The atomic cluster expansion (ACE) efficiently parametrizes complex energy surfaces of pure elements and alloys. Due to the local nature of the many-body basis, ACE is inherently local or semilocal for graph ACE. Here, we employ descriptor-constrained density-functional theory (DFT) for parametrizing ACE with charge or other degrees of freedom, thereby transfering the variational property of the density functional to ACE. The descriptors can be of scalar, vectorial, or tensorial nature. From the simplest case of scalar atomic descriptors we directly obtain charge-dependent ACE with long-range electrostatic interactions between variable charges. We observe that the variational properties of the charges greatly help in training, avoiding the need for charge-constrained DFT calculations.

We develop a quantum simulation-based approach for studying properties of strongly correlated magnetic materials at increasing scale. We consider a paradigmatic example of a quantum spin liquid (QSL) state hosted by the honeycomb Kitaev model and use a trainable, symmetry-guided ansatz for preparing its ground state. Applying the tools of quantum subspace expansion (QSE), Hamiltonian operator approximation, and overlap measurements, we simulate the QSL at zero temperature and finite magnetic field, thus moving outside of the symmetric subspace. Next, we implement a protocol for quantum subspace expansion-based measurement of spin-spin correlation functions. Finally, we perform QSE-based simulation of the dynamical structure factor obtained from Green's functions of the finite field Kitaev model. Our results show that quantum simulators offer an insight into quasiparticle properties of strongly correlated magnets and can become a valuable tool for studying material science.

Rare-earth diantimonides display intriguing ground states often associated with structural order, which can be manipulated in thin-film geometries. In this study, we report epitaxial synthesis of one such compound, ${\mathrm{YbSb}}_2$, on III-V substrates using molecular-beam epitaxy. The synthesized thin films exhibit large, nonsaturating, linear magnetoresistance across a wide magnetic field range. Additionally, they demonstrate superconducting properties, with a critical temperature of $\approx 1.025$ K and a critical field of $\approx 83.85$ Oe, consistent with the reports in bulk single crystals. While ${\mathrm{YbSb}}_2$ has been classified as a type-I superconductor in its bulk form, our findings provide evidence of a mixed state in the epitaxial thin films. This work demonstrates the realization of an epitaxial superconductor-semiconductor hybrid system and paves the way for controlling the electronic ground state in rare-earth diantimonides through thin-film engineering.

Predicting stable structures of nanoclusters is crucial yet computationally demanding. This study presents a machine learning-based methodology designed to accelerate the prediction of stable structures in nanoclusters. By integrating local environment descriptors, with dimensionality reduction, kernel-based similarity measure, and outlier detection, we efficiently screen and select promising configurations, thus accelerating identification of global and local minimum structures. The approach is validated through rigorous optimization, demonstrating its capability to identify low-energy structures while significantly reducing computational costs. This method offers a robust framework for structural screening.

We present a solid state system which spontaneously generates remarkable engraving patterns on the surface of Ge. The layered construction, with a metal film on the Ge surface, results in coupling of the metal catalyzed etching reaction with the long range stress field at the Ge-metal interface. The etching patterns generated have similarities with Turing patterns, hydrodynamic patterns, crack propagation, and biological form. We describe spirals, radial patterns, and more disordered structures. Euler buckling of the metal layer generates a characteristic wavelength for some patterns.

