Acoustic topological insulators inspired by quantum Hall effects exhibit topologically protected edge states and offer great potential for manipulating sound waves. However, most two-dimensional acoustic topological insulator systems are severely hindered by fixed geometry and frequency responses, structure scale comparable with the wavelength, and a single topological phase-transition mechanism. Few studies have investigated edge states with dispersion tunable, reconfigurable lattice, and subwavelength-scale control over sound topological states. Here, a subwavelength-scale and tunable topological metamaterial based on local resonance is realized by constructing a honeycomb lattice with open-hole hollow tube. The topological metamaterial is radically distinct from sonic crystal topological insulators based on Bragg scattering. The corresponding band structure of the proposed honeycomb cell possesses double-degenerate Dirac cones. We introduce the symmetry-breaking mechanism to establish reconfigurable topological insulators by simply rotating the side hole of the metamolecules or shrinking and expanding the metamolecule spacing to mimic the pseudospin states with opposite Chern numbers and realize topological phase transition. Results provide a basis for exploring subwavelength-scale control sound propagation and analyzing topological phase-transition mechanism.

In this paper, we numerically study a Coulomb impurity problem for interacting Dirac fermions restricted in disordered graphene quantum dots. In the presence of randomly distributed lattice defects and spatial potential fluctuations, the response of the critical coupling constant for atomic collapse is mainly investigated by local density of states calculations within the extended mean-field Hubbard model. We find that both types of disorder cause an amplification of the critical threshold. As a result, up to a 34% increase in the critical coupling constant is reported. This numerical result may explain why the Coulomb impurities remain subcritical in experiments, even if they are supercritical in theory. Our results also point to the possibility that atomic collapse can be observed in defect-rich samples such as ${\mathrm{Ar}}^{+}$ ion bombarded, ${\mathrm{He}}^{+}$ ion irradiated, and hydrogenated graphene.

We investigate the effect of a non-Markovian, structured reservoir on an open Heisenberg spin chain. We establish a coherent self-feedback mechanism as the reservoir couples frequency dependently to the spin chain. Thus, loss and driving take place due to the interaction of the spin chain with its own past. This new paradigm of non-Markovian imposed boundary driving allows discussion of a new kind of nonequilibrium steady state. We show that for certain parameters, even in the long-time limit, persistent oscillations occur within the chain. Moreover, we demonstrate that the conditions for these oscillations and excitation trapping depend on the characteristics of the chain, thus making it possible to characterize a chain by detection of its emitted signal under influence of self-feedback.

We introduce a simple model for the quantum transport of Fermi particles between two contacts connected by a lead. It generalizes the Landauer formalism by explicitly taking into account the relaxation processes in the contacts. We calculate the contact resistance and nonequilibrium quasimomentum distribution of the carriers in the lead and show that they strongly depend on the rate of relaxation processes.

Inelastic neutron scattering on a single crystal of silicon was performed at temperatures from 100 to 1500 K. These experimental data were reduced to obtain phonon spectral intensity at all wave vectors $\vec{Q}$ and frequencies $\omega$ in the first Brillouin zone. Thermal broadenings of the phonon peaks were obtained by fitting and by calculating with an iterative ab initio method that uses thermal atom displacements on an ensemble of superlattices. Agreement between the calculated and experimental broadenings was good, with possible discrepancies at the highest temperatures. Distributions of phonon widths versus phonon energy had similar shapes for computation and experiment. These distributions grew with temperature but maintained similar shapes. Parameters from the ab initio calculations were used to obtain the thermal conductivity from the Boltzmann transport equation, which was in good agreement with experimental data. Despite the high group velocities of longitudinal acoustic phonons, their shorter lifetimes reduced their contribution to the thermal conductivity, which was dominated by transverse acoustic modes.

The demagnetizing field associated with shape anisotropy originates from the magnetic field generated through magnetization of magnetic materials. The shape and demagnetizing effects must be considered as characteristics of a magnetic object in the presence of a magnetic field. In this study, we demonstrated the strong influence of the demagnetizing field on the spin current transports in Pt/YIG, including the spin Seebeck effect, spin Hall effect, spin Hall magnetoresistance, and planar Hall resistance. The calculated effective demagnetizing factors were closely related to the thickness- and width-dependent anomalous plateau behaviors of the spin-dependent electrical and thermal transports as well as magneto-optical measurements. Moreover, we demonstrated that the width of the plateau manifested from the shape of the YIG sample. Furthermore, through magnetic force microscopy, we directly visualized the evolution of the magnetic domains with an abrupt 90° magnetic rotation, which we further corroborated by the 90° resistance phase shift in the angular-dependent planar Hall effect.

