Recent studies have shown that although black phosphorene ($\alpha$-P) has promising electrical, thermal, and mechanical properties, its poor stability in the air makes its applications challenging. Very recently, monolayer puckered-phase nitrophosphorene ($\alpha$-NP) has emerged as a new two-dimensional material with improved stability compared to $\alpha$-P, while it preserves the appealing electronic and mechanical properties of $\alpha$-P. However, the phonon transport properties of $\alpha$-NP remain poorly understood. In this study, we systematically compare phonon transport and thermal conductivity of $\alpha$-NP and $\alpha$-P by means of an iterative solution for the Boltzmann transport equation and first-principles calculations. We pay particular attention to careful enforcement of the Born-Huang sum rules on the interatomic force constants so that the flexural phonon branch of $\alpha$-NP becomes perfectly quadratic close to the center of the Brillouin zone. We find that $\alpha$-NP has a less anisotropic thermal conductivity compared to $\alpha$-P, with the room-temperature values of 37.65 and $18.66{\mathrm{W}\mathrm{m}}^{-1}{\mathrm{K}}^{-1}$ in the armchair and zigzag directions, respectively. Interestingly, the anisotropy in the thermal conductivity of $\alpha$-NP is found to be reversed compared to that of $\alpha$-P, which has a higher thermal conductivity in the zigzag direction than the armchair direction. We found that as opposed to the usual understanding, considering only the directional dependence of the phonon group velocity fails to explain this reversed anisotropic thermal conductivity, and other phonon behaviors, such as avoided crossing of the acoustic and optical phonon branches, play a key role in determining the phonon relaxation time and thermal conductivity in $\alpha$-NP. Our results may provide theoretical guidance for future fundamental studies on the anisotropic phonon thermal transport in low-dimensional materials, and they may also enable efficient heat management of nanoelectronic devices based on $\alpha$-NP.

The high breakdown current densities and resilience to scaling of the metallic transition-metal trichalcogenides ${\mathrm{TaSe}}_3$ and ${\mathrm{ZrTe}}_3$ make them of interest for possible interconnect applications, and it motivates this paper of their thermal conductivities and phonon properties. These crystals consist of planes of strongly bonded one-dimensional chains more weakly bonded to neighboring chains. Phonon dispersions and the thermal conductivity tensors are calculated using density functional theory combined with an iterative solution of the phonon Boltzmann transport equation. The phonon velocities and the thermal conductivities of ${\mathrm{TaSe}}_3$ are considerably more anisotropic than those of ${\mathrm{ZrTe}}_3$. The maximum longitudinal-acoustic velocity in ${\mathrm{ZrTe}}_3$ occurs in the cross-chain direction, and this is consistent with the strong cross-chain bonding that gives rise to large Fermi velocities in that direction. The thermal conductivities are similar to those of other metallic two-dimensional transition-metal dichalcogenides. At room temperature, a significant portion of the heat is carried by the optical modes. In the low-frequency range, the phonon lifetimes and mean free paths in ${\mathrm{TaSe}}_3$ are considerably shorter than those in ${\mathrm{ZrTe}}_3$. The shorter lifetimes in ${\mathrm{TaSe}}_3$ are consistent with the presence of lower-frequency optical branches and zone-folding features in the acoustic branches that arise due to the doubling of the ${\mathrm{TaSe}}_3$ unit cell within the plane.

We report muon spin rotation ($\mu \mathrm{SR}$) experiments together with first-principles calculations on microscopic properties of superconductivity in the kagome superconductor ${\mathrm{LaRu}}_3{\mathrm{Si}}_2$ with $T_{\mathrm{c}}\simeq$ 7K. Below $T_{\mathrm{c}}$, $\mu \mathrm{SR}$ reveals type-II superconductivity with a single s-wave gap, which is robust against hydrostatic pressure up to 2 GPa. We find that the calculated normal state band structure features a kagome flat band, and Dirac as well as van Hove points formed by the Ru-$d{z}^2$ orbitals near the Fermi level. We also find that electron-phonon coupling alone can only reproduce a small fraction of $T_{\mathrm{c}}$ from calculations, which suggests other factors in enhancing $T_{\mathrm{c}}$ such as the correlation effect from the kagome flat band, the van Hove point on the kagome lattice, and the high density of states from narrow kagome bands. Our experiments and calculations taken together point to nodeless moderate coupling kagome superconductivity in ${\mathrm{LaRu}}_3{\mathrm{Si}}_2$.

