The family of van der Waals dichalcogenides (vdWDs) includes a large number of compositions and phases, exhibiting varied properties and functionalities. They have opened up a novel electronics of two-dimensional materials, characterized by higher integration and interfaces which are atomically sharper and cleaner than conventional electronics. Among these functionalities, some vdWDs possess remarkable thermoelectric properties. ${\mathrm{SnSe}}_2$ has been identified as a promising thermoelectric material on the basis of its estimated electronic and transport properties. In this work we carry out experimental measurements of the electric and thermoelectric properties of ${\mathrm{SnSe}}_2$ flakes. For a 30-μm-thick ${\mathrm{SnSe}}_2$ flake at room temperature, we measure electron mobility of 40 ${\mathrm{cm}}^2{\mathrm{V}}^{-1}{\mathrm{s}}^{-1}$, a carrier density of $4\times {10}^{18}\mathrm{c}{\mathrm{m}}^{-3}$, a Seebeck coefficient $S\approx -400\mu \mathrm{V}/\mathrm{K}$, and thermoelectric power factor $S^2\sigma \approx 0.35\mathrm{mW}{\mathrm{m}}^{-1}{\mathrm{K}}^{-2}$. The comparison of experimental results with theoretical calculations shows fair agreement and indicates that the dominant carrier scattering mechanisms are polar optical phonons at room temperature and ionized impurities below 50 K. In order to explore possible improvement of the thermoelectric properties, we carry out reversible electrostatic doping on a thinner flake, in a field effect setup. On this 75-nm-thick ${\mathrm{SnSe}}_2$ flake, we measure a field effect variation of the Seebeck coefficient of up to 290% at low temperature, and a corresponding variation of the thermoelectric power factor of up to 1050%. We find that the power factor increases with the depletion of $n$-type charge carriers. Field effect control of thermoelectric transport opens perspectives for boosting energy harvesting and novel switching technologies based on two-dimensional materials.

We fabricated two types of cobalt-ferrite (001) thin films, insulative Fe-rich cobalt-ferrite ${\mathrm{Co}}_x{\mathrm{Fe}}_{3-x}{\mathrm{O}}_{4+\delta }$ (I-CFO) and conductive Fe-rich cobalt-ferrite ${\mathrm{Co}}_y{\mathrm{Fe}}_{3-y}{\mathrm{O}}_4$ (C-CFO), with perpendicular magnetic anisotropy (PMA) on MgO (001) substrates. Although the stoichiometric cobalt ferrite is known as an insulating material, it is found that the conductivity of Fe-rich cobalt ferrites can be controlled by changing the source materials and deposition conditions in the pulsed laser deposition technique. The I-CFO and C-CFO films exhibit PMA through the in-plane lattice distortion. We investigated the Fe-ion-specific valence states in both I-CFO and C-CFO films by Mössbauer spectroscopy and x-ray magnetic circular dichroism and found that the difference in conductivity corresponds to the abundance ratio of the ${\mathrm{Fe}}^{2+}$ state at the octahedral $B$-site $\left(O_{\mathrm{h}}\right)$ in the inverse spinel structure. Furthermore, first-principles calculations reproduce the changes in the density of states at the Fermi level depending on the cation vacancies at the $B$-site, which explains the difference in conductivity between I-CFO and C-CFO.

We develop the theoretical framework of nonequilibrium ultrafast photonics in monolayer quantum spin Hall insulators supporting a multitude of topological states. In these materials, ubiquitous strong light-matter interactions in the femtosecond scale lead to nonadiabatic quantum dynamics, resulting in topology-dependent nonlinear optoelectronic transport phenomena. We investigate the mechanism driving topological Dirac fermions interacting with strong ultrashort light pulses and uncover various experimentally accessible physical quantities that encode fingerprints of the quantum material's topological electronic state from the high-harmonic generated spectrum. Our work sets the theoretical cornerstones to realize the full potential of time-resolved harmonic spectroscopy for understanding nonequilibrium processes in quantum topological systems and identifying topological invariants in two-dimensional quantum spin Hall solid state systems.

