Interfaces of complex oxides host a rich variety of phenomena and exotic phases not found in their bulk counterparts. The ${\mathrm{LaTiO}}_3/{\mathrm{SrTiO}}_3$ (LTO/STO) system is particularly interesting due to multiple conduction channels such as the LTO, interfacial, and partial reduction of STO. To date, the extent of the conducting region has not been quantified but is critical for a fundamental understanding of the LTO/STO system. Here, we quantify the 2DEG thickness in LTO/STO through Sondheimer oscillations in the longitudinal magnetoresistace. By analyzing the LTO thickness dependence of these oscillations, we estimate that the 2DEG penetrates the STO substrate by approximately 2.8 nm, indicating that the metallic bulk LTO is not the sole contributor to metallicity. This work provides the first quantitative estimate of the STO conduction depth in LTO/STO using Sondheimer oscillations and provides critical insight into the complex conduction pathways in LTO/STO previously considered to be a simple 2DEG.

In ${\mathrm{Co}}_x{\mathrm{NbSe}}_2$, crystal symmetry and cobalt site occupation drive the formation of two distinct magnetic phases. At $x=1/4$, the centrosymmetric structure ($P{6}_3/mmc$) promotes Co-Co interactions leading to the formation of an $A$-type antiferromagnetic structure phase with a transition temperature of $T_N^A=169\mathrm{K}$. At $x=1/3$, the noncentrosymmetric structure ($P{6}_{3}22$) induces a lower-temperature magnetic phase with $T_N^S=28\mathrm{K}$. We report the coexistence of both substructures within a superlattice, with a nuclear propagation vector of (1/3, 1/3, 0) relative to the host lattice. Single crystals of ${\mathrm{Co}}_{0.28}{\mathrm{NbSe}}_2$ exhibit both magnetic transitions, with $T_N^A$ corresponding to the $x\sim 1/4$ phase and $T_N^S$ corresponding to the $x\sim 1/3$ phase. Magnetic susceptibility and specific heat measurements confirm these transitions, although only the high-temperature $T_N^A$ phase significantly affects resistivity. We successfully isolate each phase in powder samples, while single crystals with an intercalation ratio of $x=0.28$ display the coexistence of both phases in a single sample. Using single-crystal neutron diffraction, we solved the magnetic structure of the high-temperature centrosymmetric phase ($T_N^A$), and neutron powder diffraction revealed the double-$q$ magnetic structure of the low-temperature noncentrosymmetric phase ($T_N^S$).

We report on the effect of tunnel barrier intercalation between a graphene monolayer (MLGr) and a ferromagnet on the structural, static, and dynamic magnetic properties of ${\mathrm{SiO}}_2/\mathrm{MLGr}/\mathrm{oxide}$ $\mathrm{barrier}/{\mathrm{Co}}_{40}{\mathrm{Fe}}_{40}{\mathrm{B}}_{20}/\mathrm{Ta}$ heterostructures. Gr-based multilayers with an oxide barrier of MgO and ${\mathrm{Al}}_2{\mathrm{O}}_3$, elaborated by molecular beam epitaxy and atomic layer deposition, respectively, were investigated. High-resolution transmission electron microscopy was conducted to investigate the interfacial structures of the heterostructures and to highlight their specific features. Raman spectroscopy and vibrating sample magnetometry measurements were employed to investigate the quality of Gr and the static magnetic properties of the material stacks, respectively. Additionally, Brillouin light-scattering and microstrip ferromagnetic resonance measurements were carried out to investigate the dynamic magnetic properties, including the perpendicular magnetic anisotropy, the magnetic damping, and the interfacial Dzyloshinskii-Moriya interaction, key parameters for spintronic applications. Our results were compared to those obtained from reference samples, without MLGr, aiming to clarify the role of the MLGr and pointing out the influence of the oxide barriers on the magnetic properties of Gr/tunnel barrier/FM heterostructures.

We present a magnetoelastic lattice in which a localized external magnetic field, generated by an assembly of fixed magnets, tunes the potential landscape to create monostable, bistable, and tristable configurations. Focusing on the tristable potential, we numerically and experimentally confirm the existence of three distinct types of transition waves, each characterized by unique amplitudes and velocities, and establish a scaling law that governs their behavior. We also examine how these transition waves interact with the system's finite boundaries. Furthermore, by adjusting the potential symmetry through the localized external field, we investigate wave collision dynamics. In lattices with asymmetric potentials, the collision of similar transition waves leads to the remote nucleation of a third phase. In symmetric potentials, the collision of dissimilar transition waves results in the formation of a stationary domain wall, with its width tuned by the shape of the tristable potential well.

