29 research outputs found
Critical angle for interfacial phonon scattering: Results from ab initio lattice dynamics calculations
Thermal boundary resistance is a critical quantity that controls heat
transfer at the nanoscale, which is primarily related to interfacial phonon
scattering. Here, we combine lattice dynamics calculations and inputs from
first principles ab initio simulations to predict phonon transmission at the
Si/Ge interface as a function of both the phonon frequency and the phonon
wavevector. This technique allows us to determine the overall thermal
transmission coefficient as a function of the phonon scattering direction and
frequency. Our results show that the thermal energy transmission is highly
anisotropic, while thermal energy reflection is almost isotropic. In addition,
we found the existence of a global critical angle of transmission beyond which
almost no thermal energy is transmitted. This critical angle around 50 degrees
is found to be almost independent of the interaction range between Si and Ge,
the interfacial bonding strength, and the temperature above 30 K. We interpret
these results by carrying out a spectral and angular analysis of the phonon
transmission coefficient and differential thermal boundary conductance
Enhancing the superconducting transition temperature of BaSi2 by structural tuning
We present a joint experimental and theoretical study of the superconducting
phase of the layered binary silicide BaSi2. Compared with the layered AlB2
structure of graphite or diboride-like superconductors, in the hexagonal
structure of binary silicides the sp3 arrangement of silicon atoms leads to
corrugated sheets. Through a high-pressure synthesis procedure we are able to
modify the buckling of these sheets, obtaining the enhancement of the
superconducting transition temperature from 4 K to 8.7 K when the silicon
planes flatten out. By performing ab-initio calculations based on density
functional theory we explain how the electronic and phononic properties of the
system are strongly affected by changes in the buckling. This mechanism is
likely present in other intercalated layered superconductors, opening the way
to the tuning of superconductivity through the control of internal structural
parameters.Comment: Submitte
Reduced phase space of heat-carrying acoustic phonons in single-crystalline InTe
Chalcogenide semiconductors and semimetals are a fertile class of efficient thermoelectric materials, which, in most cases, exhibit very low lattice thermal conductivity κph despite lacking a complex crystal structure such as the tetragonal binary compound InTe. Our measurements of κph(T) in single-crystalline InTe along the c axis show that κph exhibits a smooth temperature dependence upon cooling to about 50 K, the temperature below which a strong rise typical for dielectric compounds is observed. Using a combination of first-principles calculations, inelastic neutron scattering (INS), and low-temperature specific heat and transport properties measurements on single-crystalline InTe, we show that the phonon spectrum exhibits well-defined acoustic modes, the energy dispersions of which are constrained to low energies due to distributions of dispersionless, optical modes, which are responsible for a broad double peak structure in the low-temperature specific heat. The latter are assigned to the dynamics of In+ cations in tunnels formed by edge-sharing (In3+Te42−)− tetrahedra chains, the atomic thermal displacement parameters of which, probed as a function of temperature by means of single-crystal x-ray diffraction, suggest the existence of a complex energy potential. Indeed, the In+-weighted optical modes are not observed by INS, which is ascribed to the anharmonic broadening of their energy profiles. While the low κph value of 1.2Wm−1K−1 at 300 K originates from the limited energy range available for acoustic phonons, we show that the underlying mechanism is specific to InTe and argue that it is likely related to the presence of local disorder induced by the In+ sit
The crystal structure of cold compressed graphite
Through a systematic structural search we found an allotrope of carbon with
Cmmm symmetry which we predict to be more stable than graphite for pressures
above 10 GPa. This material, which we refer to as Z-carbon, is formed by pure
sp3 bonds and is the only carbon allotrope which provides an excellent match to
unexplained features in experimental X-ray diffraction and Raman spectra of
graphite under pressure. The transition from graphite to Z-carbon can occur
