The structural stability and phonon properties of SnSe/SnS superlattices at finite temperatures have been studied using machine learning force field molecular dynamics and the anharmonic phonon approach. The vertical SnSe/SnS superlattice undergoes a phase transition from the Pnma phase to a novel P4/nmm phase at finite temperatures, which is different from the high-temperature Cmcm phase of the SnSe and SnS systems. The stability of P4/nmm phase is determined by molecular dynamics trajectories and anharmonic phonon dispersion relations. The imaginary modes of TO modes at the q=M(1/2,1/2,0) point of the P4/nmm phase in harmonic approximation become rigid at elevated temperatures. An analysis of phonon power spectra upon temperature also confirms the dynamic stabilization. The P4/nmm phase has higher symmetry than the Pnma phase, and the phase transition between them is accompanied by competition between the Jahn-Teller effect and phonon anharmonicity. Unlike the anisotropic distribution of Sn-Se/S bonds in the Pnma phase, the P4/nmm phase forms chemical bonds with similar bond lengths both in-plane and interlayer, and their resonance effect can significantly enhance phonon scattering. The calculated phonon density of states and lifetime is strongly temperature dependent, demonstrating the heavy anharmonicity in the SnSe/SnS system. The P4/nmm phase has an extremely low lattice thermal conductivity, close to the experimental values of SnSe and SnS. Moreover, with the reduction of band gap and the enhancement of band degeneracy near the Fermi level, the P4/nmm phase exhibits superior electronic transport properties and significantly enhanced response to infrared and visible light. This makes it show great potential in thermoelectric and photovoltaic applications.
We found that the electronic transport property of SnSe single crystals was sensitive to oxygen content. Semiconducting SnSe single crystals were obtained by using Sn of grain form as a starting material while powder Sn resulted in metallic SnSe. X-ray photoelectron spectroscopy analysis revealed that the surfaces of raw Sn were oxidized, where the volume fraction was relatively low in grain Sn. This demonstrates that contamination of oxygen causes metallic behavior in grown SnSe single crystals.
SnSe is a promising thermoelectric material with record-breaking figure of merit, \textit{i.e., ZT}. As a semiconductor, optimal electrical dosage is the key challenge to maximize \textit{ZT} in SnSe. However, to date a comprehensive understanding of the electronic structure and most critically, the self-hole doping mechanism in SnSe is still absent. Here, we report the highly anisotropic electronic structure of SnSe investigated by both angle-resolved photoemission spectroscopy and quantum transport, in which a unique "\textit{pudding-mold}" shaped valence band with quasi-linear energy disper…
By means of first-principles calculations, we explore systematically the geometric, electronic and piezoelectric properties of multilayer SnSe. We find that these properties are layer-dependent, indicating that the interlayer interaction plays an important role. With increasing the number of SnSe layers from 1 to 6, we observe that the lattice constant decreases from 4.27 $\mathring{A}$ to 4.22 $\mathring{A}$ along zigzag direction, and increases from 4.41 $\mathring{A}$ to 4.51 $\mathring{A}$ along armchair direction close to the bulk limit (4.21 $\mathring{A}$ and 4.52 $\mathring{A}$, respec…
Black phosphorene and its analogs have attracted intensive attention due to their unique puckered structures, anisotropic characteristics, and negative Poisson's ratio. The van der Waals heterostructures assembly by stacking different materials may show novel physical properties which the parent materials don't possess. In this work, the first-principles calculations were performed to study the mechanical properties of the BP/SnSe van der Waals heterostructure. Interestingly, a near-zero Poisson's ratio vzx was found in BP/SnSe heterostructure. In addition, compared to the parent materials BP …
Electron-crystal and phonon-glass are regarded as two essential factors for ideal thermoelectric materials, which require both an enhanced electronic transport and a depressed phononic transport. These two characteristics usually can not coexist in complete bulk semiconductors because of the strong interaction between electrons and phonons. Introducing nanostructures into bulk thermoelectric materials has long been applying to allow for diverse phonon scattering mechanisms to reduce lattice thermal conductivity while keeping the electrical conductivity intact. However, some two-dimensional mat…
The binary compound SnSe exhibits record high thermoelectric performance, largely because of its very low thermal conductivity. The origin of the strong phonon anharmonicity leading to the low thermal conductivity of SnSe is investigated through first-principles calculations of the electronic structure and phonons. It is shown that a Jahn-Teller instability of the electronic structure is responsible for the high-temperature lattice distortion between the Cmcm and Pnma phases. The coupling of phonon modes and the phase transition mechanism are elucidated, emphasizing the connection with hybrid …
