlinear phenomena - such as optical frequency comb generation(2) via the Kerr nonlinearity – but also enhances the radiation pressure interaction. This has allowed the observation of radiation pressure phenomena in an experimental setting and constitute the fast developing research field of cavity quantum optomechanics(3, 4). I will describe a range of optomechanical phenomena studied using on-chip optical microresonators, that combine both optical and mechanical degrees of freedom in one and the same device. Radiation pressure back-action of photons is shown to lead to effective cooling(5-8) of the mechanical oscillator mode using dynamical backaction. Cooling to the quantum regime is possible using sideband resolved cooling, with passive of cryogenic precooling to ca. 700 mK, which enables cooling the oscillators such that it resides in the quantum ground state more than 1/3 of its time(9). Increasing the mutual coupling further, it is possible in this regime to observe quantum coherent coupling(9) in which the mechanical and optical mode hybridize and the coupling rate exceeds the mechanical and optical decoherence rate (7). In this regime the mechanical and optical mode form an optomechanical ‘polariton’. This enables a range of quantum optical experiments, including state transfer from light to mechanics using the phenomenon of optomechanically induced transparency(10). In addition experiments are described that utilized the optomechanical coupling for highly efficient force measurements using nanomechanical oscillators(11), as well as elements enabling switching, slowing or advancing of radiation(12). References: 1. T. J. Kippenberg et al., Analysis of Radiation-Pressure Induced Mechanical Oscillation of an Optical Microcavity. Physical Review Letters 95, 033901 (2005). 2. T. J. Kippenberg, R. Holzwarth, S. A. Diddams, Microresonator-based optical frequency combs. Science 332, 555 (Apr 29, 2011). 3. T. J. Kippenberg, K. J. Vahala, Cavity Optomechanics: Backaction at the mesoscale. Science 321, 1172 (2008, 2008). 4. M. Aspelmeyer, T. J. Kippenberg, F. M. Marquardt, Cavity Optomechanics., (2012). 5. V. B. Braginsky, S. P. Vyatchanin, Low quantum noise tranquilizer for Fabry-Perot interferometer. Physics Letters A 293, 228 (Feb 4, 2002). 6. V. B. Braginsky, Measurement of Weak Forces in Physics Experiments. (University of Chicago Press, Chicago, 1977). 7. A. Schliesser et al., Radiation pressure cooling of a micromechanical oscillator using dynamical backaction. Physical Review Letters 97, 243905 (Dec 15, 2006). 8. A. Schliesser et al., Resolved-sideband cooling of a micromechanical oscillator. Nature Physics 4, 415 (2008). 9. E. Verhagen et al., Quantum-coherent coupling of a mechanical oscillator to an optical cavity mode. Nature 482, 63 (Feb 2, 2012). 10. S. Weis et al., Optomechanically induced transparency. Science 330, 1520 (Dec 10, 2010). 11. E. Gavartin, P. Verlot, T. J. Kippenberg, A hybrid on-chip optomechanical transducer for ultrasensitive force measurements. Nature nanotechnology 7, 509 (Aug, 2012). 12. X. Zhou et al., Slowing, advancing and switching of microwave signals using circuit nanoelectromechanics. Nature Physics 9, 179 (2013).

Authors : Konstantinos TERMENTZIDIS 1, Arthur FRANCE-LANORD 1, Etienne BLANDRE 1, Samy MERABIA 2, Tristan ALBARET 2 and David LACROIX 1
Affiliations : 1 LEMTA, CNRS UMR-7563, University of Lorraine, Vondoeuvre les Nancy, France 2 ILM, CNRS, University of Lyon 1, Lyon, France

Resume : Silicon is considered as a reference material both for its crystalline and amorphous phases. While each phase separately has been investigated extensively, less attention is given to nanostructures involving both phases and especially their interfaces. Yet, almost all silicon devices contain such interfaces and the transport for both electrons and phonons through them is a crucial issue. In this study, we model amorphous/crystalline interfaces for nanowires and superlattices. The aim is to predict the thermal properties of these nanostructured materials, and explain the trends with the phonon density of states and the Kapitza resistance. The calculations have been done with molecular dynamics method, using a realistic interatomic potential. The thermal conductivity of both nanostructures is found to be close to the bulk amorphous one and almost independent the temperature. In bulk amorphous silicon, the vibrational features of the crystalline phase are present but smoothed out, due to the lack of periodicity. The localized modes and the large depression of the high frequency phonons, which have been proved to carry heat even if they are nonpropagating diffusive modes, contribute to the reduction of the thermal conductivity. We will present also the local phonon density of states at different locations on both sides of a planar interface. The phonons feel the interfaces in a distance of roughly 4Å away from them.

Authors : P. Ferrando, A. F.Lopeandía, J. Rodríguez-Viejo
Affiliations : Nanomaterials and Microsystems Group. Physics dep. Universitat Autónoma de Barcelona. 08193 Bellaterra, Spain.

