The discovery and observations of exotic, quantized optical and electrical responses have sparked renewed interest in nonmagnetic chiral crystals. Within this class of materials, six group V transition metal ditetrelides, that is, $X{Y}_2$ ($X=\text{V}$, Nb, Ta and $Y=\text{Si}$, Ge), host composite Weyl nodes on high-symmetry lines, with Kramers-Weyl fermions at time-reversal invariant momenta. In addition, at least two of these materials, $\mathrm{Nb}{\mathrm{Ge}}_2$ and $\mathrm{Nb}{\mathrm{Si}}_2$, exhibit superconducting transitions at low temperatures. The interplay of strong electron-phonon interactions and complex Fermi-surface topology presents an opportunity to study both superconductivity and hydrodynamic electron transport in these systems. Towards this broader question, we present an ab initio theoretical study of the electronic transport and electron-phonon scattering in this family of materials, with a particular focus on $\mathrm{Nb}{\mathrm{Ge}}_2$ vs $\mathrm{Nb}{\mathrm{Si}}_2$, and the other group V ditetrelides. We shed light on the microscopic origin of $\mathrm{Nb}{\mathrm{Ge}}_2$'s large and anisotropic room-temperature resistivity and contextualize its strong electron-phonon scattering with a presentation of other relevant scattering lifetimes, both momentum relaxing and momentum conserving. Our work explores the intriguing possibility of observing hydrodynamic electron transport in these chiral Weyl semimetals.

Experiments on asymmetric diblock copolymers at temperatures slightly above the order-disorder transition (ODT) temperature indicate the existence of a dense fluid of micelles. Molecular dynamics simulations are used here to identify a higher critical micelle temperature below which micelles appear. The onset of micellization occurs very near where self-consistent field theory predicts an ODT.

Optically and magnetically active point defects in semiconductors are interesting platforms for the development of solid state quantum technologies. Their optical properties are usually probed by measuring photoluminescence spectra, which provide information on excitation energies and on the interaction of electrons with lattice vibrations. We present a combined computational and experimental study of photoluminescence spectra of defects in diamond and SiC, aimed at assessing the validity of theoretical and numerical approximations used in first-principles calculations, including the use of the Franck-Condon principle and the displaced harmonic oscillator approximation. We focus on prototypical examples of solid state qubits, the divacancy centers in SiC and the nitrogen-vacancy in diamond, and we report computed photoluminescence spectra as a function of temperature that are in very good agreement with the measured ones. As expected we find that the use of hybrid functionals leads to more accurate results than semilocal functionals. Interestingly our calculations show that constrained density functional theory (CDFT) and time-dependent hybrid DFT perform equally well in describing the excited state potential energy surface of triplet states; our findings indicate that CDFT, a relatively cheap computational approach, is sufficiently accurate for the calculations of photoluminescence spectra of the defects studied here. Finally, we find that only by correcting for finite-size effects and extrapolating to the dilute limit can one obtain a good agreement between theory and experiment. Our results provide a detailed validation protocol of first-principles calculations of photoluminescence spectra, necessary both for the interpretation of experiments and for robust predictions of the electronic properties of point defects in semiconductors.

Néel skyrmions are generally realized in asymmetric multilayers made of heavy metals (HMs) and ultrathin ferromagnets possessing strong interfacial Dzyaloshinskii-Moriya interactions (iDMIs). Depending on the relative strengths of iDMIs at the interfaces, various types of Néel skyrmions have been suggested, which are typified with characteristically different topological properties and current-driven dynamics. This suggests the importance of a precise quantification of their spin chiralities. In this paper, we explore the possibility of realizing Néel skyrmions in magnetic multilayers without the direct usage of standard HMs. Specifically, through depositing a thin layer of ferrimagnetic (FIM) CoTb layer on top of an antiferromagnetic (AFM) quantum material of composition ${\mathrm{Mn}}_3\mathrm{Sn}$, the AFM exchange interaction at the asymmetric interface provides an equivalent iDMI for stabilizing FIM Néel skyrmions. Secondly, through using advanced four-dimensional Lorentz scanning transmission electron microscopy (4D LSTEM), in combination with x-ray magnetic circular dichroism photoemission electron microscopy (XMCD-PEEM), we can directly determine the spin chirality of FIM Néel skyrmions. The present findings not only broaden the phase space for chiral interfacial magnetism but also provide a possibility for future applications of heavy-metal-free skyrmionic devices.