The local atomic structure of SnSe was characterized across its orthorhmbic-to-orthorhombic structural phase transition using x-ray pair distribution function analysis. Substantial Sn off-centering distortions persist in the high symmetry high temperature phase, with symmetry different from that of ordered distortions below the transition. The analysis implies that the transition is neither order-disorder nor displacive, but rather a complex crossover where the character of coupling changes from 3D-like at low temperature to 2D-like at high temperature. Robust ferro-coupled SnSe intra-layer di…
The crystal structure of the thermoelectric material tin selenide has been investigated with angle-dispersive synchrotron x-ray powder diffraction under hydrostatic pressure up to 27 GPa. With increasing pressure, a continuous evolution of the crystal structure from the GeS type to the higher-symmetry TlI type was observed, with a critical pressure of 10.5(3) GPa. The orthorhombic high-pressure modification, beta'-SnSe, is closely related to the pseudo-tetragonal high-temperature modification at ambient pressure. The similarity between the changes of the crystal structure at elevated temperatu…
We present results of electronic band structure, Fermi surface and electron transport properties calculations in orthorhombic $n$- and $p$-type SnSe, applying Korringa-Kohn-Rostoker method and Boltzmann transport approach. The analysis accounted for temperature effect on crystallographic parameters in $Pnma$ structure as well as the phase transition to $CmCm$ structure at $T_c\sim 807 $K. Remarkable modifications of conduction and valence bands were notified upon varying crystallographic parameters within the structure before $T_c$, while the phase transition mostly leads to jump in the band g…
SnSe, a group IV-VI monochalcogenide with layered crystal structure similar to black phosphorus, has recently attracted extensive interests due to its excellent thermoelectric properties and potential device applications. Experimental electronic structure of both the valence and conduction bands is critical for understanding the effects of hole versus electron doping on the thermoelectric properties, and to further reveal possible change of the band gap upon doping. Here, we report the multivalley valence bands with a large effective mass on semiconducting SnSe crystals and reveal single-valle…
Layered tin selenide (SnSe) has recently emerged as a high-performance thermoelectric material with the current record for the figure of merit (ZT) observed in the high-temperature Cmcm phase. So far, access of the Cmcm phase has been mainly obtained via thermal equilibrium methods based on sample heating or application of external pressure, thus restricting the current understanding only to ground-state conditions. Here, we investigate the ultrafast carrier and phononic dynamics in SnSe. Our results demonstrate that optical excitations can transiently switch the point-group symmetry of the cr…
SnxSey crystalline compounds consisting of Sn and Se atoms of varying composition are systematically investigated at pressures from 0 to 100 GPa using the first-principles evolutionary crystal structure search method based on density functional theory (DFT). All known experimental phases of SnSe and SnSe2 are found without any prior input. A second order polymorphic phase transition from SnSe-Pnma phase to SnSe-Cmcm phase is predicted at 2.5 GPa. Initially being semiconducting, this phase becomes metallic at 7.3 GPa. Upon further increase of pressure up to 36.6 GPa, SnSe-Cmcm phase is transfor…
We conduct comprehensive investigations of both thermal and electrical transport properties of SnSe and SnS using first-principles calculations combined with the Boltzmann transport theory. Due to the distinct layered lattice structure, SnSe and SnS exhibit similarly anisotropic thermal and electrical behaviors. The cross-plane lattice thermal conductivity $κ_{L}$ is 40-60% lower than the in-plane values. Extremely low $κ_{L}$ is found for both materials because of high anharmonicity. It is suggested that nanostructuring would be difficult to further decrease $κ_{L}$ because of the short mean …
Unlike time-reversal topological insulators, surface metallic states with Dirac cone dispersion in the recently discovered topological crystalline insulators (TCIs) are protected by crystal symmetry. To date, TCI behaviors have been observed in SnTe and the related alloys Pb$_{1-x}$Sn$_{x}$Se/Te, which incorporate heavy elements with large spin-orbit coupling (SOC). Here, by combining first-principles and {\it ab initio} tight-binding calculations, we report the formation of a TCI in the relatively lighter rock-salt SnS and SnSe. This TCI is characterized by an even number of Dirac cones at th…