Two-dimensional frustrated quantum magnets can serve as a platform for exploring exotic quantum states. Here, we report the magnetic behavior of a rhombic lattice quantum antiferromagnet ${\mathrm{KCoPO}}_4·{\mathrm{H}}_2\mathrm{O}$. The rhombic lattice model is equivalent to a model of a square lattice with two different diagonal components, and competing exchange interactions potentially produce exotic quantum states. The magnetic and thermodynamic properties of this compound were well characterized by the $J_{\mathrm{eff}}=1/2$ state. An antiferromagnetic transition is observed at $T_{\mathrm{N}}=2.8$ K. The magnetic structure of ${\mathrm{KCoPO}}_4·{\mathrm{H}}_2\mathrm{O}$ is identified as an incommensurate helical structure, characterized by a magnetic modulation propagation vector $\mathbf{k}=\left(0.1747\right(1\left)00\right)$ with an in-plane moment rotating along the $a$ axis. Moreover, a magnetic field-induced phase transition is observed at approximately 1 T. The application of a magnetic field suppresses spin ordering and enhances spin fluctuations. Our experimental results suggest the presence of a quantum phase in the magnetic phase diagram of the spin-1/2 Heisenberg rhombic lattice antiferromagnet with easy-plane anisotropy.

Using vertex-corrected quasiparticle self-consistent $GW$ schemes, we address the electronic structure of two manganese dioxide polymorphs, $\alpha \text{−}{\mathrm{MnO}}_2$ and $\beta \text{−}{\mathrm{MnO}}_2$. In particular, we determine the fundamental band gaps, macroscopic dielectric constants, magnetic moments of the Mn atoms, band structures, and associated densities of states. Additionally, we obtain the imaginary component of the dielectric function ${\epsilon }_2\left(\omega \right)$ from the solution of the Bethe–Salpeter equation. For $\beta \text{−}{\mathrm{MnO}}_2$, we record overall good agreement when comparing the density of states with X-ray photoelectron spectroscopy and/or bremsstrahlung isochromat spectroscopy spectra and the dielectric function ${\epsilon }_2\left(\omega \right)$ with optical response spectra. Applied to $\alpha \text{−}{\mathrm{MnO}}_2$, whose pristine bulk structure is poorly characterized, our work provides a prediction at the same level of theory. The quality of the achieved description is further supported by comparisons with experimental spectra of nanostructured and doped variants. This study demonstrates that state-of-the-art $GW$ methods successfully account for key electronic-structure features of ${\mathrm{MnO}}_2$ polymorphs.

Advanced defect engineering techniques have enabled the creation of unique quantum phases from pristine materials. One-dimensional (1D) atomic defects in low-dimensional systems are particularly intriguing due to their distinct quantum properties, such as 1D Rashba states that allow for the generation of nondissipative spin currents, making them ideal for spintronic devices. Using density-functional calculations and model-based symmetry analysis, we report the emergence of 1D Rashba states in a two-dimensional ${\mathrm{Si}}_2{\mathrm{Bi}}_2$ monolayer (ML) with vacancy line defects (VLDs). We show that introducing VLDs in the ${\mathrm{Si}}_2{\mathrm{Bi}}_2$ ML induces 1D confined defect states near the Fermi level, which are strongly localized along the extended defect line. Notably, we observed 1D Rashba spin-split bands in these defect states with significant spin splitting originating mainly from the strong $p\text{−}p$ coupling orbitals between Si and Bi atoms near the defect sites. These spin-split defect states exhibit perfectly collinear spin polarization in momentum $\vec{k}$ space, which is oriented perpendicularly to the VLD orientation. Moreover, using $\vec{k}·\vec{p}$ perturbation theory supplemented with symmetry analysis, we show that the 1D Rashba states with collinear spin polarization are enforced by the lowering of symmetry of the VLDs into the $C_s$ point group, which retains the $M_{xz}$ mirror symmetry along with the 1D nature of the VLDs. The observed 1D Rashba states in this system protect carriers against spin decoherence and support an exceptionally long spin lifetime, which could be promising for developing highly efficient spintronic devices.

