Saddle point search schemes are widely used to identify the transition state of different processes, like chemical reactions, surface and bulk diffusion, surface adsorption, and many more. In solid-state materials with relatively large numbers of atoms, the minimum mode following schemes such as dimer are commonly used because they alleviate the calculation of the Hessian on the high-dimensional potential energy surface. Here, we show that the dimer search can be further accelerated by leveraging Gaussian process regression (GPR). The GPR serves as a surrogate model to feed the dimer with the required energy and force input. We test the GPR-accelerated dimer method for predicting the diffusion coefficient of vacancy-mediated self-diffusion in body-centered cubic molybdenum and sulfur diffusion in hexagonal molybdenum disulfide. We use a multitask learning approach that utilizes a shared covariance function between energy and force input, and we show that the multitask learning significantly improves the performance of the GPR surrogate model compared to previously used learning approaches. Additionally, we demonstrate that a translation-hop sampling approach is necessary to avoid overfitting the GPR surrogate model to the minimum-mode-following pathway and thus succeeding in locating the saddle point. We show that our method reduces the number of evaluations compared to a conventional dimer method.

The recent discovery of a charge density wave order at the wave vector $P(\frac{1}{3},\frac{1}{3},\frac{1}{3})$ in the kagome metal ${\mathrm{ScV}}_6{\mathrm{Sn}}_6$ has created a mystery because subsequent theoretical and experimental studies show a dominant phonon instability instead at another wave vector $H(\frac{1}{3},\frac{1}{3},\frac{1}{2})$. In this paper, I use first-principles total energy calculations to map out the landscape of the structural distortions due to the unstable phonon modes at $H$, $L(\frac{1}{2},0,\frac{1}{2})$, and $P$ present in this material. In agreement with previous results, I find that the distortions due to the $H$ instability cause the largest gain in energy relative to the parent structure, followed in order by the $L$ and $P$ instabilities. However, only two distinct structures occur due to this instability, which are separated by 6 meV/f.u. The instability at $L$ results in three distinct structures separated in energy by 5 meV/f.u. In contrast, six different distorted structures are stabilized due to the instability at $P$, and they all lie within 2 meV/f.u. of each other. Hence, despite a lower-energy gain, the condensation at $P$ could be favorable due to a larger entropy gain associated with the fluctuations within a manifold with larger multiplicity via the order-by-disorder mechanism.

Hubbard-corrected density-functional theory has proven to be successful in addressing self-interaction errors in 3D magnetic materials. However, the effectiveness of this approach for 2D magnetic materials has not been extensively explored. Here, we use $\mathrm{PBEsol}+U$ and its extensions $\mathrm{PBEsol}+U+V$ to investigate the electronic, structural, and vibrational properties of 2D antiferromagnetic ${\mathrm{FePS}}_3$ and ferromagnetic ${\mathrm{CrI}}_3$, and compare the monolayers with their bulk counterparts. Hubbard parameters (on-site $U$ and intersite $V$) are computed self-consistently using density-functional perturbation theory, thus avoiding any empirical assumptions. We show that for ${\mathrm{FePS}}_3$, the Hubbard corrections are crucial in obtaining the experimentally observed insulating state with the correct crystal symmetry, also providing vibrational frequencies in good agreement with Raman experiments. For ferromagnetic ${\mathrm{CrI}}_3$, we discuss how a straightforward application of Hubbard corrections worsens the results and introduces a spurious separation between spin-majority and minority conduction bands. Promoting the Hubbard $U$ to be a spin-resolved parameter—that is, applying different (first-principles) values to the spin-up and spin-down manifolds—recovers a more physical picture of the electronic bands and delivers the best comparison with experiments.