Fabricating and investigating freestanding membranes of materials are key approaches for exploring the intrinsic properties of those materials, even in their metastable phases stabilized by external factors. In this study, we fabricated freestanding $\mathrm{Zr}{\mathrm{O}}_2$ membranes with metastable crystal structures by exfoliating $\mathrm{Zr}{\mathrm{O}}_2$ (ZO) epitaxial layers from $\mathrm{Zr}{\mathrm{O}}_2/\mathrm{L}{\mathrm{a}}_{0.7}\mathrm{S}{\mathrm{r}}_{0.3}\mathrm{Mn}{\mathrm{O}}_3/\mathrm{SrTi}{\mathrm{O}}_3$ heterostructures. By combining the results of first-principles calculations, we found that with increasing thickness up to 15 nm, ZO membranes undergo structural relaxation from the metastable tetragonal structure to a metastable orthorhombic one. In addition, the dielectric constant of the tetragonal ZO is susceptible to the out-of-plane lattice constant of the ZO layer. The change in out-of-plane interplanar distance, which primarily stems from the strain release associated with the exfoliation of ZO layers, leads to an increase in the dielectric constant (to $\sim 25$). ZO membranes with high dielectric constants could be utilized in energy storage capacitors.

Two-dimensional materials with flat bands at the Fermi level are ideal platforms to explore novel strong correlation physics. More exotic and desirable are materials whose low-energy bands can be driven to flat bands by a handy external field. Combining analytical analysis and numerical calculations, with a tight-binding model, we show that semifunctionalized graphene have a hidden unbalanced bipartite limit supporting a flat band in the spectrum, which can be approached by a vertical electric field. The electric-field control is equally effective for the nonmagnetic state, the ferromagnetic state, and the antiferromagnetic state. The continuous tuning of the bandwidth also enables a series of electric-field-driven magnetic phase transitions. The present study uncovers an intriguing secret of semifunctionalized graphene, which is also applicable to semifunctionalized graphene analog materials, and has interesting implications for other two-dimensional materials.

Thin films of cuprate superconductors are easier to control in terms of doping as compared to bulk samples. However, they require specific substrates to facilitate epitaxial growth. These substrates are often incompatible with materials used in electronic applications. Furthermore, it is challenging to separate the substrate's properties from the material of interest. Here, we demonstrate the fabrication of an electron-doped cuprate membrane. We show that the membrane has a coherent crystal structure. Furthermore, the superconducting properties of the membrane postliftoff closely resemble those of the thin films preliftoff, as revealed by a scanning superconducting quantum interference device (SQUID) microscope. Such membranes pave the way for designing new material properties and incorporating complex superconducting materials into typically incompatible electronic devices.

Hexagonal boron nitride ($h$-BN) exhibits dominant $\pi$-bands near the Fermi level, similar to graphene. However, unlike graphene, where tight-binding (TB) models accurately reproduce band edges near the $K$ and $K^{\prime }$ points in the Brillouin zone, a wider bandgap in $h$-BN necessitates capturing the band edges at both the $K$ and $M$ points for precise bandgap calculations. We present effective TB models derived from ab initio calculations using maximally localized Wannier functions (MLWFs) centered on boron and nitrogen sites. These models consider hopping terms of up to four distant neighbors and achieve excellent agreement with ab initio results near the $K$ and $M$ points. Furthermore, we compare the band structures from our simplified models with those obtained from ab initio calculations and the full tight-binding model to assess their accuracy. To account for the effects of strains, we introduce fitting parametrizations that relate the hopping parameters of the effective TB model to the lattice constant and interlayer distance. Additionally, we utilize the two-center approximation to calculate the interlayer hopping energies based on the relative distances between sublattices to generalize the interlayer hopping parameters across different stacking configurations. We demonstrate the effectiveness of this method by comparing the electronic structure of zero-twist and twisted $h$-BN systems with ab initio calculations.

Two-dimensional (2D) Janus materials have gained increasing attention as water-splitting photocatalysts for hydrogen production. We use first-principles calculations to predict a stable 2D Janus $T\text{−}\mathrm{TiNBr}$ structure with strong near-ultraviolet sunlight absorption and band edges that align favorably with the water redox potentials for oxygen and hydrogen evolution. We show that the optical and electronic properties of $T\text{−}\mathrm{TiNBr}$ can be modulated to a certain extent by applying external uniaxial strain. Explicit calculations of the redox reactions reveal that solar-driven water splitting is viable at the N-side of $T\text{−}\mathrm{TiNBr}$ while the Br-side requires modifications such as vacancy creation, the application of an external potential, or adjustment of the pH conditions.

