We investigate the nonequilibrium dynamics of the spinless Haldane model with nearest-neighbor interactions on the honeycomb lattice by employing an unbiased numerical method. In this system, a first-order transition from the Chern insulator at weak coupling to the charge-density-wave (CDW) phase at strong coupling can be characterized by a level crossing of the lowest energy levels. Here we show that if adiabatically following the eigenstates across this level crossing, their Chern numbers are preserved, leading to the identification of a topologically nontrivial low-energy excited state in the CDW regime. By promoting a resonant energy excitation via an ultrafast circularly polarized pump pulse, we find that the system acquires a nonvanishing Hall response as a result of the large overlap enhancement between the time-dependent wave function and the topologically nontrivial excited state. This response is suggestive of a photoinduced topological phase transition via unitary dynamics, despite a proper definition of the Chern number remaining elusive for an out-of-equilibrium interacting system. We contrast these results with more common quench protocols in which such features are largely absent in the dynamics even if the postquench Hamiltonian displays a topologically nontrivial ground state.

We study LiFeP, LiFeAs, and NaFeAs in their paramagnetic metallic phase including dynamical electronic correlations within a density functional theory $+$ slave-spin mean-field framework. The three compounds are found to lie next to the crossover between a normal and a Hund's metal, where a region of enhanced electronic compressibility that may boost superconductivity is systematically present in this type of systems. We find that LiFeP lies in the normal metallic regime, LiFeAs at the crossover, and NaFeAs is in the Hund's metal regime, which possibly explains the different experimental trends for the pressure and doping dependence of superconductivity in these compounds. Our picture captures the orbitally resolved mass renormalizations measured in these materials, while an analysis of the Sommerfeld specific-heat coefficient highlights some limitations of currently used implementations of density-functional theory for the correct prediction of the details of band structures in the iron-based superconductors.

The continuous multiscale entanglement renormalization ansatz (cMERA) [J. Haegeman et al., Phys. Rev. Lett. 110, 100402 (2013)] gives a variational wave functional for ground states of quantum field-theoretic Hamiltonians. A cMERA is defined as the result of applying to a reference unentangled state a unitary evolution generated by a quasilocal operator, the entangler. This makes the extension of the formalism to the case where boundaries and defects are present nontrivial. Here we show how this generalization works, using the ($1+1$)-dimensional free boson cMERA as a proof-of-principle example, and restricting ourselves to conformal boundaries and defects. In our prescription, the presence of a boundary or defect induces a modification of the entangler localized only to its vicinity, in analogy with the so-called principle of minimal updates for the lattice tensor network MERA.

Topological soliton is a nonperturbative excitation in commensurate density wave states and connects degenerate ground states. In incommensurate density wave states, ground states are continuously degenerate and topological soliton is reckoned to be smoothly connected to the perturbative phason excitation. We study the ultrafast nonequilibrium dynamics due to a photoexcited electron-hole pair in a one-dimensional chain with an incommensurate charge density wave ground state. Time-resolved evolution reveals both perturbative excitation of collective modes and nonperturbative topological phase transition due to creating novel topological solitons, where the continuous complex order parameter with amplitude and phase is essential. We identify the nontrivial phase-winding solitons in the complex plane unique to this nonequilibrium state and capture it by a low-energy effective model. The perturbative temporal gap oscillation and the solitonic in-gap states enter the optical conductivity absorption edge and the spectral density related to spectroscopic measurement, providing concrete connections to real experiments.

We use a fully quantum mechanical approach to demonstrate control of plasmonic excitations in prototype models of topological insulators by molecule-scale perturbations. Strongly localized surface plasmons are present in the host systems, arising from the topologically nontrivial single-particle edge states. A numerical evaluation of the random phase approximation equations for the perturbed systems reveals how the positions and the internal electronic structure of the added molecules affect the degeneracy of the locally confined collective excitations, i.e., shifting the plasmonic energies of the host system and changing their spatial charge density profile. In particular, we identify conditions under which significant charge transfer from the host system to the added molecules occurs. Furthermore, the induced field energy density in the perturbed topological systems due to external electric fields is determined.

