Highly transparent superconducting contacts to a topological insulator (TI) remain a persistent challenge on the route to engineer topological superconductivity. Recently, the higher-order TI ${\mathrm{WTe}}_2$ was shown to turn superconducting when placed on palladium (Pd) bottom contacts, demonstrating a promising material system in pursuing this goal. Here, we report the diffusion of Pd into ${\mathrm{WTe}}_2$ and the formation of superconducting ${\mathrm{PdTe}}_x$ as the origin of observed superconductivity. We find an atomically sharp interface in the direction vertical to the van der Waals layers between the diffusion crystal and its host crystal, forming state-of-the-art superconducting contacts to a TI. The diffusion is discovered to be nonuniform along the width of the ${\mathrm{WTe}}_2$ crystal, with a greater extent along the edges compared to the bulk. The potential of this contacting method is highlighted in transport measurements on Josephson junctions by employing external superconducting leads.

${\mathrm{Co}}_3{\mathrm{Sn}}_2{\mathrm{S}}_2$ is a magnetic Weyl semimetal, anisotropic ferromagnet, and half-metal in a single material. These qualities modify the spin dynamics and behavior of electron-magnon scattering. We have investigated this unconventional ferromagnet for modified electron-magnon scattering behavior using temperature and magnetic field dependent resistivity. ${\mathrm{Co}}_3{\mathrm{Sn}}_2{\mathrm{S}}_2$ exhibits exponential suppression of spin-flip electron-magnon scattering below a characteristic crossover temperature which separates the regime of anomalous magnon scattering from one-magnon scattering. Interestingly, the Weyl fermion mediated spin dynamics with large magnetocrystalline anisotropy can also cause exponential suppression of electron-magnon scattering giving similar behavior in resistivity. The gap characterizing the suppression of electron-magnon scattering is 2.46(4) meV and increases linearly with magnetic field. The suppression of electron-magnon scattering could occur from half-metallic or anisotropic character but the presence of anomalous-magnon scattering at low temperatures evinces the half-metallic nature. A large anisotropy gap in magnon dispersion of half-metal can help prevent the deviation from 100% spin polarization at low temperatures.

Rutile-structured germanium oxide $\left(\mathrm{r}\text{−}{\mathrm{GeO}}_2\right)$, an ultrawide band-gap (UWBG) semiconductor, is a promising candidate for future high-power electronics because of its excellent properties, including ambipolar dopability, high carrier mobilities, and a higher thermal conductivity than $\beta \text{−}{\mathrm{Ga}}_2{\mathrm{O}}_3$. In this paper, focusing on a wide variety of its applications, we propose an UWBG alloy system based on $\mathrm{r}\text{−}{\mathrm{GeO}}_2$ and other rutile-structured oxides $\left({\mathrm{SnO}}_2\text{−}{\mathrm{GeO}}_2\text{−}{\mathrm{SiO}}_2\right)$ and clarify the electronic structure and electrical properties based on experiments and the first-principles calculations. Experimentally, (001)-oriented $\mathrm{r}\text{−}{\mathrm{Ge}}_x{\mathrm{Sn}}_{1-x}{\mathrm{O}}_2$ alloy thin films with an entire range of Ge compositions $\left(x\right)$ were grown by a mist chemical vapor deposition technique. Structural characterizations show that the fabricated $\mathrm{r}\text{−}{\mathrm{Ge}}_x{\mathrm{Sn}}_{1-x}{\mathrm{O}}_2$ alloy films with $x\le 0.96$ and the well-crystallized part of the film with $x=1.00$ have uniform chemical compositions and the same epitaxial relations with $\mathrm{r}\text{−}{\mathrm{TiO}}_2$ (001) substrates. Transmission electron microscopy observations reveal that there are few dislocations in $\mathrm{r}\text{−}{\mathrm{Ge}}_{0.66}{\mathrm{Sn}}_{0.34}{\mathrm{O}}_2$ because of relatively small in-plane lattice mismatch. In contrast, many dislocations are observed near the film/substrate interface in $\mathrm{r}\text{−}{\mathrm{Ge}}_{0.96}{\mathrm{Sn}}_{0.04}{\mathrm{O}}_2$. Lattice constants of the alloys both along the $a$ and $c$ axes decrease with increase in Ge compositions. Their band gaps were determined by spectroscopic ellipsometry analysis, indicating that the band gaps increase as Ge compositions increase $(3.81-4.44\mathrm{eV})$ with a bowing parameter of 1.2 eV. The values of lattice constants and the trend of band-gap transitions obtained by calculations are in good agreement with those of experimentally obtained each other. Then we presented the calculated natural band alignments of $\mathrm{r}\text{−}{\mathrm{Ge}}_x{\mathrm{Sn}}_{1-x}{\mathrm{O}}_2$ and $\mathrm{r}\text{−}{\mathrm{Ge}}_x{\mathrm{Si}}_{1-x}{\mathrm{O}}_2$ alloys, suggesting the possibility of $p$-type doping in $\mathrm{r}\text{−}{\mathrm{GeO}}_2$ and Ge-rich $\mathrm{r}\text{−}{\mathrm{Ge}}_x{\mathrm{Sn}}_{1-x}{\mathrm{O}}_2$ and availability of $\mathrm{r}\text{−}{\mathrm{SiO}}_2$ and Si-rich $\mathrm{r}\text{−}{\mathrm{Ge}}_x{\mathrm{Si}}_{1-x}{\mathrm{O}}_2$ as a blocking layer of other rutile-structured devices. Finally, electrical measurements demonstrated $n$-type conductivities in $\mathrm{r}\text{−}{\mathrm{Ge}}_x{\mathrm{Sn}}_{1-x}{\mathrm{O}}_2$ $(x\le 0.57)$.

