We present a promising machine-learning model, which focuses on site magnetic properties for rapid screening in materials design. We perform high-throughput first-principles calculations to predict the magnetic anisotropy energies of a variety of iron-cobalt nitrides. We illustrate the efficacy of a spatial decomposition technique that divides the total magnetic anisotropy energy into contributions from individual sites in terms of spin-orbit coupling energies. The spatial decomposition scheme that we utilized works for a wide range of magnetic anisotropy energies. We also construct a machine-learning model by combining the site-specific spin-orbit coupling energies with structural details on each atomic site. We adopt the same approach to predicting the site-specific magnetic moments. We demonstrate the capability of our machine-learning model to accelerate computational screening of candidate materials that possess high magnetizations and large magnetic anisotropy energies.

Developing simple ways to control spin states in spintronic devices is a crucial step towards increasing their functionality. MnTe is a room-temperature antiferromagnet with promising spintronic properties, including for thermospintronics and magnon-based devices. Here, we show that, in MnTe, less than 1% Li is sufficient to produce a dramatic spin reorientation as observed by neutron diffraction. The behavior of the 0001 magnetic Bragg peak reveals a significant reorientation of the Mn spins from planar in the pure material to almost completely axial with minimal Li doping. Temperature dependence of the magnetic peaks in Li-doped samples indicates that axial spins shift back to planar suddenly upon approaching the Néel temperature ($T_N=307$ K). Density functional theory calculations support the idea that a shift in the Fermi level caused by doping is responsible for switching the material between two competing magnetic ground states. These results pave the way for developing easy switching of magnetic states in functional materials such as spintronic devices and topological insulators.

In the quest for materials for ferroelectrics-based spintronics with a large spin-orbit coupling, it is essential to carefully control the ferroelectric domains structure, their spatial organization, and the domain wall type. Here we perform the growth of GeTe thin films on Si by molecular beam epitaxy in a large thickness range. We show that the volume fraction along with the size of the ferroelectric nanodomains can be controlled by finely adjusting the deposition thickness and temperature. We evidence that the formation of ${71}^{\circ }$-type domain walls and in situ measurements during thermal cycling show the hysteretic appearance and decay of ferroelectric domains. In combination with a detailed analysis of the GeTe/Si interface, we demonstrate that the interfacial misfit dislocations formed during the growth play a key role in the stability of the ferroelectric nanodomains.

We identify all symmetry-enforced band crossings in nonmagnetic orthorhombic crystals with and without spin-orbit coupling and discuss their topological properties. We find that orthorhombic crystals can host a large number of different band degeneracies, including movable Weyl and Dirac points with hourglass dispersions, fourfold double Weyl points, Weyl and Dirac nodal lines, almost movable nodal lines, nodal chains, and topological nodal planes. Interestingly, spin-orbit coupled materials in space groups 18, 36, 44, 45, and 46 can have band pairs with only two Weyl points in the entire Brillouin zone. This results in simpler connectivity of the Fermi arcs and more pronounced topological responses than in materials with four or more Weyl points. In addition, we show that the symmetries of space groups 56, 61, and 62 enforce nontrivial weak ${\mathbb{Z}}_2$ topology in materials with strong spin-orbit coupling, leading to helical surface states. With these classification results in hand, we perform extensive database searches for orthorhombic materials crystallizing in the relevant space groups. We find that ${\mathrm{Sr}}_2{\mathrm{Bi}}_3$ and ${\mathrm{Ir}}_2\mathrm{Si}$ have bands crossing the Fermi energy with a symmetry-enforced nontrivial ${\mathbb{Z}}_2$ invariant, CuIrB possesses nodal chains near the Fermi energy, ${\mathrm{Pd}}_7{\mathrm{Se}}_4$ and ${\mathrm{Ag}}_2\mathrm{Se}$ exhibit fourfold double Weyl points, the latter one even in the absence of spin-orbit coupling, whereas the fourfold degeneracies in AuTlSb are made up from intersecting nodal lines. For each of these examples, we compute the ab initio band structures, discuss their topologies, and for some cases also calculate the surface states.

We study the flow behavior of a twist-bend nematic (${\mathrm{N}}_{\mathrm{TB}}$) liquid crystal. It shows three dynamic regimes in certain temperature and shear-rate range. In region I, $\sigma \sim \sqrt{\dot{\gamma }}$, in region II, the stress shows a near plateau, characterized by a power law $\sigma \sim {\dot{\gamma }}^{\alpha }$, where $\alpha \sim 0.1–0.4$ and in region III, $\sigma \sim \dot{\gamma }$. With increasing shear rate, $\sigma$ changes continuously from region I to II, whereas it changes discontinuously with a hysteresis from region II to III. In the near plateau (region II), we observe dynamic stress fluctuations, exhibiting regular, periodic, and quasiperiodic oscillations under the application of a steady shear. The observed spatiotemporal dynamics are close to those predicted theoretically in sheared nematogenic fluids.

