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Rashba spin-orbit coupling (RSOC) facilitates spin manipulation without relying on an external magnetic field, opening up exciting possibilities for advanced spintronic devices. In this work, we examine the effects of crystal momentum ($k$) nonlinearity and anisotropy on the conventional Rashba effect, with a particular focus on their impact on the spin Hall conductivity (SHC) in a newly predicted family of 2D Janus materials, W$_2$COX (X = S, Se, Te). Using first-principles density functional theory calculations, we confirm the dynamical and mechanical stability of the studied 2D materials. Strikingly, this materials family exhibits pronounced nonlinear Rashba spin splitting at the $\Gamma$ point of Brillouin zone near the Fermi level, which cannot be adequately described by the linear-$k$ Rashba model. Therefore, third-order momentum contributions ({$k^3$}) must be incorporated into the Rashba Hamiltonian. Our analysis reveals that among the studied systems, W$_2$COS exhibits the highest {$k^3$} contribution of $-45.9$\,eV\,{\AA$^3$}, despite having the lowest linear Rashba constant. A detailed analysis of electronic structure reveals topological nontrivial behaviour in these 2D materials, yielding sizable SHC that is primarily governed by the nonlinear Rashba effect. Notably, these materials also exhibit large spin Hall angle (0.018 -- 2.5), which is comparable to that of in bulk topological insulators like Bi$_2$Se$_3$ and Bi$_2$Te$_3$, and surpassing those in narrow bandgap bulk semiconductors GeTe and SnTe, as well as heavy metals such as Pt. Sizable SHC, large spin Hall angles, and the ability to tune SHC via electric fields without altering the topological properties, rooted in the crystal field splitting, underscore the potential of these materials for spintronic applications., Comment: 11 pages, 9 figures

Kagome metals are a unique class of quantum materials characterized by their distinct atomic lattice arrangement, featuring interlocking triangles and expansive hexagonal voids. These lattice structures impart exotic properties, including superconductivity, interaction-driven topological many-body phenomena, and magnetism, among others. The kagome metal CsM3Te5 (where M = Ti, Zr, or Hf) exhibits both superconductivity and nontrivial topological electronic properties, offering a promising platform for exploring topological superconductivity. This study employs first-principles density functional theory calculations to systematically analyze the elastic, mechanical, vibrational, thermodynamic, and electronic properties of CsM3Te5 (M = Ti, Zr, Hf). Our calculations reveal that the studied compounds - CsTi3Te5, CsZr3Te5, and CsHf3Te5 - are ductile metals with elastic properties akin to the hexagonal Bi and Sb, with average elastic constants, including a bulk modulus of 27 GPa, a shear modulus of 11 GPa, and Young's modulus of 29 GPa. We observe peculiar dispersionless, flat, phonon branches in the vibrational spectra of these metals. Additionally, we thoroughly analyze the symmetries of the zone-center phonon eigenvectors and predict vibrational fingerprints of the Raman- and infrared-active phonon modes. The analysis of thermodynamic properties reveals the Einstein temperature for CsTi3Te5, CsZr3Te5, and CsHf3Te5 to be 66, 54, and 53 K, respectively. Our orbital-decomposed electronic structure calculations reveal significant in-plane steric interactions and multiple Dirac band crossings near the Fermi level. We further investigate the role of spin-orbit coupling effect on the studied properties. This theoretical investigation sheds light on the intriguing quantum behaviour of kagome metals and provides valuable insights for their potential applications in future technologies., Comment: 10 pages, 8 figures

The investigation of emerging non-toxic perovskite materials has been undertaken to advance the fabrication of environmentally sustainable lead-free perovskite solar cells. This study introduces a machine learning methodology aimed at predicting innovative halide perovskite materials that hold promise for use in photovoltaic applications. The seven newly predicted materials are as follows: CsMnCl$_4$, Rb$_3$Mn$_2$Cl$_9$, Rb$_4$MnCl$_6$, Rb$_3$MnCl$_5$, RbMn$_2$Cl$_7$, RbMn$_4$Cl$_9$, and CsIn$_2$Cl$_7$. The predicted compounds are first screened using a machine learning approach, and their validity is subsequently verified through density functional theory calculations. CsMnCl$_4$ is notable among them, displaying a bandgap of 1.37 eV, falling within the Shockley-Queisser limit, making it suitable for photovoltaic applications. Through the integration of machine learning and density functional theory, this study presents a methodology that is more effective and thorough for the discovery and design of materials.

The Rashba spin-orbit coupling effect, primarily arising from structural-inversion asymmetry in periodic crystals, has garnered considerable attention due to its tunability and potential applications in spintronics. Its capability to manipulate electron spin without an external magnetic field opens new avenues for spintronic device design, particularly in semiconductor technology. Within this framework, 2D Rashba materials hold special interest due to their inherent characteristics, which facilitate miniaturization and engineering capabilities. In this Perspective article, we provide an overview of recent advancements in the research of 2D Rashba materials, aiming to offer a comprehensive understanding of the diverse manifestations and multifaceted implications of the Rashba effect in material science. Rather than merely presenting a list of materials, our approach involves synthesizing various viewpoints, assessing current trends, and addressing challenges within the field. Our objective is to bridge the gap between fundamental research and practical applications by correlating each material with the necessary advancements required to translate theoretical concepts into tangible technologies. Furthermore, we highlight promising avenues for future research and development, drawing from insights gleaned from the current state of the field.