through simple sliding and buckling of graphene sheets. Our calculations
predict that Z-carbon is a transparent wide band gap semiconductor with a
hardness comparable to diamond.Comment: 4 pages, 5 figure
Inelastic neutron scattering study of spin excitations in the superconducting state of high temperature superconductors
Giant tuning of electronic and thermoelectric properties by epitaxial strain in p-type Sr-doped LaCrO3 transparent thin films
The impact of epitaxial strain on the structural, electronic, and thermoelectric
properties of p-type transparent Sr-doped LaCrO3 thin films has been investigated. For this
purpose, high-quality fully-strained La0.75Sr0.25CrO3 (LSCO) epitaxial thin films were grown by
molecular beam epitaxy on three different (pseudo)cubic (001)-oriented perovskite-oxide
substrates: LaAlO3, (LaAlO3)0.3(Sr2AlTaO6)0.7, and DyScO3. The lattice mismatch between the
LSCO films and the substrates induces in-plane strain ranging from -2.06% (compressive) to
+1.75% (tensile). The electric conductivity can be controlled over two orders of magnitude, σ
ranging from ~0.5 S cm-1 (tensile strain) to 35 S cm-1 (compressive strain). Consistently, the
Seebeck coefficient S can be finely tuned by a factor of almost two from ~127 μV K-1 (compressive
strain) to 208 μV K-1 (tensile strain). Interestingly, we show that the thermoelectric power factor
(PF = S2 σ) can consequently be tuned by almost two orders of magnitude. The compressive strain
yields a remarkable enhancement by a factor of three for 2% compressive strain with respect to
almost relaxed films. These results demonstrate that epitaxial strain is a powerful lever to control
the electric properties of LSCO and enhance its thermoelectric properties, which is of high interest
for various devices and key applications such as thermal energy harvesters, coolers, transparent
conductors, photo-catalyzers and spintronic memories.Financial support from the European Commission through the
project TIPS (H2020-ICT-02-2014-1-644453), the French
national research agency (ANR) through the projects MITO
(ANR-17-CE05-0018), LILIT (ANR-16-CE24-0022), DIAMWAFEL (ANR-15-CE08-0034-02), the CNRS through the
MITI interdisciplinary programs (project NOTE), IDEX
Lyon-St-Etienne through the project IPPON, the Spanish
Ministerio de Ciencia e Innovación, through the “Severo
Ochoa” Programme for Centres of Excellence in R&D (SEV2015-0496) and the MAT2017-85232-R (AEI/FEDER, EU),
PID2019-107727RB-I00 (AEI/FEDER, EU), and from Generalitat de Catalunya (2017 SGR 1377) is acknowledged. The
China Scholarship Council (CSC) is acknowledged for the
grant of Dong Han. Ignasi Fina acknowledges Ramón y Cajal
contract RYC-2017-22531. Seebeck measurements at ILM
were made within the ILMTech transport platform. The
authors are also grateful to Jean-Baptiste Goure, Philippe
Regreny, Aziz Benamrouche, and Bernat Bozzo for their
technical support and the reviewers for their valuable and
constructive comments that have improved the quality of the
manuscript.Peer reviewe
Isostructural phase transition by point defect reorganization in the binary type-I clathrate Ba7.5Si45
International audienceCompetition between microscopic point defect (vacancy and interstitial) configurations is inherent to crystalline phases of increased structural complexity. Phase transitions that preserve symmetry between them belong to a specific class of isostructural transitions. Type-I silicon clathrates are representatives of such structurally complex crystalline phases showing an intriguing structural transition at high pressure associated with an abrupt reduction of volume with no indication for any breakage of symmetry. Using isothermal high-pressure X-ray diffraction performed on a single crystal of the simplest representative type-I silicon clathrates, binary Ba 8 Si 46 , we confirm the isostructural character of the transition and identify the associated mechanism. A detailed analysis of the atomic structural parameters across the transition in combination with ab initio studies allow us to pinpoint a microscopic mechanism driven by a rearrangement of point defects initially present in the structure. An analysis based on the Landau theory gives a coherent description of the experimental observations. A discussion on the analogy between this transformation and liquid-liquid transitions is proposed
Development of High-Pressure Device, based on Paris-Edinburgh Press, Applied on SPS Process
International audienc
Localization of Propagative Phonons in a Perfectly Crystalline Solid
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