The high-pressure melting curves of metals provide simple and useful tests for theories of melting, as well as important constraints for the modeling of planetary interiors. Here, we present an experimental technique that reveals the latent heat of fusion of a metal sample compressed inside a diamond anvil cell. The technique combines microsecond-timescale pulsed electrical heating with an internally heated diamond anvil cell. Further, we use the technique to measure the melting curve of platinum to the highest pressure measured to date. Melting temperature increases from $\approx 3000$ K at 34 GPa to $\approx 4500$ K at 107 GPa, thermodynamic conditions that are between the steep and shallow experimental melting curves reported previously. The melting curve is a linear function of compression over the 0–20 % range of compression studied here, allowing a good fit to the Kraut-Kennedy empirical model with fit parameter $C=6.0$.

Temperature-dependent ferromagnetic resonance (FMR) spectroscopy was used to investigate a ${\mathrm{Ni}}_{50}{\mathrm{Mn}}_{35}{\mathrm{In}}_{15}$ single crystalline slab to understand the nature of its martensitic transition. Its magnetic anisotropy in multivariant martensitic structures depends on external stress and strain. Near the transition, the preferred orientation of a pair of twinned domains with easy axes along ${\left[100\right]}_{\mathrm{A}}$ and ${\left[010\right]}_{\mathrm{A}}$ is observed in ${\mathrm{Ni}}_{50}{\mathrm{Mn}}_{35}{\mathrm{In}}_{15}$. The temperature dependence of the $g$ value indicates a unique characteristic with an opposite shift of $g$ away from 2 in the austenite and martensite phases. This indicates a transition from quenched to unquenched states of electron orbital motion that is induced by breaking the inversion symmetry during the phase transition. The FMR result is magnetic evidence for the martensitic transition and thus it advances our scientific understanding on this unique structural transformation.

In this work, we perform steady-state, continuous wave (cw), photoluminescence (PL) measurements on $\mathrm{MAPb}{\mathrm{I}}_3$ thin films in the temperature range of 10–160 K, using excitation densities spanning over almost seven orders of magnitude, in particular investigating very low densities that are rarely used in the published literature. The temperature range used in this study is below or at the edge of the orthorhombic-tetragonal phase transition in $\mathrm{MAPb}{\mathrm{I}}_3$. In particular, we show that even in high-quality $\mathrm{MAPb}{\mathrm{I}}_3$, capable of providing high photovoltaic efficiency, the defect density is high enough to give rise to an energy level band. Furthermore, we show that the intensity ratio between the two PL components related to the two crystalline phases is a function of temperature and excitation density. At high excitation intensities, we show that amplified spontaneous emission is attainable even in cw conditions. Time-resolved PL is also performed to justify the assignments of the PL features. Finally, our systematic approach, typical for the characterization of semiconductors, suggests that it should also be applied to hybrid halide perovskites and that, under suitable conditions, the PL characteristics of $\mathrm{MAPb}{\mathrm{I}}_3$ can be reconciled with those of conventional inorganic semiconductors.

We calculate the stability of the MAX-phase materials ${\mathrm{Zr}}_{n+1}{\mathrm{AlC}}_n$ and ${\mathrm{Ti}}_{n+1}{\mathrm{AlC}}_n$ in the context of the M-A-X ternary phase diagrams and competing binary and ternary compounds, as a function of temperature, by applying density functional theory (DFT) within the quasiharmonic approximation. By examining the convex hull of free energy we find that the Zr-based MAX phases are thermodynamically unstable at room temperature and below with respect to decomposition to carbide and intermetallics, although with increasing temperature the ${\mathrm{Zr}}_3{\mathrm{AlC}}_2$ phase becomes stable. On the other hand, the pure ${\mathrm{Ti}}_2\mathrm{AlC}$ phase is thermodynamically stable at room temperature, consistent with the success in its synthesis.