${\mathrm{CO}}_2$ capture using carbon-based materials, particularly graphene and graphene-like materials, is a promising strategy to deal with ${\mathrm{CO}}_2$ emissions. However, significant gaps remain in our understanding of the molecular-level interaction between ${\mathrm{CO}}_2$ molecules and graphene, particularly in terms of chemical bonding and electron transfer. In this work, we employ random structure search and density functional theory to understand the adsorption of ${\mathrm{CO}}_2$ molecules on Ca-, Sr-, Na-, K-, and Ti-decorated graphene surfaces. Compared to the pristine material, we observe enhanced ${\mathrm{CO}}_2$ adsorption on the decorated graphene surfaces. Particularly on group 2 metals and titanium-decorated graphene, ${\mathrm{CO}}_2$ can be strongly chemisorbed as a bent ${\mathrm{CO}}_2$ anion or as an oxalate, depending on the number of ${\mathrm{CO}}_2$ molecules. Electronic structure analysis reveals that the adsorption mechanism involves a charge transfer from the metal adatom to the adsorbed ${\mathrm{CO}}_2$. Overall, this study suggests that reducing ${\mathrm{CO}}_2$ to oxalate on group 2 metals and titanium-decorated graphene surfaces is a potential strategy for ${\mathrm{CO}}_2$ storage.

Defect spinel $\gamma \text{−}{\mathrm{Ga}}_2{\mathrm{O}}_3$ is the least stable polymorph of ${\mathrm{Ga}}_2{\mathrm{O}}_3$, so its frequent appearance as a structural defect within or on the surface of monoclinic $\beta \text{−}{\mathrm{Ga}}_2{\mathrm{O}}_3$ remains a mystery. Through first-principles calculations, we explore potential pathways for the phase transition from $\beta \text{−}{\mathrm{Ga}}_2{\mathrm{O}}_3$ to $\gamma \text{−}{\mathrm{Ga}}_2{\mathrm{O}}_3$, and examine two key driving forces: tensile strain and Ga deficiency. When configurational entropy contributions to phase energies are included, the $\gamma$ phase becomes energetically competitive with the $\beta$ phase, with the free energy difference between these phases diminishing even further under Ga-deficient conditions. Notably, a stability crossover occurs at room temperature at high vacancy concentrations $(\left[{\mathrm{V}}_{\text{Ga}}^{3-}\right]>3\%)$. A simple model $\beta \to \gamma$ transformation pathway is identified, comprising two primary reactions, that enables the formation of the $\gamma$ phase via simultaneous migration of Ga atoms from tetrahedral lattice sites to octahedral interstitial positions. The transformation barriers are prohibitively large in pristine ${\mathrm{Ga}}_2{\mathrm{O}}_3$, but can be substantially reduced by: (1) the presence of Ga vacancies, (2) elongational strains along the crystallographic $a$-axis, and (3) when volumetric relaxations are possible during transformation. These results elucidate prior experimental observations, where $\gamma \text{−}{\mathrm{Ga}}_2{\mathrm{O}}_3$ is seen on damaged surfaces or in highly $n$-type $\beta \text{−}{\mathrm{Ga}}_2{\mathrm{O}}_3$ environments, which support Ga deficiency and mechanical strain. The insights into the driving forces and mechanisms of $\gamma \text{−}{\mathrm{Ga}}_2{\mathrm{O}}_3$ formation enhance understanding of how localized strain and nonequilibrium defect concentrations may facilitate its formation from the $\beta$ phase.

Grasping (electronic) structure changes during photochromic processes is crucial for fully understanding the photochromic effect in rare-earth oxyhydride films. In this study, we employ in situ UV illumination positron annihilation lifetime spectroscopy (PALS) to investigate the time evolution of open-volume defects and metallic domains during photodarkening and bleaching in yttrium oxyhydride films. The PALS depth profiles before and after a photodarkening-bleaching cycle reveal a light-induced increase in open-volume defects, that occurs homogeneously throughout the oxyhydride layer. The time-dependent PALS measurements show that upon photodarkening, a fast initial formation of metallic domains occurs, as well as a fast release of loosely bounded hydrogen from vacancy clusters and nanopores. During further photodarkening, the concentration of divacancy-like defects gradually increases due to the aggregation of light-induced hydrogen vacancies with preexisting yttrium monovacancies. After the UV illumination is stopped, two subsequent bleaching phases are observed. During the first bleaching phase, a strong correlation between the shortest positron lifetime ${\tau }_1$ and the photochromic contrast is seen in both samples, suggesting that metallic domains disappear and, correspondingly, positron trapping at yttrium monovacancies and divacancy-like defects increases. During the second bleaching phase, a subsequent correlation between ${\tau }_1$ and the photochromic contrast is observed in the more H-rich sample, which is related to the disappearance of larger metallic domains. After bleaching, most of the metallic domains and the photoexcited electrons in the matrix have disappeared, while the formed small vacancy complexes and larger vacancies remain stable. Our PALS study suggests that the formation of metallic domains is the cause of photodarkening, and the formed vacancy defects are important for understanding the memory effect.