A remarkable consequence of the Hohenberg-Kohn theorem of density functional theory is the existence of an injective map between the electronic density and any observable of the many-electron problem in an external potential. In this work, we study the problem of predicting a particular observable, the band gap of semiconductors and band insulators, from the knowledge of the local electronic density. Using state-of-the-art machine learning techniques, we predict the experimental band gaps from computationally inexpensive density functional theory calculations. We propose a modified Behler-Parrinello (BP) architecture that greatly improves the model capacity while maintaining the symmetry properties of the BP architecture. Using this scheme, we obtain band gaps at a level of accuracy comparable to those obtained with state-of-the-art and computationally intensive hybrid functionals, thus significantly reducing the computational cost of the task.

Vanadium oxides have been highly attractive for over half a century since the discovery of the metal insulator transition near room temperature. Here ${\mathrm{Na}}_x\mathrm{V}{\mathrm{O}}_2$ is studied through a systematic comparison with other layered sodium metal oxides with early $3d$ transition metals, first disclosing a unified evolution pattern of Na density waves through in situ x-ray diffraction analysis. Combining ab initio simulations and theoretical modelings, a sodium-modulated Peierls-like transition mechanism is then proposed for the bonding formation of metal ion dimers. More importantly, the unique trimer structure in ${\mathrm{Na}}_x\mathrm{V}{\mathrm{O}}_2$ is shown to be very sensitive to the on-site Coulomb repulsion value, suggesting a delicate balance between strong electronic correlations and orbital effects that can be precisely modulated by both Na compositions and atomic stackings. This unveils a unique opportunity to design strongly correlated materials with tailored electronic transitions through electrochemical modulations and crystallographic designs, to elegantly balance various competition effects. We think the understanding will also help further elucidate complicated electronic behaviors in other vanadium oxide systems, as well as help improve electrochemical performances in layered metal oxides for Na ion battery applications.

An active learning approach to train machine-learning interatomic potentials (moment tensor potentials) for multicomponent alloys to ab initio data is presented. Employing this approach, the disordered body-centered cubic (bcc) ${\mathrm{TiZrHfTa}}_x$ system with varying Ta concentration is investigated via molecular dynamics simulations. Our results show a strong interplay between elastic properties and the structural $\omega$ phase stability, strongly affecting the mechanical properties. Based on these insights we systematically screen composition space for regimes where elastic constants show little or no temperature dependence (elinvar effect).

Antimony sulfide (${\mathrm{Sb}}_2{\mathrm{S}}_3$) is a promising material for solar energy conversion. It consists of earth-abundant elements with a low toxicity and high stability. Furthermore, it shows suitable optical and electronic properties like a large absorption coefficient and a convenient band gap of 1.7 eV. Nevertheless, so far, the obtained power conversion efficiencies and the open circuit voltages are far below the theoretical limits and need to be increased for a viable application of ${\mathrm{Sb}}_2{\mathrm{S}}_3$-based cells at the large scale. To achieve this it is important to identify the dominating loss mechanism. Here time-resolved two-photon photoemission experiments are presented from the (100) surface of a cleaved ${\mathrm{Sb}}_2{\mathrm{S}}_3$ single-crystal to investigate the charge carrier dynamics after photoexcitation. Based on these measurements an ultrafast relaxation within below 150 fs towards the conduction band minimum is observed. This is followed by a decay within 1 ps into states, which are located in the band gap 0.06 eV and 0.44 eV below the conduction band minimum where the charge carriers have long lifetimes of 27 ps and 63 ps, respectively. Based on the energetic position, the energy width and the formation time of the lower-lying gap state a self-trapping mechanism of free charge carriers by optical phonons with a frequency of 1.44 THz is proposed, and the results are discussed in this context. These findings provide evidence that the poor performance of ${\mathrm{Sb}}_2{\mathrm{S}}_3$ in solar devices can be traced back to intrinsically formed traps that can hardly be avoided.

