Temperature dependence of vibrational modes in semiconductors depends on lattice thermal expansion and anharmonic phonon-phonon scattering. Evaluating the two contributions from experimental data is not straightforward, especially for epitaxial layers that present mechanical deformation and anisotropic lattice expansion. In this paper, a temperature-dependent Raman study in epitaxial Ge and ${\mathrm{Ge}}_{1-x}{\mathrm{Sn}}_x$ layers is presented. A model is introduced for the Raman mode energy shift as a function of temperature, comprising thermal expansion of the strained lattice and anharmonic corrections. With support of x-ray diffraction, the model is calibrated on experimental data of epitaxial Ge grown on Si and ${\mathrm{Ge}}_{1-x}{\mathrm{Sn}}_x$ grown on Ge/Si, finding that the main difference between bulk and epitaxial layers is related to the anisotropic lattice expansion. The phonon anharmonicity and other parameters do not depend on dislocation defect density (in the range $7\times {10}^6–4\times {10}^8{\mathrm{cm}}^{-2}$) nor on alloy composition in the range 5–14 at.%. The strain-shift coefficient for the main model of Ge and for the Ge-Ge vibrational mode of ${\mathrm{Ge}}_{1-x}{\mathrm{Sn}}_x$ is weakly dependent on temperature and is around –500 ${\mathrm{cm}}^{\text{–}1}$. In ${\mathrm{Ge}}_{1-x}{\mathrm{Sn}}_x$, the composition-shift coefficient amounts to –100 ${\mathrm{cm}}^{\text{-1}}$, independent of temperature and strain.

Multiphase $\mathrm{bcc}/B2$-based alloy systems have recently received considerable attention because their microstructures are often remarkably similar to the $\gamma /{\gamma }^{\prime }$ microstructure of Ni-based superalloys. The underlying plastic deformation mechanisms of bcc-based intermetallics, however, are not well understood across the composition space where they are thermodynamically stable. Within this contribution, we analyze deformation of $B2$ intermetallics to develop a reliable platform for efficiently predicting antiphase boundary energies and the associated fault widths as a function of elemental substitution on a particular lattice site of the intermetallic. To achieve this we extend the diffuse multilayer fault model to predict close packed structures that recreate the bonding environment within the layers adjacent to the $\frac{1}{2}\langle 111\rangle \left\{110\right\}$ antiphase boundary of the $B2$ intermetallic. Specifically, the impact of elemental substitution on both antiphase boundary energy and fault width is presented for ${\mathrm{Hf}}_{1-x}{\mathrm{Ti}}_x\mathrm{Ru}$ and ${\mathrm{Hf}}_{1-x}{\mathrm{Al}}_x\mathrm{Ru}$ and the implications of our findings are discussed. We also highlight a simple bonding model for transition metal-based $B2$ intermetallics that explains their chemical stability and large antiphase boundary energies. The results presented here offer insight into both the nature of plastic deformation within the $B2$ intermetallic and the important underlying chemical concepts that can potentially be leveraged to aid in the design of bcc-based alloy systems that rival Ni-based $\gamma /{\gamma }^{\prime }$ microstructures.