The orthorhombic monochalcogenide SnSe has attracted much attention in recent years as a promising high-temperature thermoelectric material. We present a study of its thermal conductivity and specific heat of SnSe between 2~K and 300~K and quantify its anisotropic thermal diffusivity, $D$. For both crystallographic orientations, thermal diffusivity remains above the recently identified Planckian limit ($D > v_s^2 τ_P$, where $v_s$ is the sound velocity and $τ_P= \hbar/k_BT$) and its anisotropy in $D$ is set by the anisotropy of $v_s$. Comparison with cubic members of the IV-VI family leads to …
We use density functional theory calculations to study the electronic structure of epitaxial (111) interfaces of the topological crystalline insulators SnSe and SnTe with the magnetic insulator EuS and the non-magnetic insulator CaTe, respectively. We consider both interface slab models with a vacuum region and periodic heterostructures without vacuum. We find that gaps of 21 meV at the $Γ$ point and 9 meV at the M point arise in the topological state at the SnSe/EuS interface, due to the magnetic proximity effect, which breaks the time reversal symmetry. The surface state at $Γ$ is shifted be…
SnSe is a layered material that currently holds the record for bulk thermoelectric efficiency. The primary determinant of this high efficiency is thought to be the anomalously low thermal conductivity resulting from strong anharmonic coupling within the phonon system. Here we show that the nature of the carrier system in SnSe is also determined by strong coupling to phonons by directly visualizing polaron formation in the material. We employ ultrafast electron diffraction and diffuse scattering to track the response of phonons in both momentum and time to the photodoping of free carriers acros…
Recently SnSe compound was reported to have a peak thermoelectric figure-5 of-merit (ZT) of 2.62 at 923 K, but the ZT values at temperatures below 750 K are relatively low. In this work, the electronic structures of SnSe are calculated using the density functional theory, and the electro- and thermo-transport properties upon varying chemical potential (or carrier density) are evaluated by the semi-classic Boltzmann transport theory, showing that the calculated ZT values along the a10 and c-axes below 675 K are in agreement with reported values, but that along the b-axis can be as high as 2.57 …
SnSe is a topical thermoelectric material with a low thermal conductivity which is linked to its unique crystal structure. We use low-temperature heat capacity measurements to demonstrate the presence of two characteristic vibrational energy scales in SnSe with Debye temperatures thetaD1 = 345(9) K and thetaD2 = 154(2) K. These hard and soft substructures are quantitatively linked to the strong and weak Sn-Se bonds in the crystal structure. The heat capacity model predicts the temperature evolution of the unit cell volume, confirming that this two-substructure model captures the basic thermal …
We used density functional and many-body perturbation theory to calculate the quasiparticle band structures and electronic transport parameters of p-type SnSe both for the low-temperature Pnma and high-temperature Cmcm phases. The Pnma phase has an indirect band gap of 0.829 eV while the Cmcm has a direct band gap of 0.464 eV. Both phases exhibit multiple local band extrema within an energy range comparable to the thermal energy of carriers from the global extrema. We calculated the electronic transport coefficients for single-crystal and polycrystalline materials to understand previous experi…
To gain insight into the peculiar temperature dependence of the thermoelectric material SnSe, we employ many-body perturbation theory and explore the influence of the electron-phonon interaction on its electronic and transport properties. We show that a lattice dynamics characterized by soft highly-polar phonons induces a large thermal enhancement of the Fröhlich interaction. We account for these phenomena in ab-initio calculations of the photoemission spectrum and electrical conductivity at finite temperature, unraveling the mechanisms behind recent experimental data. Our results reveal a com…
The reliable calculation of electronic structures and understanding of electrical properties depends on an accurate model of the crystal structure. Here, we have reinvestigated the crystal structure of the high-zT thermoelectric material tin selenide, SnSe, between 4 and 1000 K using high-resolution neutron powder diffraction. Symmetry analysis reveals the presence of four active structural distortion modes, one of which is found to be active over a relatively wide range of more than +/-200 K around the symmetry-breaking Pnma-Cmcm transition at 800 K. Density functional theory calculations on …
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…
