The kagome lattice (comprising geometrically frustrated corner-sharing triangles), historically used in basket weaving and religious rituals, has emerged as an exciting platform for studying exotic phases of matter in quantum physics, such as quantum spin liquids, Chern magnetism, chiral charge density waves, and topological superconductivity. Despite the immense interest in kagome compounds, the exploration of strong topological insulators within this lattice has been scarce. In this work, we propose a new family of kagome compounds, ${R\mathrm{V}}_6{\mathrm{Ge}}_6$ (R = rare-earth atom), to host such a strong topological insulator phase. This phase is characterized by backscattering resilient surface states protected by the bulk insulating gap developed due to band inversions. The topological invariants of the band structure in Hilbert space allow us to identify different classes of gapped band structures and confirm the existence of a $Z_2$ topological invariant for bands near the Fermi energy in ${R\mathrm{V}}_6{\mathrm{Ge}}_6$ through ab initio calculations. Our investigation establishes ${R\mathrm{V}}_6{\mathrm{Ge}}_6$ as a strong topological insulator family among kagome compounds, further expanding the topological possibilities in this exotic lattice geometry. Notably, the electronic structure near the Fermi energy is dominated by the vanadium kagome lattice plane, presenting an exciting opportunity to study kagome physics in isolation from trivial bands. Furthermore, the observation of the topological insulator phase in ${R\mathrm{V}}_6{\mathrm{Ge}}_6$, where the vanadium valence state is in the $d$ orbital, creates an unprecedented opportunity to introduce long-range magnetic order within the topological state via doping in the vanadium site and introducing unpaired $d$-electrons.

The pyrite-structure transition-metal disulfide $\mathrm{Ni}{\mathrm{S}}_2$ is in principle a model cubic antiferromagnetic Mott insulator that can be doped through insulator-metal transitions with both electrons and holes (in ${\mathrm{Ni}}_{1–x}{\mathrm{Cu}}_x{\mathrm{S}}_2$ and ${\mathrm{Ni}}_{1–x}{\mathrm{Co}}_x{\mathrm{S}}_2$), eventually inducing superconductivity and ferromagnetism, respectively. Magnetism and transport have proven challenging to understand in $\mathrm{Ni}{\mathrm{S}}_2$, however. The antiferromagnetic spin structure below $\sim 39\mathrm{K}$ is complex due to frustration, while unexplained weak ferromagnetism emerges below $\sim 30\mathrm{K}$. Surface conduction is also now understood to dominate in $\mathrm{Ni}{\mathrm{S}}_2$ at low temperatures, raising questions about the interpretation of decades of prior data. Here, we present a complete study of the surface magnetotransport phenomena that emerge at low temperatures in high-quality single-crystal $\mathrm{Ni}{\mathrm{S}}_2$, which turn out to be strikingly rich. On cooling, isotropic magnetoresistance due to a field-induced shift of the first-order weak ferromagnetic ordering transition is first uncovered, i.e., metamagnetic magnetoresistance. At lower temperatures, larger, anisotropic magnetoresistance effects arise due to distinct switching events associated with the weak ferromagnetism. Strong evidence is presented that this is due to a field-driven in-plane to out-of-plane reorientation of surface spins, likely correlated with surface steps and terraces. In-plane exchange bias accompanies these effects, further supporting this interpretation. At the lowest temperatures, the spin reorientation field eventually exceeds the 9-T measurement window, generating strongly field-asymmetric magnetoresistance. Some of these unusual phenomena also manifest in the Hall channel, culminating in a sizable anomalous Hall effect at low temperatures. These results significantly demystify recent magnetoresistance and magnetic microscopy observations in $\mathrm{Ni}{\mathrm{S}}_2$ crystals and nanoflakes, and constitute an important step in elucidating the complex electronic and magnetic properties of this pivotal antiferromagnetic Mott insulator.

Optical absorption in rhombohedral ${\mathrm{BiFeO}}_3$ starts at photon energies below the photoemission band gap of $\approx 3$ eV calculated from first principles. A shoulder at the absorption onset has so far been attributed to low-lying electronic transitions or to oxygen vacancies. In this paper, optical spectra are calculated ab initio to determine the nature of the optical transitions near the absorption onset of pristine ${\mathrm{BiFeO}}_3$, the effect of electron-hole interaction, and the spectroscopic signatures of typical defects, i.e., doping (excess electrons or holes), intrinsic defects (oxygen and bismuth vacancies), and low-energy structural defects (ferroelectric domain walls).