In the recent past, MnTe has proven to be a crucial component of the intrinsic magnetic topological insulator (IMTI) family ${\left[\mathrm{MnTe}\right]}_m{\left[{\mathrm{Bi}}_2{\mathrm{Te}}_3\right]}_n$, which hosts a wide range of magnetotopological properties depending on the choice of $m$ and $n$. However, bulk crystal growth allows only a few combinations of $m$ and $n$ for these IMTIs due to the strict limitations of the thermodynamic growth conditions. One way to overcome this challenge is to utilize the atomic layer-by-layer molecular beam epitaxy (MBE) technique, which allows arbitrary sequences of ${\left[\mathrm{MnTe}\right]}_m$ and ${\left[{\mathrm{Bi}}_2{\mathrm{Te}}_3\right]}_n$ to be formed beyond the thermodynamic limit. For such MBE growth, finding optimal growth templates and conditions for the parent building block, MnTe, is a key requirement. Here, we report that two different hexagonal phases of MnTe–nickeline (NC) and zinc-blende/wurtzite (ZB-WZ) structures, with distinct in-plane lattice constants of $4.20\pm 0.04$ and $4.39\pm 0.04\AA$, respectively–can be selectively grown on $c$-plane ${\mathrm{Al}}_2{\mathrm{O}}_3$ substrates using different buffer layers and growth temperatures. Moreover, we provide comparative studies of different MnTe phases using atomic-resolution scanning transmission electron microscopy, and we show that ZB- and WZ-like stacking sequences can easily alternate between the two. Surprisingly, ${\mathrm{In}}_2{\mathrm{Se}}_3$ buffer layer, despite its lattice constant ($4.02\AA$) being closer to that of the NC phase, fosters the ZB-WZ instead, whereas ${\mathrm{Bi}}_2{\mathrm{Te}}_3$, sharing the same lattice constant ($4.39\AA$) with the ZB-WZ phase, fosters the NC phase. These discoveries suggest that lattice matching is not always the most critical factor determining the preferred phase during epitaxial growth. Overall, this will deepen our understanding of epitaxial growth modes for chalcogenide materials and accelerate progress toward new IMTI phases as well as other magnetotopological applications.

In this work, the optothermal Raman technique was used to measure the local thermal conductivity of macro/single ferroelastic domains of bulk ${\mathrm{BiFeO}}_3$ (BFO) crystal and multidomain epitaxially grown thin film (∼180 nm) on $\mathrm{DySc}{\mathrm{O}}_3$ (DSO) substrate. Domain walls (DWs) were found to act as effective phonon scatterers, reducing the thermal conductivity at room temperature from 2.17 to 1.16 W/mK and from 9.89 to 3.20 W/mK at 150 K. The results indicate that BFO is a promising candidate for bidirectional thermal conductivity tuning and switchable nonlinear thermal components if the DW density can be modulated by an externally applied field.

Nanoscale strain networks are reported in $\mathrm{BaTi}{\mathrm{O}}_3$ (BTO) crystals of 300 nm size using Bragg coherent diffractive imaging. BTO nanocrystals with clear facets were chosen to identify the crystallographic directions, allowing the strain field direction and periodicity to be studied in detail. Stripes of strain were observed, which were both stable and preserved in tetragonal and cubic phases at elevated temperatures, above the tetragonal-to-cubic phase transition. A finite element analysis approach was used to simulate the domain structures inside a BTO crystal and to understand the origins of the strain stripes as piezoelectric blocks. A ${180}^{\circ }$ domain model gives a better qualitative match to the experimental images.

The stabilization and origin of antiferroelectricity in the antiferroelectric (AFE) materials have always been the tools to facilitate the AFE/FE engineering. However, the mechanistic understanding of the driving forces, especially in the electronic level, is still elusive. Here, combining density functional theory calculations and symmetry analysis, following the pseudo-Jahn-Teller effect (PJTE) theory, we investigate both the stabilization and origin of antiferroelectricity in the AFE perovskite ${\mathrm{NaNbO}}_3$. Utilizing the potential energy surface and effective Hamiltonian, it is observed that the cooperative couplings play a critical role to stabilize the AFE phase. Moreover, considering adiabatic potential energy surface cross sections at $\mathrm{\Gamma }$ on the basis of the PJTE, the origin of the cubic-AFE phase transition at $\mathrm{\Gamma }$ is observed as the coupling of ($T_{2u}, T_{2g}$) electronic states, inducing the ${\mathrm{\Delta }}_5$ mode, whereas both the ($T_{1u}, T_{2g}$) state and ($T_{2u}, T_{2g}$) state couplings via the ${\mathrm{\Gamma }}_4^{-}$ mode are the reason behind the cubic to ferroelectric phase transition at $\mathrm{\Gamma }$.