Since 2014 the layered semiconductor SnSe in the high-temperature Cmcm phase is known to be the most efficient thermoelectric material. Making use of first-principles calculations we show that its vibrational and thermal transport properties are determined by huge non-perturbative anharmonic effects. We show that the transition from the Cmcm phase to the low-symmetry Pnma is a second-order phase transition driven by the collapse of a zone border phonon, whose frequency vanishes at the transition temperature. Our calculations show that the spectral function of the in-plane vibrational modes are…
Improving the thermoelectric efficiency is one of the greatest challenges in materials science. The recent discovery of excellent thermoelectric performance in simple orthorhombic SnSe crystal offers new promise in this prospect [Zhao et al. Nature 508, 373 (2014)]. By calculating the thermoelectric properties of orthorhombic IV-VI compounds GeS,GeSe,SnS,and SnSe based on the first-principles combined with the Boltzmann transport theory, we show that the Seebeck coefficient, electrical conductivity, and thermal conductivity of orthorhombic SnSe are in agreement with the recent experiment. Impo…
We theoretically investigate how each orbital and valley play a role for high thermoelectric performance of SnSe. In the hole-doped regime, two kinds of valence band valleys contribute to its transport properties: one is the valley near the U-Z line, mainly consisting of the Se-$p_z$ orbitals, and the other is the one along the $Γ$-Y line, mainly consisting of the Se-$p_y$ orbitals. Whereas the former valley plays a major role in determining the transport properties at room temperature, the latter one also offers comparable contribution and so the band structure exhibits multi-valley character…
NaCl-Assisted Growth of Ferroelectric SnSe Nanosheets with Spin Glass-like Behavior
Beijing Key Laboratory of Optoelectronic Functional Materials & Micro-nano Devices, School of Physics, Renmin University of China（中国人民大学物理学院光电功能材料与微纳器件北京市重点实验室）；Key Laboratory of Quantum State Construction and Manipulation (Ministry of Education), Renmin University of China（中国人民大学量子态构建与操控教育部重点实验室）
Interatomic forces that bind materials are largely determined by an often complex interplay between the electronic band-structure and the atomic arrangements to form its equilibrium structure and dynamics. As these forces also determine the phonon dispersion, lattice dynamics measurements are often crucial tools for understanding how materials transform between different structures. This is the case for the mono-chalcogenides which feature a number of lattice instabilities associated with their network of resonant bonds and a large tunability in their functional properties. SnSe hosts a novel …
Excellent thermoelectric performance in the out-of-layer n-doped SnSe has been observed experimentally (Chang et al., Science 360, 778-783 (2018)). However, a first-principles investigation of the dominant scattering mechanisms governing all thermoelectric transport properties is lacking. In the present work, by applying extensive first-principles calculations of electron-phonon coupling associated with the calculation of the scattering by ionized impurities, we investigate the reasons behind the superior figure of merit as well as the enhancement of zT above 600 K in n-doped out-of-layer SnSe…
SnSe monolayer with orthorhombic Pnma GeS structure is an important two-dimensional (2D) indirect band gap material at room temperature. Based on first-principles density functional theory calculations, we present systematic studies on the electronic and magnetic properties of X (X = Ga, In, As, Sb) atoms doped SnSe monolayer. The calculated electronic structures show that Ga-doped system maintains semiconducting property while In-doped SnSe monolayer is half-metal. The As- and Sb- doped SnSe systems present the characteristics of n-type semiconductor. Moreover, all considered substitutional d…
Recent studies on different topological materials in condensed matter physics have provided the evidence of topological nature for bosonic particle like phonons by performing various theoretical calculations and experimental observations. Here, the topological behaviours of phonons of SnS, SnSe and SnTe materials in rock-salt structure are investigated using $ab$-$initio$ methodology. For all these materials, the tilted linear band touching is observed along with the presence of band inversion in direction X-W. The topological point in phonon dispersion curve along X-W direction is found to be…
The layered semiconductor SnSe is one of the highest-performing thermoelectric materials known. We demonstrate, through a first-principles lattice-dynamics study, that the high-temperature Cmcm phase is a dynamic average over lower-symmetry minima separated by very small energetic barriers. Compared to the low-temperature Pnma phase, the Cmcm phase displays a phonon softening and enhanced three-phonon scattering, leading to an anharmonic damping of the low-frequency modes and hence the thermal transport. We develop a renormalisation scheme to quantify the effect of the soft modes on the calcul…
