Layered transition-metal dichalcogenides are proposed as building blocks for van der Waals heterostructures due to their graphenelike two-dimensional structure. For this purpose, a magnetic semiconductor could represent an invaluable component for various spintronics and topotronics devices. Here, we combine different local magnetic probe spectroscopies with angle-resolved photoemission and density-functional theory calculations to show that $2H\text{−}{\mathrm{MoTe}}_2$ is on the verge of becoming magnetic. Our results present clear evidence that the magnetism can be “switched on” by a hydrogenlike impurity. We also show that this magnetic state survives up to the free surface region, demonstrating the material's potential applicability as a magnetic component for thin-film heterostructures.

We study the electronic and magnetic states of as-grown and annealed $M{\mathrm{Fe}}_2{\mathrm{O}}_4\left(111\right)/{\mathrm{Al}}_2{\mathrm{O}}_3$(111) ($M$ = Co, Ni) thin films with various thicknesses grown on Si(111) substrates with the $\gamma \text{−}{\mathrm{Al}}_2{\mathrm{O}}_3$(111) buffer layers by using x-ray absorption spectroscopy (XAS) and x-ray magnetic circular dichroism (XMCD), to investigate magnetically dead layers in these films. Although the magnetically dead layers in the as-grown samples are formed near the interface with the ${\mathrm{Al}}_2{\mathrm{O}}_3$ buffer layer, we reveal that ferrimagnetic order is partially recovered by postdeposition annealing at 973 K for 48 h in air. By analyzing the line shapes of the XAS and XMCD spectra, we conclude that, in the dead layers, there are a significant number of vacancies at the $T_d$ sites of the spinel structure, which may be the microscopic origin of the degraded ferrimagnetic order in the $M{\mathrm{Fe}}_2{\mathrm{O}}_4$ thin films.

The electrostatic screening properties of ionic fluids are of paramount importance in countless physical processes. Yet the screening behavior of ionic conductors out of thermal equilibrium has to date mainly been studied in the context of thermoelectric phenomena by virtue of direct extensions of Debye-Hückel theories. We investigate how the static response of a symmetric ionic fluid is influenced by the presence of a thermal gradient by introducing a theory of electrostatic screening under a stationary temperature profile. By borrowing mathematical methods commonly used in the semiclassical approximation of quantum particles, we find analytical solutions to the asymptotic decay of the charge density which can be used to describe the nonequilibrium response of the system to external charge perturbations and for arbitrary ionic concentrations. Notably, a transition between monotonic and oscillatory screening regimes is observed as an effect of the temperature variation which generalizes known results of thermal equilibrium to out of equilibrium conditions. A final quantitative example on the screening of charged surfaces in aqueous electrolytes shows that the deviation from thermal equilibrium predicted by our solutions is generally larger than thermoelectric effects and should therefore be taken into account for a comprehensive description of the electrical double layer. Our findings pave the way to the rigorous treatment of nonequilibrium steady states in ionic systems with potential applications to the study of energy materials, nanostructured systems, and waste-heat-recovery technologies.

It has been well documented that oxide films thinner than a few unit cells (u.c.) are often insulating, and the Anderson localization supported by the low dimensionality is one of the major mechanisms of such insulating behavior. To discriminate the effect of randomness from that of dimensionality, we examined structural fluctuation as well as the average structure of ${\mathrm{LaNiO}}_3$ thin films on ${\mathrm{LaAlO}}_3$ substrates as a function of thickness. The surface x-ray diffraction experiment revealed that there is little difference in average atomic position and atomic occupancy among 2 to 5 u.c. thick samples. The main difference occurs in the atomic displacement parameters. 4 and 5 u.c. thick samples have significantly smaller atomic displacement parameters than 2 and 3 u.c. thick samples. This result suggests that structural fluctuation plays some role in the interfacial transport properties although it is often neglected in the discussion of oxide interface physics.