Artificially layered superlattices with two distinct spin structures offer new opportunities for manipulation of magnetic properties and interfacial spin configurations. We have grown epitaxial, coherent superlattices of ferrimagnetic ${\mathrm{Mn}}_3\mathrm{Ga}$ and noncollinear antiferromagnetic ${\mathrm{Mn}}_3\mathrm{GaN}$. The out-of-plane ferrimagnetism of the ${\mathrm{Mn}}_3\mathrm{Ga}$ layer, and the Berry-phase charge to spin current generation by the noncollinear antiferromagnetic ${\mathrm{Mn}}_3\mathrm{GaN}$ layer, provide a unique combination for spintronic applications. Reactive magnetron sputtering growth resulted in abrupt transitions between the two layers through controlling the ${\mathrm{N}}_2$ flow. X-ray diffraction and cross-sectional scanning transmission electron microscopy images demonstrate clean layering and consistent modulation wavelengths, with interfacial roughness less than one unit cell. This allows investigation of the interfacial magnetic interactions. Through a combination of superconducting quantum interference device magnetometry and polarized neutron reflectometry we show that ${\mathrm{Mn}}_3\mathrm{Ga}/{\mathrm{Mn}}_3\mathrm{GaN}$ superlattice structures have the out-of-plane magnetic anisotropy decreased compared to ${\mathrm{Mn}}_3\mathrm{Ga}$ single-layer films. This softening is primarily a result of reduced anisotropy energy at the interface and is linked to the ${\mathrm{Mn}}_3\mathrm{GaN}$ layer. This superlattice structure provides a platform for devices that use out-of-plane spin torques generated from an antiferromagnetic material to switch the net magnetic moment of a ferrimagnetic material. Our results demonstrate the tunability of magnetic anisotropy to allow for optimal balancing of the switching power and thermal stability in spintronic heterostructures.

High-entropy alloys are solid solutions composed of five or more elements in near-equimolar proportions that demonstrate a number of unusual properties that have yet to be fully explained. Among these, the origin and existence of diffusion qualified as sluggish have been debated since the first measurements of diffusion in disordered systems became available. To better understand the potential role of entropy in this phenomenon, we analyze vacancy diffusion in a ternary concentrated solid-solution alloy, FeNiCr. This is done through atomistic simulations using the kinetic activation-relaxation technique, an off-lattice kinetic Monte Carlo algorithm combined with an embedded atom method potential. Through an analysis of millions of activated events, we compare the kinetics of a vacancy when the activation prefactor is computed specifically within the harmonic approximation to a system where activated prefactors are set at a constant value, regardless of the environment. This allows us to identify the role of disorder on energetic barriers and prefactor distributions, particularly in the case of defect kinetics. More precisely, through the emerging statistical evidence of a compensation between the barrier and prefactor, we show that disorder strongly perturbs the system's vibrational entropy, contributing to explain sluggish diffusion.

Reversible martensitic transformations nucleate from sparse defects that lower the transformation's nucleation energy barrier. However, most defects do not serve as potent nucleation sites and, instead, can pin boundary motion and impede phase growth. Identifying potent defects from the general defect population remains an open challenge and has important implications for engineering reversible alloys. This study considers the influence of mesoscale order-disorder domains and the phase boundaries between the $L{2}_1$ and $B2$ phases on nucleation kinetics. We use solution heat treatment and secondary annealing in ${\mathrm{Ni}}_{45}{\mathrm{Co}}_5{\mathrm{Mn}}_{36.7}{\mathrm{In}}_{13.3}$ microparticles to compare an average $L{2}_1$ domain interfacial area density of $590.6-50.6\mu {\mathrm{m}}^{-1}$, measured from transmission electron microscopy micrographs using the intercept method in ASTM standard E112-13. A total of 131 single particles with radii between 3.8 and 20.7 μm were individually characterized magnetically to measure their transformation temperatures and the transformation behavior. Overall, the undercooling in each particle ranged from 11.3 to 59.4 K, with the smallest volumes having the largest magnitude and variance. The nucleation site potency distributions between the two domain sizes were statistically alike, suggesting that $L{2}_1$ domain size is not a critical factor in initiating nucleation at the length scales of this experiment. The implications for microlevel devices include opportunities to heat treat materials to high operational temperatures (e.g., 773 K) without impacting nucleation behavior.