The formation of 2D lateral heterostructures in rippled ${\mathrm{MoS}}_2$ and similar transition metal dichalcogenides (TMDs) is studied using density functional theory. Compression of rippled TMDs beyond a threshold compression leads to the formation of a flat valence band associated with strongly localized holes. The implications for exciton manipulation and the emergence of one-dimensional heavy fermion behavior are discussed.

The $\langle a\rangle$-type screw dislocations are known to be significant mediators of plasticity in hexagonal-close-packed (HCP) metals. These dislocations have polymorphic core structures, and subtle changes in the relative energies of these core structures are known to have a large impact on the dynamics of the dislocations. This work identifies a previously neglected long-range elastic interstitial-solute/dislocation interaction that influences the core structures. Essentially, interstitial solutes induce a change in the dislocation core structure to minimize the energy of interaction between the solutes and the dislocation. Molecular dynamics simulations, continuum linear elasticity, and statistical analysis show that this long-range interaction can locally alter the dislocation cores so that many different polymorphs appear along a single dislocation not only because of direct contact between interstitials and the dislocation core but also because of this long-range elastic interaction.

Magnetoelasticity plays a crucial role in numerous magnetic phenomena, including magnetocalorics, magnon excitation via acoustic waves, and ultrafast demagnetization, or the Einstein–de Haas effect. Despite a long-standing discussion on anisotropy-mediated magnetoelastic interactions of relativistic origin, the exchange-mediated magnetoelastic parameters within an atomistic framework have only recently begun to be investigated. As a result, many of their behaviors and values for real materials remain poorly understood. Therefore, by using a proposed simple modification of the embedded cluster approach that reduces the computational complexity, we critically analyze the properties of exchange-mediated spin-lattice coupling parameters for elemental $3d$ ferromagnets (bcc Fe, fcc Ni, and fcc Co), comparing methods used for their extraction and relating their realistic values to symmetry considerations and orbitally decomposed contributions. Additionally, we investigate the effects of noncollinearity (spin temperature) and applied pressure on these parameters. For Fe, we find that single-site rotations, associated with spin temperatures around 100 K, induce significant modifications, particularly in Dzyaloshinskii-Moriya-type couplings; in contrast, such interactions in Co and Ni remain almost configuration independent. Moreover, we demonstrate a notable change in the exchange-mediated magnetoelastic constants for Fe under isotropic contraction. Finally, the conversion between atomistic, quantum-mechanically derived parameters and the phenomenological magnetoelastic theory is discussed, which can be a useful tool towards larger and more realistic dynamics simulations involving coupled subsystems.

We present a detailed x-ray magnetic circular dichroism (XMCD) study of the magnetic properties of Gd-doped EuO thin films, synthesized via molecular-beam epitaxy with Gd doping levels up to over 12%. The impact of Gd doping on the electronic and magnetic behavior of EuO is studied using XMCD and magnetometry. Gd doping significantly enhances the Curie temperature ($T_{\mathrm{C}}$) from 69 K in undoped EuO to over 120 K, driven by increased carrier density, while preserving the high quality of the single-crystalline films. At higher doping levels, a plateau in $T_{\mathrm{C}}$ is observed, which is attributed to the formation of Eu-Gd nearest-neighbor pairs that limit dopant activation. We also observe a distinctive “double-dome” structure in the temperature-dependent magnetization, which we attribute to both the ferromagnetic ordering of Eu $4f$ moments at lower temperatures and the influence of conduction electrons via $4f\text{−}5d$ exchange interactions at higher temperatures. These findings provide key insights into the mechanisms of carrier-induced magnetic transitions.