Magnetic topological semimetals present open questions regarding the interplay of crystal symmetry, magnetism, band topology, and electron correlations. Ln${\mathrm{Sb}}_x{\mathrm{Te}}_{2\text{−}x\text{−}\delta }$ (Ln denotes Lanthanide) is a family of square-net-derived topological semimetals that allows compositional control of band filling, and access to different topological states via an evolving charge density wave (CDW) distortion. Previously studied Gd and Ce members containing a CDW have shown complex magnetic phase diagrams, which implied that spins localized on Ln interact with the CDW, but to this date no magnetic structures have been solved within the CDW regime of this family of compounds. Here, we report on the interplay of the CDW with magnetism in ${\mathrm{NdSb}}_x{\mathrm{Te}}_{2\text{−}x\text{−}\delta }$ by comparing the undistorted square net member ${\mathrm{NdSb}}_{0.94}{\mathrm{Te}}_{0.92}$ with the CDW-distorted phase ${\mathrm{NdSb}}_{0.48}{\mathrm{Te}}_{1.37}$, via single-crystal x-ray diffraction, magnetometry, heat capacity, and neutron powder diffraction. ${\mathrm{NdSb}}_{0.94}{\mathrm{Te}}_{0.92}$ is a collinear antiferromagnet with $T_N\sim 2.7\mathrm{K}$, where spins align antiparallel to each other, but parallel to the square net of the nuclear structure. ${\mathrm{NdSb}}_{0.48}{\mathrm{Te}}_{1.37}$ exhibits a nearly fivefold-modulated CDW ($q_{\text{CDW}}=0.18$), isostructural to other Ln${\mathrm{Sb}}_x{\mathrm{Te}}_{2\text{−}x\text{−}\delta }$ at similar $x$. ${\mathrm{NdSb}}_{0.48}{\mathrm{Te}}_{1.37}$ displays more complex magnetism with $T_N=2.3\mathrm{K}$, additional metamagnetic transitions, and an elliptical cycloid magnetic structure with $q_{\text{mag}}=-0.41{\mathrm{b}}^{*}$. The magnitudes of $q_{\text{CDW}}$ and $q_{\text{mag}}$ exhibit an integer relationship, $1+2q_{\text{mag}}=q_{\text{CDW}}$, implying a coupling between the CDW and magnetic structure. Given that the CDW is localized within the nonmagnetic distorted square net, we propose that conduction electrons “template” the spin modulation via the Ruderman-Kittel-Kasuya-Yosida interaction.

Nonsolvent-induced phase separation (NIPS) is a complex but important phenomenon involved in the manufacture of fibers and other polymeric industrial products. We performed in situ x-ray photon correlation spectroscopy (XPCS) measurements on a cuprammonium cellulose solution immersed in aqueous acetone solution to reveal the structure and dynamics involved in the NIPS process on the microscopic scale. A combination of the structural information from the small angle x-ray scattering profiles and the dynamic information from the XPCS analysis revealed that NIPS occurs via two steps: dilution of the polymer solution by the solvent and desorption of the solvent from the polymer solution. This study will provide an opportunity for the fundamental understanding of NIPS, which is important from both the scientific and industrial viewpoints.

Alloys composed of several elements in roughly equimolar composition, often referred to as high-entropy alloys, have long been of interest for their thermodynamics and peculiar mechanical properties, and more recently for their potential application in catalysis. They are a considerable challenge to traditional atomistic modeling, and also to data-driven potentials that for the most part have memory footprint, computational effort, and data requirements which scale poorly with the number of elements included. We apply a recently proposed scheme to compress chemical information in a lower-dimensional space, which reduces dramatically the cost of the model with negligible loss of accuracy, to build a potential that can describe 25 $d$-block transition metals. The model shows semiquantitative accuracy for prototypical alloys and is remarkably stable when extrapolating to structures outside its training set. We use this framework to study element segregation in a computational experiment that simulates an equimolar alloy of all 25 elements, mimicking the seminal experiments in the groups of Yeh and Cantor, and use our observations on the short-range order relations between the elements to define a data-driven set of Hume-Rothery rules that can serve as guidance for alloy design. We conclude with a study of three prototypical alloys, CoCrFeMnNi, CoCrFeMoNi, and IrPdPtRhRu, determining their stability and the short-range order behavior of their constituents.