We present a numerical methodology to compute the Nye-tensor fingerprints of dislocation loop absorption at grain boundaries (GBs) for comparison with TEM observations of irradiated polycrystals. Our approach links atomistic simulations of self-interstitial atom (SIA) prismatic loops gliding toward and interacting with GBs in body-centered cubic iron with experimentally extracted geometrically necessary dislocation (GND) maps to facilitate the interpretation of damage processes. The Nye-tensor analysis is strongly mesh-size dependent—corresponding to resolution-dependent TEM observations. The method computes GND fingerprints from discretized dislocation line segments extracted from molecular dynamics simulations of dislocation loops being absorbed at a GB. Specifically, we perform MD simulation of prismatic loops of two diameters and monitor the three stages of the absorption process: loop glide, the partial, and full absorption of the loops at a [1 0 0] symmetric tilt GB. These methods provide a framework for future investigations of the nature of defect absorption by grain boundaries under irradiation conditions.

We use ab initio calculations to study phonon and charge-carrier transport in the Janus $\mathrm{PtSTe}$ monolayer, a lower-symmetry derivative of quasi-two-dimensional transition metal chalcogenides, with a view to characterizing its thermoelectric performance. For the sake of comparison, we also perform the same study on its parent structures, ${\mathrm{PtTe}}_2$ and ${\mathrm{PtS}}_2$. We find a significantly increased thermoelectric figure of merit, driven by modest increases in the Seebeck coefficient but, most importantly, by a drastic reduction in the thermal conductivity. We show that this decrease cannot be explained by a simple inspection of the phonon band structure; instead, it has its roots in a relaxation of the selection rules for three-phonon scattering as a result of the broken symmetry of the Janus structure. This suggests that Janus monolayers have a significant intrinsic advantage and untapped potential for thermoelectric applications among quasi-two-dimensional systems.

The highly nonlinear switching behavior of hafnia memristor actually hinders its wide application in neuromorphic computing. The theoretical understanding of its switching mechanism has been focused on the processes of conductive filament (CF) generation and rupture, but the possible phase transition and crystallization around the region of CFs due to the variation of O content have been less studied or understood. In this paper, ${\mathrm{HfO}}_{\mathrm{x}}$ structural models covering the full stoichiometries from Hf to ${\mathrm{HfO}}_2$ are established, and the crystal structure evolution during the reduction process of hafnia is obtained through first-principles calculation. The electronic structures and O vacancy migration characteristics of these structures are analyzed. A criterion is prescribed to predict the mode of abrupt binary switching or gradual conductance modulation according to the structure evolution of the CFs. In particular, factors that influence the merging of tiny conductive channels into strong CFs are discussed intensively, including the anisotropy of O vacancy migration and the size effect. The feasibility of Mg doping to achieve robust gradual switching is discussed.