We have measured soft x-ray resonant photoemission for the $n\text{−}{\mathrm{SrTiO}}_{3-\delta }/i\text{−}\mathrm{Si}\left(001\right)$ $(\delta =\sim 0.0003)$ hybrid semiconductor heterojunction with the goal of probing occupied electronic states in the band gap near the surface. By utilizing both swept energy, angle-integrated and fixed-energy, and angle-resolved spectroscopies, we identify the atomic and orbital character of the in-gap state as well as the different electronic phases that contribute to it. Specifically, we isolate the state associated with trapped charge on the $n\text{−}{\mathrm{SrTiO}}_{3-\delta }$ surface that causes surface depletion from the localized gap state resulting from electron correlation effects in the complex oxide. Using an x-ray energy close to select dipole allowed $\mathrm{Ti}{L}_3\to 3d$ x-ray absorption resonances results in a 40-fold increase in sensitivity. This feature makes photoemission possible when probing electrons from dopants at concentrations as low as a few hundredths of a percent.

Numerous body-centered cubic refractory multiprincipal element alloys (RMPEAs) exhibit high temperature strengths that surpass those of Ni-based superalloys. The superior properties of RMPEAs have been shown to stem in part from the unique dislocation glide mechanisms in compositionally disordered systems. Here, using a three-dimensional phase field dislocation dynamics approach combined with Langevin dynamics, we simulate the glide mechanisms of screw and edge dislocations at elevated temperature and low stress levels in the RMPEA MoNbTi with a body-centered cubic, random solid solution structure. For $600-1200$ K, edge-dislocation glide is smooth, temperature sensitive, and faster than screw dislocation glide. Screw dislocations exhibit three distinct glide mechanisms over the same temperature range. Ultimately, the formation of nonplanar kink pairs that ensues above 900 K reduces screw mobility with respect to temperature and leads to athermal mobility from 1100 to 1200 K.

Compounds forming the quasi-two-dimensional Shastry-Sutherland lattice (SSL) have attracted significant experimental and theoretical attention in the field of frustrated magnetism. This is primarily due to their realization of an exactly soluble $J_1\text{−}{J}_2$ orthogonal dimer model capable of hosting magnetic order, dimer singlet, and plaquette singlet phases in zero applied field and their complex magnetic phase diagrams with fractional magnetization plateaus and possible superfluid and supersolid phases found between the plateau states. The discovery and characterization of SSL compounds based on rare-earth magnetic ions provide a direct route to study the stability and properties of these exotic magnetic phases in systems with a variety of different magnetic anisotropies. In this paper, we discuss the synthesis and magnetic characterization of polycrystalline samples of the $R_2{\mathrm{Be}}_2{\mathrm{SiO}}_7$ family, where $R=\mathrm{Nd}$, Sm, and Gd-Yb. All family members crystallize in the space group $P\overline{4}{2}_{1}m$ (113) and show no signs of long-range magnetic order above 2 K, except for $R=\mathrm{Tb}$ which orders antiferromagnetically at 2.6 K.

