The discovery of polar metals, bulk band conductors which combine broken inversion symmetry and metallic conductivity, has disrupted the long-standing assumption that polar order and metallicity are incompatible. Despite recent progress, however, the circumstances which allow for this property convergence remain ambiguous. Here, we perform a first-principles analysis of the evolution in the polar distortions in perovskite ${\mathrm{BaTiO}}_3$ under electrostatic doping to ascertain the dependencies of acentricity and electrical conductivity at the microscopic level. We focus on the role of local off-centering displacements, driven by the second-order Jahn-Teller effect, rather than discussing bulk homogeneous and cooperative lifting of inversion symmetry, which relies on long-range Coulombic interactions. We show that $n$-type doping disrupts the Ti-O covalent bond and advocate for using chemical bonding arguments as a framework for understanding the interplay between local distortions and free charge carriers rather than solely principles of dielectric screening in metals. In the process, we develop several tools and electronic descriptors that we expect could enable a more direct comparison between the behavior of doped ferroelectrics, such as ${\mathrm{BaTiO}}_3$, and polar metals, such as ${\mathrm{LiOsO}}_3$, helping us to better understand their similarities and differences under external perturbation.

Elastic waveguides with time- and space-dependent material properties have received great attention as a means to realize nonreciprocal propagation of small-amplitude mechanical waves in unbounded elastic media. Previous works have shown that propagating waves in a modulated medium violate reciprocity by means of asymmetric frequency and wave number conversion between two counterpropagating modes. In the present study, we investigate nonreciprocal longitudinal and transverse vibrations in a finite elastic waveguide with time- and space-dependent material properties. A semianalytical approach based on coupled mode theory is derived, which makes use of the mode shapes of the nonmodulated beam subject to a set of imposed boundary conditions. The modulation parameter space is explored for designs that yield a large degree of nonreciprocity for low-frequency longitudinal and transverse vibrations. For the cases considered in this work, only a small subset of modulation parameters displays strong nonreciprocity, which reveals a sparse and complex design space that must be analyzed in order to create nonreciprocal wave devices.

We investigated ${\mathrm{Sr}}_3{\mathrm{Ru}}_2{\mathrm{O}}_7$, a quantum critical metal that shows a metamagnetic quantum phase transition and electronic nematicity, through density functional calculations. These predict a ferromagnetic ground state in contrast to the experimentally observed paramagnetism, raising the question of competing magnetic states and associated fluctuations that may suppress magnetic order. We did a search to identify such low-energy antiferromagnetically ordered metastable states. We find that the lowest-energy antiferromagnetic state has a striped order. This corresponds to the $E$-type order that has been shown to be induced by Mn alloying. We also note significant transport anisotropy in this $E$-type ordered state. These results are discussed in relation to experimental observations.

The anisotropy and the temperature dependence of the spin-wave stiffness constants of the permanent magnet ${\mathrm{Nd}}_2{\mathrm{Fe}}_{14}\mathrm{B}$ are determined in the range 160–440 K using single-crystal inelastic neutron scattering technique. It is found that the deviation from isotropic spin-wave dispersion, between the $a$ and the $c$ axis of the tetragonal structure, is small and eventually the anisotropy decreases when the temperature increases. The softening of the stiffness constants can be described by a linear temperature variation with an average rate of about $-0.46$ meV ${\AA }^2/\mathrm{K}$. These results are compared with the most recent atomistic and multiscale calculations.

We develop a low-temperature, long-wavelength theory for the interfacial spin Seebeck effect (SSE) in easy-axis antiferromagnets. The field-induced spin-flop (SF) transition of Néel order is associated with a qualitative change in SSE behavior: Below SF, there are two spin carriers with opposite magnetic moments, with the carriers polarized along the field forming a majority magnon band. Above SF, the low-energy, ferromagneticlike mode has magnetic moment opposite the field. This results in a sign change of the SSE across SF, which agrees with recent measurements on ${\mathrm{Cr}}_2{\mathrm{O}}_3/\mathrm{Pt}$ and ${\mathrm{Cr}}_2{\mathrm{O}}_3/\mathrm{Ta}$ devices [Li et al., Nature (London) 578, 70 (2020)]. In our theory, SSE is due to a Néel spin current below SF and a magnetic spin current above SF. Using the ratio of the associated Néel to magnetic spin-mixing conductances as a single constant fitting parameter, we reproduce the field dependence of the experimental data and partially the temperature dependence of the relative SSE jump across SF.