In Laves phases, synchroshear is the dominant basal slip mechanism. It is accomplished by the glide of synchro-Shockley dislocations. However, the atomic-scale mechanisms of motion of such zonal dislocations are still not well understood. In this paper, using atomistic simulations, two $30{}^{\circ }$ synchro-Shockley dislocations with different Burgers vectors and core structures and energies are identified. We demonstrate that nucleation and propagation of kink pairs is the energetically favorable mechanism for the motion of the synchro-Shockley dislocation (partial I). Vacancy hopping and interstitial shuffling are identified as two key mechanisms related to kink propagation, and we investigated how vacancies and antisite defects assist kink nucleation and propagation, which is crucial for kink mobility. Additionally, we identified a mechanism of nonsequential atomic shuffling for the motion of the synchro-Shockley dislocation (partial II). These findings provide insights into the dependency on temperature and chemical composition of plastic deformation induced by zonal dislocations in Laves phases and the many related topologically close-packed phases.

Herein we investigate the spectra of crystal structures and their metastability for elemental carbon and the entire set of group-IV monocarbides (SiC, GeC, and SnC) using the ab initio random structure sampling. All of the known structures of elemental C and SiC are correctly identified as high probability in the random sampling and the predicted metastabilities of polymorphs are consistent with the existing knowledge. Furthermore, we find the same four structure types, tetrahedrally bonded zincblende, wurtzite, and rhombohedral-$R\overline{3}m$, as well as octahedrally coordinated rocksalt at high pressure, as the only relevant polymorphs for all studied carbides. However, the rocksalt structure in GeC is shown to be dynamically unstable, contrary to SiC and SnC. Additionally, all GeC and SnC structures are found to have positive enthalpies of formation and are therefore unstable relative to decomposition. The tetrahedrally bonded polymorphs, especially the SnC ones, are predicted to have very high intrinsic (phonon-limited) electron mobilities comparable with the best known semiconductors. Additionally, wurtzite SnC is lattice matched to InN suggesting epitaxial growth as a possible avenue to realize wurtzite SnC.

Antiferromagnetic domain formations and behaviors are long-standing implicit questions in magnetism and are becoming more important in recently emerging antiferromagnetic spintronics. In this paper, we experimentally investigate and numerically simulate evolutions of antiferromagnetic domains in NiO of a bulk single crystal under magnetic fields. With appropriate magnetic and crystallographic potential energy considerations, we extend a micromagnetic framework with a simulation algorithm which reproduces well the domain patterns and furnishes detailed understanding of the domain evolutions. The present work provides an important theoretical methodology to predict and understand behaviors of micromagnetic structures in antiferromagnets.

Constructing an accurate atomistic model for the high-pressure phases of tin (Sn) is challenging because the properties of Sn are sensitive to pressures. We develop machine-learning-based deep potentials for Sn with pressures ranging from 0 to 50 GPa and temperatures ranging from 0 to 2000 K. In particular, we find the deep potential, which is obtained by training the ab initio data from density functional theory calculations with the state-of-the-art SCAN exchange-correlation functional, is suitable to characterize high-pressure phases of Sn. We systematically validate several structural and elastic properties of the $\alpha$ (diamond structure), $\beta$, bct, and bcc structures of Sn, as well as the structural and dynamic properties of liquid Sn. The thermodynamics integration method is further utilized to compute the free energies of the $\alpha , \beta$, bct, and liquid phases, from which the deep potential successfully predicts the phase diagram of Sn including the existence of the triple-point that qualitatively agrees with the experiment.

We investigated the stability of polymeric $\mathrm{C}{\mathrm{O}}_2$ over a wide range of pressures, temperatures, and chemical environments. We find that the $I\overline{4}2d$ polymeric structure, consisting of a three-dimensional network of corner sharing $\mathrm{C}{\mathrm{O}}_4$ tetrahedra, forms at 40–140 GPa and from a $\text{CO-}{\mathrm{N}}_2$ mixture at 39 GPa. An exceptional stability field of 0–286 GPa and 100–2500 K is documented for this structure, making it a viable candidate for planetary interiors. The stability of the tetrahedral polymeric motif of $\mathrm{C}{\mathrm{O}}_2\text{−}\mathrm{V}$ is a consequence of the rigidity of $s{p}^3$ hybridized orbitals of carbon in a closed-packed oxygen sublattice.