Topotactic transformations involve structural changes between related crystal structures due to a loss or gain of material while retaining a crystallographic relationship. The perovskite oxide ${\mathrm{La}}_{0.7}{\mathrm{Sr}}_{0.3}\mathrm{Co}{\mathrm{O}}_3$ (LSCO) is an ideal system for investigating phase transformations due to its high oxygen vacancy conductivity, relatively low oxygen vacancy formation energy, and strong coupling of the magnetic and electronic properties to the oxygen stoichiometry. While the transition between cobaltite perovskite and brownmillerite (BM) phases has been widely reported, further reduction beyond the BM phase lacks systematic studies. In this paper, we study the evolution of the physical properties of LSCO thin films upon exposure to highly reducing environments. We observe the rarely reported crystalline Ruddlesden-Popper phase, which involves the loss of both oxygen anions and cobalt cations upon annealing where the cobalt is found as isolated Co ions or Co nanoparticles. First-principles calculations confirm that the concurrent loss of oxygen and cobalt ions is thermodynamically possible through an intermediary BM phase. The strong correlation of the magnetic and electronic properties to the crystal structure highlights the potential of utilizing ion migration as a basis for emerging applications such as neuromorphic computing.

Specific heat is a central property of condensed matter systems including polymers and oligomers in their condensed phases, yet predictions of this quantity from molecular simulations and successful comparisons with experimental data are scarce if existing at all. One reason for this may be that the internal energy and thus the specific heat cannot be coarse-grained so that they defy their rigorous computation with united-atom models. Moreover, many modes in a polymer barely contribute to the specific heat because of their quantum mechanical nature. Here, we demonstrate that an analysis of the mass-weighted velocity autocorrelation function allows specific heat predictions to be corrected for quantum effects so that agreement with experimental data is on par with predictions of other routinely computed quantities. We outline how to construct corrections for both all-atom and united-atom descriptions of chain molecules. Corrections computed for 11 hydrocarbon oligomers and commodity polymers deviate by <$k_{\text{B}}/10$ within a subset of nine molecules. Our results may benefit the prediction of heat conductivity.

We demonstrate reversible voltage control of perpendicular exchange bias via ${\mathrm{H}}^{+}$ pumping in a $\mathrm{NiO}/\mathrm{Pd}/\mathrm{Co}/\mathrm{Pd}/\mathrm{Gd}{\left(\mathrm{OH}\right)}_3/\mathrm{Au}$ heterostructure at room temperature. The perpendicular exchange bias results from a tailored layer structure consisting of an antiferromagnetic NiO layer and a ferromagnetic Co layer, stabilized by an ultrathin Pd interlayer. Voltage mediated ${\mathrm{H}}^{+}$ pumping through the $\mathrm{Gd}{\left(\mathrm{OH}\right)}_3$ layer and subsequent H absorption at the Pd/Co interface leads to a decrease in the perpendicular anisotropy. In consequence, also the perpendicular exchange bias vanishes upon voltage application (3V). During voltage switch-off, this process reverses and perpendicular exchange bias recovers. The first voltage switching cycle shows relatively slow kinetics and an inverse relation of exchange bias and coercivity changes. We discuss these features with regard to an H-induced crystallization of the initially amorphous Pd/Co/Pd trilayer, which is revealed by transmission electron microscopy. With subsequent voltage switching steps, a decrease of the exchange bias field in the voltage switch-off state is observed, which levels off with increasing cycle numbers. A reversible setting of exchange bias field values is achieved when a magnetic field (+/− 2 kOe) is superposed during the H loading step. In this case, the shift of the exchange bias field can be controlled by the direction of the applied magnetic field. These results open an innovative route to electrically control exchange bias.

Ferroelectric and ferroelastic domains have been predicted to enhance metal halide perovskite (MHP) solar cell performance. While the formation of such domains can be modified by temperature, pressure, or strain, established methods lack spatial control at the level of single domains. Here, we induce the formation of ferroelastic domains in ${\mathrm{CsPbBr}}_3$ nanowires at room temperature using an atomic force microscope (AFM) tip and visualize the domains using nanofocused x-ray diffraction with a 60 nm beam. Regions scanned with a low AFM tip force show orthorhombic 004 reflections along the nanowire axis, while regions exposed to higher forces exhibit 220 reflections. The applied stress locally changes the crystal structure, leading to lattice tilts that define ferroelastic domains, which spread spatially and terminate at {112}-type domain walls. The ability to induce individual ferroelastic domains within MHPs using AFM gives new possibilities for device design and fundamental experimental studies.