We report cooperative magnetic orderings in a $6H$-perovskite multiferroic system, ${\mathrm{Ba}}_3\mathrm{Ho}{\mathrm{Ru}}_2{\mathrm{O}}_9$, via comprehensive neutron powder diffraction measurements. This system undergoes long-range antiferromagnetic ordering at $T_{N1}\sim 50\mathrm{K}$ with a propagation wave vector of $K_1=\left(0.500\right)$, a transition temperature much higher than the previously reported one at ∼10 K ($T_{N2}$). Both Ru and Ho moments order simultaneously below $T_{N1}$, followed by spin reorientations at lower temperatures, demonstrating strong $\mathrm{Ru}\left(4d\right)-\mathrm{Ho}\left(4f\right)$ magnetic correlation. Below $T_{N1}$ another magnetic phase with a propagation wave vector $K_2=\left(0.250.250\right)$ emerges and coexists with the one associated with $K_1$, which is rarely observed and suggests complex magnetism due to phase competition in the magnetic ground state. We argue that the exchange striction arising from the up-up-down-down spin structure associated with a $K_2$-wave vector below $T_{N2}$ may be responsible for the small ferroelectric polarization reported previously in this compound.

We study the transient photoluminescence (PL) of photoexcited metals by solving the Boltzmann equation considering the effects of electron-electron and electron-phonon (e-ph) collisions where the e-ph coupling function is calculated from first principles in order to account for the energy transfer rate between electrons and phonons accurately. We apply the present scheme to the transient PL of silver and demonstrate that the agreement between the theory and the experiment is good where the effect of nonequilibrium electron distribution is significant to fit the experimental data.

In this paper, we probe the possibility of high-temperature conventional superconductivity in the boron-carbon system, using ab initio screening. A database of 320 metastable structures with fixed composition ($50\%/50\%$) is generated with the minima hopping method, and characterized with electronic and vibrational descriptors. Full electron-phonon calculations on 16 representative structures allow us to identify general trends in $T_c$ across and within the four families in the energy landscape, and to construct an approximate $T_c$ predictor, based on transparently interpretable and easily computable electronic and vibrational descriptors. Based on these, we estimate that around $10\%$ of all metallic structures should exhibit $T_c$'s above 30 K. This paper is a first step toward ab initio design of new high-$T_c$ superconductors.

We present a new method for compressing matrix product operators (MPOs) which represent sums of local terms, such as Hamiltonians. Just as with area law states, such local operators may be fully specified with a small amount of information per site. Standard matrix product state (MPS) tools are ill-suited to this case, due to extensive Schmidt values that coexist with intensive ones, and Jordan blocks in the transfer matrix. We ameliorate these issues by introducing an “almost Schmidt decomposition” that respects locality. Our method is “$\varepsilon$-close” to the accuracy of MPS-based methods for finite MPOs, and extends seamlessly to the thermodynamic limit, where MPS techniques are inapplicable. In the framework of control theory, our method generalizes Kung's algorithm for model order reduction. Several examples are provided, including an all-MPO version of the operator recursion method (Lanczos algorithm) directly in the thermodynamic limit. All results are accompanied by practical algorithms, well-suited for the large MPOs that arise in DMRG for long-range or quasi-2D models.