Partially disordered antiferromagnetism (PDA) (in which one of the three magnetic ions in a triangular network remains magnetically disordered) has been known commonly among geometrically frustrated insulating materials. The 1/3 plateau in isothermal magnetization M of such materials has been of great theoretical interest. Here we report these properties in an ${\mathrm{AlB}}_2$-structure-derived metallic material, ${\mathrm{Er}}_2\mathrm{Rh}{\mathrm{Si}}_3$, in which the Er sublattice has triangular networks. The presence of a well-defined λ anomaly in the temperature T dependence of heat capacity and its magnetic-field H dependence and the loss of spin-disorder contribution to electrical resistivity ρ confirm antiferromagnetic order below $T_N=5\mathrm{K}$. On the other hand, the separation of zero-field-cooled and field-cooled dc magnetic susceptibility χ curves, the decay of isothermal remnant magnetization, and the frequency dependence of real and imaginary components of ac χ suggest the onset of spin-glass freezing concomitant with the antiferromagnetic order. In addition, interestingly, we observe the 1/3 plateau in M(H) below 20 kOe for $T<T_N$. The change in ρ as a function of H at a given temperature well below $T_N$ is also revealing, with this compound exhibiting a plateau below 20 kOe, with complexities at higher fields. Therefore, this compound serves as a prototype for theoretical understanding of transport behavior across the 1/3 plateau due to PDA magnetism in a metal without any interference from the $4f$ delocalization phenomenon.

The synthesis of thin films of magnetic topological materials is necessary to achieve novel quantized Hall effects and electrodynamic responses. $\mathrm{Eu}{\mathrm{In}}_2{\mathrm{As}}_2$ is a recently predicted topological axion insulator that has an antiferromagnetic ground state and an inverted band structure, but has only been synthesized and studied as a single crystal. We report on the synthesis of $c$-axis-oriented $\mathrm{Eu}{\mathrm{In}}_2{\mathrm{As}}_2$ films on sapphire substrates by molecular beam epitaxy. By carefully tuning the substrate temperature during growth, we stabilize the Zintl phase of $\mathrm{Eu}{\mathrm{In}}_2{\mathrm{As}}_2$ expected to be topologically nontrivial. The magnetic properties of these films reproduce those seen in single crystals, but their resistivity is enhanced when grown at lower temperatures. We additionally find that the magnetoresistance of $\mathrm{Eu}{\mathrm{In}}_2{\mathrm{As}}_2$ is negative even up to fields as high as 31 T. While it is highly anisotropic at low fields, it becomes nearly isotropic at high magnetic fields above 5 T. Overall, the transport characteristics of $\mathrm{Eu}{\mathrm{In}}_2{\mathrm{As}}_2$ appear similar to those of chalcogenide topological insulators, motivating the development of devices to gate tune the Fermi energy and reveal topological features in quantum transport.

Understanding the competition between brittleness and plasticity in refractory ceramics is of importance for aiding design of hard materials with enhanced fracture resistance. Inspired by experimental observations of crack shielding due to dislocation activity in TiN ceramics [Kumar et al., Int. J. Plast. 27, 739 (2011)], we carry out comprehensive atomistic investigations to identify mechanisms responsible for brittleness and slip-induced plasticity in Ti-N systems. First, we validate a semiempirical interatomic potential against density-functional theory results of Griffith and Rice stress intensities for cleavage ($K_{\mathrm{Ic}}$) and dislocation emission ($K_{\mathrm{Ie}}$) as well as ab initio molecular dynamics mechanical-testing simulations of pristine and defective TiN lattices at temperatures between 300 and 1200 K. The calculated $K_{\mathrm{Ic}}$ and $K_{\mathrm{Ie}}$ values indicate intrinsic brittleness, as $K_{\mathrm{Ic}}\ll K_{\mathrm{Ie}}$. However, $K_{\mathrm{I}}$-controlled molecular statics simulations—which reliably forecast macroscale mechanical properties through nanoscale modeling—reveal that slip plasticity can be promoted by a reduced sharpness of the crack and/or the presence of anion vacancies. Classical molecular dynamics simulations of notched Ti-N supercell models subject to tension provide a qualitative understanding of the competition between brittleness and plasticity at finite temperatures. Although crack growth occurs in most cases, a sufficiently rapid accumulation of shear stress at the notch tip may postpone or prevent fracture via nucleation and emission of dislocations. Furthermore, we show that the probability to observe slip-induced plasticity leading to crack blunting in flawed Ti-N lattices correlates with the ideal tensile/shear strength ratio $\left(I_{\text{plasticity}}^{\text{slip}}\right)$ of pristine Ti-N crystals. We propose that the $I_{\mathrm{plasticity}}^{\mathrm{slip}}$ descriptor should be considered for ranking the ability of ceramics to blunt cracks via dislocation-mediated plasticity at finite temperatures.

