Transition metal dichalcogenide, especially MoS2 has attracted lot of attention recently owing to its tunable visible range band gap and anisotropic electronic and transport properties. Here, we report a comprehensive inelastic light scattering measurements on CVD grown (horizontally and vertically aligned flakes) as well as single crystal flakes of MoS2, probing the anisotropic optical response via studying the polarization dependence intensity of the Raman active phonon modes as a function of different incident photon energy and flake thickness. Our polarization dependent Raman studies intriguingly revealed strong anisotropic behavior reflected in the anomalous renormalization of the modes intensity as a function of flake thickness, phonons and photon energy. Our observations reflects the strong anisotropic light-matter interaction in this high crystalline symmetric layered MoS2 system especially for the in-plane vibrations, which is crucial for understanding as well application of these materials for future application such as optoelectronic applications.
In this work, we performed temperature-dependent Raman spectroscopy studies on three layered metal thiophosphates: CuInP2S6, CuCrP2S6, and CuInP2S6-In4/3P2S6 heterostructures. These materials are emerging multiferroics, with CuInP2S6 and CuInP2S6-In4/3P2S6 exhibiting room temperature ferrielectric polarization while CuCrP2S6 is both anti-ferroelectric and antiferromagnetic at low temperatures (<145 and <30 K, respectively). We studied two prominent peaks in the Raman spectra of these compounds, namely their P-P and P-S stretching modes. First order linear fits to the mode softening yielded coe…
Resonance Raman scattering, a very effective and sensitive technique for atomically thin semiconducting transition metal dichalcogenide, can be used to observe the phonons from the entire Brillouin zone. In addition to the significance of resonance effect on the Raman spectrum it may also be used to probe the electron-phonon coupling. Our study is devoted to understand the phonons in layered MoS2, especially for very low frequency range (i.e. below 100 cm-1), as a function of temperature under the resonance effect. Understanding the phonon-phonon and electron-phonon coupling and the effects of…
The phonon and electronic properties, the Eliashberg function and the temperature dependence of resistance of electride Ca2N are investigated by the DFT-LDA plane-wave method. The phonon dispersion, the partial phonon density of states and the atomic eigenvectors of zero-center phonons are studied. The electronic band dispersion and partial density of states conclude that Ca2N is a metal and the Ca 3p, 4s and N 2p orbitals are hybridized. For the analysis of an electron - phonon interaction (EPI) and its contribution to resistance the Eliashberg function was calculated and a temperature depend…
We use a combination of polarized Raman spectroscopy experiment and model magnetism-phonon coupling calculations to study the rich magneto-Raman effect in the two-dimensional (2D) magnet $\mathrm{CrI_3}$. We reveal a novel layered-magnetism-assisted phonon scattering mechanism below the magnetic onset temperature, whose Raman excitation breaks time-reversal symmetry, has an antisymmetric Raman tensor, and follows the magnetic phase transitions across critical magnetic fields, on top of the presence of the conventional phonon scattering with symmetric Raman tensors in $N$-layer $\mathrm{CrI_3}$…
The evolution of the optical phonons in layered semiconductor alloys SnSe1-xSx is studied as a function of the composition by using polarized Raman spectroscopy with six different excitation wavelengths (784.8, 632.8, 532, 514.5, 488, and 441.6 nm). The polarization dependences of the phonon modes are compared with transmission electron diffraction measurements to determine the crystallographic orientation of the samples. Some of the Raman modes show significant variation in their polarization behavior depending on the excitation wavelengths. It is established that the maximum intensity direct…
Coherent Spin-Phonon Coupling in the Layered Ferrimagnet Mn3Si2Te6
Los Alamos National Laboratory（洛斯阿拉莫斯国家实验室）；The University of Texas at El Paso（德克萨斯大学埃尔帕索分校）；Brookhaven National Laboratory（布鲁克海文国家实验室）；University of Edinburgh（爱丁堡大学）；Instituto de Ciencia de Materiales de Madrid, CSIC（马德里材料科学研究所，西班牙国家研究委员会）；University of California Irvine（加州大学尔湾分校）
Coherent terahertz magnon-phonon three-wave mixing in a layered antiferromagnet
Nanjing University（南京大学）；Max Planck Institute for the Structure and Dynamics of Matter（物质结构与动力学马克斯·普朗克研究所）；University of Electronic Science and Technology of China（电子科技大学）；Institute of Physics, Chinese Academy of Sciences（中国科学院物理研究所）
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…
Considering electron correlation appropriately is important to predict the properties of layered transition metal oxides, which have drawn a lot of attention as cathode materials for sodium-ion batteries. Here, we explore the phonon and electronic properties of layered NaMnO2 using the recently developed strongly constrained and appropriately normed (SCAN) functional. We also introduce the Coulomb interaction U to find an accurate description of Mn 3d orbitals. The phonon dispersion curves show the structural stability with the SCAN, which is consistent with prior experimental stability at hig…