Electric currents have the intriguing ability to induce magnetization in nonmagnetic crystals with sufficiently low crystallographic symmetry. Some associated phenomena include the non-linear anomalous Hall effect in polar crystals and the nonreciprocal directional dichroism in chiral crystals when magnetic fields are applied. In this work, we demonstrate that the same underlying physics is also manifested in the electronic tunneling process between the surface of a nonmagnetic chiral material and a magnetized scanning probe. In the paramagnetic but chiral metallic compound Co$_{1/3}$NbS$_2$, the magnetization induced by the tunneling current is shown to become detectable by its coupling to the magnetization of the tip itself. This results in a contrast across different chiral domains, achieving atomic-scale spatial resolution of structural chirality. To support the proposed mechanism, we used first-principles theory to compute the chirality-dependent current-induced magnetization and Berry curvature in the bulk of the material. Our demonstration of this magnetochiral tunneling effect opens up a new avenue for investigating atomic-scale variations in the local crystallographic symmetry and electronic structure across the structural domain boundaries of low-symmetry nonmagnetic crystals., Comment: 4 figures

Kagome metals are reported to exhibit remarkable properties, including superconductivity, charge density wave order, and a large anomalous Hall conductivity, which facilitate the implementation of spintronic devices. In this work, we study a novel kagome metal based on Mn magnetic sites in a KMn$_3$Sb$_5$ stoichiometry. By means of first-principles density functional theory calculations, we demonstrate that the studied compound is dynamically stable, locking the ferromagnetic order as the ground state configuration, thus preventing the charge-density-wave state as reported in its vanadium-based counterpart KV$_3$Sb$_5$. Our calculations predict that KMn$_3$Sb$_5$ exhibits an out-of-plane (001) ferromagnetic response as the ground state, allowing for the emergence of topologically protected Weyl nodes near the Fermi level and nonzero anomalous Hall conductivity ($\sigma_{ij}$) in this centrosymmetric system. We obtain a tangible $\sigma_{xy} = 314$ S$\cdot$cm$^{-1}$ component, which is comparable to that of other kagome metals. Finally, we explore the effect of the on-site Coulomb repulsion ($+U$) on the structural and electronic properties and find that, although the lattice parameters and $\sigma_{xy}$ moderately vary with increasing $+U$, KMn$_3$Sb$_5$ stands as an ideal stable ferromagnetic kagome metal with a large anomalous Hall conductivity response.

A new mechanism for memristive switching in 2D materials is through electric-field controllable electronic/structural phase transitions, but these devices have not outperformed status quo 2D memristors. Here, we report a high-performance bipolar phase change memristor from strain engineered multilayer 1T'-MoTe$_{2}$ that now surpasses the performance metrics (on/off ratio, switching voltage, switching speed) of all 2D memristive devices, achieved without forming steps. Using process-induced strain engineering, we directly pattern stressed metallic contacts to induce a semimetallic to semiconducting phase transition in MoTe2 forming a self-aligned vertical transport memristor with semiconducting MoTe$_{2}$ as the active region. These devices utilize strain to bring them closer to the phase transition boundary and achieve ultra-low ~90 mV switching voltage, ultra-high ~10$^8$ on/off ratio, 5 ns switching, and retention of over 10$^5$ s. Engineered tunability of the device switching voltage and on/off ratio is also achieved by varying the single process parameter of contact metal film force (film stress $\times$ film thickness).

This work presents a machine learning approach to predict novel perovskite oxide materials in the Pr-Al-O and Pr-Sc-O compound families with the potential for photoluminescence applications. The predicted materials exhibit a large bandgap and high Debye temperature, and have remained unexplored thus far. The predicted compounds (Pr$_3$AlO$_6$, Pr$_4$Al$_2$O$_9$, Pr$_3$ScO$_6$ and Pr$_3$Sc$_5$O$_{12}$) are screened using machine learning approach, which are then confirmed by density functional theory calculations. The study includes the calculation of the bandgap and density of states to determine electronic properties, and the optical absorption and emission spectra to determine optical properties. Mechanical stability of the predicted compounds, as demonstrated by satisfying the Born-Huang criterion. By combining machine learning and density functional theory, this work offers a more efficient and comprehensive approach to materials discovery and design.

The van der Waals layered ferroelectric CuInP$_2$S$_6$ has been found to exhibit a variety of intriguing properties arising from the fact that the Cu ions are unusually mobile in this system. While the polarization switching mechanism is usually understood to arise from Cu ion motion within the monolayers, a second switching path involving Cu motion across the van der Waals gaps has been suggested. In this work, we perform zero-temperature first-principles calculations on such switching paths, focusing on two types that preserve the periodicity of the primitive unit cell: ``cooperative" paths preserving the system's glide mirror symmetry, and ``sequential" paths in which the two Cu ions in the unit cell move independently of each other. We find that CuInP$_2$S$_6$ features a rich and varied energy landscape, and that sequential paths are clearly favored energetically both for cross-gap and through-layer paths. Importantly, these segments can be assembled to comprise a globally insulating cycle with the out-of-plane polarization evolving by a quantum as the Cu ions shift to neighboring layers. In this sense, we argue that CuInP$_2$S$_6$ embodies the physics of a quantized adiabatic charge pump.

Twisted bilayer graphene exhibits electronic properties that are highly correlated with the size and arrangement of moir\'e patterns. While rigid rotation of two layers creates the topology of moir\'e patterns, local rearrangements of the atoms due to interlayer van der Waals interactions result in atomic reconstruction within the moir\'e cells. The ability to manipulate these patterns by controlling twist angle and/or externally applied strain provides a promising route to tune their properties. While this phenomenon has been extensively studied for angles close to or smaller than the magic angle ({\theta}m=1.1{\deg}), its extent for higher angles and how it evolves with strain is unknown and is believed to be mostly absent at high angles. We use theoretical and numerical analyses to resolve reconstruction in angles above {\theta}m using interpretive and fundamental physical measures. In addition, we propose a method to identify local regions within moir\'e cells and track their evolution with strain for a range of representative high twist angles. Our results show that reconstruction is actively present beyond the magic angle and its contribution to the evolution of the moir\'e cells is major. Our theoretical method to correlate local and global phonon behavior provides further validation on the role of reconstruction at higher angles. Our findings provide a better understanding of moir\'e reconstruction in large twist angles and the evolution of moir\'e cells in the presence of strain, that might be very crucial for twistronics-based applications.