The solidification of alloys is typically controlled by solute diffusion due to the solute partitioning happening at the solid-liquid interface. In this study, we show that the switching from solute diffusion-controlled growth to thermal diffusion-controlled growth may happen at the solidification front during rapid solidification processes of alloys such as additive manufacturing using a phase-field model. The switching is found to be triggered by the cooling of the solid-liquid interface when it becomes colder than the solidus temperature. The switching introduces a sudden jump of growth velocity, an increase in solute concentration, and the refining of the resulting microstructures. All those changes predicted by the phase-field simulations agree with experimental observations quantitatively. The switching of control mechanisms can be exploited by manipulating the processing conditions to form refined microstructures or layered structures for improved mechanical properties.

The accurate description of electronic properties and optical absorption spectra is a long-standing challenge for density functional theory. Recently, the introduction of screened range-separated hybrid (SRSH) functionals for solid-state materials has allowed for the calculation of fundamental band gaps and optical absorption spectra that are in very good agreement with many-body perturbation theory. However, since solid-state SRSH functionals are typically tuned to reproduce the properties of bulk phases, their transferability to low-dimensional structures, which experience substantially different screening than in the bulk, remains an open question. In this work, we explore the transferability of SRSH functionals to several prototypical van der Waals materials, including transition-metal sulfides and selenides, indium selenide, black phosphorus, and hexagonal boron nitride. Considering the bulk and a monolayer of these materials as limiting cases, we show that the parameters of the SRSH functional can be determined systematically, using only the band-edge quasiparticle energies of these extremal structural phases as fitting targets. The resulting SRSH functionals can describe both electronic band structures and optical absorption spectra with accuracy comparable to more demanding ab initio many-body perturbation theory (GW and Bethe-Salpeter equation) approaches. Selected examples also demonstrate that the SRSH parameters, obtained from the bulk and monolayer reference structures, display good accuracy for band structures and optical spectra of bilayers, indicating a degree of transferability that is independent of the fitting procedure.

Here we study the stability of charge order in the kagome metal ${\mathrm{ScV}}_6{\mathrm{Sn}}_6$. Synchrotron x-ray diffraction measurements reveal high-temperature, short-range charge correlations at the wave vectors along $\mathbf{q}=(\frac{1}{3},\frac{1}{3},\frac{1}{2})$ whose interlayer correlation lengths diverge upon cooling. At the charge order transition, this divergence is interrupted, and long-range order freezes in along $\mathbf{q}=(\frac{1}{3},\frac{1}{3},\frac{1}{3})$, as previously reported, while disorder enables the charge correlations to persist at the $\mathbf{q}=(\frac{1}{3},\frac{1}{3},\frac{1}{2})$ wave vector down to the lowest temperatures measured. Both short-range and long-range charge correlations seemingly arise from the same instability and both are rapidly quenched upon the introduction of larger Y ions onto the Sc sites. Our results validate the theoretical prediction of the primary lattice instability at $\mathbf{q}=(\frac{1}{3},\frac{1}{3},\frac{1}{2})$, and we present a heuristic picture for viewing the frustration of charge order in this compound.

$\beta \text{−}\mathrm{G}{\mathrm{a}}_2{\mathrm{O}}_3$ is an ultrawide band-gap semiconductor with importance for various technological applications. We investigated the tracer diffusion of oxygen and aluminum (as a substitute tracer for gallium) in $\left(-201\right)$ oriented $\beta \text{−}\mathrm{G}{\mathrm{a}}_2{\mathrm{O}}_3$ single crystals between 1100 and $1600{}^{\circ }\mathrm{C}$. Isotope enriched ${}^{18}\mathrm{O}_2$ gas or ion-beam sputtered thin films of ${\mathrm{Al}}_2{\mathrm{O}}_3$ were used as a tracer source. The isotope depth profiles were analyzed by secondary ion mass spectrometry. The Al diffusivities are by two orders of magnitude higher than the O diffusivities. Activation enthalpies of diffusion of about 4.2 eV for Al and 5.0 eV for O were derived. Possible diffusion mechanisms are discussed in the framework of density functional theory calculations as known from the literature. As a result, O interstitials and Ga vacancies are suggested as defects governing diffusion.

