We focus on the many-body eigenstates across a localization-delocalization phase transition. To characterize the robustness of the eigenstates, we introduce eigenstate overlaps $\mathcal{O}$ with respect to different boundary conditions. In the ergodic phase, the average of eigenstate overlaps $\overline{\mathcal{O}}$ exponentially decays with an increase in the system size, indicating the fragility of its eigenstates,and this can be considered as an eigenstate version of the butterfly effect of chaotic systems. For localized systems, $\overline{\mathcal{O}}$ is almost size independent, showing the strong robustness of the eigenstates and the inconsistency of the eigenstate thermalization hypothesis. In addition, we find that the response of eigenstates to the change in the boundary conditions in many-body localized systems is identified with the single-particle wave functions in Anderson localized systems. This indicates that the eigenstates of many-body localized systems, as many-body wave functions, may be independent of each other. We demonstrate that this is consistent with the existence of a large number of quasilocal integrals of motion in the many-body localized phase. Our results provide another method to study localized and delocalized systems from the perspective of eigenstates.

To estimate the Curie temperature of metallic magnets from first principles, we develop a local force method for the tight-binding model having spin-dependent hopping derived from spin-density-functional theory. While spin-dependent hopping is crucial for the self-consistent mapping to the effective spin model, the numerical cost to treat such nonlocal terms in the conventional Green's function scheme is formidably expensive. Here, we propose a formalism based on the kernel polynomial method (KPM), which makes the calculation dramatically efficient. We perform a benchmark calculation for bcc-Fe, fcc-Co, and fcc-Ni and find that the effect of the magnetic nonlocal terms is particularly prominent for bcc-Fe. We also present several local approximations to the magnetic nonlocal terms for which we can apply the Green's function method and reduce the numerical cost further by exploiting the intermediate representation of the Green's function. By comparing the results of the KPM and local methods, we discuss which local method works most successfully. Our approach provides an efficient way to estimate the Curie temperature of metallic magnets with a complex spin configuration.

Spin waves are promising candidates to carry, transport, and process information. Controlling the propagation characteristics of spin waves in magnetic materials is an essential ingredient for designing spin-wave-based computing architectures. Here, we study the influence of surface inhomogeneities on the spin-wave signals transmitted through thin films. We use micromagnetic simulations to study the spin-wave dynamics in an in-plane magnetized yttrium iron garnet thin film with thickness in the nanometer range in the presence of surface defects in the form of locally introduced uniaxial anisotropies. These defects are used to demonstrate that the backward volume magnetostatic spin waves (BVMSW) are more responsive to backscattering in comparison to magnetostatic surface spin waves (MSSWs). For this particular defect type, the reason for this behavior can be quantitatively related to the difference in the magnon band structures for the two types of spin waves. To demonstrate this, we develop a quasianalytical theory for the scattering process. It shows an excellent agreement with the micromagnetic simulations, sheds light on the backscattering processes, and provides a new way to analyze the spin-wave transmission rates in the presence of surface inhomogeneities in sufficiently thin films, for which the role of exchange energy in the spin wave dynamics is significant. Our study paves the way to designing magnonic logic devices for data processing which rely on a designed control of spin-wave transmission.

By applying a microscopic gauge-invariant kinetic theory in $d$-wave superconductors, we analytically derive the energy spectra of the breathing Higgs mode and, in particular, rotating Higgs mode that is unique for the $d$-wave order parameter. An analytical investigation on their dynamic properties is also revealed. We show that the breathing Higgs mode is optically visible in the second-order regime. Whereas the rotating Higgs mode is optically inactive, we show that this mode can be detected in the pseudogap phase by a magnetic resonance experiment. It is interesting to find that with a longitudinal temperature gradient, the charge-neutral rotating Higgs mode generates a thermal Hall current by a magnetic field in the pseudogap phase, providing a unique scheme for its detection. This finding is likely to capture a very recent experimental observation of negative thermal Hall signal in cuprate superconductors [G. Grissonnanche et al., Nature (London) 571, 376 (2019)] for the overdoped pseudogap phase.