Increasing resistivity of metal wires with reducing nanoscale dimensions is a major performance bottleneck of semiconductor computing technologies. We show that metals with suitably anisotropic Fermi velocity distributions can strongly suppress electron scattering by surfaces and outperform isotropic conductors such as copper in nanoscale wires. We derive a corresponding descriptor for the resistivity scaling of anisotropic conductors, screen thousands of metals using first-principles calculations of this descriptor, and identify the most promising materials for nanoscale interconnects. Previously-proposed layered conductors such as MAX phases and delafossites show promise in thin films, but not in narrow wires due to increased scattering from side walls. We find that certain intermetallics (notably CoSn) and borides (such as $\mathrm{Y}{\mathrm{Co}}_3{\mathrm{B}}_2$) with one-dimensionally anisotropic Fermi velocities are most promising for narrow wires. Combined with first-principles electron-phonon scattering predictions, we show that the proposed materials exhibit $2\text{–}\text{3}\times$ lower resistivity than copper at 5-nm wire dimensions.

Achieving room-temperature valley polarization in two-dimensional (2D) atomic layers (2D materials) by substitutional doping opens new avenues of applications. Here, monolayer ${\mathrm{MoS}}_2$, when doped with vanadium at low (0.1 atomic %) concentrations, is shown to exhibit high spin-valley coupling, and hence a high degree of valley polarization at room-temperature. The atomic layers of ${\mathrm{MoS}}_2$ (MS) and V-doped ${\mathrm{MoS}}_2$ (VMS) are grown via the chemical vapor deposition-assisted method. The formation of new energy states near the valence band is confirmed from band gap calculations and also from the density functional theory–based band structure analyses. Time-reversal symmetry broken energy shift in the equivalent valleys is predicted in VMS, and the room-temperature chirality-controlled photoluminescent (PL) excitation measurements indicate such a shift in valley exciton energies (∼35 meV). An enhanced valley polarization in VMS (∼42%) is observed in comparison to that in MS (<12%), while in MS, the chirality-controlled excitations did not show the difference in emission energies. Spin Hall effect of light–based optical rotation measurements indicate the asymmetric absorption among the two different chiralities of the incident light, hence supporting the existence of room-temperature valley polarization. This study opens possibilities of room-temperature opto-spintronics using stable 2D materials.

In search of new prospects for room temperature thermoelectric applications, we computationally investigated the electronic structure and transport properties of Zintl phases ${\mathrm{Na}}_2\mathrm{AuBi}$ and ${\mathrm{Na}}_2\mathrm{AuSb}$. We find remarkably low room temperature lattice thermal conductivity of 0.46 and 0.73 W ${\mathrm{m}}^{-1}{\mathrm{K}}^{-1}$ for ${\mathrm{Na}}_2\mathrm{AuBi}$ and ${\mathrm{Na}}_2\mathrm{AuSb}$, respectively. The proposed values are competitive with the record figure of merit material SnSe. We have demonstrated that such low thermal conductivity finds its origin majorly in low phonon group velocities and short phonon lifetime, arising from atomic displacements and lattice anharmonicity. We further predict a high figure of merit, i.e., $ZT\sim$ 1 at 300 K for $p$-type ${\mathrm{Na}}_2\mathrm{AuBi}$. The value is further improved to $ZT\sim$ 1.8 with spin-orbit coupling. We hope through this work we could shed light on two potential Zintl phases which would motivate further experimental investigations.

Two-dimensional chromium ditelluride (${\mathrm{CrTe}}_2$) is a promising ferromagnetic layered material that exhibits long-range ferromagnetic ordering in the monolayer limit. The formation energies of the different possible structural phases (1T, 1H, 2H) calculated from density functional theory (DFT) show that the 1T phase is the ground state, and the energetic transition barriers between the phases, calculated by the nudged elastic band method, are large, on the order of 0.5 eV. The self-consistent Hubbard $U$ correction parameters are calculated for all the phases of ${\mathrm{CrTe}}_2$. The calculated magnetic moment of 1T-${\mathrm{CrTe}}_2$ with $\ge 2$ layers lies in the plane, whereas the magnetic moment of a monolayer is out-of-plane. Band filling and tensile biaxial strain cause the magnetic moment of a monolayer to switch from out-of-plane to in-plane, and compressive biaxial strain in a bilayer causes the magnetic moment to switch from in-plane to out-of-plane. The magnetic anisotropy is shown to originate from the large spin-orbit coupling (SOC) of the Te atoms and the anisotropy of the exchange coupling constants $J_{xy}$ and $J_z$ in an XXZ type Hamiltonian. Renormalized spin wave theory using experimental values for the magnetic anisotropy energy and Curie temperatures provides a range of values for the nearest neighbor exchange coupling.