We present a comprehensive temperature dependent Raman measurements for horizontally aligned CVD grown layered MoS2 in a temperature range of 4 to 330 K under resonance condition. Our analysis of temperature dependent phonon frequency shift and linewidth suggest a finite role of three and four phonon anharmonic effect. We observed Davydov splitting of the out-of-plane (A1g ) and in-plane (E12g ) modes, attributed to the weak interlayer interaction, and reflected in the appearance of additional modes with decreasing temperature for both 3 layers and few layers system. We also observed that the …
The discovery of fascinating ways to control and manipulate antiferromagnetic materials have garnered considerable attention as an attractive platform to explore novel spintronic phenomena and functionalities. Layered antiferromagnets (AFMs) exhibiting interesting magnetic structures, can serve as an attractive starting point to establish novel functionalities down to the two-dimensional limit. In this work, we explore the magnetoresistive properties of the spin-ladder AFM TaFe$_{1.25}$Te$_3$. Magnetization studies reveal an anisotropic magnetic behavior resulting in the stabilization of a spi…
Time-resolved diffuse scattering experiments have gained increasing attention due to their potential to reveal non-equilibrium dynamics of crystal lattice vibrations with full momentum resolution. Although progress has been made in interpreting experimental data on the basis of one-phonon scattering, understanding the role of individual phonons can be sometimes hindered by multi-phonon excitations. In Ref. [{\it arXiv:2103.10108}] we have introduced a rigorous approach for the calculation of the all-phonon inelastic scattering intensity of solids from first-principles. In the present work, we …
Efficient prediction of highly anisotropic excitonic properties in the layered antiferromagnet CrSBr via time-dependent density functional theory
University of Massachusetts, Amherst（马萨诸塞大学阿默斯特分校）；CIC nanoGUNE BRTA（纳米技术联合研究中心 nanoGUNE）；Universidad de Cantabria（坎塔布里亚大学）；Centro de Física de Materiales (CFM-MPC), CSIC-UPV/EHU（材料物理中心 (CFM-MPC)，西班牙国家研究委员会-巴斯克地区大学）
We use first-principles theory to show that the ingredients assumed to be essential to the occurrence of negative thermal expansion (NTE) -- rigid unit phonon modes with negative Grüneisen parameters -- are neither sufficient nor necessary for a material to undergo NTE. Instead, we find that NTE in PbTiO$_3$ involves a delicate interplay between the phonon properties of a material (Grüneisen parameters) and its anisotropic elasticity. These unique insights open new avenues in our fundamental understanding of the thermal properties of materials, and in the search for NTE in new materials classe…
Transition metal dichalcogenide (TMD) alloys have attracted great interests in recent years due to their tunable electronic properties, especially the semiconductor-metal phase transition, along with their potential applications in solid-state memories and thermoelectrics. However, the thermal conductivity of layered two-dimensional (2D) TMD alloys remains largely unexplored despite that it plays a critical role in the reliability and functionality of TMD-enabled devices. In this work, we study the temperature-dependent anisotropic thermal conductivity of the phase-transition 2D TMD alloys WSe…
We report high pressure X-ray diffraction and a detailed systematic Raman measurements on $ReS_2$ sample, which is mechanically exfoliated from a single crystal. A few new Bragg peaks are observed to emerge above 6 GPa indicating a structural transition from distorted $1T$ to distorted $1T$$^{\prime}$ in triclinic structure. The same is corroborated by appearance of new Raman modes in the same pressure range. Softening of the Raman modes corresponding to $Re$ atom vibrations are observed in the distorted $1T$$^{\prime}$ phase in the pressure range 15-25 GPa. In the same pressure range the anom…
Length dependent lattice thermal conductivity of single \& multi layered hexagonal boron nitride: A first-principles study using the Callaway-Klemens \& real space super cell methods
Intertwined atomic-nanoscale-microscale structures via intralayer anisotropic Fe-chains in the layered ferromagnet FePd2Te2
Key Laboratory of Quantum State Construction and Manipulation (Ministry of Education), School of Physics, Renmin University of China（量子态构建与调控教育部重点实验室，物理学院，中国人民大学）；Beijing Key Laboratory of Optoelectronic Functional Materials & Micro-nano Devices, School of Physics, Renmin University of China（光电功能材料与微纳器件北京市重点实验室，物理学院，中国人民大学）；Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences（凝聚态物理北京实验室，中国科学院物理研究所）；Institute of Quantum Materials and Physics, Henan Academy of Sciences（量子物质与物理研究所，河南省科学院）
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 i…
Understanding the interplay between various design strategies (for instance, bonding heterogeneity and lone pair induced anharmonicity) to achieve ultralow lattice thermal conductivity ($κ_l$) is indispensable for discovering novel functional materials for thermal energy applications. In the present study, we investigate layered PbXF (X = Cl, Br, I), which offers bonding heterogeneity through the layered crystal structure, anharmonicity through the Pb$^{2+}$ $6s^2$ lone pair, and phonon softening through the mass difference between F and Pb/X. The weak inter-layer van der Waals bonding and the…