We studied the electrical transport, Hall effect, and magnetic properties of monoclinic layered ferromagnet ${\mathrm{Cr}}_{2.76}{\mathrm{Te}}_4$. Our studies demonstrate ${\mathrm{Cr}}_{2.76}{\mathrm{Te}}_4$ to be a soft ferromagnet with strong magnetocrystalline anisotropy. Below 50 K, the system shows an antiferromagneticlike transition. Interestingly, between 50 and 150 K, we observe fluctuating magnetic moments between in-plane and out-of-plane orientations, leading to noncoplanar spin structure. On the other hand, the electrical resistivity data suggest it to be metallic throughout the measured temperature range, except a $kink$ at around 50 K due to antiferromagnetic ordering. The Rhodes-Wohlfarth ratio $\frac{{\mu }_{\mathrm{eff}}}{{\mu }_s}=1.89(>1)$ calculated from our magnetic studies confirms that ${\mathrm{Cr}}_{2.76}{\mathrm{Te}}_4$ is an itinerant ferromagnet. Large anomalous Hall effect has been observed due to the skew scattering of impurities and the topological Hall effect has been observed due to noncoplanar spin structure in the presence of strong magnetocrystalline anisotropy. We examined the mechanism of anomalous Hall effect by employing the first-principles calculations.

The alkaline-earth metal orthovanadate ${\mathrm{Sr}}_3{\left({\mathrm{VO}}_4\right)}_2$ with the palmierite structure is reported to host a dielectric anomaly as well as a structural phase transition above the room temperature. With ${\mathrm{V}}^{5+}$ ions and tetrahedral oxygen coordination, the crystal structure of this compound is not studied in detail from first principles yet. In this work, we perform a detailed analysis of the crystal structure and instabilities of $M_3{\left({\mathrm{VO}}_4\right)}_2$ ($M=\text{Ca,}\text{Sr,}\text{Ba}$) orthovanadates with the palmierite structure using first-principles density functional theory. We find that as the $M^{2+}$ cation size decreases, a significant structural distortion that changes the symmetry from $R\overline{3}m$ to $C2/c$ emerges. This change is accompanied with a rotation of the oxygen tetrahedra. Our calculations also indicate that the polar instability in these compounds is suppressed by these tetrahedral rotations.

Crystals and glasses differ by the amplitude and the temperature dependence of their thermal conductivity. However, there are crystals known to display glasslike thermal conductivity. Here, we show that ${\mathrm{EuTiO}}_3$, a quantum paraelectric known to order antiferromagnetically at 5.5 K, is one such system. The temperature dependence of resistivity and Seebeck coefficient yield an insulating band gap of $\sim 0.22$ eV. Thermal conductivity is drastically reduced. Its amplitude and temperature dependence are akin to what is seen in amorphous silica. Comparison with nonmagnetic perovskite solids, ${\mathrm{SrTiO}}_3$, ${\mathrm{KTaO}}_3$, and ${\mathrm{EuCoO}}_3$, shows that what impedes heat transport are $4f$ spins at ${\mathrm{Eu}}^{2+}$ sites, which couple to phonons well above the ordering temperature. Thus, in this case, superexchange and valence fluctuations, not magnetic frustration, are the drivers of the glasslike thermal conductivity.

We pursue the premise that plasmon-phonon coupling and hybrid plasmon-phonon modes can be broadly tailored in van der Waals (vdW) heterostructures. While the coupling between optical plasmons in graphene and phonons of substrate materials has already been widely investigated, the coupling of acoustic plasmons to phonons has remained elusive to date. Here we demonstrate that double-layer graphene intercalated with a transition-metal dichalcogenide (TMD) can harbor acoustic plasmon-phonon resonances with particularly high coupling strength. Using the quantum-electrostatic heterostructure method, which takes into account the contribution of each vdW monolayer at the ab initio level, we present the dependence of the plasmon-phonon coupling strength on the thickness of the TMD, as well as on the graphene doping. Our results reveal optimal and experimentally feasible conditions to achieve ultrastrong plasmon-phonon coupling, and thus enable further advances in nanoscale thermal and optical devices of high sensitivity.