Studies of competing orders in one-dimensional magnetic chains have attracted considerable attention in recent years, as the presence of distant-neighbor Heisenberg interactions is found to allow interesting quantum phase transitions. We investigate here the role of spin-orbit effects by considering spin-1/2 chains in the presence of both collinear and noncollinear interactions. By employing exact diagonalization and density matrix renormalization group calculations we investigate the rich phase diagram of this system. We find transitions from collinear to transverse magnetic correlated order as the strength of the noncollinear coupling increases, accompanied by a jump in the vector spin chirality order parameter of the system. This shows that tuning interactions allows control of the onset of sizable vector spin chirality in a system, which may be used to transmit information down the chain. We investigate the characteristic structure of the distinct phases and explore their possible physical implementation in different materials systems.

In this paper I report on a numerical investigation of nonlinear spin dynamics in a magnetic thin film made of yttrium iron garnet (YIG). This film is exposed to a small in-plane oriented magnetic field and strong spin currents. The rich variety of findings encompasses dynamic regimes hosting localized, nonpropagating solitons, as well as a turbulent chaotic regime, which condenses into a quasistatic phase featuring a noncollinear spin texture. Eventually, at the highest spin current, a homogeneously switched state is established.

Square-root topological states are new topological phases, whose topological property is inherited from the square of the Hamiltonian. Because of the square-root procedure, the bulk gap of the squared Hamiltonian is doubled. We realize the first-order and second-order square-root topological insulators in phononic crystals by putting additional cavities on the connecting tubes in the acoustic Su-Schrieffer-Heeger model and the honeycomb lattice, respectively. In the doubled bulk gaps, multiple acoustic localized modes, i.e., the end and corner states, are evidently confirmed by our calculations and experimental observations. We further propose a second-order square-root topological semimetal by stacking the decorated honeycomb lattice to three dimensions.

We demonstrate that the recently discovered triple-q (3q) magnetic structure, when embedded in a magnet-superconductor hybrid (MSH) system, gives rise to the emergence of topological superconductivity. We investigate the structure of chiral Majorana edge modes at domain walls, and show that they can be distinguished from trivial in-gap modes through the spatial distribution of the induced supercurrents. Finally, we show that topological superconductivity in 3q MSH systems is a robust phenomenon that does not depend on the relative alignment of the magnetic and superconducting layers, or on the presence of electronic degrees of freedom in the magnetic layer.

The equation of state (EOS) and shock compression of bulk vanadium were investigated using canonical ab initio molecular dynamic simulations, with experimental validation to 865 GPa from shock data collected at Sandia's Z Pulsed Power Facility. In simulations the phase space was sampled along isotherms ranging from 3000 K to $50000$ K, for densities between $\rho =3$ and $15\mathrm{g}/{\mathrm{cm}}^3$, with a focus on the liquid regime and the body-centered-cubic phase in the vicinity of the melting limit. The principal Hugoniot predicted from first principles is overall consistent with shock data, while it showed that current multiphase SESAME-type EOS for vanadium needed revision in the liquid regime. A more accurate SESAME EOS was developed using constraints from experiments and simulations. This work emphasizes the need to use a combined theoretical and experimental approach to develop high-fidelity EOS models for extreme conditions.

The magnetic properties of $\kappa \text{−}{\left(\mathrm{BEDT}\text{−}\mathrm{TTF}\right)}_2\mathrm{Hg}{\left(\mathrm{SCN}\right)}_2\mathrm{Br}$ are examined using ${}^{13}\mathrm{C}$ NMR methods. Measurements of the spin lattice relaxation rate indicate a first-order metal-insulator transition at $T_{MI}=90\mathrm{K}$ and a pronounced maximum at $5\text{K}$, ascribed to dynamical slowing of magnetic fluctuations. $T_{MI}$ is also associated with the onset of weak line broadening, indicative of a disordered form of charge disproportionation. The line width increases strongly for $T<40\mathrm{K}$, exceeding 4000 ppm at 2 K and applied magnetic field $B=9.50\mathrm{T}$. The lack of long-range magnetic order is attributed to the disordered charge disproportionation onsetting at higher temperature.