Although electrostatic gating with liquid electrolytes has been thoroughly investigated to enhance electrical transport in two-dimensional (2D) materials, solid electrolyte alternatives are now actively being researched to overcome the limitations of liquid dielectrics. Here, we report direct growth of few-layer ($3-4L$) molybdenum disulfide (${\mathrm{MoS}}_2$), a prototypical 2D transition metal dichalcogenide (TMD), on lithium-ion solid electrolyte substrate by chemical vapor deposition (CVD), and demonstrate a transfer-free device fabrication method. The growth resulted in 5–10 μm sized triangular ${\mathrm{MoS}}_2$ single crystals as confirmed by Raman spectroscopy, x-ray photoelectron spectroscopy, and scanning electron microscopy. Field-effect transistors (FETs) fabricated on the as-grown few-layer crystals show near-ideal gating performance with room temperature subthreshold swings around 65 mV/decade while maintaining an ON/OFF ratio around ${10}^5$. Field-effect mobility in the range of $42–49\mathrm{c}{\mathrm{m}}^2{\mathrm{V}}^{–1}{\mathrm{s}}^{–1}$ and current densities as high as 120 μA/μm with 0.5 μm channel length has been achieved, back-gated by the solid electrolyte. This is the highest reported mobility among comparable FETs on as-grown single/few-layer CVD ${\mathrm{MoS}}_2$. This growth and transfer-free device fabrication method on solid electrolyte substrates can be applied to other 2D TMDs for studying advanced thin-film transistors and interesting physics, and is amenable to diverse surface science experiments, otherwise difficult to realize with liquid electrolytes.

The electronic structure of $3^{\circ }$-twisted bilayer graphene (TBG) is studied by angle-resolved photoelectron spectroscopy (ARPES). Sub-mm-sized TBG prepared by direct bonding in a high vacuum enabled us to use conventional ARPES band mapping with synchrotron light. The results indicate that strong interlayer coupling makes a moiré potential for the Dirac electrons and significantly modifies the graphene bands around the $\overline{K}$ points such as the band splitting and electron velocity reduction. The observed electronic structure is consistently reproduced by tight-binding calculations combined with a band unfolding method.

The electrode materials of Na-ion batteries must exhibit fast ion transport, a property that is often negatively impacted when mobile cations order. Many layered Na (and K) intercalation compounds adopt the P3 crystal structure, which tends to stabilize cation-vacancy orderings over a wide range of Na concentrations. The ordered phases have been predicted to consist of domains that are periodically separated by antiphase boundaries (APBs) to accommodate variations in Na concentration. The mechanisms and rates with which Na diffuses within such ordered phases of P3 are not understood. We have conducted a first-principles study of Na diffusion in the P3 structure, using ${\mathrm{Na}}_x{\mathrm{CoO}}_2$ as a model system. We find that most simple nearest-neighbor hops in Na-vacancy ordered phases in P3 result in unstable configurations and are therefore forbidden. We identify an alternative mechanism of Na transport that is mediated by APB migration via the formation and expansion of kinks along domain boundaries. The limiting kinetic barriers for this mechanism are quite low (0.03 to 0.30 eV). Vacancy defects are not required, and furthermore have formation energies that are higher than those of the APB kink defects. Our results suggest that APB migration is a fundamental diffusion mechanism in the P3 structure.

Defect engineering plays an important role in tailoring the electronic transport properties of van der Waals materials. However, it is usually achieved through tuning the type and concentration of defects, rather than dynamically reconfiguring their spatial distribution. Here, we report temperature-sensitive spatial redistribution of defects in $\mathrm{Pd}{\mathrm{Se}}_2$ thin flakes through scanning tunneling microscopy. We observe that the spatial distribution of Se vacancies in $\mathrm{Pd}{\mathrm{Se}}_2$ flakes exhibits a strong anisotropic characteristic at 80 K, and that this orientation-dependent feature is weakened when temperature is raised. Moreover, we carry out transport measurements on $\mathrm{Pd}{\mathrm{Se}}_2$ thin flakes and show that the anisotropic features of carrier mobility and phase coherent length are also sensitive to temperature. Combining with theoretical analysis, we conclude that temperature-driven defect spatial redistribution could interpret the temperature-sensitive electrical transport behaviors in $\mathrm{Pd}{\mathrm{Se}}_2$ thin flakes. Our work highlights that engineering spatial distribution of defects in the van der Waals materials, which has been overlooked before, may open up an avenue to tailor the physical properties of materials and explore different device functionalities.

