Metal Chalcogenides have been known for important technological applications and have attracted continuous interest in their structure, electronic, thermal and transport properties. Here we present first principles calculations of the vibrational and thermodynamic properties of NiX (X = S, Se, Te) compounds along with inelastic neutron scattering measurements of the phonon spectrum in NiSe. The measured phonon spectrum is in very good agreement with the computed result. We also report the measurement of thermal expansion behavior of NiSe using X-ray diffraction from 13 K to 300 K. The change in the hexagonal c lattice parameter in NiSe is considerably greater as compared to a parameter. The ab-initio calculated anisotropic Grüneisen parameters of the different phonon modes in all the chalcogenides along with the elastic constants are used to compute anisotropic thermal expansion behviour, which is found in good agreement with experiments. The displacement pattern of phonons indicate that difference in amplitudes of Ni and X atoms follow the anisotropy of thermal expansion behavior along c- and a-axis.
Superlattices provide a great platform for studying coherent transportation of low-frequency phonons, which are the main issues in mastering the manipulation of heat conduction. Studies have shown that the dominating characteristics in the thermal conductivity of superlattice can be adjusted between wave-like and particle-like phonon properties depending on the superlattice period. However, the phonon coherence length and the phonon mean free path from Umklapp processes have not been defined in one superlattice system, and the transition from wave-like and particle-like behavior is not clear t…
Anisotropic electron-phonon coupling and chiral phonons in van der Waals room temperature ferromagnet Fe$_{5}$GeTe$_{2}$
Indian Institute of Technology Kharagpur（印度理工学院卡拉格普尔分校）；Institute of Nuclear Physics, Polish Academy of Sciences（波兰科学院核物理研究所）；Jawaharlal Nehru University（贾瓦哈拉尔·尼赫鲁大学）；Sungkyunkwan University（成均馆大学）
Anharmonic Lattice Dynamics and Anisotropic Electron-Phonon Coupling in Quasi-1-Dimensional Charge Density Wave Ta2NiSe7
Indian Institute of Technology Mandi（印度技术学院曼迪分校）；SRM Institute of Science and Technology（SRM科学与技术学院）；Jawaharlal Nehru Centre for Advanced Scientific Research（贾瓦哈拉尔·尼赫鲁高级科学研究中心）
Interactions of charge carriers with lattice vibrations, or phonons, play a critical role in unconventional electronic transport of metals and semimetals. Recent observations of phonon-mediated collective electron flow in bulk semimetals, termed electron hydrodynamics, present new opportunities in the search for strong electron-electron interactions in high carrier density materials. Here we present the general transport signatures of such a second-order scattering mechanism, along with analytical limits at the Eliashberg level of theory. We study electronic transport, using $ab$ $initio$ calc…
We outline a numerical procedure to incorporate the crystal symmetries in the Helmholtz Fermi-surface harmonics basis set, which are the solutions of the Helmholtz equation defined on the Fermi surface. This improvement allows for an optimal representation of anisotropic quantities defined on the Fermi surface in terms of few symmetric elements of the set. We demonstrate the general validity of our approach by identifying the fully symmetric Helmholtz Fermi-surface harmonics subset for several representative systems with different crystal structures, namely, FCC-Cu, HEX-MgB$_2$, and BCC-YH$_6$…
Starting from recent advances in the first-principles modeling of polarons, variational polaron equations in the strong-coupling adiabatic approximation are formulated in Bloch space. In this framework, polaron formation energy as well as individual electron, phonon and electron-phonon contributions are obtained. We suggest an efficient gradient-based optimization algorithm and apply these equations to the generalized Fröhlich model with anisotropic non-degenerate electronic bands, both in two- and three-dimensional cases. The effect of the divergence of Fröhlich electron-phonon matrix element…
Active tuning of highly anisotropic phonon polaritons in van der Waals crystal slabs by gated graphene
University of Oviedo（奥维耶多大学）；Center of Research on Nanomaterials and Nanotechnology, CINN (CSIC-Universidad de Oviedo)（纳米材料与纳米技术研究中心，CINN（CSIC-奥维耶多大学））；Donostia International Physics Center (DIPC)（圣塞巴斯蒂安国际物理中心）；Moscow Institute of Physics and Technology（莫斯科物理与技术学院）；IKERBASQUE, Basque Foundation for Science（伊克尔巴斯基克，巴斯克科学基金会）
Identifying and understanding the mechanisms behind strong phonon-phonon scattering in condensed matter systems is critical to maximizing the efficiency of thermoelectric devices. To date, the leading method to address this has been to meticulously survey the full phonon dispersion of the material in order to isolate modes with anomalously large linewidth and temperature-dependence. Here we combine quantitative MeV ultrafast electron diffraction (UED) analysis with Monte Carlo based dynamic diffraction simulation and first-principles calculations to directly unveil the soft, anharmonic lattice…