The double perovskite oxide ${\mathrm{Sr}}_2\mathrm{CrOs}{\mathrm{O}}_6$ with a $3d^3\text{−}5d^3$ configuration exhibits very high-$T_C$ ferrimagnetism (∼725 K) at the end point of half-metallicity. Many substitution studies have been conducted theoretically and experimentally over the last two decades to shed more light on the open issue of how the $3d^3\text{−}5d^3$ configuration generates the high-$T_C$ ferrimagnetic state and to accelerate development toward applications. We have succeeded in synthesizing a solid solution of ${\mathrm{Sr}}_2{\mathrm{Cr}}_{1–x}{\mathrm{Ni}}_x\mathrm{Os}{\mathrm{O}}_6$ under high-pressure and high-temperature conditions. ${\mathrm{Sr}}_2{\mathrm{Cr}}_{0.5}{\mathrm{Ni}}_{0.5}\mathrm{Os}{\mathrm{O}}_6$ exhibits magnetization sixfold greater ($\sim 1.2{\mu }_{\mathrm{B}}$/formula unit at 5 K) than that of ${\mathrm{Sr}}_2\mathrm{CrOs}{\mathrm{O}}_6$. This enhancement is preserved even at room temperature. X-ray absorption spectroscopy revealed that the electronic configuration is ${\mathrm{Sr}}_2{\left({{\mathrm{Cr}}^{3+}}_{2/3}{{\mathrm{Cr}}^{6+}}_{1/3}\right)}_{0.5}{{\mathrm{Ni}}^{2+}}_{0.5}{\mathrm{Os}}^{5+}{\mathrm{O}}_6$, indicating that the valence state of Os does not change from the host state $\left[{\mathrm{Os}}^{5+}\left(5d^3\right)\right]$. Instead, nonmagnetic ${\mathrm{Cr}}^{6+}\left(3d^0\right)$ is partly generated among coexisting ${\mathrm{Cr}}^{3+}\left(3d^3\right)$. X-ray magnetic circular dichroism measurements showed that the Os ions are antiferromagnetically coupled to the Cr and ferromagnetically to the Ni. The replacement of antiferromagnetic Cr by ferromagnetic Ni explains the increase of the net magnetism in this ferrimagnetic system. We infer that the strong antiferromagnetic exchange interaction of the $3d^3\text{−}5d^3$ configuration associated with the ${\mathrm{Cr}}^{3+}\text{−}\mathrm{O}\text{−}{\mathrm{Os}}^{5+}$ bond still accounts for the robust high-$T_C$ ferrimagnetism of the Ni-substituted series. We deduce from the experiments that the ferromagnetic exchange interaction of the $3d^8\text{−}5d^3$ configuration of the ${\mathrm{Ni}}^{2+}\text{−}\mathrm{O}\text{−}{\mathrm{Os}}^{5+}$ is stronger than that of the $3d^8\text{−}3d^3$ configuration of the ${\mathrm{Ni}}^{2+}\text{−}\mathrm{O}\text{−}{\mathrm{Cr}}^{3+}$, suggesting that the larger 5d orbital of the Os allows for a stronger virtual hopping from the Ni than the smaller $3d$ orbital of the Cr. The present results can help to further develop practical materials and to resolve open issues concerning the relative strengths of the various exchange interactions.

Bipartite quantum and thermal entanglement is quantified within pure and mixed states of a mixed spin-($1/2,1$) Heisenberg dimer with the help of negativity. It is shown that the negativity, which may serve as a measure of the bipartite entanglement at zero as well as nonzero temperatures, strongly depends on intrinsic parameters—for instance, exchange and uniaxial single-ion anisotropy—in addition to extrinsic parameters such as temperature and magnetic field. It turns out that a rising magnetic field unexpectedly reinforces the bipartite entanglement due to the Zeeman splitting of energy levels, which lifts the twofold degeneracy of the quantum ferrimagnetic ground state. The maximal bipartite entanglement is thus reached within a quantum ferrimagnetic phase at sufficiently low but nonzero magnetic fields under the assumption that the gyromagnetic $g$ factors of the spin-1/2 and spin-1 magnetic ions are equal and the uniaxial single-ion anisotropy is half of the exchange constant. It is suggested that the heterodinuclear complex $\left[\mathrm{Ni}\left(\mathrm{dpt}\right)\left({\mathrm{H}}_2\mathrm{O}\right)\mathrm{Cu}\left(\mathrm{pba}\right)\right]·2{\mathrm{H}}_2\mathrm{O}$ [pba = 1,3-propylenebis(oxamato) and dpt = bis-(3-aminopropyl)amine], which affords an experimental realization of the mixed spin-($1/2,1$) Heisenberg dimer, remains strongly entangled up to relatively high temperatures (about 115 K) and magnetic fields (about 140 T) that are comparable with the relevant exchange constant.

A lithium-doped magnesium hydride ${\mathrm{Li}}_2{\mathrm{MgH}}_{16}$ was recently reported [Y. Sun et al., Phys. Rev. Lett. 123, 097001 (2019)] to exhibit the highest ever predicted superconducting transition temperature $T_{\mathrm{c}}$ under high pressure. Based on first-principles density-functional theory calculations, we reveal that the Li dopants locating in the pyroclore lattice sites give rise to the excess electrons distributed outside Li atoms. Such loosely bound anionic electrons are easily captured to stabilize a clathrate structure consisting of H cages. This addition of anionic electrons to H cages enhances the H-derived electronic density of states at the Fermi level, thereby leading to a high-$T_{\mathrm{c}}$ superconductivity. We thus propose that the electride nature of Li dopants is an essential ingredient in the charge transfer between Li dopants and H atoms. Our findings offer a deeper understanding of the underlying mechanism of charge transfer in ${\mathrm{Li}}_2{\mathrm{MgH}}_{16}$ at high pressure.