The effect of dampinglike spin-orbit torque (DL SOT) on magnetic domain walls (DWs) in in-plane magnetized soft tracks is studied analytically and with micromagnetic simulations. We find that DL SOT drives vortex DWs, whereas transverse DWs, the other typical DW structure in soft tracks, propagate only if the Dzyaloshinskii-Moriya interaction (DMI) is present. The SOT drive can add to, and be more efficient than, spin-transfer torque (STT), and so may greatly benefit applications that require in-plane DWs. Our analysis based on the Thiele equation shows that the driving force arises from a cycloidal distortion of the DW structure caused by geometrical confinement or DMI. This distortion is higher, and the SOT more efficient, in narrower, thinner tracks. These results show that the effects of SOT cannot be understood by simply considering the effective field at the center of the structure, an ill-founded but often-used estimation. We also show that the relative magnitude of STT and DL SOT can be determined by comparing the motion of different vortex DW structures in the same track.

Here we introduce a Landau-Lifshitz-based diagonalization (LLD) method, and use this approach to calculate the effects of the interfacial Dzyaloshinskii-Moriya interactions (DMI) on the radial-type spin wave modes of magnetic vortices in circular disks. The LLD method is a semianalytical approach that involves the diagonalization of the magnetostatic kernel, exchange, and DMI contributions to extract the system eigenfrequencies and eigenmodes. The magnetic vortex state provides a convenient model system in which to investigate the effects of the DMI on the dynamics of magnetic structures with confined geometries. Our calculations show that the DMI leads to shifts of the mode frequencies that are similar in magnitude to what is observed for spin waves of a comparable wavelength in extended films. However, unlike what is found in thin films, only the down-shifted modes are observed in the disks, and these correspond to modes that propagate either radially outward or inward, depending on the vortex circulation. The semianalytical calculations agree well with full micromagnetic simulations. This technique also applies to other systems with cylindrical symmetry, for example, magnetic skyrmions.

We investigate wave packet spreading after a single spin flip in prototypical two-dimensional ferromagnetic and antiferromagnetic quantum spin systems. We find characteristic spatial magnon density profiles: While the ferromagnet shows a square-shaped pattern reflecting the underlying lattice structure, as exhibited by quantum walkers, the antiferromagnet shows a circular-shaped pattern which hides the lattice structure and instead resembles a classical wave pattern. We trace these fundamentally different behaviors back to the distinctly different magnon energy-momentum dispersion relations and also provide a real-space interpretation. Our findings point to opportunities for real-time, real-space imaging of quantum magnets both in materials science and in quantum simulators.

The magnetic and electronic phase diagram of a model for the quasi-one-dimensional alkali-metal iron selenide compound ${\mathrm{Na}}_2{\mathrm{FeSe}}_2$ is presented. The novelty of this material is that the valence of iron is ${\mathrm{Fe}}^{2+}$, contrary to most other iron-chain compounds with valence ${\mathrm{Fe}}^{3+}$. Using first-principles techniques, we developed a three-orbital tight-binding model that reproduces the ab initio band structure near the Fermi level. Including Hubbard and Hund couplings and studying the model via the density-matrix renormalization group and Lanczos methods, we constructed the ground-state phase diagram. A robust region where the block state $\uparrow \uparrow \downarrow \downarrow \uparrow \uparrow \downarrow \downarrow$ is stabilized was unveiled. The analog state in iron ladders, employing $2\times 2$ ferromagnetic blocks, is by now well established, but in chains a block magnetic order has not been observed yet in real materials. The phase diagram also contains a large region of canonical staggered spin order $\uparrow \downarrow \uparrow \downarrow \uparrow \downarrow \uparrow$ at very large Hubbard repulsion. At the block-to-staggered transition region, an exotic phase is stabilized with a mixture of both states: an inhomogeneous orbital-selective charge density wave with the exotic spin configuration $\uparrow \uparrow \downarrow \uparrow \downarrow \downarrow \uparrow \downarrow$. Our predictions for ${\mathrm{Na}}_2{\mathrm{FeSe}}_2$ may guide crystal growers and neutron-scattering experimentalists towards the realization of block states in one-dimensional iron selenide chain materials.