Van der Waals superlattices are important for tailoring the electronic structures and properties of layered materials. Here we report the superconducting properties and electronic structure of a natural van der Waals superlattice (${\mathrm{PbSe})}_{1.14}{\mathrm{NbSe}}_2$. Anisotropic superconductivity with a transition temperature $T_c$ = 5.6 $\pm$ 0.1 K, which is higher than monolayer ${\mathrm{NbSe}}_2$, is revealed by transport measurements on high-quality samples. Angle-resolved photoemission spectroscopy (ARPES) measurements reveal the two-dimensional electronic structure and a charge transfer of 0.43 electrons per ${\mathrm{NbSe}}_2$ unit cell from the blocking PbSe layer. In addition, polarization-dependent ARPES measurements reveal a significant circular dichroism with opposite contrast at K and ${\mathrm{K}}^{\prime }$ valleys, suggesting a significant spin-orbital coupling and distinct orbital angular momentum. Our work suggests natural van der Waals superlattice as an effective pathway for achieving intriguing properties distinct from both the bulk and monolayer samples.

The semiconductor ${\mathrm{ReSe}}_2$ is characterized by a strongly anisotropic optical absorption and is therefore promising as an optically active component in two-dimensional heterostructures. However, the underlying femtosecond dynamics of photoinduced excitations in such materials has not been sufficiently explored. Here, we apply an infrared optical excitation to single-layer ${\mathrm{ReSe}}_2$ grown on a bilayer graphene substrate and monitor the temporal evolution of the excited state signal using time- and angle-resolved photoemission spectroscopy. We measure an optical gap of $(1.53\pm 0.02)\mathrm{eV}$, consistent with resonant excitation of the lowest exciton state. The exciton distribution is tunable via the linear polarization of the pump pulse and exhibits a biexponential decay with time constants given by ${\tau }_1=(110\pm 10)$ fs and ${\tau }_2=(650\pm 70)$ fs, facilitated by interlayer charge transfer to the underlying bilayer graphene and recombination via an in-gap state that is pinned at the Fermi level. By extracting the momentum-resolved exciton distribution we estimate its real-space radial extent to be greater than $(17\pm 1)$ Å, implying significant spatial broadening of the distribution due to screening from the bilayer graphene substrate.

In this study, we employed molecular dynamics simulations, both traditional and machine learned, to emulate spherical nanoindentation experiments of crystalline W matrices at different temperatures and loading rates using different approaches, such as EAM, EAM with Ziegler, Biersack, and Littmark corrections, modified EAM, analytic bond-order approach, and a recently developed machine-learned tabulated Gaussian approximation potential (tabGAP) framework for describing the W-W interaction and plastic deformation mechanisms. Results showed similarities between the recorded load-displacement curves and dislocation densities, for different interatomic potentials and crystal orientations at low and room temperature. However, we observe concrete differences in the early stages of elastic-to-plastic deformation transition, revealing different mechanisms for dislocation nucleation and dynamics during loading, especially at higher temperatures. This is attributed to the particular features of orientation dependence in crystal plasticity mechanisms and, characteristically, the stacking fault and dislocation glide energies information in the interatomic potentials, with tabGAP being the one with the most well-trained results compared to density functional theory calculations and experimental data.

