While most of the two-dimensional (2D) topological crystalline insulators (TCIs) belong to group IV-VI narrow-band-gap semiconductors in a square lattice, in the present work we predict a TCI family based on transition metal intercalated compounds in a hexagonal lattice. First-principles calculations combined with a substrate-fixed globally optimal structural search technique show that a layer of Os prefers a uniform distribution between two graphene sheets. Band dispersion calculations reveal a Dirac point and a Dirac nodal ring near the Fermi level. The Dirac point is ascribed to the hybridization of $e_2$ and $e_2{}^{*}$ orbitals, and the Dirac ring is formed due to dispersion of $s$ and $e_1{}^{*}$ orbitals. Upon inclusion of spin-orbit coupling, these Dirac states open topologically nontrivial local band gaps, which are characterized by nonzero mirror Chern numbers. The quantum spin Hall effect is also observed by integrating the spin Berry curvature in the Brillouin zone. In contrast to the 2D group IV-VI TCIs whose band inversions at $X$ and $Y$ points are “locked” by $C_4$ rotation symmetry, here the relative energy of two local band gaps can be manipulated by in-plane biaxial strains. Some other similar intercalation compounds are also shown to be topologically nontrivial. Our work extends the 2D TCI family into a hexagonal lattice composed of transition metals.

Atomic force microscopy (AFM) measurements have reported contrast inversions for systems such as ${\mathrm{Cu}}_2\mathrm{N}$ and graphene that can hamper image interpretation and characterization. Here, we apply a simulation method based on ab initio real-space pseudopotentials to gain an understanding of the tip-sample interactions that influence the inversion. We find that chemically reactive tips induce an attractive binding force that results in the contrast inversion. We find that the inversion is tip height dependent and not observed when using less reactive CO-functionalized tips.

Charge and energy fractionalization are among the most intriguing features of interacting one-dimensional fermion systems. In this work we determine how these phenomena are modified in the presence of an interaction quench. Charge and energy are injected into the system suddenly after the quench, by means of tunneling processes with a noninteracting one-dimensional probe. Here, we demonstrate that the system settles to a steady state in which the charge fractionalization ratio is unaffected by the prequenched parameters. On the contrary, due to the postquench nonequilibrium spectral function, the energy partitioning ratio is strongly modified, reaching values larger than 1. This is a peculiar feature of the nonequilibrium dynamics of the quench process and it is in sharp contrast with the nonquenched case, where the ratio is bounded by 1.

We study a system of dipolar bosons in a one-dimensional optical lattice using exact diagonalization and density matrix renormalization group methods. In particular, we analyze low energy properties of the system at an average filling of 3/2 atoms per lattice site. We identify the region of the parameter space where the system has non-Abelian Fibonacci anyon excitations that correspond to fractional domain walls between different charge-density waves. When such one-dimensional systems are combined into a two-dimensional network, braiding of Fibonacci anyon excitations has potential application for fault tolerant, universal, topological quantum computation. Contrary to previous calculations, our results also demonstrate that super-solid phases are not present in the phase diagram for the discussed 3/2 average filling. Instead, decreasing the value of the nearest-neighbor tunneling strength leads to a direct, Berezinskii-Kosterlitz-Thouless, superfluid to charge-density-wave quantum phase transition.

In the present work, a theoretical study of electron-phonon (electron-ion) coupling rates in semiconductors driven out of equilibrium is performed. Transient change of optical coefficients reflects the band gap shrinkage in covalently bonded materials, and thus, the heating of atomic lattice. Utilizing this dependence, we test various models of electron-ion coupling. The simulation technique is based on tight-binding molecular dynamics. Our simulations with the dedicated hybrid approach (XTANT) indicate that the widely used Fermi's golden rule can break down describing material excitation on femtosecond time scales. In contrast, dynamical coupling proposed in this work yields a reasonably good agreement of simulation results with available experimental data.

Magnetic topological states are investigated theoretically and numerically. It is shown that ferromagnetically interacting spins on a two-dimensional honeycomb lattice with nearest-neighbor interactions, which is governed by the Landau-Lifshitz-Gilbert equation, can be topologically nontrivial with gapped bulk spin waves and topologically protected gapless edge spin waves. These edge spin waves are robust against defects and perturbations, and should be superb channels of processing and manipulating spin waves, in contrast to the normal spin waves that are very sensitive to defects as well as sample geometry. Because of the unidirectional nature of these topologically protected edge spin waves, a spin-wave beam splitter can be made out of a domain wall in a strip. It is shown that an incoming spin-wave beam along one edge splits into two spin-wave beams propagating along two opposite directions on the other edge after passing through a domain wall.