The anharmonic phonon properties of SnSe in the Pnma phase were investigated with a combination of experiments and first-principles simulations. Using inelastic neutron scattering (INS) and nuclear resonant inelastic X-ray scattering (NRIXS), we have measured the phonon dispersions and density of states (DOS) and their temperature dependence, which revealed a strong, inhomogeneous shift and broadening of the spectrum on warming. First-principles simulations were performed to rationalize these measurements, and to explain the previously reported anisotropic thermal expansion, in particular the …
Topological electronic structures are key to the topological classification of quantum materials and play an important role in their physical properties and applications. Recently, SnSe has attracted great research interests due to its superior thermoelectric performance. However, it's topological nature has long been ignored. In this work, by combining synchrotron-based angle-resolved photoemission spectroscopy and ab-initio calculations, we systematically investigated the topological electronic structure of orthorhombic SnSe. By identifying the continuous gap in the valence bands due to the …
Both SnS and SnSe have been experimentally and theoretically confirmed as topological crystalline insulators in native rocksalt structure. Here, phononic structure, thermodynamical properties and temperature dependent electron-phonon interaction (EPI) have investigated for both the materials in rocksalt phase. Previously performed theoretical studies have predicted the phase instability of SnS in this crystal structure at ambient condition. But, after a detailed study performing on the phonon calculation of SnS, we have predicted the phase stability of SnS with considering the Sn 4$p$ orbitals…
Materials with reduced dimensionality often exhibit exceptional properties that are different from their bulk counterparts. Here we report the emergence of a commensurate 2 $\times$ 2 charge density wave (CDW) in monolayer and bilayer SnSe$_2$ films by scanning tunneling microscope. The visualized spatial modulation of CDW phase becomes prominent near the Fermi level, which is pinned inside the semiconductor band gap of SnSe$_2$. We show that both CDW and Fermi level pinning are intimately correlated with band bending and virtual induced gap states at the semiconductor heterointerface. Through…
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…
We experimentally investigate transport properties of a hybrid structure, which consists of a thin single crystal SnSe flake on a top of 5~$μ$m spaced Au leads. The structure initially is in highly-conductive state, while it can be switched to low-conductive one at high currents due to the Joule heating of the sample, which should be identified as $α$-$Pnma$ -- $β$-$Cmcm$ diffusionless martensitic phase transition in SnSe. For highly-conductive state, there is significant hysteresis in $dI/dV(V)$ curves at low biases, so the sample conductance depends on the sign of the applied bias change. Th…
Two-dimensional (2D) materials may have potential applications in thermoelectric devices. In this work, we systematically investigate the thermoelectric properties of orthorhombic group IV-VI monolayers $\mathrm{AB}$ (A=Ge and Sn; B=S and Se) by the first-principles calculations and semiclassical Boltzmann transport theory. The spin-orbit coupling (SOC) is included to investigate their electronic transport, which produces observable effects on power factor, especially for n-type doping. According to calculated $ZT$, the four monolayers exhibit diverse anisotropic thermoelectric properties, alt…
The interlayer bonding in two dimensional materials is particularly important because it is not only related to their physical and chemical stability but also affects their mechanical, thermal, electronic, optical, and other properties. To address this issue, we report the direct characterization of the interlayer bonding in 2D SnSe using contact-resonance atomic force microscopy in this study. Site specific CR spectroscopy and CR force spectroscopy measurements are performed on both SnSe and its supporting SiO2 substrate comparatively. Based on the cantilever and contact mechanic models, the …
SnS and SnSe are isoelectronic materials with a common phase diagram. Recently, SnSe was found to be the most efficient intrinsic thermoelectric material in its high-temperature Cmcm phase above 800 K. Making use of first-principles calculations, here we show that the electronic and vibrational properties of both materials are very similar in this phase and, consequently, SnS is also expected to have a high thermoelectric figure of merit at high temperature in its Cmcm phase. In fact, the electronic power factor and lattice thermal conductivity are comparable for both materials, which ensures …
The tin-selenide and tin-sulfide classes of materials undergo multiple structural transitions under high pressure leading to periodic lattice distortions, superconductivity, and topologically non-trivial phases, yet a number of controversies exist regarding the structural transformations in these systems. We perform first-principles calculations within the framework of density functional theory and a careful comparison of our results with available experiments on SnSe$_2$ reveals that the apparent contradictions among high-pressure results can be attributed to differences in experimental condi…
