Real-time dynamics techniques have proven increasingly useful in theoretically and experimentally understanding strongly correlated systems. By employing unbiased time-resolved exact diagonalization, we study pump dynamics in the two-dimensional plaquette Hubbard model, where distinct hopping integrals $t_h$ and $t_h^{\prime }$ are present within and between plaquettes. In the intermediate coupling regime, a significant enhancement of $d$-wave superconductivity is observed and compared with that obtained by simply examining expectation values with the eigenstates of the Hamiltonian. Our work provides a further understanding of superconductivity in the Hubbard model, extends the description of the pairing amplitude to the frequency-anisotropy plane, and offers a promising approach for experimentally engineering emergent out-of-equilibrium states.

The adiabatic Born-Oppenheimer approximation is considered to be a robust approach that very rarely breaks down. Consequently, it is predominantly utilized to address various electron-phonon properties in condensed matter physics. By combining many-body perturbation and density functional theories we demonstrate the importance of dynamical (nonadiabatic) effects in estimating superconducting properties in various bulk and two-dimensional materials. Apart from the expected long-wavelength nonadiabatic effects, we found sizable nonadiabatic Kohn anomalies away from the Brillouin zone center for materials with strong intervalley electron-phonon scatterings. Compared to the adiabatic result, these dynamical phonon anomalies can significantly modify electron-phonon coupling strength $\lambda$ and superconducting transition temperature $T_c$. Further, the dynamically induced modifications of $\lambda$ have a strong impact on transport properties, where probably the most interesting is the rescaling of the low-temperature and low-frequency regime of the scattering time $1/\tau$ from about $T^3$ to about $T^2$, resembling the Fermi liquid result for electron-electron scattering. Our goal is to point out the potential implications of these nonadiabatic effects and reestablish their pivotal role in computational estimations of electron-phonon properties.

The universal theory of critical phase transitions describes the critical behavior at second-order phase transitions in infinitely large systems. With the increased contemporary interest in nanoscale materials, we investigated CoO nanoparticles by means of neutron scattering and found how the theory of critical phenomena breaks down in the nanoscale regime. Using CoO as a model system, we have identified a size-dependent nanocritical temperature region close to the antiferromagnetic phase transition where the magnetic correlation length of the nanoparticles converges to a constant value, which is significantly smaller than that of the saturated state found at low temperatures. This is in clear contrast to the divergence around $T_{\mathrm{N}}$ observed for bulk systems. Our finding of nanocriticality in the magnetic phase transition is of great importance for the understanding of phase transitions at the nanoscale.

The dynamical structure factor is an important observable of quantum magnets but due to numerical and theoretical limitations, it remains a challenge to make predictions for Hubbard-like models beyond one dimension. In this work, we study the magnetic excitations of the triangular lattice Hubbard model including next-nearest-neighbor hopping. Starting from the expected ${120}^{\circ }$ and stripe magnetic orders, we compute the magnon spectra within a self-consistent random phase approximation. In the stripe phase, we generically find accidental zero modes related to a classical degeneracy known from the corresponding $J_1\text{−}{J}_2$ Heisenberg model. We extend the order-by-disorder mechanism to Hubbard systems and show how quantum fluctuations stabilize the stripe order. In addition, the frustration-induced condensation of magnon modes allows us to map out the entire phase diagram which is in remarkable agreement with recent numerical works. We discuss connections to experiments on triangular lattice compounds and the relation of our results to the proposed chiral spin liquid phase.

We performed an inelastic neutron-scattering experiment on a polycrystalline sample of a helimagnet ${\mathrm{Ni}}_2{\mathrm{InSbO}}_6$ to construct the spin Hamiltonian. Well-defined spin-wave excitation with a band energy of 20 meV was observed below $T_N=76$ K. Using the linear spin-wave theory, the spectrum was reasonably reproduced with honeycomb spin layers coupled along the stacking axis (the $c$ axis). The proposed spin model reproduces the soliton lattice induced by a magnetic field applied perpendicular to the $c$ axis.

Nonunitary superconductivity has attracted renewed interest as a novel gapless phase of matter. In this study, we investigate the superconducting gap structure of nonunitary odd-parity chiral pairing states in a superconductor involving strong spin-orbit interactions. By applying a group theoretical classification of chiral states in terms of discrete rotation symmetry, we categorized all possible point-nodal gap structures in nonunitary chiral states into four types in terms of the topological number of nodes and node positions relative to the rotation axis. In addition to conventional Dirac and Weyl point nodes, we identify a type of Dirac point nodes unique to nonunitary chiral superconducting states. Such a Dirac point node is a nodal point of two pseudospin excitations, but the dispersions around it depend on the pseudospin. The node type can be identified experimentally based on the temperature dependence of the nuclear magnetic resonance longitudinal relaxation rate. The implication of our results for a nonunitary odd-parity superconductor in ${\mathrm{UTe}}_2$ is also discussed.