In two-dimensional materials, structure difference induces the difference in phonon dispersions, leading to the anisotropy of in-plane thermal transport. Here, we report an exceptional case in layered PdSe2, where the bonding, force constants, and lattice constants are nearly-equal along the in-plane crystallographic axis directions. The phonon dispersions show significant differences between the Gamma-X and Gamma-Y directions, leading to the anisotropy of in-plane thermal conductivity with a ratio up to 1.8. Such anisotropy is not only unexpected in equilaterally structured (in-plane) materia…
Chiral Phonons and Giant Anisotropic Photoresponse in Quasi-1D van der Waals Semiconductor ZrSnS3
National Key Laboratory of Spintronics, Hangzhou International Innovation Institute, Beihang University（北京航空航天大学杭州创新研究院）；Department of Physics, Binghamton University-SUNY（宾汉姆顿大学）；University of Warsaw, Faculty of Physics（华沙大学物理系）；School of Materials Science and Engineering, Institutes of Physical Science and Information Technology, Anhui University（安徽大学物质科学与信息技术研究院）；Nanoscale Synthesis & Research Laboratory, Department of Applied Physics, University of Karachi（卡拉奇大学应用物理系纳米合成与研究实验室）
Microscopic Origin of Temperature-Dependent Anisotropic Heat Transport in Ultrawide-Bandgap Rutile GeO2
Technische Universität Berlin（柏林工业大学）；Department of Physics, Universidade Federal do Ceará（塞阿拉联邦大学物理系）；Leibniz-Institut für Kristallzüchtung（莱布尼茨晶体生长研究所）
Discovery and observations of exotic, quantized optical and electrical responses have sparked renewed interest in nonmagnetic chiral crystals. Within this class of materials, six group V transition metal ditetrelides, that is, XY$_2$ (X = V, Nb, Ta and Y = Si, Ge), host composite Weyl nodes on high-symmetry lines, with Kramers-Weyl fermions at time-reversal invariant momenta. In addition, at least two of these materials, NbGe$_2$ and NbSi$_2$, exhibit superconducting transitions at low temperatures. The interplay of strong electron-phonon interaction and complex Fermi surface topology present …
Anisotropic and isotropic elasticity and thermal transport in monolayer C$_{24}$ networks from machine-learning molecular dynamics
College of Physical Science and Technology, Bohai University（渤海大学物理科学与技术学院）；Department of Physical Chemistry, School of Chemistry, Tel Aviv University（特拉维夫大学化学学院物理化学系）
Anisotropic in-plane thermal conductivity observed in few-layer black phosphorus
黑磷作为一种新型的二维材料，在电子学和光电子学领域有着潜在的应用前景。在这里，我们报告的各向异性面内导热系数的悬浮少层黑磷测量的显微拉曼光谱。对于厚度大于15 nm的黑磷膜，扶手椅型和锯齿型热导率分别为~20和~40 W m $^{-1}$ K $^{-1}$，并且随着膜厚度减小，热导率降低至~10和~20 W m $^{-1}$ K $^{-1}$，表现出显著的各向异性。发现对于厚的黑磷膜，热导率各向异性比为~2，对于最薄的9.5 nm厚的膜，热导率各向异性比下降到~1.5。理论模型表明，所观察到的各向异性主要与各向异性声子色散有关，而本征声子散射率被发现是类似的扶手椅和锯齿形方向。黑磷薄膜中的表面散射强烈抑制长平均自由程声学声子的贡献。
Metal-organic frameworks often display large positive or negative thermal expansion coefficients. MOF-74, a material envisioned for many applications does not display such a behavior. For this system, temperature-dependent x-ray diffraction reveals particularly small negative thermal expansion coefficients perpendicular and positive ones parallel to the hexagonally arranged pores. The observed trends are explained by combining state-of-the-art density-functional theory calculations with the Grüneisen theory of thermal expansion, which allows tracing back thermal expansion to contributions of i…
Spin-dependent anisotropic electron-phonon coupling in KTaO$_3$
University of Geneva（日内瓦大学）；University of Modena and Reggio Emilia（摩德纳和雷焦艾米利亚大学）；SPIN-CNR Institute for Superconducting and other Innovative Materials and Devices（超导及其他创新材料器件研究所（SPIN-CNR））；ISC-CNR Institute for Complex Systems（复杂系统研究所（ISC-CNR））；Sapienza University of Rome（罗马智慧大学）；Donostia International Physics Center（圣塞巴斯蒂安国际物理中心）
The highly anisotropic thermal conductivity in layered materials is crucial for a broad range of applications such as thermal management of electronic devices, thermal insulation, and thermoelectrics. Understanding of anisotropic thermal transport in layered materials largely depends on atomistic simulations based on density functional theory (DFT) or empirical potentials, which however suffer either low computational efficiency or accuracy. Recently, machine learning interatomic potentials (MLIPs) are emerging as a powerful tool to bridge the gap. Despite the recent progress in developing MLI…
Bulk layered MX2 transition metal chalcogenides (M = Mo, W and X = S, Se) are known to exhibit an indirect to direct band gap transition as the number of layers is reduced. Previous time-resolved work has principally focused on the investigation of the transient evolution of the band structure after photo-excitation, but additional information on the dynamics of the concomitant lattice rearrangement is needed to fully understand this phenomenon. Here, ultrafast electron diffraction is used to probe the atomic motion and bond dilation in monolayer WSe2 with femtosecond temporal resolution. The …