We report efficient electron doping into a (111)-oriented ${\mathrm{KTaO}}_3$ single crystal via low-energy (2.5 keV) hydrogen ion beam irradiation. The hydrogen-doped ${\mathrm{KTaO}}_3$ exhibits metallic behavior with a quadratic temperature dependence of resistivity at low temperatures down to 4 K. The hydrogen concentration reaches up to ${\mathrm{H}}_{1.4}{\mathrm{KTaO}}_3$ as confirmed by low-temperature (100 K) hydrogen ion beam irradiation and in situ nuclear reaction analysis. The doped hydrogens act as electron donors because ex situ Hall effect measurements indicate $n$-type conduction in the hydrogen-doped ${\mathrm{KTaO}}_3$. This is consistent with our first-principles calculations of the defect formation energy, which indicate the formation of interstitial hydrogens. We find that partial desorption of excess hydrogen upon warming to room temperature gives rise to unusual thermal hysteresis in resistance. These results provide an important step forward in the search for superconductivity in bulk ${\mathrm{KTaO}}_3$.

Density Functional Tight Binding (DFTB), an approximative approach derived from Density Functional Theory (DFT), has the potential to pave the way for simulations of large periodic or nonperiodic systems. We have specifically tailored DFTB parameters to enhance the accuracy of electronic band gap calculations in both three-dimensional and two-dimensional lead-iodide perovskites, at a significantly reduced computational cost relative to state-of-the-art ab initio calculations. Our electronic DFTB parameters allow computing not only the band gap but also effective masses of perovskite materials with reasonable accuracy compared to existing experimental data and state-of-the-art DFT calculations. The electronic band structures of vacancy-ordered and lead- and iodide-deficient perovskites are also explored. Additionally, we demonstrate the efficiency of DFTB in computing electronic band alignments in perovskite heterostructures. The DFTB-based approach is anticipated to be beneficial for studying large-scale systems such as heterostructures and nanocrystals.

The family of hexagonal manganites is studied intensively for its multiferroicity, magnetoelectric coupling, improper ferroelectricity, functional domain walls, and topology-related scaling behaviors. It is established that these physical properties are codetermined by the cation sublattices and that aliovalent doping can readily be leveraged to modify them. The doping, however, also impacts the anion defect chemistry and semiconducting properties, which makes the system highly sensitive to the synthesis and processing conditions. Here, we study the electronic properties of $\mathrm{YMn}{\mathrm{O}}_3$ as a function of aliovalent cation doping and thermoatmospheric history, combining density functional theory calculations with thermopower and thermogravimetric measurements. We show that the charge-carrier concentration and transport properties can be controlled via both aliovalent cation dopants and anion defects, enabling reversible switching between $n$-type and $p$-type conductivity. This tunability is of importance for envisaged applications of hexagonal manganites in, e.g., next-generation capacitors and domain-wall nanoelectronics, or as catalysts or electrodes in fuel cells or electrolyzers. Furthermore, our approach is transferrable to other transition-metal oxides, providing general guidelines for controlling their semiconducting properties.

Carbon biphenylene network (C-BPN), which is an ultrathin material consisting of carbon atoms arranged in square-hexagonal-octagonal (4-6-8) periodic rings, has intriguing properties for nano-scale device design due to its unique crystal structure. Here, using the Landauer formalism in combination with first-principles calculations, we show that C-BPN is a highly stable thermoelectric material at elevated temperatures under mechanical strain, where its thermoelectric efficiency can be anisotropically engineered. Transport calculations reveal that C-BPN's transmission spectrum has significant degrees of directional anisotropy and it undergoes a metal-insulator transition under strain, which leads to an increase in its Seebeck coefficient. C-BPN's lattice thermal conductance can be selectively tuned up to $35\%$ bidirectionally at room temperature by strain engineering. Enhancement in its power factor and the suppression of its lattice thermal conductance improves the p-type figure of merit up to 0.31 and 0.46 at 300 and 500 K, respectively. Our findings reveal that C-BPN has high potency to be used in thermoelectric nanodevices with selective anisotropic properties at elevated temperatures.