The layered material ${\mathrm{ZrTe}}_5$ appears to exhibit several exotic behaviors, which resulted in significant interest recently, although the exact properties are still highly debated. Among these we find a Dirac/Weyl semimetallic behavior, nontrivial spin textures revealed by low-temperature transport, and a potential weak or strong topological phase. The anomalous behavior of resistivity has been recently elucidated as originating from band shifting in the electronic structure. Our work examines magnetotransport behavior in ${\mathrm{ZrTe}}_5$ samples in the context of multicarrier transport. The results, in conjunction with ab initio band structure calculations, indicate that many of the transport features of ${\mathrm{ZrTe}}_5$ across the majority of the temperature range can be adequately explained by the semiclassical multicarrier transport model originating from a complex Fermi surface.

We discuss different properties and the potential of several topological invariants based on position operators to identify phase transitions, and compare with more accurate methods, such as crossing of excited energy levels and jumps in Berry phases. The invariants have the form $\text{Im}\text{ln}\langle exp\left[i(2\pi /L){\mathrm{\Sigma }}_j{x}_j(m_{{}_{\uparrow }}{\widehat{n}}_{j\uparrow }+m_{\downarrow }{\widehat{n}}_{j\downarrow })\right]\rangle$, where $L$ is the length of the system, $x_j$ the position of site $j$, and ${\widehat{n}}_{j\sigma }$ the operator of the number of particles at site $j$ with spin $\sigma$. We show that $m_{\sigma }$ should be integers, and in some cases of magnitude larger than 1, to lead to well-defined expectation values. For the interacting Rice-Mele model (which contains the interacting Su-Schrieffer-Heeger and the ionic Hubbard model as specific cases), we show that three different invariants give complementary information and are necessary and sufficient to construct the phase diagrams in the regions where the invariants are protected by inversion symmetry. We also discuss the consequences for pumping of charge and spin, and the effect of an Ising spin-spin interaction or a staggered magnetic field.

We present a theory of Mott memristors whose working principle is the nonlinear carrier avalanche multiplication in Mott insulators subject to strong electric fields. The internal state of the memristor, which determines its resistance, is encoded in the density of doublon and hole excitations in the Mott insulator. In the current-voltage characteristic, insulating and conducting states are separated by a negative-differential-resistance region, leading to hysteretic behavior. Under oscillating voltage, the response of a voltage-controlled, nonpolar memristive system is obtained, with retarded current and pinched hysteresis loop. As a first step towards neuromorphic applications, we demonstrate self-sustained spiking oscillations in a circuit with a parallel capacitor. Being based on electronic excitations only, this memristor is up to several orders of magnitude faster than previous proposals relying on Joule heating or ionic drift.

We propose a nondissipative transport effect and vortical response in Weyl semimetals in the presence of spatial inhomogeneities, namely, a spatially varying tilt of the Weyl cones. We show that when the spectrum is anisotropic and tilted due to spatial lattice variations, one is confronted with generalized quantum anomalies due to the effective fields stemming from the tilt structure. In particular, we demonstrate that the position-dependent tilt parameter induces local vorticity, thus generating a chiral vortical effect even in the absence of rotation or magnetic fields. As a consequence, it couples to the electric field and thus contributes to the anomalous Hall effect.

We study the correlated insulating phases of twisted bilayer graphene (TBG) in the absence of lattice strain at integer filling $\nu =\pm 3$. Using the self-consistent Hartree-Fock method on a particle-hole symmetric model and allowing translation symmetry breaking terms, we obtain the phase diagram with respect to the ratio of $AA$ interlayer hopping $\left(w_0\right)$ and $AB$ interlayer hopping $\left(w_1\right)$. When the interlayer hopping ratio is close to the chiral limit ($w_0/w_1\lesssim 0.5$), a quantum anomalous Hall state with Chern number ${\nu }_c=\pm 1$ can be observed consistent with previous studies. Around the realistic value $w_0/w_1\approx 0.8$, we find a spin and valley polarized, translation symmetry breaking, state with $C_{2z}T$ symmetry, a charge gap and a doubling of the moiré unit cell, dubbed the $C_{2z}T$ stripe phase. The real-space total charge distribution of this $C_{2z}T$ stripe phase in the flat band limit does not have modulation between different moiré unit cells, although the charge density in each layer is modulated, and the translation symmetry is strongly broken. Other symmetries, including $C_{2z}, C_{2x}$, particle-hole symmetry $P$, and the topology of the $C_{2z}T$ stripe phase, are also discussed in detail. We observed braiding and annihilation of the Dirac nodes by continuously turning on the order parameter to its fully self-consistent value, and provide a detailed explanation of the mechanism for the charge gap opening despite preserving $C_{2z}T$ and valley $\mathrm{U}\left(1\right)$ symmetries. In the transition region between the quantum anomalous Hall phase and the $C_{2z}T$ stripe phase, we find an additional competing state with comparable energy corresponding to a phase with a tripling of the moiré unit cell.