Grain boundary (GB) migration exhibits intriguing antithermal behavior (or non-Arrhenius behavior), with the temperature and driving force playing crucial roles. Through atomistic simulations on nickel bicrystals, we investigate the change in GB mobility with variations in both temperature and driving force. Our results reveal that the GB mobility initially increases with temperature and subsequently decreases after reaching the transition temperature ($T_{\mathrm{trans}}$), and, notably, $T_{\mathrm{trans}}$ exhibits a linear relationship with the activation energy ($Q$) associated with GB migration. By modulating the driving force, we found that the driving force could effectively lower $Q$, resulting in the shift of $T_{\mathrm{trans}}$ towards lower temperatures. Additionally, higher driving forces were found to activate more migration modes at lower temperatures, potentially leading to a transition in the thermal behavior of GB migration. Our work supports the existing theoretical models for GB migration based on both classical thermal activation and disconnection nucleation. Furthermore, we refined the existing model by incorporating the influence of the driving force. The modified model can not only describe the effect of driving force on the thermal behavior of GB migration but also accounts for the observed “antidriving force” phenomenon in GB migration. Our research has the potential to offer valuable insights for investigating realistic GB migration under more intricate constraints and environments.

Structural, morphological, and optical properties of (${\mathrm{In}}_{1-x}{\mathrm{Ga}}_x{)}_2{\mathrm{O}}_3$ thin films are reported as a function of the cation composition. A material library with $0.1\le x\le 0.64$ was fabricated by discrete combinatorial synthesis on $r$-plane sapphire substrates using pulsed laser deposition. The samples crystallize in the cubic bixbyite phase for $x\le 0.35$. The lattice constant and absorption edge energy systematically decrease and increase, respectively, with increasing Ga content up to $x=0.2$. For higher Ga admixtures, both saturate. In addition, a significant change in surface morphology occurs at $x\sim 0.2$. Transmission electron microscopy examinations of selected samples show a homogeneous incorporation of ${\mathrm{Ga}}_2{\mathrm{O}}_3$ into cubic ${\mathrm{In}}_2{\mathrm{O}}_3$ for $x=0.11$, while a segregation of Ga-rich and In-rich regions can be seen for $x=0.22$ and $x=0.35$. In the sample with $x=0.35$, the Ga-rich regions exhibit a preferred orientation with an angle of ${45}^{\circ }$–${55}^{\circ }$ with respect to the substrate normal, which has been shown to result from a correspondingly faceted In-rich bixbyite layer at the substrate–thin film interface.

Quasicrystals represent a class of ordered materials that have diffraction symmetry forbidden in periodic crystals. Since the first discovery of quasicrystals in 1984, approximately 100 thermodynamically stable quasicrystals have been synthesized. The discovery of new quasicrystals has led to the observation of novel physical phenomena, such as robust quantum criticality, fractal superconductivity, and peculiar long-range magnetic ordering. However, the pace of discovery of new quasicrystals has significantly slowed down, which is attributed to the lack of design principles for exploring new quasicrystals. Here, we demonstrate that machine learning can greatly accelerate the process of material discovery. Our model can predict stable quasicrystalline phases with high accuracy. With this model, we discovered three stable decagonal quasicrystals through an exhaustive screening of more than 1000 ternary aluminum alloy systems.

Two-dimensional (2D) van der Waals materials are promising for spintronics due to their unique physical properties down to the monolayer thickness. Tin monosulfide (SnS) is a van der Waals material belonging to the monochalcogenide family, lacking mirror symmetry in its crystal structure. Monolayer SnS also exhibits spin-splitting band structure as well as in-plane ferroelectricity. In this work, we investigate the potential of SnS for spintronic applications by optimizing the growth of large-area 2D thin films of SnS on a CMOS-compatible $\mathrm{Si}/\mathrm{Si}{\mathrm{O}}_2$ substrate using pulsed laser deposition. We perform a comprehensive SnS thickness-dependent spin pumping study in $\mathrm{Sn}\mathrm{S}/{\mathrm{Ni}}_{80}{\mathrm{Fe}}_{20}$ system for SnS films grown at 300 and 473 K. The thickness dependence of the damping constant of ${\mathrm{Ni}}_{80}{\mathrm{Fe}}_{20}$ agreed with the theory of spin pumping only for optimized SnS layers grown at 300 K, exhibiting low and thickness-independent surface roughness. The effective spin mixing conductance of $\mathrm{Sn}\mathrm{S}/{\mathrm{Ni}}_{80}{\mathrm{Fe}}_{20}$ for optimized SnS layers grown at 300 K is estimated to be $\left(8.83\pm 1.15\right){\mathrm{nm}}^{-2}$. We also performed spin-torque ferromagnetic resonance (STFMR) measurements on the $\mathrm{Sn}\mathrm{S}\left(1.3\mathrm{nm}\right)/{\mathrm{Ni}}_{80}{\mathrm{Fe}}_{20}$ devices for the optimized SnS layers grown at 300 K. Through angle-resolved STFMR measurements, we show the presence of unconventional spin-orbit torques in SnS. Our findings suggest that SnS is promising for emerging spintronic devices.