${\mathrm{Ga}}_2{\mathrm{O}}_3$ is a wide-band-gap material of interest for a wide variety of devices, many of these requiring heterostructures, for instance, to achieve carrier confinement. A common method to create such heterostructures is to alloy with ${\mathrm{In}}_2{\mathrm{O}}_3$ or ${\mathrm{Al}}_2{\mathrm{O}}_3$. However, the lattice constants of these materials are significantly different from those of ${\mathrm{Ga}}_2{\mathrm{O}}_3$, leading to large amounts of strain in the resulting heterostructure. If the thickness of the heterostructure is increased, this can lead to cracking. By considering alloys of ${\mathrm{In}}_2{\mathrm{O}}_3$ and ${\mathrm{Al}}_2{\mathrm{O}}_3$, the lattice constants can be tailored to those of ${\mathrm{Ga}}_2{\mathrm{O}}_3$, while still keeping a sizable conduction-band offset. We use density functional theory with hybrid functionals to investigate the structural and electronic properties of ${\mathrm{In}}_2{\mathrm{O}}_3$ and ${\mathrm{Al}}_2{\mathrm{O}}_3$ alloys in the bixbyite, corundum, and monoclinic structures. We find that the lattice constants increase with In incorporation. Band gaps decrease nonlinearly with increasing In concentration. We find the ${\left({\mathrm{In}}_{0.25}{\mathrm{Al}}_{0.75}\right)}_2{\mathrm{O}}_3$ monoclinic structure to be of particular interest, as it closely matches the ${\mathrm{Ga}}_2{\mathrm{O}}_3$ lattice constants while providing an indirect/direct band gap of 5.94/5.70 eV and a conduction-band offset of 1 eV compared to ${\mathrm{Ga}}_2{\mathrm{O}}_3$.

We present multicode calculations for the spin Hall effect in binary Pt-based alloys, where we explore the viability of alloying the archetype spin Hall material Pt with a large set of metals [Al, Ag, Au, Cu, Hf (hcp), Hf (fcc), Ir, Pd] in order to optimize the charge to spin current conversion for practical applications. To capture intrinsic and extrinsic mechanisms in material-specific calculations, we employ different first-principles codes based on density functional theory in the framework of Green's-function-based multiple scattering approaches. Capturing the transport properties within the relativistic and fully quantum mechanical Kubo-Bastin formalism as well as the semiclassical Boltzmann approach allows for a better understanding of the microscopic physics as well as a larger set of reliable data for the key transport parameters. If available, we compare our results to experimental data, where we generally find good agreement. As we access the full concentration range, we are able to identify the optimal doping regime, which will depend on the binary alloy but generally falls within a region of 60–90 at.% of Pt. When including the unavoidable experimental residual resistivities, the maximum spin Hall angle that we identified is $13\%$ in ${\mathrm{Al}}_{0.2}{\mathrm{Pt}}_{0.8}$ and ${\mathrm{Hf}}_{0.1}{\mathrm{Pt}}_{0.9}$, which is comparable to the best spin Hall angles experimentally found in good metal systems. The longitudinal resistivities in this regime go beyond $70\mu \mathrm{\Omega }$ cm, which is compatible with metallic-based magnetic random access memory devices.

Interatomic potentials provide a means to simulate extended length and time scales that are outside the reach of ab initio calculations. The development of an interatomic potential for a particular material requires the optimization of the parameters of the functional form of the potential. We present a parametrization protocol for analytic bond-order potentials (BOPs) that provides a physically transparent and computationally efficient description of the interatomic interaction. The parametrization protocol of the BOP follows the derivation of the BOP along the coarse-graining of the electronic structure from density-functional theory (DFT) to the tight-binding (TB) bond model to analytic BOPs. In particular, it starts from TB parameters that are obtained by downfolding DFT eigenstates of two-atomic molecules to an $sd$-valent minimal basis. This $sd$-valent Hamiltonian is combined with a pairwise repulsion to obtain an initial binding energy relation. The $s$ electrons are then removed from the Hamiltonian and instead represented by an isotropic embedding term. In the final step, the parameters of the remaining $d\text{−}d$ interaction, the pair repulsion, and the embedding term are optimized simultaneously. We demonstrate that the application of this parametrization protocol leads to a basic BOP for Re with good transferability. We discuss different strategies to refine the basic BOP towards global transferability or towards local accuracy. We demonstrate that homogeneous samplings of the structural phase space in a map of local atomic environments can be used to systematically increase the global transferability. We also demonstrate the influence of training data weighting on local accuracy refinements with a Pareto-front analysis, and we suggest further requirements to select a final BOP. The local accuracy and global transferability of the final BOP is also shown and compared to DFT.