We report high-resolution angular magnetoresistance (AMR) experiments performed on crystals of ${\mathrm{ReO}}_3$ with high mobility $(>100000{\mathrm{cm}}^2/\mathrm{V}$ s at 2 K) and extremely low residual resistivity $(5–8 \mathrm{n}\mathrm{\Omega }\mathrm{cm})$. The Fermi surface, comprised of intersecting cylinders, supports open orbits. The resistivity ${\rho }_{xx}$ in a magnetic field $B=9$ T displays a singular pattern of behavior. With $\mathbf{E}\parallel \widehat{\mathbf{x}}$ and $\mathbf{B}$ initially $\parallel \widehat{\mathbf{z}}$, tilting $\mathbf{B}$ in the longitudinal $k_z\text{−}{k}_x$ plane leads to a steep decrease in ${\rho }_{xx}$ by a factor of 40. However, if $\mathbf{B}$ is tilted in the transverse $k_y\text{−}{k}_z$ plane, ${\rho }_{xx}$ increases steeply by a factor of 8. Using the Shockley-Chambers tube integral approach, we show that, in ${\mathrm{ReO}}_3$, the singular behavior results from the rapid conversion of closed to open orbits, resulting in opposite signs for AMR in orthogonal planes. The floor values of ${\rho }_{xx}$ in both AMR scans are identified with specific sets of open and closed orbits. Also, the “completion angle” ${\gamma }_c$ detected in the AMR is shown to be an intrinsic geometric feature that provides a new way to measure the Fermi radius $k_F$. However, additional sharp resonant features that appear at very small tilt angles in the longitudinal AMR scans are not explained by the tube integral approach.

Databases of density functional theory (DFT) calculations, such as the Open Quantum Materials Database (OQMD), have paved the way for accelerated materials discovery. DFT calculations require crystal structure information as input; however, due to inherent challenges in solving a compound's structure from powder diffraction data alone, there are thousands of experimentally synthesized compounds whose structures remain unsolved. We present a rapid DFT-based structure solution method capable of resolving numerous outstanding structure solution problems at low computational cost. The method involves (1) searching inorganic compound databases for all prototypes that match known structural characteristics of the compound, such as stoichiometry, space group, and number of atoms per cell, (2) performing DFT calculations of the target composition in each of the structural prototypes, and (3) evaluating these prototypes as candidates using a combination of DFT energy and match between calculated and experimental diffraction pattern. As this approach is straightforward and inexpensive, we employ it to solve 521 previously unsolved compounds from the Powder Diffraction File, resulting in a 1.4% expansion of the set of all experimental compounds in the OQMD. DFT calculations of these compounds could yield valuable properties.

Based on first-principles density functional calculations, we report the existence of both nontrivial band topology and superconductivity in three metallic Si allotropes, termed $Cmcm\text{−}{\mathrm{Si}}_4$, $Cmmm\text{−}{\mathrm{Si}}_4$, and $I4/mmm\text{−}{\mathrm{Si}}_4$. $Cmcm\text{−}{\mathrm{Si}}_4$ and $I4/mmm\text{−}{\mathrm{Si}}_4$ are predicted in this study, whereas $Cmmm\text{−}{\mathrm{Si}}_4$ was proposed in previous theoretical calculations. These metastable allotropes retain their crystalline structure at ambient pressure and exhibit superconductivity at the critical temperatures of 1.2–11.4 K. We investigate the topological characteristics of the electronic states and find the weak topological nature for all three allotropes. For $Cmcm\text{−}{\mathrm{Si}}_4$, in particular, the formation of the surface Dirac point is clearly identified. Our result provides a promising platform for realizing a topological superconducting state in all-Si systems at ambient pressure.

Understanding the interaction between polar and superconducting order parameters may hold the key to several classes of superconductors that remain poorly understood, including $\mathrm{SrTi}{\mathrm{O}}_3$ and several tellurides. Here we show that doped, strained $\mathrm{SrTi}{\mathrm{O}}_3$ films can exhibit both global or local polar order, respectively, depending on the amount of epitaxial mismatch strain, thereby providing a platform to understand how inversion symmetry breaking affects superconductivity. We find that the superconducting critical temperature correlates with the length scale of polar order. In particular, the transition temperature is enhanced when polar nanodomains are sufficiently large or, in the extreme limit, films are globally ferroelectric. In these cases, the Cooper pairs reside in a noncentrosymmetric environment. Conversely, low transition temperatures are found when the nanodomains are small. The findings point to the length scale of polar nanodomains and spin-orbit coupling as important parameters controlling the superconductivity of $\mathrm{SrTi}{\mathrm{O}}_3$. The ability to control the size of the polar domains opens up new opportunities to design and control the nature of superconductivity in a wide range of materials.

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.