$\mathrm{DFT}+U$ provides a convenient, cost-effective correction for the self-interaction error (SIE) that arises when describing correlated electronic states using conventional approximate density functional theory (DFT). The success of a $\mathrm{DFT}+U$(+$J$) calculation hinges on the accurate determination of its Hubbard $U$ and Hund $J$ parameters, and the linear response (LR) methodology has proven to be computationally effective and accurate for calculating these parameters. This study provides a high-throughput computational analysis of the $U$ and $J$ values for transition metal $d$-electron states in a representative set of over 1000 magnetic transition metal oxides (TMOs), providing a frame of reference for researchers who use $\mathrm{DFT}+U$ to study transition metal oxides. In order to perform this high-throughput study, an atomate workflow is developed for calculating $U$ and $J$ values automatically on massively parallel supercomputing architectures. To demonstrate an application of this workflow, the spin-canting magnetic structure and unit cell parameters of the multiferroic olivine ${\mathrm{LiNiPO}}_4$ are calculated using the computed Hubbard $U$ and Hund $J$ values for $\mathrm{Ni}\text{−}d$ and $\mathrm{O}\text{−}p$ states, and are compared with experiment. Both the $\mathrm{Ni}\text{−}dU$ and $J$ corrections have a strong effect on the Ni-moment canting angle. Additionally, including a $\mathrm{O}\text{−}pU$ value results in a significantly improved agreement between the computed lattice parameters and experiment.

Associating thin ferromagnetic (FM) films with two-dimensional (2D) materials appears to be a promising approach for the next-generation spintronic devices. Consequently, the control of the magnetic phenomena in the FM/2D systems is of the utmost importance. We investigate in this paper the structural, static, and dynamic magnetic properties of $\mathrm{Si}{\mathrm{O}}_2/\mathrm{single}-\mathrm{layer}$ graphene (SLG)/CoFeB ($3\mathrm{nm}\le t_{\mathrm{CFB}}\le 20\mathrm{nm})/\mathrm{Cu}$ (3 nm) structures and compared them with control samples, without the SLG underlayer. The single-layer characteristics of the synthesized graphene are deduced from Raman spectroscopy investigations. Vibrating sample magnetometry measurements allowed deducting the magnetization at saturation $M_s$, the coercive field $H_c$, and the magnetic dead layer thickness $t_d$. In the absence of SLG, the nominal thickness $t_{\mathrm{CFB}}$ is almost preserved, while $t_d\approx 2\mathrm{nm}$ was determined in presence of graphene. Here, $H_c$ was found to increase in presence of SLG compared with the control structures, while $M_s$ are comparable. Brillouin light scattering and microstrip FM resonance measurements were employed to investigate the dynamic magnetic properties of the samples. It was found that the magnetic anisotropy results from interface and volume contributions and that the largest interface anisotropy was observed in presence of SLG. Measurements of the spin waves nonreciprocity, related to the interfacial Dzyloshinskii-Moriya interaction (iDMI), revealed weak iDMI in our samples. The magnetic damping in the SLG/CFB films was found to be dominated by the two-magnon contribution. No spin pumping was observed either without or with graphene. Our results highlight the influence of the graphene ripples on the magnetic properties of graphene/FM-based heterostructures.

The structural, electronic, mechanical, and thermoelectric properties of alkali-metal-based bismuth selenides (${ABiSe}_2$; $A$ = Na, K) are investigated using a combination of ab initio density functional theory and semiclassical Boltzmann transport theory. The computed lattice constants are in close agreement with experimental results obtained from the Crystallography Open Database. The calculation of the phonon dispersion, elastic tensor, and formation energy confirm that both compounds are dynamically, mechanically, and thermodynamically stable. The band gaps of $\mathrm{KBi}{\mathrm{Se}}_2$ and $\mathrm{NaBi}{\mathrm{Se}}_2$ are indirect, and their room-temperature values computed at the HSE06 level including spin-orbit coupling (SOC) are 1.20 and 1.13 eV, respectively. SOC interactions impact the band gaps of these compounds which in turn influence the optical and thermoelectric properties, respectively. The absorption coefficients and figure-of-merit values obtained from optical and thermoelectric calculations suggest these materials as potential candidates for photovoltaic and thermoelectric applications.