The group-IV diamond-structure elements are known to host a variety of planar defects, including {001} planar defects in C and {001}, {111} and {113} planar defects in Si and Ge. Among the {001} planar defects, the Humble defect, known for some time to occur in Ge, has recently also been observed in Si/Ge alloys, but the details of its electronic structure remain poorly understood. Here we perform first-principles density functional calculations to study Humble defects in both Ge and Ge$_{0.8}$Si$_{0.2}$. We also measure the Si L$_{2,3}$-edge electron energy loss spectra both at the defect and in a bulk-like region far from the defect, and compare with theoretical calculations on corresponding Si sites in our first-principles calculations. We find that inclusion of core-hole effects in the theory is essential for reproducing the observed L$_{2,3}$ edge spectra, and that once they are included, the results provide a set of fingerprints for different types of local atomic bonding environments in Ge$_{0.8}$Si$_{0.2}$. Our first-principles calculations reveal that the Humble defects have a tendency to enlarge the electronic band gap, which may have potential uses in band engineering. The use of hybrid functionals for an improved description of the band gap in these systems is also discussed., Comment: 11 pages, 10 figures; Comments are welcome

In order to explore the properties of a two-sublattice ferroelectric, we measured the infrared and Raman scattering response of CuInP2S6 across the ferroelectric and glassy transitions and compared our findings to a symmetry analysis, calculations of phase stability, and lattice dynamics. In addition to uncovering displacive character and a large hysteresis region surrounding the ferroelectric transition temperature T_C, we identify the vibrational modes that stabilize the polar phase and confirm the presence of two ferroelectric variants with opposite polarizations. Below TC, a poorly understood relaxational or glassy transition at Tg is characterized by local structure changes in the form of subtle peak shifting and activation of low frequency out-of-plane Cu- and In-containing modes. The latter are due to changes in the Cu/In coordination environments and associated order-disorder processes. Moreover, Tg takes place in two steps with another large hysteresis region and significant underlying scattering. Combined with imaging of the room temperature phase separation, this effort lays the groundwork for studying CuInP2S6 under external stimuli and in the ultra-thin limit.

Hafnia (HfO2) is a promising material for emerging chip applications due to its high-k dielectric behaviour, suitability for negative capacitance heterostructures, scalable ferroelectricity, and silicon compatibility. The lattice dynamics along with phononic properties such as thermal conductivity, contraction, and heat capacity are under-explored, primarily due to the absence of high quality single crystals. Herein, we report the vibrational properties of a series of HfO2 crystals stabilized with yttrium (chemical formula HfO2:xY, where x = 20, 12, 11, 8, and 0%) and compare our findings with a symmetry analysis and lattice dynamics calculations. We untangle the effects of Y by testing our calculations against the measured Raman and infrared spectra of the cubic, antipolar orthorhombic, and monoclinic phases and then proceed to reveal the signature modes of polar orthorhombic hafnia. This work provides a spectroscopic fingerprint for several different phases of HfO2 and paves the way for an analysis of mode contributions to high-k dielectric and ferroelectric properties for chip technologies.

We report a new \{001\} planar defect found in SiGe nanopillars. The defect structure, determined by atomic resolution electron microscopy, matches the Humble defect model proposed for diamond. We also investigated several possible variants of the Humble structure using first principles calculations and found that the one lowest in energy was also in best agreement with the STEM images. The pillar composition has been analyzed with electron energy loss spectroscopy, which hints at how the defect is formed. Our results show that the structure and formation process of defects in nanostructured group IV semiconductors can be different from their bulk counterparts.

Orientation dependent electronic properties of wurtzite zinc oxide nanorods are characterized by aloof beam electron energy-loss spectroscopy (EELS) carried out in a scanning transmission electron microscope (STEM). The two key crystal orientation differentiating transitions specific to the in-plane (13.0 eV) and out-of-plane (11.2 eV) directions with respect to the wurtzite structure are examined by first principles density-functional theory calculations. We note some degree of orientation dependence at the onset of direct band gap transition near 3.4 eV. We demonstrate that good polarization selectivity can be achieved by placing the electron probe at different locations around the specimen with increasing impact parameter while keeping the beam-specimen orientation fixed. The observed results are qualitatively elucidated in terms of the perpendicular electric fields generated by the fast electron (60 kV) used in the microscope. The fact that good polarization selectivity can be achieved by aloof beam EELS without the requirement of sample reorientation is an attractive aspect from the characterization method point of view in the STEM-EELS community., Comment: 9 pages and 9 figures

A new two-dimensional material { Mg2B4C2, belonging to the family of the conventional superconductor MgB2, is theoretically predicted to exhibit superconductivity with critical temperature Tc estimated in the 47-48 K range (predicted using the McMillian-Allen-Dynes formula) without any tuning of external parameters such as doping, strain, or substrate-induced effects. The origin of such a high intrinsic Tc is ascribed to the presence of strong electron-phonon coupling and topological Dirac states (which are absent in MgB2) yielding a large density of states at the Fermi level. This material also features a nontrivial electronic band topology exhibiting Dirac points, practically gapless Dirac nodal lines, and topological nontrivial edge states. Consequently, it is a potential candidate for realization of topological superconductivity in 2D. This system is obtained after replacing the chemically active boron layers in MgB2 by chemically inactive boron-carbon layers. Hence, the surfaces of this material are inert. Our calculations confirm the stability of 2D Mg2B4C2. We also find that the key features of this material remain essentially unchanged when its thickness is increased by modestly increasing the number of inner MgB2 layers., Comment: 11 pages, 3 figures