In simple ionic crystals, intrinsic point defects must satisfy electrical neutrality and exist as Schottky defects. In magnesium oxide (MgO), a Schottky defect is then a combination of anionic and cationic vacancies. Since vacancies are charged, the stable configuration of the Schottky defect corresponds to a bound pair of vacancies of opposite signs. In this study, we investigate the kinetics of formation and migration of such a bound pair on long timescales reaching in some cases thousands of seconds using the kinetic activation-relaxation technique, an off-lattice kinetic Monte Carlo method with an event catalog built on-the-fly during static molecular simulations. We show that the diffusion of this bound Schottky defect involves the migration of vacancies bounded to the first and third neighbor sites of the crystal structure with an apparent migration energy which cannot be inferred from the migration energies expected from isolated defects. Overall, this study gives insights and constraints on the oxygen diffusion mechanism reported experimentally in high-purity MgO samples.

The prediction of electron inelastic mean free paths (IMFPs) from simple material parameters is a challenging problem in studies using electron spectroscopy and microscopy. Herein, we propose a machine learning (ML) approach to predict IMFPs from some basic material property data. The ML model showed excellent performance based on the calculated IMFPs for a group of 41 elemental materials [Li, Be, C (graphite), C (diamond), C (glassy), Na, Mg, Al, Si, K, Sc, Ti, V, Cr, Fe, Co, Ni, Cu, Ge, Y, Nb, Mo, Ru, Rh, Pd, Ag, In, Sn, Cs, Gd, Tb, Dy, Hf, Ta, W, Re, Os, Ir, Pt, Au, and Bi] from a previous paper [Shinotsuka et al., Surf. Interface Anal. 47, 871 (2015); Surf. Interface Anal. 47, 1132 (2015)]. which was comparable with that of the robust Tanuma-Powell-Penn (TPP-2M) formula. The developed ML model was then extended to materials that do not have reported IMFPs in the database. The IMFPs for 18 transition and lanthanide metals (Mn, Zn, Zr, Tc, Cd, La, Ce, Pr, Nd, Pm, Sm, Eu, Ho, Er, Tm, Yb, Lu, and Hg) were predicted by the ML model. In the comparison with full-Penn algorithm-calculated IMFPs through two newly found experimental energy loss functions (ELFs), i.e., Mn and Zr, the Gaussian process regression-predicted IMFPs not only agreed well with those calculated using the TPP-2M formula in the energy range >50 eV but were also consistent with the trend of IMFPs calculated through experimental ELFs in the range of 2.7–50 eV, where the TPP-2M formula cannot be used. Our findings suggest that ML is very powerful and efficient and has great potential to complete a database of IMFPs for materials that can prove solutions closer to reality than empirical models on materials with similar physical and chemical properties and can be applied to other different situations for correlated information prediction.

We demonstrate room-temperature ferromagnetism with in-plane magnetic anisotropy in thin flakes of the $\mathrm{Cr}{\mathrm{Te}}_2$ van der Waals ferromagnet. Using quantitative magnetic imaging with a single-spin magnetometer based on a nitrogen-vacancy defect in diamond, we infer a room-temperature in-plane magnetization in the range of $M\sim 27$ kA/m for flakes with thicknesses down to 20 nm. In addition, our measurements indicate that the orientation of the magnetization is not determined solely by shape anisotropy in micron-sized $\mathrm{Cr}{\mathrm{Te}}_2$ flakes, which suggests the existence of a non-negligible magnetocrystalline anisotropy. These results make $\mathrm{Cr}{\mathrm{Te}}_2$ a unique system in the growing family of van der Waals ferromagnets, as it is the only material platform known to date that offers an intrinsic in-plane magnetization and a Curie temperature above 300 K in thin flakes.

Indium-doped cadmium oxide (In:CdO) thin films exhibit tunable epsilon-near-zero (ENZ) modal frequencies across a wide spectral range, bridging the mid-wave and near-infrared (IR). In:CdO thin films are prepared by reactive cosputtering from metallic Cd and In targets using high-power impulse magnetron sputtering (HiPIMS) and radio frequency sputtering, respectively. Using this approach, CdO thin films with carrier concentrations ranging from $2.3\times {10}^{19}$ to $4.0\times {10}^{20}\mathrm{c}{\mathrm{m}}^{\text{–}3}$ and mobilities ranging from 300 to $400\mathrm{c}{\mathrm{m}}^2/\mathrm{V}\mathrm{s}$ are readily achieved. UV-VIS absorption spectra are used to measure optical bandgap, revealing a Burstein-Moss shift of 0.58 eV across the doping range investigated. Optical measurements demonstrate the tunability of near-perfect plasmonic ENZ absorption across the mid-wave and into the near-IR spectral ranges by controlling the carrier concentration through doping, while tuning the film thickness for impedance matching. In comparison to other dopants that can be introduced to HiPIMS-deposited CdO, In offers the largest range of carrier concentrations while maintaining high mobility, thus allowing for the widest accessibility of the IR spectrum of a single plasmonic material grown by sputtering.