We simulate dynamic mechanical analysis experiments for the Kob-Andersen binary Lennard-Jones system. For this, the SLLOD algorithm with time-dependent strain rates is applied to give a sinusoidally varying strain at different densities and temperatures. The starting point is a temperature scan at a fixed reference density. Isomorph theory predicts that for other densities, corresponding temperatures can be identified at which the mechanical properties are unchanged when scaled appropriately. We determine the isomorphically equivalent temperatures by analyzing how particle forces change upon scaling configurations to the new density. Loss moduli expressed in suitable reduced units are compared for isomorphic state points. When plotted against the unscaled temperatures, these reduced loss curves are observed to collapse indicating the validity of isomorph theory for dynamic mechanical analysis experiments. Two different methods to determine isomorphic temperatures are considered. While one of them breaks down for the largest density rescalings considered in this paper, the other one is still applicable in this region. The decorrelation of force vectors upon rescaling is investigated as a possible origin of this effect. Our results demonstrate that the simplification of the phase diagram entailed by isomorph theory for a wide class of systems is relevant also for the mechanical properties of glasses.

Here we report the discovery of pressure-induced superconductivity in quasi-one-dimensional ${\mathrm{Ta}}_2{\mathrm{PdSe}}_6$, through a combination of electrical transport, synchrotron x-ray diffraction, and theoretical calculations. Our transport measurements show that the superconductivity appears at a critical pressure $P_{\mathrm{c}}\sim 18.3\mathrm{GPa}$ and is robust upon further compression up to 62.6 GPa. The estimated upper critical field ${\mu }_0{H}_{\mathrm{c}2}\left(0\right)$ in the pressurized ${\mathrm{Ta}}_2{\mathrm{PdSe}}_6$ is much lower than the Pauli limiting field, in contrast to the case in its isostructural analogs $M_2{\mathrm{Pd}}_{\mathrm{x}}{X}_5$ $(M=\mathrm{Nb}, \mathrm{Ta}; X=\mathrm{S}, \mathrm{Se})$. Concomitant with the occurrence of superconductivity, anomalies in pressure-dependent transport properties are observed, including sign reversal of Hall coefficient, abnormally enhanced resistance, and dramatically suppressed magnetoresistance. Meanwhile, room-temperature synchrotron x-ray diffraction experiments reveal the stability of the pristine monoclinic structure (space group $C2$/$m$) upon compression. Combined with the density functional theory calculations, we argue that a pressure-induced Lifshitz transition could be the electronic origin of the emergent superconductivity in ${\mathrm{Ta}}_2{\mathrm{PdSe}}_6$.