The interfacial effect in nanosystems is crucial for diverse fields of physics. Here we explore in detail the interface associated Dresselhaus spin-orbit (SO) coupling, which comprises the usual linear ${\beta }_{\nu ,\mathrm{u}}$ (${\mathrm{\Gamma }}_{\mathrm{u}}$) and cubic ${\beta }_{\nu ,3}$ (${\mathrm{\Gamma }}_3$) contributions as well as the interface-induced term ${\beta }_{\nu ,\mathrm{int}}$ (${\mathrm{\Gamma }}_{\mathrm{int}}$) of intrasubband (intersubband) kinds. Focusing on ordinary ${\mathrm{Al}}_{0.48}{\mathrm{In}}_{0.52}\mathrm{As}/{\mathrm{Ga}}_{0.47}{\mathrm{In}}_{0.53}\mathrm{As}$ heterostructures with either one or two occupied subbands, we perform a self-consistent Poisson-Schrödinger calculation to determine all the relevant SO contributions. We observe that the interface SO term becomes intensifying for heterostructures with either a weaker bulk Dresselhaus strength of the barrier layer or a lower interface smoothness. Remarkably, it is found that the renormalized linear Dresselhaus coefficient, which accounts for the interfacial contribution, may change sign as the interface smoothness varies, opening up the feasibility of the interface-engineered topological matter of persistent skyrmion lattice [J. Y. Fu, P. H. Penteado, M. O. Hachiya, D. Loss, and J. C. Egues, Phys. Rev. Lett. 117, 226401 (2016)]. Moreover, we also determine the intersubband Dresselhaus contributions including an emergent quadratic term (${\mathrm{\Gamma }}_2$) depending on the interfacial effect and the parity of wave functions. As opposed to intersubband terms of the usual linear (${\mathrm{\Gamma }}_{\mathrm{u}}$) and cubic (${\mathrm{\Gamma }}_3$) kinds, the quadratic contribution leads to unusual avoided crossings of the band dispersion and thus may hybridize spin textures of distinct spin branches. Our results should stimulate experiments probing interface-mediated intra- and intersubband Dresselhaus SO effects and provide an extra leverage for extracting reliable bulk Dresselhaus SO parameters.

The electric structure of twisted bilayer GeSe, which shows a rectangular moiré pattern, is analyzed using a $\mathit{k}·\mathit{p}$ type effective continuum model. The effective model is constructed on the basis of the the local approximation method, where the local lattice structure of a twisted bilayer system is approximated by its untwisted bilayer with parallel displacement, and the required parameters are fixed with the help of the first-principles method. By inspecting the twist angle dependence of the physical properties, we reveal a relation between the effective potential under moiré pattern and the alignment of the Ge atoms, and also the resultant one-dimensional flat band, where the band is flattened stronger in a specific direction than the perpendicular direction. Due to the relatively large effective mass of the original monolayers, the flat band with its band width as small as a few meV appear in a relatively large angle. This gives us an opportunity to explore the dimensional crossover in the twisted bilayer platform.

We propose a Corbino-disk geometry of a graphene membrane under out-of-plane strain deformations as a convenient path to detect pseudomagnetic and electric fields via electronic transport. The threefold symmetric pseudomagnetic field changes sign six times as a function of angle, leading to snake states connecting the inner and outer contacts and to nearly quantized transport. For dynamical strain obtained upon AC gating, the system supports an AC pseudoelectric field which, in the presence of the pseudomagnetic field, produces a net electronic charge current in the absence of an external voltage via a pseudo-Hall effect.

With the emergence of the van der Waals two-dimensional (2D) ferromagnetic materials, ${\mathrm{Fe}}_3{\mathrm{GeTe}}_2$ (FGT) with a high Curie temperature ($T_{\mathrm{C}}$) and the perpendicular magnetic anisotropy (PMA) provides a rich platform for the next generation spintronic devices. Up to now, in most 2D magnets-based devices, the easiest and the most convenient way to obtain thin flakes is still the mechanical exfoliation method. However, samples with thickness variation occur inevitably during this process, which are either ignored due to the thickness-inhomogeneity-induced complicated phenomena or far from been fully reached. Herein, unlike conventional symmetric magnetoresistance with respect to the magnetic field observed in thickness-uniform FGT flakes, we demonstrate a hitherto rarely observed antisymmetric magnetoresistance in thickness-inhomogeneous nanodevices. Since the $T_{\mathrm{C}}$ and coercive field ($H_{\mathrm{C}}$) of FGT are thickness dependent, thickness variations in FGT flake lead to different regions with distinct $H_{\mathrm{C}}\mathrm{s}$. Together with the thickness variations-induced differences in $H_{\mathrm{C}}\mathrm{s}$ and the PMA, an eddy current generates in the proximity of the thickness boundary during the magnetic switching process. This eddy current perturbs the longitudinal resistance and results in the unexpected antisymmetry. Our work provides a new understanding and the device application in thickness-variation 2D ferromagnetic materials, which are more experimentally common but have been neglected thus far.