The relaxation of atomic positions to their optimal structural arrangement is crucial for understanding the emergence of new physical behavior in long scale superstructures in twisted bilayers of two-dimensional materials. The amount of deviation from a rigid moiré structure will depend on the elastic properties of the constituent monolayers which, for the twisted bilayer, the more flexible the monolayers are, the lower the energy required to deform the layers to maximize the areas with an energetically optimal interlayer arrangement of atoms. We investigate this atomic reconstruction for twisted bilayers of highly flexible InSe. Results using two methods are demonstrated: first we train a machine-learned interatomic potential (MLIP) to enable fully atomistic relaxations of small-twist-angle large-length-scale moiré supercells while retaining density functional theory (DFT) level accuracy. We find substantial out-of-plane corrugation and in-plane domain formation for a wide range of twist angles and moiré length scales. We then adapt an existing continuum approach and show that it can reproduce some, but not all, features of the fully atomistic calculations.

Using density functional theory, we characterize crystal and electronic structures as well as the topology of the $X{\mathrm{Sb}}_4{\mathrm{Te}}_4$ family of materials $(X=\mathrm{Ge}, \mathrm{Sn}, \mathrm{Pb})$ that are composed of the $X{\mathrm{Sb}}_2{\mathrm{Te}}_4$ septuple layers interleaved by Sb bilayers. We first demonstrate that all these compounds are dynamically stable by means of phonon spectra calculations. Then, our electronic structure calculations, performed within the modified Becke-Johnson potential approximation, predict ${\mathrm{GeSb}}_4{\mathrm{Te}}_4$ and ${\mathrm{SnSb}}_4{\mathrm{Te}}_4$ to be semimetals, while ${\mathrm{PbSb}}_4{\mathrm{Te}}_4$ to be an insulator with a narrow gap of about 50 meV. Further, we find all $X{\mathrm{Sb}}_4{\mathrm{Te}}_4$ to show both the strong ${\mathbb{Z}}_2$ topology and the crystalline topology provided by the time-reversal and mirror reflection symmetries, respectively. We demonstrate the stability of this dual topological nature with respect to a considerable $X\text{−}\mathrm{Sb}$ intermixing previously found experimentally.

A comprehensive study of the low-temperature properties of ${\mathrm{YbNi}}_4\mathrm{Mg}$ has revealed evidence of a superheavy-fermion state, characterized by a large electronic specific-heat coefficient ${\gamma }_0\approx 5.65\mathrm{J}{\mathrm{mol}}^{-1}{\mathrm{K}}^{-2}$ and an elevated Wilson ratio $R_W=32.1$. No magnetic ordering was observed down to 70 mK; however, a broad maximum appears in the specific heat at $T^{*}=0.3\mathrm{K}$, along with a shoulder in the derivative of susceptibility $\mathrm{d}\chi /\mathrm{d}T$ and resistivity $\mathrm{d}\rho /\mathrm{d}T$. These features indicate a cooperative yet short-ranged magnetism entwined with the superheavy Fermi liquid. The large Wilson ratio, which is also detected in other superheavy-fermion compounds lacking long-range order, might be a signature of residual spin fluctuations. Applying a weak magnetic field of $\sim 0.1\mathrm{T}$ induces a metamagnetic-like crossover, as demonstrated by the quasi-adiabatic demagnetization measurements showing a broad minimum in the temperature-field trace. Here, an enhanced magnetocaloric cooling effect stemming from the field-sensitive superheavy-fermion state is observed, rivaling that of well-established insulating magnetic coolants like the rare-earth garnet ${\mathrm{Gd}}_3{\mathrm{Ga}}_5{\mathrm{O}}_{12}$.