Nanotubes and nanorods have been recently established as very good materials to work as electron sources in a field emission (FE) process. These are one-dimensional materials and electron-electron interactions are expected to play a crucial role in their physics. Here we study the influence of electron-electron interactions on the field emission. We study the problem in the low energy regime; thus we need to abandon the antiadiabatic approximation and derive tunneling amplitude for a finite duration of the tunneling process. In this work we identified the parameters when exact analytic expression for tunneling current can be given. We obtained formalism that enables one to capture at the same time the collective effects due to electron-electron interactions and thermionic emission. Our results reveal that different types of nanotubes, and their minigap/compressibility parameters, can be easily distinguished based on FE measurements on these materials.

The critical behavior of laser-induced changes in magnetic ordering is studied experimentally in two-dimensional zigzag antiferromagnets XY-like ${\mathrm{NiPS}}_3$ and Ising-like ${\mathrm{FePS}}_3$. To examine laser-induced dynamics in flakes of these compounds, we employ time-resolved exchange linear dichroism effect sensitive to zigzag magnetic ordering and independent of the orientation of the antiferromagnetic vector. In both compounds laser excitation in the vicinity of the absorption edge induces partial quenching of the antiferromagnetic ordering manifested by exchange linear dichroism reduction. The amplitude of the effect varies with temperature as the derivative of the antiferromagnetic vector and exhibits a critical behavior with the exponents corresponding to XY and Ising models for ${\mathrm{NiPS}}_3$ and ${\mathrm{FePS}}_3$, respectively. Critical slowing down of the demagnetization in the vicinity of Néel temperature is found, however, only in ${\mathrm{FePS}}_3$. In contrast, the increase of the demagnetization time near the ordering temperature in ${\mathrm{NiPS}}_3$ is minor. We show that the difference in the demagnetization times correlates well with the spin specific heat in both compounds. Beyond the range of slowing down, the demagnetization times in ${\mathrm{NiPS}}_3$ and ${\mathrm{FePS}}_3$ are comparable, about 5–10 ps, and are longer than those reported earlier for ${\mathrm{CoPS}}_3$ and considerably shorter than for ${\mathrm{MnPS}}_3$. This points to the importance of the unquenched angular momentum of transition-metal ions in laser-induced demagnetization process.

High thermal conductivity and ambipolar mobility are highly desirable for semiconductors in electronics and have been observed in bulk boron arsenide (BAs). In this work, we explore the scaling behavior of a monolayer hydrogenated BAs field-effect transistor (ML H-BAs FET) by employing ab initio quantum transport methods. Both the armchair- and zigzag-directed ML H-BAs FETs can well satisfy the requirements of the International Technology Roadmap for Semiconductors even if the gate length is scaled down to $2\sim 3$ nm for high-performance applications. The excellent n- and $p\text{−}\mathrm{type}$ symmetry of bulk BAs is well preserved in the ML H-BAs FET along with the zigzag direction but is lost in the armchair direction. However, such asymmetry can be suppressed by applying uniaxial compressive strain owing to the broken valence band degeneracy. Our findings provide important theoretical insights into transport symmetry and the scaling behavior of ML H-BAs FETs.

In this paper, we report a systematic study on how vibrational spectra mismatch affects the degree of phonon nonequilibrium near an interface, how fast it is relaxed as the phonons diffuse into a lead, and the overall interfacial thermal resistance from the nonequilibrium phonons. Our discussion is based on the solution of the Peierls-Boltzmann transport equation with ab initio inputs for 36 interfaces between semi-infinite group-III (Al, Ga, In) and group-V (P, As, Sb) compound semiconductor leads. The simulation reveals that the nonequilibrium phonons cause significant interfacial thermal resistance for all 36 interfaces, making the overall interfacial thermal resistance two to three times larger than that predicted by the Landauer formalism. We observe a clear trend that the degree of phonon nonequilibrium near an interface and the interfacial thermal resistance from the nonequilibrium phonons increase as the mismatch of the Debye temperature of two lead materials increases. This contrasts with the Landauer formalism's predictions, which show no correlation with the Debye temperature mismatch. The relaxation length of the phonon nonequilibrium varies significantly from 50 nm to 1.5 µm depending on the combination of the materials. The relaxation length is proportional to the phonon mean free path of the corresponding lead material but also largely depends on the material in the opposite lead. This suggests the relaxation length cannot be considered an intrinsic property of the corresponding lead material. These findings offer vital insights for understanding nonequilibrium effects on the interfacial thermal transport and optimizing thermal design in devices.