Resume : Suspended structures are widely used to measure thermal conductivity (k) on nanowires and thin films. Here we present a detailed analysis of the uncertainties in the measurement of thermal conductivity using suspended structures. These structures are often modeled using a 1D heat diffusion equation in order to extract the thermal conductivity of the sample under test. As we illustrate, the use of a 1D simplified model may give incorrect values of the thermal conductivity. To show this, we first calculate in a 1D approximation the uncertainty of the measured k value of the nanostructure for a wide range of thermal conductances. We prove the existence of a plateau with low uncertainties depending on the properties of the structure. Tuning these properties allows the fabrication of structures intended for the measurement of samples with specific thermal conductance. We have also carried out finite element modeling of the structure in 3D, to account for some inaccuracies that appear when k is calculated using the 1D equation for highly-conductive samples. In this framework, correct solutions of the thermal conductivity can be obtained for any geometry and/or thermal conductance.

Affiliations : 1 Plasma Laboratory - Faculty of Sciences – Department of Physics- University of Batna- Algeria 2 Laboratory of chemistry and environmental chemistry L.C.C.E - University of Batna- Algeria,

Resume : The aim of this work was on the one hand the development of theoretical models ( mathematical) to numerically simulate the experimental results of variations in the electrical conductivity as a function of various parameters, and on the other hand, the study of boundary conditions necessary for the numerical resolution of the heat propagation equation. Note that all obtained theoretical models are of the linear or sigmoidal form, Boltzmann type. Most obtained models allowed finding relationships more or less simple between the different parameters with an acceptable relative error of calculation, which allowed us to calculate theoretical values very close to the experimental values for the physical variables studied. Some of our models have given incorrect values , this can be explained by the fact that our calculation method is not reliable for this kind of data (values in small intervals). Note that this type of calculation can be improved by using other interpolation methods in particular the method of cubic splines. The model we have developed for solving the heat propagation equation, can serve in the research on the estimation of the isomerization temperature of PA by laser effect, which is widely used in current studies on the polyacetylene. Finally, it should be noted that our model for solving the heat propagation equation can be more efficient when the semi -empirical method is used to set the condition at the lower boundary of the domain.

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Nanoscale Thermal Transport II : Gyaneshwar Srivastava (U Exeter)
Authors : Davide Donadio
Affiliations : MPI for Polymer Research Ackermannweg 10 55128 Mainz – Germany

Resume : properties of materials over a very wide range, and give access to physical phenomena that would not occur in three-dimensional systems. This is especially true for the vibrational porperties and the lattice thermal conductivity of materials. In this talk I will illustrate case studies of atomistic calculations of phonon transport in two-dimensional materials, namely graphene and ultrathin silicon membranes, in which the thermal conductivity can be modified over several orders of magnitude with respect to the respective bulk counterparts. In particular, non-Fourier thermal transport in graphene at non-equilibrium conditions, and how heat conduction may be affected by mechanical strain will be discussed. Whereas in the case of graphene dimensionality reduction leads to extremely high thermal conductivity, it will be also shown how modulating thickness and surface structure turn silicon nano-membranes into low-thermal conductivity systems, attractive for thermoelectric applications.

Authors : Nikolaos Cheimarios 1,2 ; Xanthippi Zianni 1,3,4; Patrice Chantrenne 4
Affiliations : 1 Dept. of Applied Sciences, Technological Educational Institution of Central Greece, 34 400 Psachna, Greece 2 School of Chemical Engineering, National Technical University of Athens, 15780, Athens, Greece 3 Dept. of Microelectronics, IAMPPNM, NCSR ‘Demokritos’, 153 10 Aghia Paraskevi, Greece 4 Universite de Lyon, INSA de Lyon, MATEIS UMR CNRS 5510, Villeurbanne 69621, France

Resume : Good thermoelectric materials require high electron power factor and low thermal conductivity. Recent theoretical predictions and observations have provided evidence that inhomogeneities can favor the thermoelectric properties of nanowires. Enhanced thermoelectric power factor has been observed in InAs nanowires at low temperatures and has been attributed to quantum effects in the presence of inhomogeneities. The thermal conductivity of nanowires is reduced by inhomogeneities. In an earlier work, we showed theoretically that the thermal conductivity of a nanowire can be more significantly reduced by diameter modulation than by diameter decrease. The calculations were done using a kinetic theory model based on the Boltzmann transport equation and the relaxation time approximation. The mean free path was estimated using the Matthiessen rule with the bulk mean free path and an average phonon-boundary scattering length. Here, we use a method earlier proposed to study the thermal conductivity of a nanostructure with arbitrary geometry through Monte Carlo sampling of the free paths associated with phonon-phonon and phonon-boundary scattering. The two methods are compared for various nanowire dimensions and geometry modulations.


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