Two-dimensional (2D) van der Waals (vdW) heterostructures fabricated by combining 2D materials with unique properties into one ultimate unit can offer a plethora of fundamental phenomena and practical applications. Recently, proximity-induced quantum and spintronic effects have been realized in heterostructures of graphene (Gr) with 2D semiconductors and their twisted systems. However, these studies are so far limited to exfoliated flake-based devices, limiting their potential for scalable practical applications. Here, we report spin-valley coupling and spin-relaxation anisotropy in Gr-${\mathrm{MoS}}_2$ heterostructure devices prepared from scalable chemical vapor-deposited (CVD) 2D materials. Spin precession and dynamics measurements reveal an enhanced spin-orbit coupling strength in the Gr-${\mathrm{MoS}}_2$ heterostructure in comparison with pristine Gr at room temperature. Consequently, large spin-relaxation anisotropy is observed in the heterostructure, providing a method for spin filtering due to spin-valley coupling. These findings open a scalable platform for all-CVD 2D vdW heterostructures design and their device applications.

The magnetic properties of the quasi-two-dimensional van der Waals magnet ${\mathrm{Fe}}_{5-\delta }{\mathrm{GeTe}}_2$ (F5GT), which has a high ferromagnetic ordering temperature $T_{\text{C}}\sim 315$ K, remain to be better understood. It has been demonstrated that the magnetization of F5GT is sensitive to both the Fe deficiency $\delta$ and the thermal-cycling history. Here, we investigate the structural and magnetic properties of F5GT single crystals with a minimal Fe deficiency $\left(\right|\delta |\le 0.1)$, utilizing combined x-ray and neutron scattering techniques. Our study reveals that the quenched F5GT single crystals experience an irreversible, first-order transition at $T_{\text{S}}\sim 110$ K upon first cooling, where the stacking order partly or entirely converts from ABC-stacking order to AA-stacking order. Importantly, the magnetic properties, including the magnetic moment direction and the enhanced $T_{\text{C}}$ after the thermal cycling, are intimately related to the alteration of the stacking order. Our work highlights the significant influence of the lattice symmetry to the magnetism in F5GT.

The recent observation of ferroelectricity in the metastable phases of binary metal oxides, such as ${\mathrm{HfO}}_2,$ ${\mathrm{ZrO}}_2,$ ${\mathrm{Hf}}_{0.5}{\mathrm{Zr}}_{0.5}{\mathrm{O}}_2,$ and ${\mathrm{Ga}}_2{\mathrm{O}}_3$, has garnered a lot of attention. These metastable ferroelectric phases are typically stabilized using epitaxial strain, alloying, or defect engineering. Here, we propose that hole doping plays a key role in the stabilization of polar phases in binary metal oxides. Using first-principles density-functional-theory calculations, we show that holes in these oxides mainly occupy one of the two oxygen sublattices. This hole localization, which is more pronounced in the polar phase than in the nonpolar phase, lowers the electrostatic energy of the system, and makes the polar phase more stable at sufficiently large concentrations. We demonstrate that this electrostatic mechanism is responsible for stabilization of the ferroelectric phase of ${\mathrm{HfO}}_2$ aliovalently doped with elements that introduce holes to the system, such as La and N. Finally, we show that spontaneous polarization in ${\mathrm{HfO}}_2$ is robust to hole doping, and a large polarization persists even under a high concentration of holes.

The oxide layer is important for the processing and applications of silicon nanoparticles as it provides chemical stability to the material. Herein, the thermal oxidation of sub-10-nm crystalline and amorphous silicon particles in dry air is investigated with nonisothermal kinetic analysis including Kissinger and Flynn-Wall-Ozawa methods at various heating rates. The results indicate that amorphous silicon nanoparticles have a considerably lower activation energy for oxidation (98 kJ/mol) compared to their crystalline counterparts (182 kJ/mol). In situ heating diffuse reflectance infrared Fourier transform spectroscopy was employed to monitor the evolution of surface species of Si nanoparticles during oxidation. The results suggest that backbond oxidation of higher hydrides contribute to the low onset temperature for oxidation of amorphous silicon nanoparticles. Besides, surface ${\mathrm{O}}_3\mathrm{Si}\text{−}\mathrm{H}$ and Si-OH species likely prevent the formation of a dense silicon oxide layer, resulting in the low temperature oxidation behavior of amorphous silicon nanoparticles. These results provide insight into kinetics and chemistry of amorphous silicon oxidation in the thin film regime.