${\mathrm{YbCdCu}}_4$ has been characterized as a heavy-fermion Kondo lattice system without long-range magnetic order. Thermodynamic and transport data of this compound have been partially analyzed, and the reported Kondo temperatures $T_K$ are highly dependent on samples and estimation methods. In particular, the magnetic susceptibility $\chi \left(T\right)$ of this compound does not show the maximum corresponding to the Kondo resonance with a large degeneracy of the ground state, partly due to the sample quality. Thus, single crystals of ${\mathrm{YbCdCu}}_4$ are grown to reinvestigate their physical properties by means of magnetization, specific heat, electrical resistivity, Hall resistivity, and thermoelectric power measurements. Unlike earlier studies, we found consistencies regarding the $T_K\sim 160$ K measurement estimated from thermodynamic and transport property measurements. The magnetic part of the specific heat $C_m$ displays a broad maximum near 70 K, which follows well with the prediction made by the Coqblin-Schrieffer (C-S) model for whole $J=7/2$ multiplets involved in the formation of the Kondo state. However, the local maximum in $\chi \left(T\right)$ does not match well with the C-S model prediction due to the crystalline electric field (CEF) effect, where the $T_K$ value is still greater than the overall CEF splittings: $T_K>{\mathrm{\Delta }}_{CEF}$. Electrical resistivity and Hall coefficient measurements indicate an extremum near $\sim 70$ K, while a negative minimum at $\sim 130$ K is observed in thermoelectric power.

The mechanism of the vacancy ordering in metastable cubic Ge-Sb-Te (c-GST) that underlies the ultrafast phase-change dynamics and prominent thermoelectric properties remains elusive. Achieving a comprehensive understanding of the vacancy-ordering process at an atomic level is challenging because of enormous computational demands required to simulate disordered structures on large temporal and spatial scales. In this study, we investigate the vacancy ordering in c-GST by performing large-scale molecular dynamics simulations using machine learning potentials. The initial c-GST structure with randomly distributed vacancies rearranges to develop a semiordered cubic structure with layerlike ordered vacancies after annealing at 700 K for 100 ns. The vacancy ordering significantly affects the lattice dynamical properties of c-GST. In the initial structure with fully disordered vacancies, we observe a boson peak, usually associated with amorphous solids, that consists of localized modes at ∼0.575 THz. The boson peak modes are highly localized around specific atomic arrangements of straight vacancy-Te-vacancy trios. As vacancies become ordered, the boson peak disappears, and the Debye-Waller thermal $B$ factor of Te decreases substantially. This finding indicates that the c-GST undergoes a transition from amorphous-like to crystalline-like solid state by thermal annealing in low-frequency dynamics.

Variations with oxygen concentration of titanium lattice parameters are obtained by means of ab initio calculations, considering the impact of oxygen ordering. The quasiharmonic approximation is used to take into account the thermal expansion at finite temperature. Results show that lattice parameters depend mainly on oxygen concentration and, to a lesser extent, on the ordering state. Knowing these theoretical variations, one can get insights into the composition of ordered compounds existing in Ti-O binary alloys from their lattice mismatch measured experimentally by x-ray diffraction. The approach is used in a binary alloy containing 6000 ppm in weight of oxygen. It is concluded that the ordered compounds, which are observed after a recrystallization heat treatment, do not have the expected ${\mathrm{Ti}}_6\mathrm{O}$ stoichiometry but have a composition close to the nominal concentration. Oxygen ordering proceeds, therefore, before oxygen partitioning in titanium.