The origin of nematicity and its relation to superconductivity in iron pnictide high-temperature superconductors remains unclear. One of its possible sources is the uneven occupation of Fe $d_{xz}$ and $d_{yz}$ orbitals. Here, by using x-ray linear dichroism technique, we show that such an imbalance is present in $\mathrm{Eu}{\left({\mathrm{Fe}}_{1-x}{\mathrm{Co}}_x\right)}_2{\mathrm{As}}_2$ single crystals deep in the tetragonal phase and also in the superconducting state, where we find that the $d_{xz}$ orbital has a higher occupation as indicated by an x-ray linear dichroism magnitude of 1.5%. Our result shows the importance of orbital polarization in the theoretical description of nematicity and superconductivity, particularly for determination of the superconducting gap symmetry, which is affected by orbital fluctuations.

We show that the entire x-ray Thomson scattering (XRTS) spectrum, including both elastic and inelastic features, can be calculated from first principles in the framework of quantum perturbation theories. Our derivation shows that the elastic scattering feature in a warm dense regime is different from that of condensed matter at low temperature. In addition to the contribution from spatial fluctuations of electronic density, which dominates elastic scattering at low temperature, there is an extra contribution from partially occupied inner-shell states, which is important in the warm dense regime. Calculated XRTS of isochorically heated beryllium agrees well with previous experimental measurements, which may give this method a further edge on interpreting XRTS spectra compared to empirical modeling methods.

Quasiparticle self-consistent $GW$ (G Green's function, W screened Coulomb interaction) band structure calculations are presented for the $\alpha , \beta$, and $\gamma$ phases of ${\mathrm{Ge}}_3{\mathrm{N}}_4$. The $GW$ corrections to the gap are substantial and the gaps are found to be 3.85, 3.86, and 3.56 eV for $\alpha , \beta , \gamma$ respectively, at the experimental lattice constants. Only the $\beta$ phase has a direct gap. The $\alpha$ and $\beta$ phases, which are closely related to each other in crystal structure, are consequently also found to have rather similar band structures. Nonetheless, some significant differences are found near the valence band maximum, which are related to the different interlayer interactions in the $c$ direction. In the cubic spinel phase, the conduction band minima of $s$ and $p$ character are found close to each other. Effective masses are calculated for the relevant valence and conduction band edges and for the cubic case analyzed in terms of the Kohn-Luttinger effective Hamiltonian. The optical response functions from direct interband transitions are calculated.

The interaction between electronic and nuclear degrees of freedom in single-molecule junctions is an essential mechanism, which may result in the current-induced rupture of chemical bonds. As such, it is fundamental for the stability of molecular junctions and for the applicability of molecular electronic devices. In this publication, we study current-induced bond rupture in molecular junctions using a numerically exact scheme, which is based on the hierarchical quantum master equation (HQME) method in combination with a discrete variable representation for the nuclear degrees of freedom. Employing generic models for molecular junctions with dissociative nuclear potentials, we identify distinct mechanisms leading to dissociation, namely, the electronic population of antibonding electronic states and the current-induced heating of vibrational modes. Our results reveal that the latter plays a negligible role whenever the electronic population of antibonding states is energetically possible. Consequently, the significance of current-induced heating as a source for dissociation in molecular junctions involving an active antibonding state is restricted to the nonresonant transport regime, which reframes the predominant paradigm in the field of molecular electronics.

Normal-incidence 1-keV ${\mathrm{Ar}}^{+}$ ion bombardment leads to amorphization and ultrasmoothing of Ge at room temperature, but at elevated temperatures the Ge surface remains crystalline and is unstable to the formation of self-organized nanoscale patterns of ordered pyramid-shaped pits. The physical phenomenon distinguishing the high-temperature patterning from room-temperature ultrasmoothing is believed to be a surface instability due to the Ehrlich-Schwoebel barrier for diffusing vacancies and adatoms, which is not present on the amorphous material. This real-time grazing-incidence small-angle x-ray scattering study compares smoothing of a prepatterned Ge sample at room temperature with patterning of an initially flat Ge sample at an elevated temperature. In both experiments, when the nanoscale structures are relatively small in height, the average kinetics can be explained by a linear theory. The linear theory coefficients, indicating surface stability or instability, were extracted for both experiments. A comparison between the two measurements allows estimation of the contribution of the Ehrlich-Schwoebel barrier to the self-organized formation of ordered nanoscale patterns on crystalline Ge surfaces.