Understanding the formation of a polaron near vacancies in oxides is of vital importance for controlling defect properties and related functionalities. We examine the energetics and local structure of polarons in the vicinity of oxygen vacancies using density functional theory with the Hubbard on-site Coulombic correction. We systematically consider several $\mathrm{ATi}{\mathrm{O}}_3$ perovskite systems: cubic $\mathrm{SrTi}{\mathrm{O}}_3$ (c-STO), cubic $\mathrm{BaTi}{\mathrm{O}}_3$ (c-BTO), cubic $\mathrm{PbTi}{\mathrm{O}}_3$ (c-PTO), tetragonal $\mathrm{BaTi}{\mathrm{O}}_3$ (t-BTO), and tetragonal $\mathrm{PbTi}{\mathrm{O}}_3$ (t-PTO). The polaron formation energies vary by several orders of magnitude across the systems, where PTO systems have large polaron formation energies $\left(\sim -1.688\mathrm{eV}\right)$, c-BTO is intermediate (−0.234 eV), and c-STO and t-BTO have considerably smaller formation energies $(<-0.01\mathrm{eV})$. The formation of a polaron is found to influence the charge density, atomic displacement, and electronic structure, with higher polaron energies corresponding to larger charge density changes and greater displacements of the ions surrounding the oxygen vacancy, especially the displacement of the first nearest neighbor A-site. Finally, a comparison of the projected band structures shows fictitious in-gap states present in the delocalized picture of the system moving to the conduction and valence bands in the polaron solution of the system, indicating an incompleteness of the description of a vacancy without considering charge localization as in the polaron picture. In this paper, we provide fundamental properties of polarons near vacancies, which can be used to control and tune defect, charge transport, magnetoelectric, and multiferroic properties of perovskite oxides.

We demonstrate the presence of a hysteresis in tunneling spectra acquired at 4.3 K on cleaved MA-Br terminated (001) surfaces of ${\mathrm{MAPbBr}}_3$ single crystals. Simulations of the tunneling spectra reveal an underlying polarization-voltage ($P\text{−}V$) hysteresis, which is caused by an interplay of subsurface field-induced rotation and alignment of the MA molecules, stabilized by dipole-dipole interactions, and an ion-lattice relaxation. The resulting subsurface polarization is counteracted by a compensating surface polarization, detectable by surface-sensitive tunneling spectroscopy.

Transition-metal dichalcogenide (TMDC) monolayers exhibit strong many-body Coulomb interaction, which induces rich excitonic states, including neutral exciton, trion (charged exciton), biexciton, and even charged biexciton. Despite the fact that the biexcitons in pristine two-dimensional (2D) TMDCs have great potential in forming correlated exciton pairs at distinct valleys, they lack efficient emission and optical tunability at room temperature, hindering their applications in photonic and optoelectronic devices. Here, we report room-temperature strong emission of the charged biexcitons from a gold nanosphere capped ${\mathrm{WS}}_2$ monolayer through localized surface plasmon (LSP) induced hot-electron injection, and demonstrate all-optical control of the emission intensity through on- or off-resonant LSP excitation. Under on-resonant LSP excitation at 2.08 eV, the photoluminescence (PL) intensity shows a superlinear scaling with the laser power density, signifying the existence of charged biexcitons; On the contrary, the off-resonant LSP excitation at 2.33 eV results in no detectable PL signal of the charged biexcitons. By calculating the energy difference between the charged biexciton and trion, we further determine the binding energy of the former to be 25 meV. Our findings offer an opportunity for understanding the many-body effect associated with the charged biexcitons of 2D TMDCs, and could facilitate their potential quantum and optoelectronic applications under ambient environment.

We report on the pressure-tuning of the physical properties of a metal thiophosphate ${\mathrm{Pb}}_2{\mathrm{P}}_2{\mathrm{S}}_6$. Resistance measurements evidence a semiconducting-to-metallic transition at around 39 GPa and immediate emergence of superconductivity. The superconducting critical temperature $T_C$ monotonously increases from 3.2 to 5.1 K until the highest studied pressure 54 GPa. Over the whole pressure range, the pristine monoclinic lattice symmetry is not changed. Moreover, the resonance Raman scattering is observed in the range of 8–12 GPa depending on the excitation energy used. These findings establish ${\mathrm{Pb}}_2{\mathrm{P}}_2{\mathrm{S}}_6$ as a new platform to explore intrinsic metallization and superconductivity, which may promote developments of relevant applications.

We investigate the effect of spatial doping of the Mott insulator ${\mathrm{LaVO}}_3$ by inserting a few layers of the correlated metal ${\mathrm{SrVO}}_3$ in multilayer geometries. Using DFT in combination with DMFT, we demonstrate that this leads to a geometrically confined and robust metallic layer that stabilizes the metallicity in ${\mathrm{SrVO}}_3$ even in the ultrathin layer limit, suppressing a potential dimensionality-induced metal-insulator transition. For a thicker ${\mathrm{SrVO}}_3$ layer, we find a continuous transition of both structural and electronic properties across the interface between the two materials, with bulk properties reestablished on a length scale of $2\mathrm{to}3$ unit cells away from the interface. We show that a strain modulation applied along the growth direction can lead to asymmetric charge reconstruction at chemically symmetric interfaces. However, we find that this effect is rather weak, implying that fractional occupancy, and thus metallicity, persists at the interfaces.