By means of first-principles density-functional theory calculations, we investigate the role of hydrostatic pressure on the electronic structure of Td (Pmn21 ) and 1T" (Pm) phases of Weyl semimetal Mo0.5 W0.5 Te2 , which is a promising material for phase-change memory technology and superconductivity. We particularly focus on changes occurring in the distribution of the gapless Weyl points (WPs) within 0 to 45 GPa pressure range. We further investigate the structural phase transition and lattice dynamics of the Td and 1T" phases within the aforementioned pressure range. Our calculations suggest that both the Td and 1T" phases of Mo0.5 W0.5 Te2 host four WPs in their full Brillouin zone at zero pressure. The total number of WPs increases to 44 (36) with increasing pressure via pair creation up to 20 (15) GPa for the T d (1T 00 ) phase, and beyond this pressure pair annihilation of WPs starts occurring leaving only 16 WPs at 45 GPa in both phases. The enthalpy versus pressure data reveal that the 1T 00 phase is more favorable below the critical pressure of 7.5 GPa, however, beyond this critical pressure the Td phase becomes enthalpically favorable. We also provide the calculated x-ray diffraction spectra along with the calculated Raman- and infrared-active phonon frequencies to facilitate the experimental identification of the studied phases., Comment: 10 pages, 5 figures

Density-functional theory is widely used to predict the physical properties of materials. However, it usually fails for strongly correlated materials. A popular solution is to use the Hubbard corrections to treat strongly correlated electronic states. Unfortunately, the exact values of the Hubbard $U$ and $J$ parameters are initially unknown, and they can vary from one material to another. In this semi-empirical study, we explore the $U$ and $J$ parameter space of a group of iron-based compounds to simultaneously improve the prediction of physical properties (volume, magnetic moment, and bandgap). We used a Bayesian calibration assisted by Markov chain Monte Carlo sampling for three different exchange-correlation functionals (LDA, PBE, and PBEsol). We found that LDA requires the largest $U$ correction. PBE has the smallest standard deviation and its $U$ and $J$ parameters are the most transferable to other iron-based compounds. Lastly, PBE predicts lattice parameters reasonably well without the Hubbard correction.

In this work, we carry out first-principles calculations and lattice mode analysis to investigate the polarization switching mechanism in HfO$_2$. Because the stability of the polar orthorhombic $Pca2_1$ phase of HfO$_2$ arises from a trilinear coupling, polarization switching requires the flipping of not only the polar $\Gamma_{15}^Z$ mode, but also at least one zone-boundary anti-polar mode. The coupling between the polar and anti-polar modes thus leads to substantial differences among different polarization switching paths. Specifically, our lattice-mode-coupling analysis shows that paths in which the $X_2^-$ mode is reversed involve a large activation energy, which because the $X_2^-$ mode is nonpolar cannot be directly overcome by applying an electric field. Our results show that the anti-polar $Pbca$ phase, whose structure is locally quite similar to that of the $Pca2_1$ phase, similarly cannot be transformed to this phase by an electric field as this would require local reversal of the $X_2^-$ mode pattern. Moreover, for the domain wall structure most widely considered, propagation also requires the reversal of the $X_2^-$ mode, leading to a much larger activation energy compared with that for the propagation of domain wall structures with a single sign for the $X_2^-$ mode. Finally, these first-principles results for domain wall propagation in HfO$_2$ have implications to many experimental observations, such as sluggish domain wall motion and robust ferroelectricity in thin films, and lattice mode analysis deepens our understanding of these distinctive properties of ferroelectric HfO$_2$.

GeCo$_2$O$_4$ is a unique system in the family of cobalt spinels ACo$_2$O$_4$ (A= Sn, Ti, Ru, Mn, Al, Zn, Fe, etc.) in which magnetic Co ions stabilize on the pyrochlore lattice exhibiting a large degree of orbital frustration. Due to the complexity of the low-temperature antiferromagnetic (AFM) ordering and long-range magnetic exchange interactions, the lattice dynamics and magnetic structure of GeCo$_2$O$_4$ spinel has remained puzzling. To address this issue, here we present theoretical and experimental investigations of the highly frustrated magnetic structure, and the infrared (IR) and Raman-active phonon modes in the spinel GeCo$_2$O$_4$, which exhibits an AFM ordering below the N\'eel temperature $T_N$ ~21 K, followed by a cubic ($Fd{\bar 3}m$) to tetragonal ($I4_{1}/amd$) structural phase transition at $T_S$ ~16 K. Our density-functional theory (DFT+U) calculations reveal that one needs to consider magnetic-exchange interactions up to the third nearest neighbors to get an accurate description of the low-temperature AFM order in GeCo$_2$O$_4$. At room temperature three distinct IR-active modes ($T_{1u}$) are observed at frequencies 680, 413, and 325 cm$^{-1}$ along with four Raman-active modes $A_{1g}$, $T_{2g}(1)$, $T_{2g}(2)$, and $E_{g}$ at frequencies 760, 647, 550, and 308 cm$^{-1}$, respectively, which match reasonably well with our DFT+U calculated values. All the IR-active and Raman-active phonon modes exhibit signatures of moderate spin-phonon coupling. The temperature dependence of various parameters, such as the shift, width, and intensity, of the Raman-active modes, is also discussed. Noticeable changes around $T_N$ and $T_S$ are observed in the Raman line parameters of the $E_{g}$ and $T_{2g}$ modes, which are associated with the modulation of the Co-O bonds in CoO$_6$ octahedra during the excitations of these modes., Comment: 9 figures and 11 pages