It has been shown recently that the coexistence of Van Hove singularity and geometric phase in ${\mathrm{Pb}}_3\mathrm{Bi}\text{/}\mathrm{Ge}\left(111\right)$ as a prototypical Rashba system can result in chiral topological superconductivity [W. Qin, L. Li, and Z. Y. Zhang, Nat. Phys. 15, 796 (2019)]. Here, we use first-principles calculations to systematically investigate the structural, electronic, and topological properties of the ${\mathrm{Pb}}_3\mathrm{Bi}\text{/}\mathrm{Ge}\left(111\right)$ alloyed systems by considering not only the configuration that harbors topological superconductivity considered previously (labeled as the ${\mathrm{T}}_1$ phase) but also two other energetically nearly degenerate configurations with high symmetries (labeled as ${\mathrm{H}}_3$ and ${\mathrm{T}}_4$). We first show that all the three structural configurations have giant Rashba-type spin splittings, resulting from the strong spin-orbit coupling effect of the heavy elements (Pb, Bi) together with inversion symmetry breaking. Next, we demonstrate that each of the saddle- or paraboliclike band dispersions can lead to the emergence of Van Hove singularities, whose positions can be substantially tuned by the giant Rashba effect to the vicinity of the Fermi level. Furthermore, we reveal the existence of topological states in both the ${\mathrm{H}}_3$ and ${\mathrm{T}}_4$ phases, and the corresponding Rashba-induced band gaps are given by 25 and 50 meV, respectively. Given the energetic affinity of the three phases, these findings suggest the feasibility of realizing quantum phase transitions between quantum spin Hall effect and topological superconductivity within the same materials platform. The present study is expected to give new insights in searching for topological quantum states including 2D topological superconductivity based on Rashba systems.

The combination of magnetotransport and topological properties has brought great attention to rare-earth monopnictides semimetals. For some of them, like LaSb, it is unclear whether they show nontrivial topology or not based on density functional theory calculations and angular resolved photoemission spectroscopy measurements. Here we use hybrid density functional theory to demonstrate that LaSb is in fact a trivial topological semimetal, in agreement with experiments, but on the verge of a transition to a topological phase. We show that under compressive epitaxial strain, the La $d$ band crosses the Sb $p$ band near the $Z$ point in the Brillouin zone, stabilizing a topologically nontrivial phase, opening unique opportunities to probe the inter-relation between magnetotransport properties and the effects of band topology by examining epitaxially strained and unstrained thin films of the same material.

Photonic crystal slabs (PCSs) are a well-studied class of devices known to support optical Fano resonances for light normally incident to the slab, useful for narrow-band filters, modulators, and nonlinear photonic devices. In shallow-etched PCSs the linewidth of the resonances is easily controlled by tuning the etching depth. This design strength comes at the cost of large device footprints due to the poor in-plane localization of optical energy. In fully-etched PCSs realized in high-index-contrast material systems, the in-plane localization is greatly improved, but the command over linewidth suffers. This disadvantage in fully-etched PCSs, also known as high contrast gratings (HCGs), can be overcome by accessing symmetry-protected bound states in the continuum (BICs). By perturbing an HCG, the BIC may be excited from the free space with quality factor showing an inverse squared dependence on the magnitude of the perturbation, while inheriting the excellent in-plane localization of their unperturbed counterparts. Here, we report an exhaustive catalog of the selection rules (if and to which free space polarization coupling occurs) of symmetry-protected BICs controlled by in-plane symmetry breaking in six types of two-dimensional PCS lattices. The chosen lattices allow access to the three highest symmetry mode classes of unperturbed square and hexagonal PCSs. The restriction to in-plane symmetry breaking allows for manufacturing devices with simple lithographic fabrication techniques in comparison to out-of-plane symmetry breaking, useful for practical applications. The approach reported provides a high-level road map for designing PCSs supporting controllable sharp spectral features with minimal device footprints using a mature fabrication platform. To demonstrate the use of the resulting alphabet of structures, we numerically demonstrate nonlocal metasurface platforms for terahertz generation, mechanically tunable optical lifetimes, and wavefront shaping exclusively at resonance.