Electromagnetic waves in vacuum and most materials have transverse polarization. Longitudinal electromagnetic waves with an electric field parallel to the wave vector are very rare and appear under special conditions in a limited class of media, for example, in plasmas. In this Letter, we study the dispersion properties of an easy-to-manufacture metamaterial consisting of two three-dimensional cubic lattices of connected metallic wires inserted one into another, also known as an interlaced wire medium. It is shown that the metamaterial supports longitudinal waves at an extremely wide frequency band from very low frequencies up to the Bragg resonances of the structure. The waves feature unprecedentedly short wavelengths comparable to the period of the material. The revealed effects highlight a spatially dispersive response of the interlaced wire medium and provide a route toward generating electromagnetic fields with strong spatial variations.

A low-disorder, two-dimensional electron system (2DES) subjected to a large perpendicular magnetic field and cooled to very low temperatures provides a rich platform for studies of many-body quantum phases. The magnetic field quenches the electrons' kinetic energy and quantizes the energy into a set of Landau levels, allowing the Coulomb interaction to dominate. In excited Landau levels, the fine interplay between short- and long-range interactions stabilizes bubble phases, Wigner crystals with more than one electron per unit cell. Here, we present the screening properties of bubble phases, probed via a simple capacitance technique where the 2DES is placed between a top and a bottom gate and the electric field penetrating through the 2DES is measured. The bubbles formed at very low temperatures screen the electric field poorly as they are pinned by the residual disorder potential, allowing a large electric field to reach the top gate. As the temperature is increased, the penetrating electric field decreases and, surprisingly, exhibits a pronounced minimum at a temperature that appears to coincide with the melting temperature of the bubble phase. We deduce a quantitative phase diagram, as a function of Landau level filling factor (ν) and temperature, for the transition from the bubble to liquid phases for $4\le \nu \le 5$.

Topological fermions have mainly been addressed in the limit of weakly correlated systems, while the realization of Kondo-like physics arising from Dirac-Weyl fermions and implying strong correlations is rare. The noncentrosymmetric Weyl fermion $R\mathrm{AlGe}$ compounds (with $R=\mathrm{La} \mathrm{and} \mathrm{Ce}$) provide a promising playground for revealing the impact of electronic correlations driven by $f$-electron states on Weyl fermions. Here, we tackle their charge dynamics as a function of temperature. Besides spotting typical optical signatures of a type-II Weyl semimetal in both materials, we discover that electronic correlations at low temperatures renormalize the nontrivial bands hosting the Weyl nodes and lead to a reduction of the Fermi velocity as well as an enhancement of the charge carriers effective mass in CeAlGe with respect to LaAlGe, both being a fingerprint of a so-called Weyl-Kondo system.

It is shown that the convective instability in electron fluids in three- and two-dimensional (3D and 2D) Dirac and Weyl semimetals is strongly inhibited. The major obstacles for electron convection are the effects of the Coulomb forces and the momentum relaxation related to the interaction with impurities and phonons. The effect of the Coulomb forces is less pronounced in 2D materials, such as graphene. However, momentum relaxation still noticeably inhibits convection, making it very difficult to achieve in practice.

Finding minimum energy distribution of $N$ charges on a sphere is known as the Thomson problem. Here, we study the vibrational properties of the $N$ charges in the lowest energy state within the harmonic approximation for $10\le N\le 200$ and for selected sizes up to $N=372$. The maximum frequency ${\omega }_{\max }$ increases with $N^{3/4}$, which is rationalized by studying the lattice dynamics of a two-dimensional triangular lattice. The $N$ dependence of ${\omega }_{\max }$ identifies magic numbers of $N=12,32,72,132,192,212,272,282$, and 372, reflecting both a strong degeneracy of one-particle energies and an icosahedral structure that the $N$ charges form. $N=122$ is not identified as a magic number for ${\omega }_{\max }$ because the former condition is not satisfied. The magic number concept can hold even when an average of high frequencies is considered. The maximum frequency mode at the magic numbers has no anomalously large oscillation amplitude (i.e., not a defect mode).