HfO2, a simple binary oxide, holds ultra-scalable ferroelectricity integrable into silicon technology. Polar orthorhombic (Pbc21) form in ultra-thin-films ascribes as the plausible root-cause of the astonishing ferroelectricity, which has thought not attainable in bulk crystals. Though, perplexities remain primarily due to the polymorphic nature and the characterization challenges at small-length scales. Herein, utilizing a state-of-the-art Laser-Diode-heated Floating Zone technique, we report ferroelectricity in bulk single-crystalline HfO2:Y as well as the presence of anti-polar Pbca phase at different Y concentrations. Neutron diffraction and atomic imaging demonstrate (anti-)polar crystallographic signatures and abundant 90o/180o ferroelectric domains in addition to the switchable polarization with little wake-up effects. Density-functional theory calculations suggest that the Yttrium doping and rapid cooling are the key factors for the desired phase. Our observations provide new insights into the polymorphic nature and phase controlling of HfO2, remove the upper size limit for ferroelectricity, and also pave a new road toward the next-generation ferroelectric devices., Comment: 34 pages. This is the original preprint before peer review. The final version has been accepted by Nature Materials

The MechElastic Python package evaluates the mechanical and elastic properties of bulk and 2D materials using the elastic coefficient matrix ($C_{ij}$) obtained from any ab-initio density-functional theory (DFT) code. The current version of this package reads the output of VASP, ABINIT, and Quantum Espresso codes (but it can be easily generalized to any other DFT code) and performs the appropriate post-processing of elastic constants as per the requirement of the user. This program can also detect the input structure's crystal symmetry and test the mechanical stability of all crystal classes using the Born-Huang criteria. Various useful material-specific properties such as elastic moduli, longitudinal and transverse elastic wave velocities, Debye temperature, elastic anisotropy, 2D layer modulus, hardness, Pugh's ratio, Cauchy's pressure, Kleinman parameter, and Lame's coefficients, can be estimated using this program. Another existing feature of this program is to employ the ELATE package [J. Phys.: Condens. Matter 28, 275201 (2016)] and plot the spatial variation of several elastic properties such as Poisson's ratio, linear compressibility, shear modulus, and Young's modulus in three dimensions. Further, the MechElastic package can plot the equation of state (EOS) curves for energy and pressure for a variety of EOS models such as Murnaghan, Birch, Birch-Murnaghan, and Vinet, by reading the inputted energy/pressure versus volume data obtained via numerical calculations or experiments. This package is particularly useful for the high-throughput analysis of elastic and mechanical properties of materials.

Hafnia (HfO$_2$)-based thin films have promising applications in nanoscale electronic devices due to their robust ferroelectricity and integration with silicon. However, HfO$_2$ has various stable and metastable polymorphs with quite similar structures and energies. Identifying and stabilizing the ferroelectric functional phases of HfO$_2$ have attracted intensive research interest in recent years. In this work, first-principles calculations on (111)-oriented HfO$_2$ are used to discover that imposing an in-plane shear strain on the tetragonal phase induces a nonpolar to polar phase transition. This in-plane shear-induced polar phase is shown to be an epitaxial distortion of a known metastable ferroelectric $Pnm2_1$ phase of HfO$_2$. It is proposed that this ferroelectric $Pnm2_1$ phase can account for the recently observed ferroelectricity in the (111)-oriented HfO$_2$-based thin film [Nature Materials 17, 1095-1100 (2018)]. Further investigation of this second functional ferroelectric phase in HfO$_2$ could potentially improve the performances of HfO$_2$-based films in logic and memory devices., Comment: 6 pages, 4 figures

MoTe$_2$ has recently attracted much attention due to the observation of pressure-induced superconductivity, exotic topological phase transitions, and nonlinear quantum effects. However, there has been debate on the intriguing structural phase transitions among various observed phases of MoTe$_2$, and their connection to the underlying topological electronic properties. In this work, by means of density-functional theory (DFT+U) calculations, we investigate the structural phase transition between the polar T$_d$ and nonpolar 1T$'$ phases of MoTe$_2$ in reference to a hypothetical high-symmetry T$_0$ phase that exhibits higher-order topological features. In the T$_d$ phase we obtain a total of 12 Weyl points, which can be created/annihilated, dynamically manipulated, and switched by tuning a polar phonon mode. We also report the existence of a tunable nonlinear Hall effect in T$_d$-MoTe$_2$, and propose the use of this effect as a probe for the detection of polarity orientation in polar (semi)metals. By studying the role of dimensionality, we identify a configuration in which a nonlinear surface response current emerges. The potential technological applications of the tunable Weyl phase and the nonlinear Hall effect are discussed.