Charge trapping study at 300 and 77 K in ferroelectric (annealed Al- or Si-doped) and nonferroelectric (unannealed and/or undoped) ${\mathrm{HfO}}_2$ films grown by atomic layer deposition reveals the presence of “deep” and “shallow” electron traps with volume concentrations in the ${10}^{19}\text{−}{\mathrm{cm}}^{-3}$ range. The concentration of deep traps responsible for electron trapping at 300 K is virtually insensitive to the oxide doping by Al or Si but slightly decreases in films crystallized by high-temperature annealing in oxygen-free ambient. This behavior indicates that the trapping sites are intrinsic and probably related to disorder in ${\mathrm{HfO}}_2$ rather than to the oxygen deficiency of the film. Electron injection at 77 K allowed us to fill shallow electron traps energetically distributed at ∼0.2 eV. These electrons are mobile and populate states with thermal ionization energies in the range ∼0.6–0.7 eV below the ${\mathrm{HfO}}_2$ conduction band (CB). The trap energy depth and marginal sensitivity of their concentration to crystallization annealing or film doping with Si or Al suggests that these traps are associated with boundaries between crystalline grains and interfaces between crystalline and amorphous regions in ${\mathrm{HfO}}_2$ films. This hypothesis is supported by density functional theory calculations of electron trapping at surfaces of monoclinic, tetragonal, and orthorhombic phases of ${\mathrm{HfO}}_2$. The calculated trap states are consistent with the observed thermal ionization (0.7–1.0 eV below the ${\mathrm{HfO}}_2$ CB) and photoionization energies (in the range of 2.0–3.5 eV below the ${\mathrm{HfO}}_2$ CB) and support their intrinsic polaronic nature.

The magnetocaloric effect (MCE), magnetization, specific heat, and magnetostriction measurements were performed in both pulsed and steady high magnetic fields to investigate the magnetocaloric properties of Heusler alloys ${\mathrm{Ni}}_{50-x}{\mathrm{Co}}_x{\mathrm{Mn}}_{31.5}{\mathrm{Ga}}_{18.5}$ $(x=9 \mathrm{and} 9.7)$. From direct MCE measurements for ${\mathrm{Ni}}_{41}{\mathrm{Co}}_9{\mathrm{Mn}}_{31.5}{\mathrm{Ga}}_{18.5}$ up to 56 T, a steep temperature drop was observed for magnetic-field-induced martensitic transformation (MFIMT), designated as inverse MCE. Remarkably, this inverse MCE is apparent not only with MFIMT, but also in the magnetic-field-induced austenite phase. Specific heat measurements under steady high magnetic fields revealed that the magnetic field variation of the electronic entropy plays a dominant role in the unconventional magnetocaloric properties of these materials. First-principles based calculations performed for ${\mathrm{Ni}}_{41}{\mathrm{Co}}_9{\mathrm{Mn}}_{31.5}{\mathrm{Ga}}_{18.5}$ and ${\mathrm{Ni}}_{45}{\mathrm{Co}}_5{\mathrm{Mn}}_{36.7}{\mathrm{In}}_{13.3}$ revealed that the magnetic-field-induced austenite phase of ${\mathrm{Ni}}_{41}{\mathrm{Co}}_9{\mathrm{Mn}}_{31.5}{\mathrm{Ga}}_{18.5}$ is more unstable than that of ${\mathrm{Ni}}_{45}{\mathrm{Co}}_5{\mathrm{Mn}}_{36.7}{\mathrm{In}}_{13.3}$ and that it is sensitive to slight tetragonal distortion. We conclude that the inverse MCE in the magnetic-field-induced austenite phase is realized by marked change in the electronic entropy through tetragonal distortion induced by the externally applied magnetic field.