Fatigue failure of crystalline materials is a difficult problem in science and engineering, and recent results have shown that fatigue crack growth can occur in intermittent jumps that have fat-tailed distributions. As fatigue crack propagation is known to leave markings—called striations—on the fracture surface, the distances between these should also have fat-tailed distributions if the crack propagation is intermittent. Here, we combine macroscale crack tip tracking in fatigue crack growth measurements of aluminum 5005 samples with postmortem scanning electron microscopy imaging of the striation lines. We introduce two different methods for extracting the striation line spacing from the images. What we find is a similar distribution of striation spacings as jump sizes using one of our methods, but the average striation spacing does not correlate with the crack growth rate. We conclude that we observe avalanchelike crack propagation, reflected in both the macroscale crack tip tracking as well as the analysis of the fracture surfaces. Our results show that the fracture surfaces can be used to study the intermittency of fatigue crack propagation and in development of crack-resistant materials. The advantages and disadvantages of the two methods introduced are discussed.

As a single-phase alloy, CrFeCoNi is a face centered cubic (fcc) material related to the archetypical high-entropy Cantor alloy CrFeCoNiMn. For thin films, CrFeCoNi of approximately equimolar composition tends to assume an fcc structure when grown at room temperature by magnetron sputtering. However, the single-phase solid solution state is typically not achieved for thin films grown at higher temperatures. The same holds true for Cantor alloy-based ceramics (nitrides and oxides), where phase formation is extremely sensitive to process parameters such as the amount of reactive gas. This study combines theoretical and experimental methods to understand the phase formation in nitrogen-containing CrFeCoNi thin films. Density functional theory calculations considering three competing phases (CrN, Fe-Ni and Co) show that the free energy of mixing, Δ$G$ of ${\left(\mathrm{CrFeCoNi}\right)}_{1-x}{\mathrm{N}}_x$ solid solutions has a maximum at $x=0.20-0.25$, and Δ$G$ becomes lower when $x<0.20$ and $x>0.25$. Thin films of ${\left(\mathrm{CrFeCoNi}\right)}_{1-x}{\mathrm{N}}_x$ ($0.14\ge x\le 0.41$) grown by magnetron sputtering show stabilization of the metallic fcc when $x\le 0.22$ and the stabilization of the NaCl B1 structure when $x>0.33$, consistent with the theoretical prediction. In contrast, films with intermediate amounts of nitrogen $(x=0.22)$ grown at higher temperatures show segregation into multiple phases of CrN, Fe-Ni-rich and Co. These results offer an explanation for the requirement of kinetically limited growth conditions at low temperature for obtaining single-phase CrFeCoNi Cantor-like nitrogen-containing thin films and are of importance for understanding the phase-formation mechanisms in multicomponent ceramics. The results from the study further aid in making correlations between the observed mechanical properties and the crystal structure of the films.

The dielectric effects of a substrate have been shown to be important for modulating the electronic properties of an adsorbate, especially in van der Waals heterostructures. Here, using the first-principles dielectric embedding $GW$ approach within the framework of many-body perturbation theory, we perform a case study on the dielectric screening effects of hexagonal boron nitride (hBN) on various transition-metal dichalcogenides (TMDs). We consider three systems, monolayer ${\mathrm{MoS}}_2$, bilayer ${\mathrm{MoS}}_2$, and a mixed ${\mathrm{WS}}_2/{\mathrm{MoS}}_2$ bilayer adsorbed on hBN, and examine three aspects of the substrate dielectric screening: (i) the thickness dependence and the monolayer-to-bulk transition, for which we consider the effects of one-, two-, three-, and four-layer hBN; (ii) the local-field effect, for which we numerically assess a common approximation of neglecting the in-plane local-field components of the substrate polarizability; and (iii) the spatial dependence, for which we consider the mixed ${\mathrm{WS}}_2/{\mathrm{MoS}}_2$ bilayer adsorbed on hBN with either side facing the substrate. Our results provide quantitative insight into how the substrate screening effects can be leveraged for band structure engineering.