${R\mathrm{V}}_6{\mathrm{Sn}}_6$ $(R=\mathrm{Y}, \mathrm{Gd}\text{−}\mathrm{Tm}, \mathrm{Lu})$ single crystals are synthesized by the Sn-flux method and their physical properties are characterized by magnetization, resistivity, and specific heat measurements. Powder x-ray diffraction patterns of all samples can be well indexed with the hexagonal ${\mathrm{HfFe}}_6{\mathrm{Ge}}_6$-type structure, where rare-earth atoms form hexagonal layers and vanadium atoms form kagome layers. At high temperatures, magnetic susceptibility measurements of moment-bearing rare-earth ions $(R=\mathrm{Gd}\text{−}\mathrm{Tm})$ follow Curie-Weiss behavior. Effective moments estimated from the polycrystalline average of magnetic susceptibility curves are consistent with the values for free $R^{3+}$ ions. Strong magnetic anisotropy due to crystalline electric field effects is observed for moment-bearing rare-earth ions, except ${\mathrm{GdV}}_6{\mathrm{Sn}}_6$. The easy magnetization direction is determined to be the $c$ axis for $R=\mathrm{Tb}\text{−}\mathrm{Ho}$ and $ab$ plane for $R=\mathrm{Er}$ and Tm. The vanadium ions in ${R\mathrm{V}}_6{\mathrm{Sn}}_6$ possess no magnetic moment. The compounds for $R=\mathrm{Y}$ and Lu exhibit typical characteristics of paramagnetic metals. At low temperatures, the magnetic ordering is confirmed from magnetization, specific heat, and resistivity: the highest $T_N=4.9$ K for ${\mathrm{GdV}}_6{\mathrm{Sn}}_6$ and the lowest $T_N=2.3$ K for ${\mathrm{HoV}}_6{\mathrm{Sn}}_6$. No magnetic ordering is observed down to 1.8 K for $R=\mathrm{Er}$ and Tm. A slight deviation of the magnetic ordering temperature from the de Gennes scaling suggests the dominant Ruderman-Kittel-Kasuya-Yosida exchange interaction between rare-earth moments in metallic ${R\mathrm{V}}_6{\mathrm{Sn}}_6$ compounds.

Based on first-principles phonon calculations within the framework of quasiharmonic approximation, we study the effect of twin grain boundary (TGB) on the thermal expansion of insulating materials. We select diamond-structure solids (C, Si, and Ge) and rutile $\left({\mathrm{SnO}}_2\right)$ as representatives of the covalent and ionic bonding materials that are important systems commonly accompanied with TGB. We found distinct effects of TGB on thermal expansion in two types of materials. For diamond-structure solids, the thermal expansion of (111)-oriented twinning structure varies subtly from that of pristine material within the temperature range studied, which is primarily induced by the modification of vibrational phonon mode by the TGB effect. Distinctly, the thermal expansion of $\mathrm{SnO}_2$ twinnings increase substantially by comparison with the pristine case and depend strongly on the twin orientation. The (101) twinning shows much larger thermal expansion than the (301) twinning. Further analysis indicates the physical mechanism can be mainly attributed to the effect of rotational degree of freedom of the $\mathrm{SnO}_6$ octahedron motif and the stress induced by TGB. Our work reveals the physical mechanism underlying the distinct effect of TGB on thermal expansion caused by different chemical bonding, and meanwhile provides an insightful understanding of the relationship between specific structural features and thermal expansion in solid-phase materials.

We report on comprehensive investigations of ${\mathrm{Er}}_3{\mathrm{Al}}_2$ exhibiting a number of magnetic states. We observe three spontaneous antiferromagnetic transitions at 10, 14, and 24 K. Furthermore, our high-field magnetization and sound-velocity measurements reveal a number of crystal-electric-field (CEF) transitions. Additionally, the velocity of transverse acoustic waves shows a pronounced softening with decreasing temperature in the paramagnetic state. A CEF model that includes quadrupolar interactions explains the observed magnetic and elastic properties and provides a CEF scheme of ${\mathrm{Er}}_3{\mathrm{Al}}_2$. However, to explain some of the observations, a refined, more sophisticated model needs to be developed.

Vibrational and elastic properties of the $R{\mathrm{Fe}}_4{\mathrm{Sb}}_{12}$ skutterudites are investigated by, respectively, temperature $\left(T\right)$ dependent extended x-ray absorption fine structure (EXAFS) and pressure $\left(P\right)$ dependent x-ray diffraction (XRD) experiments. The Fe $K$-edge EXAFS experiments of the $R=\mathrm{K}$, Ca, and Ba materials were performed in the $T$ interval $6<T<300\mathrm{K}$ and XRD experiments of the $R=$ Na, K, Ca, Sr, and Ba materials were performed in the $P$ interval $1\text{atm}<P<16\mathrm{GPa}$. From EXAFS, we obtained the correlated Debye-Waller parameters that were thus analyzed to extract effective spring constants connected with the Fe-$Y$ (where $Y=$ either $R$, Fe or Sb) scattering paths. Our findings suggest that in the case of the light cations, $R=\mathrm{K}$ or Ca, the $R$ atoms are relatively weakly coupled to the cage, in a scenario reminiscent to the Einstein oscillators. From the XRD experiments, we obtained the bulk modulus $B_0$ for all $R=\mathrm{Na}$, K, Ca, Sr, and Ba materials, with values ranging from 77 GPa ($R=\mathrm{K}$) to $R=99\mathrm{GPa}$ ($R=$ Ba) as well as the compressibility $\beta$ as a function of $P$. The trend in $\beta$ as a function of the $R$ filler is discussed and it is shown that it does not correlate with simple geometrical considerations but rather with the filler-cage bonding properties.