Important phenomena such as magnetostriction, magnetocaloric, and magnetoelectric effects arise from, or could be enhanced by, the coupling of magnetic and structural degrees of freedom. The coupling of spin and lattice also influence transport and structural properties in magnetic materials, in particular around phase transitions. In this paper we propose a method for screening materials for a strong magnetostructural coupling by assessing the effect of the local magnetic configuration on the atomic forces using density functional theory. We have employed the disordered local moment approach in a supercell formulation to probe different magnetic local configurations and their forces and performed a high-throughput search on binary and ternary compounds available in the Crystallography Open Database. We identify a list of materials with a strong spin-lattice coupling out of which several are already known to display magnetolattice coupling phenomena such as ${\mathrm{Fe}}_3{\mathrm{O}}_4$ and CrN. Others, such as ${\mathrm{Mn}}_2{\mathrm{CrO}}_4$ and $\mathrm{Ca}{\mathrm{Fe}}_7{\mathrm{O}}_{11}$, have been less studied and have yet to reveal their potentials in experiments and applications.

Hydrogenated amorphous dielectric thin films are critical materials in a wide array of technologies. In this work, we present a thorough investigation of the thermal conductivity of hydrogenated amorphous silicon nitride $(a\text{−}{\mathrm{SiN}}_x:\mathrm{H})$, a ubiquitously used material in which the stoichiometry plays a direct role in its functionality and application. In particular, through chemical, vibrational, and structural analysis in tandem with thermal conductivity measurements on chemically variant silicon nitride films, we show that hydrogen incorporation into silicon nitride disrupts the bonding among silicon and nitrogen atoms, and directly impacts the thermal conductivity, leading to as much as a factor of 2.5 variation in heat transfer. This variability, driven by the change in hydrogen content, is fundamentally related to the changes in the average atomic distances, as we experimentally measure with selected-area electron diffraction and computationally show with molecular dynamics simulations. This, combined with our evidence of chemical and spatial fluctuations on the order of average atomic pair distances, leads us to conclude that the vibrational heat transport in $a\text{−}{\mathrm{SiN}}_x:\mathrm{H}$ is primarily dominated by diffusonlike modes. The results presented in this work combined with our extensive review of prior reports on the thermal conductivity of $a\text{−}{\mathrm{SiN}}_x:\mathrm{H}$ films resolves discrepancies in decades of prior literature and facilitates a more universal understanding of the vibrational heat transport processes in hydrogenated amorphous silicon nitride.

Low-fluence light-ion $\left({}^{11}\mathrm{B}^{+}\right)$ medium-energy (150 keV/ion) implantation preprocessing of double-walled carbon nanotubes (DWCNTs) has been effected to “decorate” them with defects. These are intended to serve as nucleation sites for potential $s{p}^3$ interlinking between tube walls in close proximity, following on strong deformation of the tube cross sections under cold compression to 20–25 GPa in diamond anvil cells. The pressure response of such implanted DWCNTs has been monitored in situ using Raman spectroscopy, and compared with those of unimplanted reference DWCNTs. Pressure dependences of the $G^{+}$ mode frequency and D to $G^{+}$ band intensity ratio, and radial breathing modes, have been monitored. These Raman signatures show that some degree of mechanical softening occurs in the implanted tube bundles, without major disruption to tube integrity in both samples. Consequently, the collapse pressure to racetrack or peanut shaped cross-sectional profiles is lowered substantially, from $P_{\mathrm{c}}\sim 18\mathrm{GPa}$ in the reference tube bundles to $P_{\mathrm{c}}\sim 11\mathrm{GPa}$ in the implanted case. Defect structures also proliferate more readily in the implanted sample under pressure. Therefore, the light-ion low-fluence implantation lowers the threshold pressure for deformation of tube cross sections to high-curvature profiles decorated with defects. Considerations of whether irreversible $s{p}^3$ interlinking at low volume fractions is discerned in the Raman data from implanted tube bundles under compression, and the stability of such bonding is discussed.