We propose a design scheme for potential electrides derived from conventional materials. Starting with rare-earth-based ternary halides, we exclude halogen anions and perform global structure optimization to obtain thermodynamically stable or metastable phases but having an excess of electrons confined inside interstitial cavities. Then, spin-polarized interstitial states are induced by chemical substitution with magnetic lanthanides. To demonstrate the capability of our approach, we test with 11 ternary halides and successfully predict 30 stable and metastable phases of nonmagnetic electrides subject to three different stoichiometric categories, and successively 28 magnetic electrides via chemical substitution with Gd. 56 out of these 58 designed electrides are discovered for the first time. Two electride systems, the monoclinic $A\mathrm{C}$ ($A=$ La, Gd) and the orthorhombic $A_2\mathrm{Ge}$ ($A=$ Y, Gd), are thoroughly studied to exemplify the set of predicted crystals. Interestingly, both systems turn out to be topological nodal line electrides in the absence of spin-orbit coupling and manifest spin-polarized interstitial states in the case of $A=$ Gd. Our work establishes a novel computational approach of functional electrides design and highlights the magnetism and topological phases embedded in electrides.

By considering the valence-electron concentration of $3d$ transition-metal alloys and compounds, we develop $3d$ high-entropy alloy ${\mathrm{Mn}}_{12.1}{\mathrm{Fe}}_{34.2}{\mathrm{Co}}_{33.5}{\mathrm{Ni}}_{12.3}{\mathrm{Cu}}_{7.9}$ with 8.7 electrons per atom, which is identical to that of ${\mathrm{Fe}}_{65}{\mathrm{Ni}}_{35}$ Invar. We carry out x-ray diffraction, scanning electron microscopy, magnetization, thermal expansion, and elastic modulus measurements, by which we show that the HEA alloy indeed carries Invar properties. This is evidenced particularly by the observed spontaneous volume magnetostriction and the lattice softening covering a broad temperature-range around the ferromagnetic Curie temperature.

In computational material design, ionic radius is one of the most important physical parameters used to predict material properties. Motivated by the progress in computational materials science and material informatics, we extend the renowned Shannon's table from 475 ions to 987 ions. Accordingly, a rigorous machine learning (ML) approach is employed to extend the ionic radii table using all possible combinations of oxidation states (OS) and coordination numbers (CN) available in crystallographic repositories. An ionic-radius regression model for Shannon's database is developed as a function of the period number, the valence orbital configuration, OS, CN, and ionization potential. In the Gaussian process regression (GPR) model, the reached $R^2$ accuracy is 99% while the root mean square error of radii is 0.0332 Å. The optimized GPR model is then employed for predicting a new set of ionic radii for uncommon combinations of OS and CN extracted by harnessing crystal structures from materials project databases. The generated data are consolidated with the reputable Shannon's data and are made available online in a database repository.

Two-dimensional (2D) and quasi-2D ferroelectric materials have demonstrated their potential for enabling device miniaturization. The fundamental understanding of the ferroelectric domain structure at atomic scale is limited, however, hindering development of functional device units at the microscopic level. In this paper, we employ a Dion–Jacobson layered oxide $\mathrm{CsBi}{\mathrm{Nb}}_2{\mathrm{O}}_7$ (CBNO), which has been predicted to be ferroelectric with high Curie temperature and large in-plane polarization, as a model system to study its ferroelectric domain structure with atomic scale analysis. We reveal the existence of unit-cell-thick ferroelectric domain size as well as both 180° and 90° domain walls in free-standing CBNO. In an epitaxial CBNO film grown on a $\mathrm{LaAl}{\mathrm{O}}_3$ substrate, we discover multiple stacking structures resembling the Aurivillius and the Ruddlesden–Popper phases. We also analyze the interfacial dislocations and their associated lattice distortion fields in the epitaxial film. This paper may inspire design concepts for ferroelectric devices demanding unit-cell-thick domain size.