Much of the dramatic growth in research on topological materials has focused on topologically protected surface states. While the domain walls of topological materials such as Weyl semimetals with broken inversion or time-reversal symmetry can provide a hunting ground for exploring topological interfacial states, such investigations have received little attention to date. Here, utilizing in-situ cryogenic transmission electron microscopy combined with first-principles calculations, we discover intriguing domain-wall structures in MoTe2, both between polar variants of the low-temperature(T) Weyl phase, and between this and the high-T high-order topological phase. We demonstrate how polar domain walls can be manipulated with electron beams and show that phase domain walls tend to form superlattice-like structures along the c axis. Scanning tunneling microscopy indicates a possible signature of a conducting hinge state at phase domain walls. Our results open avenues for investigating topological interfacial states and unveiling multifunctional aspects of domain walls in topological materials., Comment: 6 figures

The PyProcar Python package plots the band structure and the Fermi surface as a function of site and/or s,p,d,f - projected wavefunctions obtained for each $k$-point in the Brillouin zone and band in an electronic structure calculation. This can be performed on top of any electronic structure code, as long as the band and projection information is written in the PROCAR format, as done by the VASP and ABINIT codes. PyProcar can be easily modified to read other formats as well. This package is particularly suitable for understanding atomic effects into the band structure, Fermi surface, spin texture, etc. PyProcar can be conveniently used in a command line mode, where each one of the parameters define a plot property. In the case of Fermi-surfaces, the package is able to plot the surface with colors depending on other properties such as the electron velocity or spin projection. The mesh used to calculate the property does not need to be the same as the one used to obtain the Fermi surface. A file with a specific property evaluated for each $k$-point in a $k-$mesh and for each band can be used to project other properties such as electron-phonon mean path, Fermi velocity, electron effective mass, etc. Another existing feature refers to the band unfolding of supercell calculations into predefined unit cells., Comment: 32 pages, 14 figures, Submitted to "Computer Physics Communications", project url: "https://github.com/romerogroup/pyprocar"

We discuss and illustrate the appearance of topological fermions and bosons in triple-point metals, where a band crossing of three electronic bands occurs close to the Fermi level. Topological bosons appear in the phonon spectrum of certain triple-point metals, depending on the mass of atoms that form the binary triple-point metal. We first provide a classification of possible triple-point electronic topological phases possible in crystalline compounds and discuss the consequences of these topological phases, seen in Fermi arcs, topological Lifshitz transitions and transport anomalies. Then we show how the topological phase of phonon modes can be extracted and proven for relevant compounds. Finally, we show how the interplay of electronic and phononic topologies in triple-point metals puts these \textit{metallic} materials into the list of the most efficient \textit{metallic} thermoelectrics known to date., Comment: arXiv admin note: substantial text overlap with arXiv:1605.04653

We employ an ab-initio structure search algorithm to explore the configurational space of Bi in quasi two dimensions. A confinement potential restricts the movement of atoms within a pre-defined thickness during structure search calculations within the minima hopping method to find the stable and metastable forms of bilayer Bi. In addition to recovering the two known low-energy structures (puckered monoclinic and buckled hexagonal), our calculations predict three new structures of bilayer Bi. We call these structures the $\alpha$, $\beta$, and $\gamma$ phases of bilayer Bi, which are, respectively, 63, 72, and 83 meV/atom higher in energy than that of the monoclinic ground state, and thus potentially synthesizable using appropriate substrates. We also compare the structural, electronic, and vibrational properties of the different phases. The puckered monoclinic, buckled hexagonal, and $\beta$ phases exhibit a semiconducting energy gap, whereas $\alpha$ and $\gamma$ phases are metallic. We notice an unusual Mexican-hat type band dispersion leading to a van Hove singularity in the buckled hexagonal bilayer Bi. Notably, we find symmetry-protected topological Dirac points in the electronic spectrum of the $\gamma$ phase. The new structures suggest that bilayer Bi provides a novel playground to study distortion-mediated metal-insulator phase transitions.

Recently published discoveries of acoustic and optical mode inversion in the phonon spectrum of certain metals became the first realistic example of non-interacting topological bosonic excitations in existing materials. However, the observable physical and technological use of such topological phonon phases remained unclear. In this work we provide a strong theoretical and numerical evidence that for a class of metallic compounds (known as triple point topological metals), the points in the phonon spectrum, at which three (two optical and one acoustic) phonon modes (bands) cross, represent a well-defined topological material phase, in which the hosting metals have very strong thermoelectric response. The triple point bosonic collective excitations appearing due to these topological phonon band-crossing points significantly suppress the lattice thermal conductivity, making such metals phonon-glass like. At the same time, the topological triple-point and Weyl fermionic quasiparticle excitations present in these metals yield good electrical transport (electron-crystal) and cause a local enhancement in the electronic density of states near the Fermi level, which considerably improves the thermopower. This combination of "phonon-glass" and "electron-crystal" is the key for high thermoelectric performance in metals. We call these materials topological thermoelectric metals and propose several newly predicted compounds for this phase (TaSb and TaBi). We hope that this work will lead researchers in physics and materials science to the detailed study of topological phonon phases in electronic materials, and the possibility of these phases to introduce novel and more efficient use of thermoelectric materials in many everyday technological applications.

Graphene/MoS2 heterostructures are formed by combining the nanosheets of graphene and monolayer MoS2. The electronic features of both constituent monolayers are rather well-preserved in the resultant heterostructure due to the weak van der Waals interaction between the layers. However, the proximity of MoS2 induces strong spin orbit coupling effect of strength ~1 meV in graphene, which is nearly three orders of magnitude larger than the intrinsic spin orbit coupling of pristine graphene. This opens a bandgap in graphene and further causes anticrossings of the spin-nondegenerate bands near the Dirac point. Lattice incommensurate graphene/MoS2 heterostructure exhibits interesting moire' patterns which have been observed in experiments. The electronic bandstructure of heterostructure is very sensitive to biaxial strain and interlayer twist. Although the Dirac cone of graphene remains intact and no charge-transfer between graphene and MoS2 layers occurs at ambient conditions, a strain-induced charge-transfer can be realized in graphene/MoS2 heterostructure. Application of a gate voltage reveals the occurrence of a topological phase transition in graphene/MoS2 heterostructure. In this chapter, we discuss the crystal structure, interlayer effects, electronic structure, spin states, and effects due to strain and substrate proximity on the electronic properties of graphene/MoS2 heterostructure. We further present an overview of the distinct topological quantum phases of graphene/MoS2 heterostructure and review the recent advancements in this field., Comment: 31 pages, 12 figures