Excited configurations of hydrogen in the oxyhydride ${\mathrm{BaTiO}}_{3-x}{\mathrm{H}}_x$ ($x=0.1–0.5$), which are considered to be involved in its hydrogen transport and exchange processes, were investigated by positive muon spin relaxation spectroscopy using muonium (Mu) as a pseudoisotope of hydrogen. Muons implanted into the ${\mathrm{BaTiO}}_{3-x}{\mathrm{H}}_x$ perovskite lattice were mainly found in two qualitatively different metastable states. One was assigned to a highly mobile interstitial protonic state, which is commonly observed in perovskite oxides. The other was found to form an entangled two spin-$\frac{1}{2}$ system with the nuclear spin of an ${\mathrm{H}}^{-}$ ion at the anion site. The structure of the (H,Mu) complex agrees well with that of a neutralized center containing two ${\mathrm{H}}^{-}$ ions at a doubly charged oxygen vacancy, which was predicted to form in the ${\mathrm{SrTiO}}_{3-\delta }$ perovskite lattice by a computational study [Y. Iwazaki et al., APL Mater. 2, 012103 (2014)]. Above 100 K, interstitial ${\mathrm{Mu}}^{+}$ diffusion and retrapping to a deep defect were observed, which could be a rate-limiting step of macroscopic Mu/H transport in the ${\mathrm{BaTiO}}_{3-x}{\mathrm{H}}_x$ lattice.

Structural and magnetic properties of rare-earth iron garnets (RIG), which contain 160 atoms per unit cell, are systematically investigated for rare-earth elements varying from La to Lu (and including Y), by performing spin polarized density-functional calculations. The effects of $4f$ electrons (as core or as valence electrons) on the lattice constant, internal coordinates, and bond lengths are found to be rather small, with these predicted structural properties agreeing rather well with available experiments. On the other hand, treating such electrons as valence electrons is essential to interpret the total magnetization measured in some RIG at low temperature, the different orientation and magnitude of the magnetizations that Fe and rare-earth ions can adopt and to also explain why some RIG have a compensation temperature while others do not. The magnetic exchange couplings and orbital-projected density of states are also reported for two representative materials, namely ${\mathrm{Gd}}_3{\mathrm{Fe}}_5{\mathrm{O}}_{12}$ and ${\mathrm{Nd}}_3{\mathrm{Fe}}_5{\mathrm{O}}_{12}$, when accounting for their $4f$ electrons.

Employing quasiclassical theory of superconductivity combined with Keldysh technique we investigate large thermoelectric effect in multiterminal ballistic normal-superconducting hybrid structures. We argue that this effect is caused by electron-hole asymmetry generated by coherent Andreev reflection of quasiparticles at interfaces of two different superconductors with nonzero phase difference. Within our model we derive a general expression for thermoelectric voltages $V_{T1,2}$ induced in two different normal terminals exposed to a thermal gradient. Our results apply at any temperature difference in the subgap regime and allow us to explicitly analyze both temperature and phase dependencies of $V_{T1,2}$, demonstrating that in general there exists no fundamental relation between these voltages and the equilibrium Josephson current in superconducting-normal-superconducting junction.

The real-time broadening of density profiles starting from nonequilibrium states is at the center of transport in condensed-matter systems and dynamics in ultracold atomic gases. Initial profiles close to equilibrium are expected to evolve according to the linear response, e.g., as given by the current correlator evaluated exactly at equilibrium. Significantly off equilibrium, the linear response is expected to break down and even a description in terms of canonical ensembles is questionable. We unveil that single pure states with density profiles of maximum amplitude yield a broadening in perfect agreement with the linear response, if the structure of these states involves randomness in terms of decoherent off-diagonal density-matrix elements. While these states allow for spin diffusion in the XXZ spin-$1/2$ chain at large exchange anisotropies, coherences yield entirely different behavior.

We determine, experimentally and numerically, the electric and magnetic Purcell factors in a fishnet metamaterial in the frequency range 5–15 GHz by measuring the impedance of a dipole antenna. We compare measurements and numerical simulations of the Purcell factor for transverse electric (${\mathrm{TE}}_{\mathrm{z}}$) and transverse magnetic (${\mathrm{TM}}_{\mathrm{z}}$) polarizations. For ${\mathrm{TM}}_{\mathrm{z}}$ polarization, the dispersion relation of the structure is hyperbolic and enhances the Purcell factor. For ${\mathrm{TE}}_{\mathrm{z}}$ polarization, the dispersion relation does not allow any propagating solutions and decreases the Purcell factor below the effective plasma frequency. Eigenmode calculations of the periodic unit cell of the metamaterial are used to obtain the band structure and confirm the presence of hyperbolic isofrequency surfaces. The isofrequency surfaces are used to calculate the density of states (DOS). We also use the impedance method to obtain the DOS by averaging the Purcell factor obtained at different locations over the periodic unit cell and find good agreement with DOS calculated from eigenmode calculations.