Experiments on the acoustoelectric transformation in superconducting compounds based on FeSe reveal the manifestation of piezomagnetism in them. It allows us to refer the family of superconducting chalcogenides to the systems with a hidden magnetic order and broken time-reversal symmetry. The anisotropy of the piezomagnetic response indicates that the crystal structure of these compounds belongs to the trigonal syngony, in the low-temperature phase.

The metallic $\mathrm{LiOs}{\mathrm{O}}_3$ undergoes a transition to a polar phase below $T_s\approx 140\mathrm{K}$. This unusual property provides a unique opportunity to study the interaction between free electrons and electric dipoles. We report a suite of measurements of physical properties in $\mathrm{LiOs}{\mathrm{O}}_3$ as a function of temperature, including resistivity and magnetoresistance down to 0.16 K, thermoelectric power, high-precision magnetization, specific heat, and thermal conductivity on textured single-crystal samples. Enhancements from the electron contribution to the specific heat and the paramagnetic susceptibility indicate that electrons in $\mathrm{LiOs}{\mathrm{O}}_3$ are highly correlated. An anomalously large Kadowaki-Woods ratio also supports the argument of strongly correlated electrons in $\mathrm{LiOs}{\mathrm{O}}_3$. In the nonpolar phase above $T_s$, electrons are coupled strongly to the lattice vibrations, which leads to the resistivity saturation at high temperatures and eventually a crossover to the hopping conduction. The data of thermal conductivity and specific heat are consistent with an order-disorder transition at $T_s$. The analysis of critical behavior in the resistivity, specific heat, and the thermal conductivity provides useful information for understanding the electron-dipole interaction in $\mathrm{LiOs}{\mathrm{O}}_3$.

The iterative Green's function, based on a cyclic reduction of block-tridiagonal matrices, has been the ideal algorithm, through tight-binding models, to compute the surface density of states of semi-infinite topological electronic materials. In this paper, we apply this method to photonic and acoustic crystals, using finite-element discretizations and a generalized eigenvalue formulation, to calculate the local density of states on a single surface of semi-infinite lattices. Three-dimensional examples of gapless helicoidal surface states in Weyl and Dirac crystals are shown and the computational cost, convergence, and accuracy are analyzed.

This paper presents a fully ab initio many-body photoemission framework that includes coherent three-body electron-photon-phonon scattering to predict the transverse momentum distributions and the mean transverse energies (MTEs) of bulk photoelectrons from single-crystal photocathodes. The need to develop such a theory stems from the lack of studies that provide complete understanding of the underlying fundamental processes governing the transverse momentum distribution of photoelectrons emitted from single crystals. For example, initial predictions based on density-functional theory calculations of effective electron masses suggested that the (111) surface of PbTe would produce very small MTEs $(\le 15 \mathrm{meV})$, whereas our experiments yielded MTEs 10 to 20 times larger than these predictions and also exhibited a lower photoemission threshold than predicted. The ab initio framework presented in this paper correctly reproduces both the magnitude of the MTEs from our measurements in PbTe(111) and the observed photoemission below the predicted threshold. Our results show that both photoexcitations into states that propagate in the bulk of the material and coherent many-body electron-photon-phonon scattering processes, which initial predictions ignored, play surprisingly important roles in photoemission from PbTe(111). Finally, from the lessons learned, we recommend a procedure for rapid computational screening of potential single-crystal photocathodes for applications in next-generation ultrafast electron diffraction and x-ray free-electron lasers, which will enable significant advances in condensed matter research.

The second-order optical response of centrosymmetric materials manifests itself mostly at their surface, being strongly suppressed in their bulk. However, the overall surface response is also suppressed in nanoparticles with a centrosymmetric geometry subjected to homogeneous fields. Nevertheless, nanoparticles with a noncentrosymmetric geometry do exhibit second-order optical properties. We develop an analytical theory to investigate the second-order optical response of a noncentrosymmetric thin nanowire with a slightly deformed cross section made up of a centrosymmetric material subjected to two monochromatic fields. We calculate the linear and nonlinear near fields perturbatively, using the extent of the deformation away from a circular cross section as the perturbation parameter. We obtain expressions for the quadratic hyperpolarizabilities in terms of the linear response evaluated at the three frequencies involved. We analyze the spectral features of the nonlinear response functions and explore their resonant structure for a model dielectric nanowire. Furthermore, we evaluate the second-order radiated fields, the radiation patterns, and efficiency of the different quadratic processes. We obtain a strong competition between electric dipolar, magnetic dipolar, and electric quadrupolar contributions even for very small deformations.