Graphene/MoS$_2$ van der Waals (vdW) heterostructures have promising technological applications due to their unique properties and functionalities. Many experimental and theoretical research groups across the globe have made outstanding contributions to benchmark the properties of graphene/MoS$_2$ heterostructures. Even though some research groups have already made an attempt to model the graphene/MoS$_2$ heterostructures using {\it first-principles} calculations, there exists several discrepancies in the results from different theoretical research groups and the experimental findings. In the present work, we revisit this problem by first principles approach and address the existing discrepancies about the interlayer spacing between graphene and MoS$_2$ monolayers in graphene/MoS$_2$ heterostructures, and the location of Dirac points near Fermi-level. We find that the Tkatchenko--Scheffler method efficiently evaluates the long-range vdW interactions and accurately predicts interlayer spacing between graphene and MoS$_2$ sheets. We further investigate the electronic, mechanical and vibrational properties of the optimized graphene/MoS$_2$ heterostructures created using 5$\times$5/4$\times$4 and 4$\times$4/3$\times$3 supercell geometries having different magnitudes of lattice mismatch. The effect of the varying interlayer spacing on the electronic properties of heterostructures is discussed. Our phonon calculations reveal that the interlayer shear and breathing phonon modes, which are very sensitive to the weak vdW interactions, play vital role in describing the thermal properties of the studied systems. The thermodynamic and elastic properties of heterostructures are further discussed. A comparison between our results and the results reported from other research groups is presented., Comment: 13 pages, 7 figures

Using first principles calculations, we systematically study the elastic stiffness constants, mechanical properties, elastic wave velocities, Debye temperature, melting temperature, and specific heat of several thermodynamically stable crystal structures of Bi$_{x}$Sb$_{1-x}$ ($0 < x < 1$) binaries, which are of great interest due to their numerous inherent rich properties, such as thermoelectricity, thermomagnetic cooling, strong spin-orbit coupling (SOC) effects, and topological features in the electronic bandstructure. We analyze the bulk modulus ($B$), Young's modulus ($E$), shear modulus ($G$), $B/G$ ratio, and Poisson's ratio ($\nu$) as a function of the Bi concentration in Bi$_{x}$Sb$_{1-x}$. The effect of SOC on above mentioned properties is further investigated. In general, we observe that the SOC effects cause elastic softening in most of the studied structures. Three monoclinic structures of Bi-Sb binaries are found to exhibit significantly large auxeticity. The Debye temperature and the magnitude of the elastic wave velocities monotonically decrease with increasing Bi-concentration. We also discuss the specific heat capacity versus temperature data for all studied binaries. Our theoretical results are in excellent agreement with the existing experimental and theoretical data. The comprehensive understanding of the material properties such as hardness, mechanical strength, melting temperature, propagation of the elastic waves, auxeticity, and heat capacity is vital for practical applications of the studied binaries., Comment: 11 pages, 5 figures

The stability of the stored information in recording media depends on the anisotropy energy Ea = KeffV of nanoparticles (NPs) of volume V or diameter D. Therefore, the knowledge of how the effective magnetic anisotropy Keff varies with D for a given system is important for technological applications. In a recent paper [Appl. Phys. Lett. 110 (22), 222409 (2017)], the variation of Keff versus D in NPs of maghemite ({\gamma}-Fe2O3) was best described by the Eq.: Keff = Kb + (6KS/D) +Ksh{[1-(2d/D)]-3 -1}, where Kb, KS, and Ksh are the anisotropy constants of spins in the core, surface layer, and a shell of thickness d, respectively. This core-shell-surface layer (CSSL) model is an extension of the often used core-surface layer (CSL) model described by Keff = Kb + (6KS/D) [Phys. Rev. Lett. 72, 282 (1994)]. The additional term involving Ksh was found to be necessary to fit the data for smaller NPs of {\gamma}-Fe2O3 with D < 5 nm. Here we check the validity of the CSSL model for metallic magnetic NPs of Co, Ni, Fe and magnetite using Keff vs. D data from published literature. Care was taken in selecting data only for those NPs for which the effects of interparticle interactions has been taken into account in determining Keff. The importance of the new CSSL model is that it describes well the Keff vs. D variation for all particles sizes whereas the core-surface layer model often fails for smaller particles with the notable exception of Fe NPs. The verification of the CSSL model for metallic NPs of Co, Ni, and magnetite along with NPs of NiO and {\gamma}-Fe2O3 validates its general applicability.