We present a combined study based on the experimental measurements of an infrared (IR) dielectric function and first-principles calculations of IR spectra and the vibrational density of states (VDOS) of amorphous alumina $\left(\mathrm{am}\text{−}{\mathrm{Al}}_2{\mathrm{O}}_3\right)$. In particular, we show that the main features of the imaginary part of the dielectric function ${\epsilon }_2\left(\omega \right)$ at $\sim 380$ and 630 ${\mathrm{cm}}^{-1}$ are related to the motions of threefold-coordinated oxygen atoms, which are the vast majority of oxygen atoms in am-${\mathrm{Al}}_2{\mathrm{O}}_3$. Our analysis provides an alternative point of view with respect to an earlier suggested assignment of the vibrational modes, which relates them to the stretching and bending vibrational modes of ${\mathrm{AlO}}_n$ ($n=4$, 5, and 6) polyhedra. Our assignment is based on the additive decomposition of the VDOS and ${\epsilon }_2\left(\omega \right)$ spectra, which shows that (i) the band at $\sim 380{\mathrm{cm}}^{-1}$ features oxygen motions occurring in a direction normal to the plane defined by the three nearest-neighbor aluminum atoms, i.e., out-of-plane motions of oxygen atoms; (ii) Al-O stretching vibrations (i.e., in-plane motions of oxygen atoms) appear at frequencies above $\sim 500{\mathrm{cm}}^{-1}$, which characterize the vibrational modes underlying the band at $\sim 630{\mathrm{cm}}^{-1}$. Aluminum and fourfold-coordinated oxygen atoms contribute uniformly to the VDOS and ${\epsilon }_2\left(\omega \right)$ spectra in the frequency region $\sim 350–650$ ${\mathrm{cm}}^{-1}$ without causing specific features. Our numerical results are in good agreement with the previous and presently obtained experimental data on the IR dielectric function of $\mathrm{am}\text{−}{\mathrm{Al}}_2{\mathrm{O}}_3$ films. Finally, we show that the IR spectrum can be modeled successfully by assuming isotropic Born charges for aluminum atoms and fourfold-coordinated oxygen atoms, while requiring the use of three parameters, defined in a local reference frame, for the anisotropic Born charges of threefold-coordinated oxygen atoms.

A key challenge in materials informatics is to decode the decision-making process of machine learning (ML) models that have been trained to predict material properties. The existing methods usually rank alloy features based on importance metrics and do not provide material-specific fundamental insights. Here, we present the Compositional Stimulus and Model Response (CoSMoR) framework that can be applied to any composition-based ML model (irrespective of the algorithm used) to calculate the exact contribution of each feature towards the manifestation of target material property along a continuous compositional pathway. CoSMoR utilizes the local partial dependencies of target property with respect to each feature and combines it with feature variations associated with discretized compositional variations to measure exact feature contributions. We showcase the importance of CoSMoR through implementation on phase-selection problem in multiprincipal element alloys (MPEAs), wherein it leads to physical insights into phase transitions. A detailed overview of the framework, along with the codes and step-by-step implementation of the algorithm, has been provided to enable extension to new or preexistent models.