In recent years, complex concentrated alloys (CCAs), also referred to as medium- or high-entropy alloys, have attracted substantial research interest due to their excellent mechanical properties including high strength, ductility, and toughness. It is known that the chemical inhomogeneity of CCAs gives rise to spatial variations in local properties such as the generalized stacking fault energy (GSFE) surface, which in turn affect their mechanical properties, but how such an inhomogeneity affects dislocation nucleation and incipient plasticity remains largely unknown and unexplored. Here, we develop a physics-informed statistical model for incipient plasticity in CCAs by combining elasticity theory for dislocation nucleation and statistical modeling of nanoindentation. Our model connects a material's fundamental properties to the statistics of incipient plasticity and is validated by the excellent agreement with the statistical data from molecular dynamics simulations of nanoindentation of CrCoNi CCA samples. By accounting for the spatial variation in the local generalized stacking fault energy surface in CCAs, our model captures the key difference in the nanoindentation-induced incipient plasticity response of CCAs compared with a conventional metal (fcc Cu) and also reproduces the trends across CCA samples with different degrees of short-range ordering. Our model also reveals a critical length scale for the underlying GSFE fluctuations which controls the overall statistics of incipient plasticity during nanoindentation of CCAs, which reflects the critical loop size of the underlying dislocation nucleation mechanism.

We investigate how the spin-orbit-coupling-induced lifting of the spin degeneracy depends indirectly on constituting organic motifs in two-dimensional hybrid organic-inorganic perovskites. We examine experimentally synthesized structures using first-principles electronic structure theory. In particular, the effect of the organic spacer cation packing and of the methyl ammonium orientation within the inorganic layer are elucidated. Both the organic spacer packing and subtle changes in methyl ammonium orientations can cause enough distortions of lead iodide octahedral cages to induce the spin splitting. Interestingly, the momentum-independent persistent spin texture splitting rather than the traditional Rashba/Dresselhaus splitting is observed for a structure in which organic spacer molecules are modified with fluorine substitution. Our calculation on these experimentally reported structures highlights the important role of the organic constituents in these hybrid organic inorganic perovskites for the spin splitting.

The impact of magnetism on predicted atomic short-range order in three medium- and high-entropy alloys is studied using a first-principles all-electron Landau-type linear-response theory, coupled with lattice-based atomistic modeling. We perform two sets of linear-response calculations: One in which the paramagnetic state is modeled within the disordered local moment picture, and one in which systems are modeled in a magnetically ordered state, which is ferrimagnetic for the alloys considered in this paper. We show that the treatment of magnetism can have a significant impact both on the predicted temperature of atomic ordering and the nature of atomic order itself. In CrCoNi, we find that the nature of atomic order changes from being $\mathrm{L}{1}_2$-like when modeled in the paramagnetic state to ${\mathrm{MoPt}}_2$-like when modeled assuming the system has magnetically ordered. In CrFeCoNi, atomic correlations between Fe and other elements present are dramatically strengthened when we switch from treating the system as magnetically disordered to magnetically ordered. Our results show it is necessary to consider the magnetic state when modeling multicomponent alloys containing mid- to late-$3d$ elements. Furthermore, we suggest that there may be high-entropy alloy compositions containing $3d$ transition metals that will exhibit specific atomic short-range order when thermally treated in an applied magnetic field. This has the potential to provide a route for tuning physical and mechanical properties in this class of materials.

Magnetite $\left({\mathrm{Fe}}_3{\mathrm{O}}_4\right)$ doped with earth-abundant metals has become a promising catalyst material. In particular, Ni-doped magnetite $(\mathrm{Ni}/{\mathrm{Fe}}_3{\mathrm{O}}_4)$ has been demonstrated as a cheap, robust, and catalytically active material in photocatalytic and electrochemical water oxidation. Recently, the incorporation of Ni atoms in ${\mathrm{Fe}}_3{\mathrm{O}}_4$ single crystalline surfaces was studied extensively with scanning tunneling microscopy and density-functional theory calculations. However, because of its importance for catalytic activity, this incorporation process and the determination of the lattice sites occupied by the Ni atoms require further experimental study. In this work, we investigated the surface structure of as-grown and annealed $\mathrm{Ni}/{\mathrm{Fe}}_3{\mathrm{O}}_4$(001) as a function of Ni coverage under ultra-high vacuum conditions with temperature-dependent x-ray and ultraviolet photoelectron spectroscopy, x-ray photoelectron diffraction, and low-energy electron diffraction. We observe that octahedrally coordinated subsurface cation vacancy sites are already occupied upon Ni deposition at room temperature and that Ni atoms start to diffuse further into the octahedral subsurface sites with increasing temperature.