We study quantum-charge and spin-transport properties and the effects of in-plane strain on the charge and spin currents in a phosphorene monolayer using an adiabatic pumping regime. To achieve this goal, we proposed a device with three external ac gate voltages as oscillating potential barriers that are responsible for the generation of dc pumped current. Using exchange magnetic field induced by proximity effect of a ferromagnetic insulator, we determine the conditions in which fully spin-polarized current and pure spin current (with zero charge current) can be obtained. It is shown that the pumped current in the three-barrier case is about two times greater than the pumped current in a two-barrier system. The effect of strain is investigated and it is found that the spin current increases up to two orders of magnitude by applying the uniaxial strain which shows that the proposed device has high sensitivity to strain and could be used as straintronic devices such as strain switches and strain sensors. Also, in the same conditions, the pumped current in phosphorene is two, four, and five orders of magnitude greater than the pumped current in ${\mathrm{MoS}}_2$, silicene, and graphene, respectively. These properties show that phosphorene can be considered as a two- dimensional (2D) semiconductor with great potential for the fabrication of novel spintronic and straintronic devices that can overcome some of the limitations exhibited by conventional 2D materials.

We analyze theoretically four-terminal electronic devices composed of two crossed graphene nanoribbons (GNRs) and show that they can function as beam splitters or mirrors. These features are identified for electrons in the low-energy region where a single valence or conduction band is present. Our modeling is based on $p_z$ orbital tight binding with Slater-Koster-type matrix elements fitted to accurately reproduce the low-energy bands from density functional theory calculations. We analyze systematically all devices that can be constructed with either zigzag or armchair GNRs in AA and AB stackings. From Green's function theory the elastic electron transport properties are quantified as a function of the ribbon width. We find that devices composed of relatively narrow zigzag GNRs and AA-stacked armchair GNRs are the most interesting candidates to realize electron beam splitters with a close to 50:50 ratio in the two outgoing terminals. Structures with wider ribbons instead provide electron mirrors, where the electron wave is mostly transferred into the outgoing terminal of the other ribbon, or electron filters where the scattering depends sensitively on the wavelength of the propagating electron. We also test the robustness of these transport properties against variations in the intersection angle, stacking pattern, lattice deformation (uniaxial strain), inter-GNR separation, and electrostatic potential differences between the layers. These generic features show that GNRs are interesting basic components to construct electronic quantum optical setups.

Quantum coherence quantifies the amount of superposition a quantum state can have in a given basis. Since there is a difference in the structure of eigenstates of the ergodic and many-body localized systems, we expect them also to differ in terms of their coherences in a given basis. Here, we numerically calculate different measures of quantum coherence in the excited eigenstates of an interacting disordered Hamiltonian as a function of the disorder. We show that quantum coherence can be used as an order parameter to detect the well-studied ergodic to many-body localized phase transition. We also perform quantum quench studies to distinguish the behavior of coherence in thermalized and localized phases. We then present a protocol to calculate measurement-based localizable coherence to investigate the thermal and many-body localized phases. The protocol allows one to investigate quantum correlations experimentally in a nondestructive way, in contrast to measures that require tracing out a subsystem, which always destroys coherence and correlation.

The switching polarization of a ferroelectric is determined by the current that flows as the system is switched between two variants. Computation of the switching polarization in crystal systems has been enabled by the modern theory of polarization, where it is expressed in terms of a change in Berry phase from the initial state to the final state. It is straightforward to compute this change of phase modulo $2\pi$, thus requiring a branch choice to specify the predicted switching polarization. The measured switching polarization depends on the actual path along which the system is switched, which in general involves nucleation and growth of domains and is therefore quite complex. In this work we present a first-principles approach for predicting the switching polarization that requires only knowledge of the initial and final states based on the empirical observation that for most ferroelectrics, the observed polarization change is the same as that for a path involving minimal evolution of the state. To compute the change along a generic minimal path, we decompose the change of Berry phase into many small contributions, each much less than $2\pi$, allowing for a natural resolution of the branch choice. We show that for typical ferroelectrics, including those that would have otherwise required a densely sampled path, this technique allows the switching polarization to be computed without any need for intermediate sampling between oppositely polarized states.