Understanding the superconductivity at the interface of $\mathrm{FeSe}/{\mathrm{SrTiO}}_3$ is a problem of great contemporary interest due to the significant increase in critical temperature $\left(T_{\mathrm{c}}\right)$ compared to that of bulk FeSe, as well as the possibility of an unconventional pairing mechanism and topological superconductivity. We report a study of the influence of a capping layer on superconductivity in thin films of FeSe grown on ${\mathrm{SrTiO}}_3$ using molecular beam epitaxy. We used in vacuo four-probe electrical resistance measurements and ex situ magnetotransport measurements to examine the effect of three capping layers that provide distinct charge transfer into FeSe: insulating FeTe, nonmetallic Te, and metallic Zr. Our results show that FeTe provides an optimal cap that barely influences the inherent $T_{\mathrm{c}}$ found in pristine $\mathrm{FeSe}/{\mathrm{SrTiO}}_3$, while the transfer of holes from a nonmetallic Te cap completely suppresses superconductivity and leads to insulating behavior. Finally, we used ex situ magnetoresistance measurements in FeTe capped FeSe films to extract the angular dependence of the in-plane upper critical magnetic field. Our observations reveal an almost isotropic in-plane upper critical field, providing insight into the symmetry and pairing mechanism of high-temperature superconductivity in FeSe.

Half-Heusler semiconductors satisfying 18-electron rule typically display promising characteristics for thermoelectric applications. A persistent inconsistency between the type of charge carriers in some of these alloys as obtained from experiment and theory however casts serious doubt on the computational prediction of new and efficient half-Heusler alloys. To gain insights into the origin of this disparity, we have investigated the effect of intrinsic point defects on the electronic structure of four frequently studied half-Heusler alloys of the form $XYZ$ with $Y$ being Ni or Co. Using state-of-the-art ab initio calculations, our study reveals that interstitial Ni and Co are energetically most stable point defects in these alloys. Remarkably, interstitial defect modifies the location of the Fermi level inside the band gap as well as the value of the band gap, thereby bringing in close agreement with the corresponding experimental result. This work thus highlights the decisive role played by interstitial defects in thermoelectric half-Heusler alloys, which may open a new avenue for deliberately utilizing these defects as a strategy for tailoring electronic structure and hence the corresponding thermoelectric properties.

We highlight fast Li-ion diffusion in hypothetical tetragonal ${\mathrm{Li}}_{10}{\mathrm{GeP}}_2{\mathrm{O}}_{12}$ (LGPO), as a counterpart to the well-known phases of orthorhombic (LISICON) LGPO, thio-LISICON, and tetragonal ${\mathrm{Li}}_{10}{\mathrm{GeP}}_2{\mathrm{S}}_{12}$ (LGPS). We use extended Car-Parrinello molecular dynamics in the canonical and isobaric-isothermal ensembles, finding first that tetragonal LGPO is dynamically stable, albeit 0.04 Ha/f.u. above LISICON LGPO. The calculated activation energy for Li-ion diffusion is 0.22 eV, well below the value calculated for LISICON LGPO (0.34 eV), and similar to those for thio-LISICON (0.23 eV) and tetragonal LGPS (0.18 eV). These results indicate that hypothetical tetragonal LGPO, although less stable than its orthorhombic allotrope, shows a room-temperature conductivity comparable to LGPS, and, if synthesized, could make a very attractive Li-ion conductor.

Because inorganic solid electrolytes are one of the key components for application to all-solid-state batteries, high-ionic-conductivity materials must be developed. Therefore we propose a method of efficiently evaluating the activation energy of ionic diffusion by calculating a potential-energy surface (PES), searching for the optimal diffusion path by an algorithm developed using dynamic programming (DP), and calculating the corresponding activation energy by the nudged elastic band (NEB) method. Taking $\beta \text{−}{\mathrm{Li}}_3{\mathrm{PS}}_4$ as an example, the activation energy of Li-ion diffusion was calculated as 0.43, 0.25, and 0.40 eV in the $a$-, $b$-, and $c$-axis directions, respectively, which is in good agreement with previously reported values. By comprehensively searching for the lowest energy path by PES-DP, the arbitrariness of the path selection can be eliminated, and the activation energy must only be calculated using the NEB method a few times, which greatly reduces the computational cost required for evaluating activation energy and enables the high-throughput screening of solid-state electrolytes.