We predict enormous piezoelectric effects in intrinsic monolayer group IV monochalcogenides (MX, M=Sn or Ge, X=Se or S), including SnSe, SnS, GeSe and GeS. Using first-principle simulations based on the modern theory of polarization, we find that their characteristic piezoelectric coefficients are about two orders of magnitude larger than those of other 2D materials, such as MoS2 and GaSe, and bulk quartz and AlN which are widely used in industry. This enhancement is a result of the unique "puckered" C2v symmetry and weaker chemical bonds of monolayer group IV monochalcogenides. Given the achi…
Stable structures of layered SnS and SnSe and their associated electronic and vibrational spectra are predicted using first-principles DFT calculations. The calculations show that both materials undergo a phase transformation upon thinning whereby the in-plane lattice parameters converge to a pseudo-cubic phase, similar to the high-temperature behaviour observed for their bulk counterparts. The electronic properties of layered SnS and SnSe evolve to an almost symmetric dispersion whilst the gap changes from indirect to direct. Characteristic signatures in the phonon dispersion curves and surfa…
Anharmonic Phonon Renormalization and Defect Tolerance of the Thermoelectric Power Factor in Monolayer SnSe
Faculty of Physics and Physics Engineering, University of Science（物理与物理工程学院，科学大学）；Viet Nam National University Ho Chi Minh City（越南国立大学胡志明市分校）；Advanced Materials Technology Institute Vietnam National University Ho Chi Minh City（先进材料技术研究所，越南国立大学胡志明市分校）；University of Health Sciences (UHS), Viet Nam National University Ho Chi Minh City（卫生科学学院（UHS），越南国立大学胡志明市分校）；Frontier Research Institute for Interdisciplinary Sciences, Tohoku University（前沿跨学科学研究院，东北大学）
Two-dimensional (2D) van der Waals ferroelectrics provide an unprecedented architectural freedom for the creation of artificial multiferroics and non-volatile electronic devices based on vertical and co-planar heterojunctions of 2D ferroic materials. Nevertheless, controlled microscopic manipulation of ferroelectric domains is still rare in monolayer-thick 2D ferroelectrics with in-plane polarization. Here we report the discovery of robust ferroelectricity with a critical temperature close to 400 K in SnSe monolayer plates grown on graphene, and the demonstration of controlled room temperature…
The search for topological systems has recently broadened to include random substitutional alloys, which lack the specific crystalline symmetries that protect topological phases, raising the question whether topological properties can be preserved, or are modified by disorder. To address this question, we avoid methods that assumed at the outset high (averaged) symmetry, using instead a fully-atomistic, topological description of alloy. Application to PbSe-SnSe alloy reveals that topology survives in an interesting fashion: (a) spatial randomness removes the valley degeneracy (splitting larger…
We compute the jerk current tensor of GaAs, Si, and ferroelectric single-layer GeS, GeSe, SnS, and SnSe. We find peak values of the order of $10^{14}$ mA/V$^3$s$^2$ in GaAs and Si within the visible energy spectrum and an order of magnitude larger in single-layer GeS, GeSe, SnSe and SnS. We show that the detailed knowledge of this tensor and its large value in single-layer GeS, GeSe, SnSe and SnS make it possible to predict the magnitude and angle of rotation of polarization of intense terahertz pulses generated in photoconductive switches and point to alternative functionalities of these devi…
We performed high pressure x-ray diffraction (XRD), Raman, and transport measurements combined with first-principles calculations to investigate the behavior of tin diselenide (SnSe$_2$) under compression. The obtained single-crystal XRD data indicate the formation of a $(1/3,1/3,1)$-type superlattice above 17 GPa. According to our density functional theory results, the pressure-induced transition to the commensurate periodic lattice distortion (PLD) phase is due to the combined effect of strong Fermi surface nesting and electron-phonon coupling at a momentum wave vector $\mathbf{q}=(1/3,1/3,1…
Efficient ab initio computational methods for the calculation of thermoelectric transport properties of materials are of great avail for energy harvesting technologies. The BoltzTraP code has been largely used to efficiently calculate thermoelectric coefficients. However, its current version that is publicly available is based only on the constant relaxation time (RT) approximation, which usually does not hold for real materials. Here, we extended the implementation of the BoltzTraP code by incorporating realistic k-dependent RT models of the temperature dependence of the main scattering proce…