The microscopic structure of the high-index $\mathrm{Si}\left(331\right)-(12\times 1)$ surface is investigated combining scanning tunneling microscopy with ab initio calculations. We present a structural model of the Si(331) surface, employing a reconstruction element composed of six pentagons integrated to the structure of the adjacent pentamer with an interstitial atom. We demonstrate that appropriately arranged additional pentagons significantly lower the surface energy of the high-index surface. The model predicts the existence of multiple Si(331) buckled configurations with similar energies.

The fate of a local two-hole doublon excitation in the one-dimensional Fermi-Hubbard model is systematically studied for strong Hubbard interaction $U$ in the entire filling range using the density-matrix renormalization group (DMRG) and the Bethe ansatz. For strong $U$, two holes at the same site form a compound object whose decay is impeded by the lack of phase space. Still, a partial decay is possible on an extremely short time scale where phase-space arguments do not yet apply. We argue that the initial decay and the resulting intermediate state are relevant for experiments performed with ultracold atoms loaded into an optical lattice as well as for (time-resolved) CVV Auger-electron spectroscopy. The detailed discussion comprises the mixed ballistic-diffusive real-time propagation of the doublon through the lattice, its partial decay on the short time scale as a function of filling and interaction strength, as well as the analysis of the decay products, which are metastable on the intermediate time scale that is numerically accessible and which show up in the two-hole excitation (Auger) spectrum. The ambivalent role of singly occupied sites is key to understanding the doublon physics; for high fillings, ground-state configurations with single occupancies are recognized to strongly relax the kinematic constraints and to open up decay channels. For fillings close to half-filling, however, their presence actually blocks the doublon decay. Finally, the analysis of the continua in the two-hole spectrum excludes a picture where the doublon decays into unbound electron holes for generic fillings, different from the limiting case of the completely filled band. We demonstrate that the decay products as well as the doublon propagation should rather be understood in terms of Bethe ansatz eigenstates.

A systematic study of polycrystalline $1T-{\mathrm{TiSe}}_{2-\delta }$ with controlled Se loss indicates that the unconventional charge density wave (CDW) phase is found to be most pronounced in samples with $\delta \sim 0.12$, instead of being Se vacancy free. The level of Se vacancy defects and temperature determines whether $1T-{\mathrm{TiSe}}_{2-\delta }$ should be categorized as a semiconductor, a semimetal, or an excitonic insulator. An interpretation using a general band picture of $p$-type doped narrow-band-gap semiconductor with an impurity band (IB) in proximity to the valence band (VB) is proposed to explain the evolution of electronic structures for $1T-{\mathrm{TiSe}}_{2-\delta }$, from the intermediate doping of $\delta \sim 0.08$, to the critical doping of $\delta \sim 0.12$ showing an anomalous resistivity peak between $\sim 100–200\mathrm{K}$, and to the heavily doped of $\delta \sim 0.17$ as an $n$-type degenerate semiconductor. Integrated chemical analysis and physical property characterization, including electron probe microanalysis (EPMA), synchrotron x-ray diffraction, resistivity, and Seebeck coefficient measurement results are provided for the polycrystalline samples prepared via vacuum-sealed high temperature annealing route.

We theoretically investigate the effects of long-range disorder and electron-electron interactions on the optical properties of hexagonal armchair graphene quantum dots consisting of up to 10 806 atoms. The numerical calculations are performed using a combination of tight-binding, mean-field Hubbard, and configuration interaction methods. Imperfections in the graphene quantum dots are modeled as a long-range random potential landscape, giving rise to electron-hole puddles. We show that, when the electron-hole puddles are present, the tight-binding method gives a poor description of the low-energy absorption spectra compared to mean-field and configuration interaction calculation results. As the size of the graphene quantum dot is increased, the universal optical conductivity limit can be observed in the absorption spectrum. When disorder is present, the calculated absorption spectrum approaches the experimental results for isolated monolayers of graphene sheets.