In this work, we carry out molecular dynamics simulations based on an extended version of the bond valence force field to investigate the underlying physics of relaxor dielectrics with static temperature-stable dielectric response. We focus on the perovskite solid solution $(1-x){\mathrm{Ba}}_{0.95}{\mathrm{Ca}}_{0.05}{\mathrm{TiO}}_3-\left(x\right){\mathrm{Bi}}_{0.5}{\mathrm{MeO}}_3(\mathrm{Me}={\mathrm{Mg}}_{1/2}{\mathrm{Ti}}_{1/2},\mathrm{Sc},{\mathrm{Zn}}_{1/2}{\mathrm{Ti}}_{1/2})$, which has a flat dielectric response over a wide temperature range which was recently observed experimentally. Our molecular dynamics simulations can not only reproduce the temperature-dielectric relationship observed in experiments, but also provide an atomic scale explanation. Specifically, doping $\mathrm{Bi}\left({\mathrm{Mg}}_{1/2}{\mathrm{Ti}}_{1/2}\right){\mathrm{O}}_3$ into the $\left(\mathrm{BaCa}\right){\mathrm{TiO}}_3$ crystal reduces the spatial correlation length of electric dipole orientations to the minimum distance, which is only within the neighboring unit cells. This nearest-neighbor correlation even persists at high temperatures. As a result, in this perovskite solid solution, the correlation length and macroscopic polarization change little with the temperature, leading to temperature-stable relaxor dielectric responses. The relationships between our theory and previously proposed models, such as the independent local mode and correlated rattling cations model, are also discussed. This theoretical work aims to deepen our understanding of the theory of connecting between atomic composition and temperature-stable relaxor dielectric response.

We report a study of the structural, magnetic, transport, and thermodynamic properties of polycrystalline samples of ${\mathrm{CeRhC}}_2$ and ${\mathrm{LaRhC}}_2$. ${\mathrm{CeRhC}}_2$ crystallizes in a tetragonal structure with space group $P{4}_1$, and it orders antiferromagnetically below $T_{\text{N1}}\approx 1.8\mathrm{K}$. Powder neutron diffraction measurements reveal a chiral magnetic structure with a single propagation vector $Q_m=(1/2,1/2,0.228\left(5\right))$, indicating an antiferromagnetic arrangement of Ce magnetic moments in the $ab$ plane and incommensurate order along the $c$ axis with a root-mean-square ordered moment of $m_{\text{ord}}=0.68{\mu }_{\text{B}}$. A second antiferromagnetic phase $\left(T_{\text{N2}}\right)$ becomes apparent in electrical resistivity, Hall, and heat capacity measurements in fields above 0.3 T. Electrical resistivity measurements reveal that ${\mathrm{LaRhC}}_2$ is a semiconductor with a band gap of $E_{\text{g}}\sim 24$ meV, whereas resistivity and Hall measurements indicate that ${\mathrm{CeRhC}}_2$ is a semimetal with a carrier concentration of $n\sim {10}^{20}{\mathrm{cm}}^{-3}$. With applied hydrostatic pressure, the zero-field antiferromagnetic transition of ${\mathrm{CeRhC}}_2$ is slightly enhanced, and ${\mathrm{CeRhC}}_2$ becomes notably more metallic up to 1.36 GPa. The trend toward metallicity is in line with density-functional calculations that indicate that both ${\mathrm{LaRhC}}_2$ and ${\mathrm{CeRhC}}_2$ are semimetals, but the band overlap is larger for ${\mathrm{CeRhC}}_2$, which has a smaller unit cell volume than that of its La counterpart.