For noninteracting Weyl semimetals (WSMs), Fermi-arc surface states can arise when there exist gapless Weyl nodes in the bulk single-particle spectrum. However, in the presence of electronic correlations, it is not clear whether this bulk-boundary correspondence still holds or not. Recently, novel correlated phases are predicted to appear in WSMs with long-ranged interactions, in which the bulk Weyl nodes can be gapped but without destroying their topological properties. Here, we explore the fate of the Fermi-arc states under the influence of such long-ranged interactions. After mapping the system onto a one-dimensional interacting Su-Schrieffer-Heeger (SSH) model with two open ends, we employ numerical exact diagonalizations to address the issue whether the Fermi-arc states will be modified. By extrapolating our data to the thermodynamic limit, we find that the zero-energy edge states of the corresponding SSH model still exist for those momenta giving the noninteracting Fermi-arc states. Moreover, this observation applies to both the single-particle and the collective edge excitations. Since the locus of these edge states constitutes the Fermi arcs, the robustness of the Fermi-arc states against long-ranged interactions is thus demonstrated. In particular, the Fermi arcs of single-particle nature can survive even when the single-particle gaps at the Weyl nodes are opened by interactions. Our results illustrate the subtlety in identifying the topological phases of interacting WSMs and show the limitation of the approaches simply by examining the nodal structure of the single-particle spectrum.

Quasinormal mode (QNM) expansion is a popular tool to analyze light-matter interaction in nanoresonators. However, expanding far-field quantities such as the energy flux is an open problem because QNMs diverge with an increasing distance to the resonant systems. We introduce a theory to compute modal expansions of far-field quantities rigorously. The presented approach is based on the complex eigenfrequencies of QNMs. The divergence problem is circumvented by using contour integration with an analytical continuation of the far-field quantity into the complex frequency plane. We demonstrate the approach by computing the angular resolved modal energy flux in the far field of a nanophotonic device.

In the present article the radiative thermal current and radiative resistance are introduced and investigated in a system of parallel slabs. The system is placed in an environment with a constant temperature and subjected to a constant temperature gradient, which causes a radiative energy flux through the system. We have calculated the steady-state temperature profile of the system, assuming that the material and thickness of the middle slab could be different from the other slabs. We propose the exact formulation for calculating the thermal current and thermal resistances in both linear and nonlinear regimes. According to our results, the middle slab acts as a thermal barrier, and depending on the width of this barrier, an extreme thermal isolation is achievable. Simulation results indicate that the thermal resistance of the barrier is an increasing function of the thickness for near-field separation distances but is virtually insensitive to the barrier width in the far-field regime. The long-range character of the radiative heat transfer which occurs in systems with identical slabs is also discussed.

Here we provide a general methodology to directly measure the topological currents emerging in the optical lattice implementation of the Haldane model. Alongside the edge currents supported by gapless edge states, transverse currents can emerge in the bulk of the system whenever the local potential is varied in space, even if it does not cause a phase transition. In optical lattice implementations the overall harmonic potential that traps the atoms provides the boundaries of the topological phase that supports the edge currents, as well as providing the potential gradient across the topological phase that gives rise to the bulk current. Both the edge and bulk currents are resilient to several experimental parameters such as trapping potential, temperature, and disorder. We propose to investigate the properties of these currents directly from time-of-flight images with both short-time and long-time expansions.

Electron-phonon coupling leads to intriguing effects in the spectra of materials. Current approximations to calculate spectra most often describe this coupling insufficiently. Starting from basic equations of many-body perturbation theory, we derived a cumulant formulation for neutral excitation spectra that contains excitonic effects and the coupling between excitons and phonons. The cumulant approach allows us to include dynamical effects arising from the electron-phonon coupling in a simple and intuitive way. It can be implemented as a postprocessing of state-of-the-art $GW$-plus-Bethe-Salpeter calculation of excitonic states and a density functional perturbation theory calculation of phonons and electron-phonon coupling. We demonstrate that, in order to obtain a consistent treatment of exciton-phonon coupling, diagrams have to be taken into account that can be neglected when the effect of lattice vibrations is treated in a static or quasistatic approximation. From the application of this approach to a model system, we analyzed the main features of the exciton-phonon interaction and provided a general picture of their link with the properties of materials such as exciton mass and exciton Bohr radius.