Embrittling potency is a thermodynamic metric that assesses the influence of solute segregation to a grain boundary (GB) on intergranular fracture. Historically, authors of studies have reported embrittling potency as a single scalar value, assuming a single segregation site of importance at a GB and a particular cleavage plane. However, the topography of intergranular fracture surfaces is not generally known a priori. Accordingly, in this paper, we present a statistical ensemble approach to compute embrittling potency, where many free surface (FS) permutations are systematically considered to model fracture of a GB. The result is a statistical description of the thermodynamics of GB embrittlement. As a specific example, embrittling potency distributions are presented for Cr segregation to sites at two Ni $\langle 111\rangle$ symmetric tilt GBs using atomistic simulations. We show that the average embrittling potency for a particular GB site, considering an ensemble of FS permutations, is not equal to the embrittling potency computed using the lowest energy pair of FSs. A mean GB embrittlement is proposed, considering both the likelihood of formation of a particular FS and the probability of solute occupancy at each GB site, to compare the relative embrittling behavior of two distinct GBs.

We present a robust technique for computationally studying surface polariton modes in hybrid materials. We use a semiclassical model that allows us to understand the physics behind the interactions between collective excitations of the hybrid system and develop a scattering and transfer matrix method that imposes the proper boundary conditions to solve Maxwell's equations and derive a general equation describing the surface polariton in a heterostructure consisting of N constituent materials. We apply this method to a test structure composed of a topological insulator (TI) and an antiferromagnetic material (AFM) to study the resulting surface Dirac plasmon-phonon-magnon polariton (DPPMP). We find that interactions between the excitations of the two constituents result in the formation of hybridized modes and the emergence of avoided-crossing points in the dispersion relations for the DPPMP. For the specific case of a $\mathrm{Bi}_2\mathrm{Se}_3$ TI material combined with a 3D AFM such as NiO, $\mathrm{MnF}_2$, or $\mathrm{FeF}_2$, the polariton branch with low frequency below 2 THz redshifts upon increasing the thickness of TI thin film, which leads to an upper bound on the thickness of the TI layer that will allow an observable signature of strong coupling and the emergence of hybridized states. We also find that the strength of the coupling between the TI and the AFM, which is parametrized by the amplitude of the avoided-crossing splitting between the two polariton branches at the magnon resonance frequency, depends on the magnitude of the magnetic dipole and the linewidth of the magnon in the AFM material as well as on the Fermi energy of Dirac plasmon in the TI. Finally, we show that materials with extremely high quality, i.e., low scattering loss rate, are essential to achieve an experimentally observable strong coupling between a TI and 3D AFM material. The overall analysis identifies the material properties that are necessary to achieve experimentally observable strong coupling for the interaction between THz excitations in a TI/AFM heterostructure and can thereby guide experimental efforts.

We use large-scale simulations to investigate the dynamic fracture of silica and sodium-silicate glasses under uniaxial tension. The stress-strain curves demonstrate that silica glass is brittle whereas the glasses rich in Na show pronounced ductility. A strong composition dependence is also seen in the crack velocity which, for the strain rate considered, is on the order of 1800 m/s for glasses with low Na concentration and decreases to 650 m/s if the concentration is high. We find that during the fracture of Na-rich glasses very irregular cavities as large as 3–4 nm form ahead of the crack front, indicating the presence of nanoductility in these glasses. Before fracture occurs, the local composition, structure, and mechanical properties are heterogeneous in space and show a strong dependence on the applied strain. Further analysis of the correlations between these local properties allows to obtain a better microscopic understanding of the deformation and fracture of oxide glasses and how the local heating close to the crack tip, up to several hundred degrees, permits the structure to relax.