Various out-of-equilibrium processes produce a supersaturation of vacancies in the material. When these defects show attractive binding energy with solutes, they will form stable point defect-solute clusters which will stabilize solutes in the solid solution with respect to the equilibrium case. Hence the out-of-equilibrium dynamic solubility limit increases, which can lead to the dissociation of solute precipitates. This vacancy-induced precipitate (VID) dissolution mechanism is an alternative to the well-known ballistic mixing effect (BAL) under irradiation, and it is also relevant for quenching, ball milling, and severe plastic deformation. Under irradiation, a BAL is efficient at low temperature only, whereas a VID is expected to be effective at intermediate temperatures. A quantitative and consistent prediction of the interstitial solute solubility limit increase generated by both BAL and VID mechanisms is presented starting from ab initio binding energies and migration energies of solutes and point defects, and using a low-temperature expansion of the free energy of the solid solution. These results are discussed for three alloys: FeC, FeN, and FeO, the latter being relevant to discuss the stability of oxide dispersed strengthened alloys microstructure under irradiation. We also suggest an experiment that would be able to determine steady-state vacancy supersaturations from the measure of solute partial pressures in out-of-equilibrium systems.

Recent application of neural networks (NNs) to modeling interatomic interactions has shown the learning machines' encouragingly accurate performance for select elemental and multicomponent systems. In this study we explore the possibility of building a library of NN-based models by introducing a hierarchical NN training. In such a stratified procedure NNs for multicomponent systems are obtained by sequential training from the bottom up: first unaries, then binaries, and so on. Advantages of constructing NN sets with shared parameters include acceleration of the training process and intact description of the constituent systems. We use an automated generation of diverse structure sets for NN training on density functional theory-level reference energies. In the test case of Cu, Pd, Ag, Cu-Pd, Cu-Ag, Pd-Ag, and Cu-Pd-Ag systems, NNs trained in the traditional and stratified fashions are found to have essentially identical accuracy for defect energies, phonon dispersions, formation energies, etc. The models' robustness is further illustrated via unconstrained evolutionary structure searches in which the NN is used for the local optimization of crystal unit cells.

We investigate the magnetic structure and its temperature dependence of Fe thin films grown on MgO(001) and ${\mathrm{Al}}_2{\mathrm{O}}_3$(0001) surfaces by means of nuclear resonant x-ray scattering. By fabricating Fe films $\delta \text{−}\mathrm{doped}$ with ${}^{57}\mathrm{Fe}$ at the interface and middle of the film, depth-resolved analysis of the magnetic structure is performed. On MgO(001), the magnetization is dominantly out-of-plane at the interface, whereas it is mainly in-plane at the middle of the film, indicating that the Fe film has a noncollinear magnetic structure. On ${\mathrm{Al}}_2{\mathrm{O}}_3$(0001), on the other hand, the magnetization is mainly in-plane in the entire film. The noncollinear magnetic structure on MgO(001) is confirmed to be energetically stable with the aid of first-principles calculations. We also investigate the temperature dependence of the internal magnetic field in a depth-resolved way. The experimental data suggest the internal magnetic field at the interface is smaller than that of the middle of the film on both substrates, suggesting that the magnetic phase transition starts at the interface at a lower temperature than the entire film.

In confined helimagnetic nanostructures, skyrmionic states in the form of incomplete and isolated skyrmion states can emerge as the ground state in absence of both external magnetic field and magnetocrystalline anisotropy. In this work, we study the dynamic properties (resonance frequencies and corresponding eigenmodes) of skyrmionic states in thin film FeGe disk samples. We employ two different methods in finite-element based micromagnetic simulation: eigenvalue and ringdown method. The eigenvalue method allows us to identify all resonance frequencies and corresponding eigenmodes that can exist in the simulated system. However, using a particular experimentally feasible excitation can excite only a limited set of eigenmodes. Because of that, we perform ringdown simulations that resemble the experimental setup using both in-plane and out-of-plane excitations. In addition, we report the nonlinear dependence of resonance frequencies on the external magnetic bias field and disk sample diameter and discuss the possible reversal mode of skyrmionic states. We compare the power spectral densities of incomplete skyrmion and isolated skyrmion states and observe several key differences that can contribute to the experimental identification of the state present in the sample. We measure the FeGe Gilbert damping, and using its value we determine what eigenmodes can be expected to be observed in experiments. Finally, we show that neglecting the demagnetization energy contribution or ignoring the magnetization variation in the out-of-film direction—although not changing the eigenmode's magnetization dynamics significantly—changes their resonance frequencies substantially. Apart from contributing to the understanding of skyrmionic states physics, this systematic work can be used as a guide for the experimental identification of skyrmionic states in confined helimagnetic nanostructures.