We present a detailed study of correlation-induced electronic reconstruction in baddeleyite-type ${\mathrm{NbO}}_2$, a distorted ${\mathrm{ZrO}}_2$-type structure that is found at pressures above 8.0 GPa. Based on density-functional plus dynamical mean-field theory (DFT$+$DMFT), we stress the importance of multiorbital Coulomb interactions in concert with first-principles band-structure calculations for a consistent understanding of emergent Mottness and pseudogap behavior in pressurized ${\mathrm{NbO}}_2$ and related $d^1$ systems. After a proper treatment of multiorbital electron-electron interactions, we find a nearly universal Mott behavior for the peak position of the lower Hubbard band that is independent of crystal- and band-structure details. We explain the nature of the metal-pseudogap-insulator transition to be seen in experiment, showing a first-order transition between two metallic states: a correlated metal and a pseudogap state with a deep density of states at the Fermi level. This emergent pseudogap phenomena is expected to play a central role toward quantum criticality in pressurized ${\mathrm{NbO}}_2$.

Effects of the spin-orbit coupling (SOC) and magnetic field on excitonic insulating (EI) states are investigated. We introduce the two-orbital Hubbard model with the crystalline field splitting, which is a minimal model for discussing the exciton condensation in strongly correlated electron systems, and analyze its effective Hamiltonian in the strong correlation limit by using the mean-field theory. In the absence of the SOC and magnetic field, the ground state changes from the nonmagnetic band-insulating state to the EI state by increasing the Hund coupling. In an applied magnetic field, the magnetic moment appears in the EI state, which is continuously connected to the forced ferromagnetic state. On the other hand, in the presence of the SOC, they are separated by a phase boundary. We find that the magnetic susceptibility is strongly enhanced in the EI phase near the boundary with a small SOC. This peculiar behavior is attributed to the low-energy fluctuation of the spin nematicity inherent in the high-spin local state stabilized by the Hund coupling. The present study not only reveals the impact of the SOC for the EI state but also sheds light on the role of quantum fluctuations of the spin nematicity for the EI state.

We analyze theoretically the phase diagram of a triangular triple quantum dot with strong on-site repulsion coupled to ferromagnetic leads. This model includes the competition of magnetic ordering of local or itinerant magnetic moments, geometric frustration, and Kondo screening. We identify all the phases resulting from this competition. We find that three Kondo phases—the conventional one, the two-stage underscreened one, and the one resulting from the ferromagnetic Kondo effect—can be realized at zero temperature, and all are very susceptible to the proximity of ferromagnetic leads. In particular, we find that the quantum dots are spin polarized in each of these phases. Further, we discuss the fate of the phases at nonzero temperatures, where a plethora of competing energy scales gives rise to a complex landscape of crossovers. Each Kondo regime splits into a pair of phases, one not magnetized and one comprising magnetically polarized quantum dots. We discuss our results in the context of heavy-fermion physics in frustrated Kondo lattices.

We have performed a detailed investigation into the intersubband scattering within InAs/GaSb coupled quantum wells in the electron dominated regime. By considering the carrier mobilities and the quantum lifetime as a function of carrier density, we find that the occupation of higher order electronlike subbands are inhibited by anticrossing with the hole subbands. We also find that, by applying a gate bias to the GaSb layer, we are able to move the electron-hole anticrossing point in energy, modulating the electronlike states that should be localized within the InAs layer.

${\mathrm{UH}}_7$ and ${\mathrm{UH}}_8$ have been synthesized in a pure form by direct reaction of uranium with excess hydrogen in a laser-heating diamond anvil cell up to $96\mathrm{GPa}$. The sequence ${\mathrm{UH}}_3$, ${\mathrm{UH}}_7$, and ${\mathrm{UH}}_8$ is observed under increasing pressure using synchrotron x-ray diffraction. ${\mathrm{UH}}_7$ is stable above $35\mathrm{GPa}$ and ${\mathrm{UH}}_8$ above $87\mathrm{GPa}$. The crystal structures of these two superhydrides correspond to the ab initio predicted structures with their cagelike sublattice of atomic hydrogen. Their volume expansion per H atom falls on the hypothetical atomic metal hydrogen compression curve. A critical survey of the literature data for other recently observed superhydrides enables one to generalize such a trend in the partial volume occupied by the hydrogen atom. Hence, superhydrides made only of atomic hydrogen can be considered additive-volume alloys of atomic metal hydrogen and the metal. With its lowest equilibrium pressure, ${\mathrm{UH}}_7$ is the superhydride for which the concept of chemical precompression to stabilize an atomic metal hydrogen analog works the most effectively so far.