This study explores the relationship between phonon dynamics and negative thermal expansion (NTE) in Cu$_x$Ni$_{2-x}$(CN)$_4$. The partial replacement of nickel (II) by copper (II) in Ni(CN)$_2$ leads to a line phase, CuNi(CN)$_4$ (x = 1), and a solid solution, Cu$_x$Ni$_{2-x}$(CN)$_4$ (0 $\leq$ x $\leq$ 0.5). CuNi(CN)$_4$ adopts a layered structure related to that of Ni(CN)$_2$ (x = 0), and interestingly exhibits 2D NTE which is about 1.5 times larger. Inelastic neutron scattering (INS) measurements combined with first principles lattice dynamical calculations provide insights into the effect…
Black phosphorus has recently attracted significant attention for its highly anisotropic properties. A variety of ultrafast optical spectroscopies has been applied to probe the carrier response to photoexcitation, but the complementary lattice response has remained unaddressed. Here we employ femtosecond electron diffraction to explore how the structural anisotropy impacts the lattice dynamics after photoexcitation. We observe two timescales in the lattice response, which we attribute to electron-phonon and phonon-phonon thermalization. Pronounced differences between armchair and zigzag direct…
Despite several first principles studies, the relative stability of BN polymorphs remains controversial. The stable polymorph varies between the cubic (c-BN) and hexagonal (h-BN) depending on the van der Waals (vdW) dispersion approximation used. These studies are unable to explain the main experimental results, c-BN is stable, the relative stability order and the large energy difference between h-BN and c-BN (greater than 150 meV/formula unit). In this study, we introduce contributions from electron-phonon interactions (EPI) to the total energy of BN polymorphs. This clearly establishes c-BN …
Recent developments of the pump-probe thermoreflectance methods (such as the beam-offset and elliptical-beam approaches of the time-domain and frequency-domain thermoreflectance techniques) enabled measurements of the thermal conductivities of in-plane anisotropic materials. Estimating the temperature rise of anisotropic layered structures under surface heating is critically important to make sure that the temperature rise is not too high to alias the signals in these experiments. However, a simple formula to estimate the temperature rise in three-dimensional (3D) anisotropic layered systems h…
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）
Magnetic layered materials have emerged recently as promising systems to introduce magnetism in structures based on two-dimensional (2D) materials and to investigate exotic magnetic ground states in the 2D limit. In this work, we apply high hydrostatic pressures up to P = 8.7 GPa to the bulk layered antiferromagnet FePS$_3$ to tune the collective lattice excitations (phonons) in resonance with magnetic excitations (magnons). Close to P = 4 GPa, the magnon-phonon resonance is achieved and the strong coupling between these collective modes leads to the formation of new quasi-particles, the magno…
High performance thermoelectric devices requires materials with low lattice thermal conductivities. Many strategies, such as phonon engineering, have been made to reduce lattice thermal conductivity without simultaneously decrease of the charge transport performance. It is a simple and effective approach to use materials with heavy element to reduce the lattice thermal conductivity. Here, based on the first-principles calculations and phonon Boltzmann transport equations, we find the replacement of Zr with heavy element Hf in ZrNCl doesn't reduce the lattice thermal conductivity, instead, it s…
Anisotropic phonon transport along different lattice directions of two-dimensional (2D) materials has been observed, however, the effect decreases with increasing the thickness beyond a few atomic layers. Here we establish a novel mechanism to induce anisotropic phonon transport in quasi-2D materials with isotropic symmetry. The phonon propagation is guided by resonance hybridization with surface nanostructures. We demonstrate that the thermal conductivity of 3 nm-thick silicon membrane with surface nanofins is greater by $\sim50\%$ parallel to the fins than that perpendicular to the fins.
The zone-center $E_{2g}$ modes play a crucial role in MgB$_2$, controlling the scattering mechanisms in the normal state as well the superconducting pairing. Here, we demonstrate via first-principles quantum-field theory calculations that, due to the anisotropic electron-phonon interaction, a $hot$-$phonon$ regime where the $E_{2g}$ phonons can achieve significantly larger effective populations than other modes, is triggered in MgB$_2$ by the interaction with an ultra-short laser pulse. Spectral signatures of this scenario in ultrafast pump-probe Raman spectroscopy are discussed in detail, rev…
Hexagonal and kagome lattices exhibit extraordinary electronic properties. It is a natural consequence of additional discrete degree of freedom associated with a valley or the occurence of electronic flat-bands. Combination of both types of lattices, observed in CoSn-like compounds, leads not only to the topological electronic behavior, but also to the emergence of chiral phonon modes. Here, we study CoSn-like compounds in the context of realization of chiral phonons. Previous theoretical studies demonstrated that the chiral phonons can be found in ideal two-dimensional hexagonal or kagome lat…