We report the growth of high-quality single crystals of ${\mathrm{ThCr}}_2{\mathrm{Si}}_2$-type tetragonal ${\mathrm{BaMn}}_2{\mathrm{P}}_2$ and investigation of its structural, electrical transport, thermal, and magnetic properties. Our results of basal plane electrical resistivity and heat capacity measurements show that the compound has an insulating ground state with a small band gap. Anisotropic susceptibility ${\chi }_{ab,c}\left(T\right)$ data infer a collinear local-moment Néel-type antiferromagnetic (AFM) ground state below the ordering temperature $T_{\mathrm{N}}=795\left(15\right)\mathrm{K}$, which is highest among all the ${\mathrm{ThCr}}_2{\mathrm{Si}}_2$- and ${\mathrm{CaAl}}_2{\mathrm{Si}}_2$-type 122-pnictide compounds reported so far suggesting that the strength of magnetic exchange interactions is strongest in this material. The magnetic transition temperatures of ${\mathrm{BaMn}}_{2}P{n}_2$ ($Pn=\mathrm{P}$, As, Sb, Bi) compounds exhibit a monotonic decrease with the increase of tetragonal unit cell parameters $a$ and $c$, suggesting a strong dependence of the strength of the decisive magnetic exchange interactions on the separation between the localized spins residing on the Mn ions. The observed monotonic increase of both ${\chi }_{ab}$ and ${\chi }_c$ for $T>T_{\mathrm{N}}$ suggests that short-range dynamic quasi-two-dimensional AFM correlations persist above the $T_{\mathrm{N}}$ up to the highest temperature of the measurements. The large $T_{\mathrm{N}}$ of ${\mathrm{BaMn}}_2{\mathrm{P}}_2$ demands for systematic hole-doping studies on this material as similar investigations on related ${\mathrm{BaMn}}_2{\mathrm{As}}_2$ with $T_{\mathrm{N}}=618\mathrm{K}$ have led to the discovery of an outstanding ground state where AFM of localized Mn spins and itinerant half-metallic ferromagnetism with $T_{\mathrm{c}}\approx 100\mathrm{K}$ originating from the doped holes coexist together.

Lanthanide ($Ln$)-based systems in $\mathrm{ZrSiS}$-type nodal-line semimetals have been subjects of research investigations as grounds for studying the interplay of topology with possible magnetic ordering and electronic correlations that may originate from the presence of $Ln4f$ electrons. In this study, we carried out a thorough study of a $Ln\mathrm{SbTe}$ system, $\mathrm{NdSbTe}$, by using angle-resolved photoemission spectroscopy along with first-principles calculations and thermodynamic measurements. We experimentally detect the presence of multiple gapless nodal-line states, which is well supported by first-principles calculations. A dispersive and an almost nondispersive nodal line exist along the bulk $X\text{−}R$ direction. Another nodal line is present well below the Fermi level across the $\mathrm{\Gamma }\text{−}M$ direction, which is formed by bands with high Fermi velocity that seem to be sensitive to light polarization. Our study provides insight into the electronic structure of an alternative $Ln\mathrm{SbTe}$ material system that will aid towards understanding the connection of $Ln$ elements with topological electronic structures in these systems.

We numerically study strong coupling between terahertz excitations in a hybrid material consisting of a three-dimensional (3D) topological insulator (TI) and a quasi-two-dimensional (2D) van der Waals antiferromagnet. We find that the interaction between a surface Dirac plasmon polariton in the 3D TI and a magnon polariton in the 2D antiferromagnet is mediated by the phonon coupling in the 3D TI material and can result in emergence of a new hybridized mode, namely, a surface Dirac plasmon-phonon-magnon polariton. We numerically study the dependence of the strong coupling on a variety of structural parameters of the 3D-TI/2D-antiferromagnetic (AFM) hybrid material. Our results reveal that the strength of the coupling depends primarily on the anisotropy constant of the 2D AFM material, as well as on its thickness, and reaches a maximum when the AFM layer is sufficiently thick to be considered a half-infinite slab. We show that the extremely large anisotropy constant reported for certain 2D van der Waals antiferromagnets results in a coupling strength that should be experimentally observable even in the presence of realistic scattering losses.