The density evolution of the physical properties of the transition-metal oxide ${\mathrm{RuO}}_2$ coupled with a deeper understanding of underlying metastable phases is necessary for correlating universality between similar binary systems. Here, we report the pressure-temperature electrical resistance dependency with the structural evolution of ${\mathrm{RuO}}_2$. Conducting quasi-four-probe electrical transport measurements in a diamond anvil cell, a low-temperature loss of metallicity is observed above 28 GPa. This insulative transition is accompanied by a significant drop in pressure, suggesting the electronic transition is linked to a first-order structural phase transition. This is supported by the observation that the insulative electronic state is retained upon warming to room temperature. Density functional theory simulations indicate that the insulative fluorite-type phase can be favorable around these conditions and would exhibit a similar pressure difference through an isochoric transformation from the metallic HP-${\mathrm{PdF}}_2$-type phase, however there is insufficient experimental evidence to confirm the presence of the fluorite-type phase. Furthermore, a unique arsenopyrite-type phase of ${\mathrm{RuO}}_2$ is observed with x-ray diffraction of a post-laser-heated sample at 62 GPa.

Spin-current density functional theory (SCDFT) is a formally exact framework designed to handle the treatment of interacting many-electron systems including spin-orbit coupling (SOC) at the level of the Pauli equation. In practice, robust and accurate calculations of the electronic structure of these systems call for functional approximations that depend not only on the densities and currents but also on spinors explicitly. Here we extend the generalized Kohn-Sham (GKS) approach of [Seidl et al., Generalized Kohn-Sham schemes and the band-gap problem, Phys. Rev. B 53, 3764 (1996)] to SCDFT. This framework entails the prominent cases of hybrid forms and meta-generalized-gradient-approximations. We clarify that the exchange-correlation potentials conjugate to the currents need to be computed within the GKS approach only when the spin currents are included in the functional form explicitly. We analyze the consequence of this fact for various approximations and numerical procedures for the evaluation of SOC effects. The practical power of the extended approach is demonstrated by calculating the spin-orbit induced/enhanced band splittings of inversion-asymmetric single-layer ${\mathrm{MoSe}}_2$ and inversion-symmetric bulk $\alpha -{\mathrm{MoTe}}_2$. Key to these results is the capacity to account for SOC self-consistently while employing energy functionals and effective potentials that depend (implicitly or explicitly) on spin currents.

High-temperature superconductivity in monolayer FeSe/Fe(Te, Se) grown on $\mathrm{SrTi}{\mathrm{O}}_3$ (STO) substrates is closely related to the interface effects from STO. Using in situ low-temperature scanning tunneling microscopy/spectroscopy, we study the surface flatness and reconstructions of STO from different pre-treatment methods, and the influence of STO reconstructions on the interfacial superconductivity. Besides, the impacts from postgrowth annealing conditions on the morphology and superconducting properties of monolayer FeSe/Fe(Te, Se) are investigated. These results pave the way for the growth of high-temperature interface superconductors in a detailed level, and provide useful references for better interfacial engineering to foster superconductivity.

Acoustic crystals have emerged as a versatile platform for studying topological phases, showcasing their universality and diversity. The properties of topological insulators and their boundary states depend on the space dimension and symmetries of materials, as evidenced by the periodic table of topological invariants. Studying and observing different topological classes require precise enforcement of topological symmetries and this poses a big challenge for acoustic metamaterials. In this paper, we propose and demonstrate an effective experimental coupling strategy for acoustic crystals, while maintaining the integrity of chiral symmetry. We employed the SSH model for testing purposes to ascertain whether various types of connections support topologically edge and interface modes. We observed that a modular platform, where the resonators are coupled through the bottom, provides support for topologically protected edge and interface states. This platform exhibits exceptional versatility, including reconfigurability, as well as preservation of chiral symmetry.