We consider two mutually interacting fermionic particle species on a one-dimensional lattice and study how the mass ratio $\eta$ between the two species affects the (equilibration) dynamics of the particles. Focusing on the regime of strong interactions and high temperatures, two well-studied points of reference are given by (i) the case of equal masses $\eta =1$, i.e., the standard Fermi-Hubbard chain, where initial nonequilibrium density distributions are known to decay, and (ii) the case of one particle species being infinitely heavy, $\eta =0$, leading to a localization of the lighter particles in an effective disorder potential. Given these two opposing cases, the dynamics in the case of intermediate mass ratios $0<\eta <1$ is of particular interest. To this end, we study the real-time dynamics of pure states featuring a sharp initial nonequilibrium density profile. Relying on the concept of dynamical quantum typicality, the resulting nonequilibrium dynamics can be related to equilibrium correlation functions. Summarizing our main results, we observe that diffusive transport occurs for moderate values of the mass imbalance and manifests itself in a Gaussian spreading of real-space density profiles and an exponential decay of density modes in momentum space. For stronger imbalances, we provide evidence that transport becomes anomalous on intermediate timescales, and in particular, our results are consistent with the absence of strict localization in the long-time limit for any $\eta >0$. Based on our numerical analysis, we provide an estimate for the “lifetime” of the effective localization as a function of $\eta$.

The unified mathematical theory of gapped and gapless edges of two-dimensional (2d) topological orders was developed by two of the authors. According to this theory, the critical point of a purely edge topological phase transition of a 2d topological order can be mathematically characterized by an enriched fusion category. In this work, we provide a physical proof of this fact in a concrete example: the 2d ${\mathbb{Z}}_2$ topological order. In particular, we construct an enriched fusion category, which describes a gappable nonchiral gapless edge of the 2d ${\mathbb{Z}}_2$ topological order. Then, we use an explicit lattice model construction to realize a topological phase transition between the two well-known gapped edges of the 2d ${\mathbb{Z}}_2$ topological order, and show that all the ingredients of the above enriched fusion category can be realized explicitly in this lattice model.

Electron transport across a tunneling barrier is governed by intricate and diverse causes such as interface conditions, material properties, and device geometries. Here, by measuring the shot noise, we investigate electron transport in a (Ga,Mn)As/GaAs-based spin Esaki diode junction over a wide range of bias voltage. The asymmetric electronic band profile across the junction allows us to tune the types of tunneling process. By changing the bias voltage in a single device, we successively address the conventional direct tunneling, the excess current conduction through the mid-gap localized states, and the thermal excitation current conduction. These observations lead to a proper comparison of the bias dependent Fano factors. While the Fano factor is unity for the direct tunneling, it is pronouncedly reduced in the excess current region. Thus, we have succeeded in evaluating several types of conduction process with the Fano factor in a single junction.

We introduce the phenomenon of spiraling vortices in driven-dissipative (nonequilibrium) exciton-polariton condensates excited by a nonresonant pump beam. At suitable low pump intensities, these vortices are shown to spiral along circular trajectories whose diameter is inversely proportional to the effective mass of the polaritons, while the rotation period is mass independent. Both the diameter and rotation period are inversely proportional to the pump intensity. Stable spiraling patterns in the form of complexes of multiple mutually interacting vortices are also found. At elevated pump intensities, which create a stronger homogeneous background, we observe more complex vortex trajectories resembling Spirograph patterns.

We provide evidence that, alongside topologically protected edge states, two-dimensional Chern insulators also support localized bulk states deep in their valence and conduction bands. These states manifest when local potential gradients are applied to the bulk, while all parts of the system remain adiabatically connected to the same phase. In turn, the bulk states produce bulk current transverse to the potential difference. This occurs even when the potential is always below the energy gap, where one expects only edge currents to appear. Bulk currents are topologically protected and behave as edge currents under an external influence, such as temperature or local disorder. Detecting topologically resilient bulk currents offers a direct means to probe the localized bulk states.