The particle size (D) dependence of the effective magnetic anisotropy Keff of magnetic nanoparticles (NPs) usually shows Keff increasing with decreasing D. This dependence is often interpreted using the Eq.: Keff = Kb + (6Ks/D) where Kb and Ks are the anisotropy constants of the spins in the bulk-like core and surface layer, respectively. Here, we show that this model is inadequate to explain the observed size-dependency of Keff for smaller nanoparticles with D < 5 nm. Instead the results in NPs of maghemite ({\gamma}-Fe2O3), NiO and Ni are best described by an extension of the above model leading to the variation given by Keff = Kb + (6Ks/D) +Ksh{[1-(2d/D)]^(-3) -1}, where the last term is due to the spins in a shell of thickness d with anisotropy Ksh. The validation of this core-shell-surface layer (CSSL) model for three different magnetic NPs systems viz. ferrimagnetic {\gamma}-Fe2O3, ferromagnetic Ni and antiferromagnetic NiO suggests its possible applicability for all magnetic nanoparticles., Comment: 5 pages, 6 figures

Experimental results and a model are presented to explain the observed unusual enhancement of the effective magnetic anisotropy Keff with decreasing particle size D from 15 nm to 2.5 nm in {\gamma}-Fe2O3 nanoparticles (NPs). The samples include oleic acid-coated NPs with D = 2.5, 3.4, 6.3 and 7.0 nm investigated here, with the results on other sizes taken from literature. Keff is determined from the analysis of the frequency dependence of the blocking temperature TB after considering the effects of interparticle interactions on TB. The data of Keff vs. D is fit to the derived core-shell-based relation: Keff = Kb + (6KS/D) +Ksh{[1-(2d/D)]^(-3) -1}, with Kb = 1.9 x10^5 ergs/cm^3 as the bulk-like contribution of the core, KS = 0.035 ergs/cm^2 as the contribution of the surface layer, and Ksh = 1.057 x10^4 ergs/cm^3 as the contributions of spins in the shell of thickness d = 1.1 nm. This equation represents an extension of the often-used Eq.: Keff = Kb + (6KS/D). Significance of this new result is discussed., Comment: 6 pages, 6 figures, and supplemental information

Search of novel two-dimensional giant Rashba semiconductors is a crucial step in the development of the forthcoming nano-spintronics technology. Using first-principle calculations, we study a stable two-dimensional crystal phase of BiSb having buckled honeycomb lattice geometry, which is yet unexplored. The phonon, room temperature molecular dynamics and elastic constant calculations verify the dynamical and mechanical stability of the monolayer at 0~K and at room temperature. The calculated electronic bandstructure reveals the direct bandgap semiconducting nature of BiSb monolayer with presence of highly mobile two-dimensional electron gas (2DEG) near Fermi-level. Inclusion of spin-orbit coupling (SOC) yields the giant Rashba spin-splitting of 2DEG near Fermi-level. The calculated Rashba energy and Rashba splitting constant are 13 meV and 2.3 eV\AA, respectively. The strength of the Rashba splitting is amongst the largest yet known 2D Rashba semiconductors. We demonstrate that the strength of the Rashba spin-splitting can be significantly tuned by applying in-plane bi-axial strain on the BiSb monolayer. Presence of the giant Rashba spin-splitting together with the large electronic bandgap (1.6 eV) makes this system of peculiar interest for optoelectronics applications. Furthermore, we study the electronic properties of BiSb/AlN heterostructures having a lattice mismatch of 1.3\% at the interface. Our results suggest that BiSb monolayer and heterostructure systems could be potentially used to develop highly efficient spin field-effect transistors, optoelectronics and nano-spintronics devices. Thus, this comprehensive study of two-dimensional BiSb systems can expand the range of possible applications in the future spintronics technology.

Recent experimental realization of long sought Weyl fermions in non-magnetic crystals has greatly motivated condensed matter physicists to search for materials supporting Weyl fermions. Weyl fermions appear to be very promising for future electronics, often referred as Weyltronics. Here, by means of first-principle calculations, we report a stoichiometric crystal structure of BiSb with broken space-inversion symmetry. This structure is insulating in bulk and has non-trivial band topology. We observe a pressure driven Weyl semi-metallic phase transition in this crystal structure. The obtained Weyl semi-metallic phase exists in the 4.0 - 6.0 GPa pressure range. We find that a total 6 pairs of Weyl points, 6 monopoles and 6 antimonopoles, exist in the Brillouin zone. The Weyl points with opposite chirality are located at different energy values yielding separate electron and hole Fermi-surfaces. Additionally, the spin-texture of the bulk BiSb compound appears to be electrically controllable when the interlink between pressure and an electric field is exploited. This produces novel manipulable topological transport properties in this system which are very promising for implementation of this kind of materials in next-generation Weyltronics and spintronic devices., Comment: 8 Pages, 4 Figures

Structural and magnetic properties of Fe@Pt core-shell nanostructure prepared by a sequential reduction process are reported. Transmission electron microscopy (TEM) shows nearly spherical particles fitting a lognormal size distribution with $D$$_o$= 3.0 nm and distribution width $\lambda$$_D$= 0.31. In x-ray diffraction, Bragg lines due to Pt shell only are clearly identified with line-widths yielding crystallite size =3.1 nm. Measurements of magnetization $M$ vs. $T$ (2 K - 350 K) in magnetic fields up to 90 kOe show a blocking temperature $T$$_B$ = 13 K below which hysteresis loops are observed with coercivity $H$$_C$ increasing with decreasing $T$ reaching $H$$_C$ = 750 Oe at 2 K. Temperature dependence of the ac susceptibilities at frequencies $f$$_m$ = 10 Hz to 5 kHz is measured to determine the change in $T$$_B$ with $f$$_m$ using Vogel-Fulcher law. This analysis shows the presence of significant interparticle interaction, the N\'{e}el-Brown relaxation frequency $f$$_o$ = 5.3 x 10$^{10}$ Hz and anisotropy constant $K$$_a$ =3.6 x10$^6$ ergs/cm$^3$. A fit of the $M$ vs. $H$ data up to $H$ = 90 kOe for $T$ $>$ $T$$_B$ to the modified Langevin function taking particle size distribution into account yields magnetic moment per particle consistent with the proposed core-shell structure; Fe core of 2.2 nm diameter and Pt shell of 0.4 nm thickness., Comment: Accepted for publication: Journal of Applied Physics