The numerical precision of density-functional-theory (DFT) calculations depends on a variety of computational parameters, one of the most critical being the basis-set size. The ultimate precision is reached in the limit of a complete basis set (CBS). Our aim in this work is to find a machine-learning model that extrapolates finite basis-size calculations to the CBS limit for periodic crystal structures. We start with a data set of 63 binary solids investigated with two all-electron DFT codes, exciting and FHI-aims, which employ very different types of basis sets. A quantile-random-forest model and a symbolic regression approach using the SISSO model are used to estimate the total-energy correction with respect to a fully converged calculation as a function of the basis-set size. The random-forest model achieves a symmetric mean absolute percentage error of lower than 25% for both codes and outperforms previous approaches in the literature. SISSO outperforms the random forest model for the exciting code. Our approach also provides prediction intervals, which quantify the uncertainty of the models' predictions.

Interlayer sliding ferroelectricity has been discovered in a variety of two-dimensional (2D) materials with superb features such as atomic thickness, fast response, and fatigue resistance. So far, research on this phenomenon has been limited to fundamental physics and electronic applications, leaving its potential for electromechanical actuation unexplored. In this work, we design an atomic-scale actuator based on sliding ferroelectricity and field-tunable interfacial friction. With a prototype based on parallelly stacked bilayer h-BN sandwiched between gold contacts, we show how an alternating electric field can drive the bilayer into controlled crawling motions and how uniaxial strain can steer the crawl direction. Using numerical simulations, we demonstrate the actuator's robust operation under a wide range of drive signals, friction scales, and frictional variations. We further provide experimental directions on how to realize field-tunable friction on h-BN interfaces. The wireless-ready actuation mechanism can be generalized to many 2D material systems possessing sliding ferroelectricity and integrated into flexible-electronics platforms, opening new avenues in the development of nanorobotics.

The structure of an $\mathrm{Al}\left(111\right)3\times 3\text{-Si}$ surface was examined by combining data from positron diffraction and core-level photoemission spectroscopy. Analysis of the diffraction rocking curves indicated that the overlayer had a flat honeycomb lattice structure. Simulations of Si core-level spectra calculated via the first principles indicated that one of the Si atoms in the unit cell was replaced by an Al atom. The surface superstructure was thus a two-dimensional layer of Al-embedded silicene on Al(111).

Two-dimensional materials, such as transition metal dichalcogenides (TMD), are highly promising for future electronics applications. The metal ion migration within the layered structures plays an important role during device fabrication and can dramatically affect the device functionality, e.g., by changing the resistivity. While often harmful, the resistance switching has been harnessed in the TMD-based memristors. While metal atom migration in these materials has been previously investigated using first-principles calculations, surprisingly little work has been devoted to understanding the drift under applied electric field as is important in electronics applications. Here, we used density-functional theory calculations to study metal atoms and ions in ${\mathrm{MoS}}_2$-based devices. We investigate the formation and migration of Ag, Au, Cu, and Li metals using first principles and extend the results into macroscopic properties using kinetic Monte Carlo simulations. Our results point out the most favorable defects along with their charge states and provide their diffusion and drift speeds under different temperatures and electric fields. We compare our results to experimentally realized electrochemical metallization cell memristors based on vertically aligned ${\mathrm{MoS}}_2$ and provide insight on the switching mechanism. In detail, silver and copper are found to be viable active electrode materials and the choice between the two could lead to significantly different memristive behavior. Even as our focus is on vertically grown ${\mathrm{MoS}}_2$ memristors, the results of this study are also applicable to other types of electronic devices.

The flexoelectric field generated by the strain gradient possesses the capability to modify and manipulate various material characteristics. However, achieving abundant and tunable strain gradients in thin films remains a formidable challenge. In this study, we successfully transferred $\mathrm{SrRu}{\mathrm{O}}_3$ (SRO) free-standing single-crystalline films onto a flexible PDMS substrate using a water-soluble sacrificial layer. By applying prestrain to the substrate, the transferred film exhibited a modifiable one-dimensional (1D) wrinkled structure. Through the design of a 1D wrinkled structure, a strain gradient of up to $9.6\times {10}^5{\mathrm{m}}^{-1}$ was introduced in the SRO film. Simultaneously, we can precisely manipulate the morphology of the wrinkles and the magnitude of the strain gradient by adjusting both the thickness of the SRO film and the level of prestrain state, thereby achieving a highly stable and flexible SRO film with a substantial strain gradient. Furthermore, we investigate the impact of strain gradient on the electrical transport properties of SRO wrinkles. It is observed that the electrical conductivity of the wrinkled films is determined by the interplay between the thickness effect and the strain gradient effect. Flexible wrinkle films serve as an optimal platform for investigating fundamental strain-related phenomena, thus exhibiting immense potential in the realm of flexible electronics.