We study entanglement properties in a nonequilibrium steady state of a free-fermion chain, that emerges after connecting two half-chains prepared at different temperatures. The entanglement negativity and the Rényi mutual information between two adjacent intervals scale logarithmically in the system size, with prefactors that we calculate analytically as a function of the bath temperatures. In particular, we show that the negativity and the Rényi mutual information with index $\alpha =1/2$ are described by different prefactors, and thus the two quantities provide inequivalent information about the state. Furthermore, we show that the logarithmic growth of the negativity during time evolution is also governed by the steady-state prefactor.

Two-dimensional (2D) magnetic materials hosting nontrivial topological states are interesting for fundamental research as well as practical applications. Recently, the topological state of 2D Weyl half-semimetal (WHS) was proposed, which hosts fully spin polarized Weyl points robust against spin-orbit coupling in a 2D ferromagnetic system, and single-layer ${\mathrm{PtCl}}_3$ was predicted as a platform for realizing this state. Here, we perform an extensive search of 2D ${\mathrm{PtCl}}_3$ structures, by using the particle swarm optimization technique and density-functional theory calculation. We show that the desired ${\mathrm{PtCl}}_3$ phase corresponds to the most stable one at its stoichiometry. The 2D structure also possesses good thermal stability up to 600 K. We suggest ${\mathrm{SnS}}_2$ as a substrate for the growth of 2D ${\mathrm{PtCl}}_3$, which has excellent lattice matching and preserves the WHS state in ${\mathrm{PtCl}}_3$. We find that uniaxial strains along the zigzag direction maintain the WHS state, whereas small strains along the armchair direction drives a topological phase transition from the WHS to a quantum anomalous Hall (QAH) insulator phase. Furthermore, we study bilayer ${\mathrm{PtCl}}_3$ and show that the stacking configuration has strong impact on the magnetism and the electronic band structure. Particularly, the $A{A}^{\prime }$ stacked bilayer ${\mathrm{PtCl}}_3$ realizes an interesting topological state—the 2D antiferromagnetic mirror Chern insulator, which has a pair of topological gapless edge bands. Our work provides guidance for the experimental realization of 2D ${\mathrm{PtCl}}_3$ and will facilitate the study of 2D magnetic topological states, including WHS, QAH insulator, and magnetic mirror Chern insulator states.

We propose a realization of a quantum Hall effect (QHE) in a second-order topological insulator (SOTI) in three dimensions, which is mediated by hinge states on a torus surface. It results from the nontrivial interplay of the material structure, the Zeeman effect, and the surface curvature. In contrast to the conventional two-dimensional (2D)- and 3D-QHE, we show that the 3D-SOTI QHE is not affected by orbital effects of the applied magnetic field, and it exists in the presence of a Zeeman term only, induced, e.g., by magnetic doping. To explain the 3D-SOTI QHE, we analyze the boundary charge for a 3D-SOTI and establish its universal dependence on the Aharonov-Bohm flux threading through the torus hole. Exploiting the fundamental relation between the boundary charge and the Hall conductance, we demonstrate the universal quantization of the latter, as well as its stability against random disorder potentials and continuous deformations of the torus surface.

We study general features of the excitation spectrum of a system of one-dimensional chiral spinless fermions with short-range interactions. We show that the nature of the elementary excitations of such a system depends strongly on the nonlinearity of the underlying dispersion of the fermions. In the case of quadratic nonlinearity, the low-momentum excitations are essentially fermionic quasiparticles and quasiholes, whereas the high-momentum ones are classical harmonic waves and solitons. In the case of cubic nonlinearity, the nature of the elementary excitations does not depend on momentum and is determined by the strength of the interactions. At a certain critical value of the interaction strength, the excitation spectrum changes qualitatively, pointing to a dynamic phase transition in the system.

A field effect transistor based on graphene was introduced to generate a hybrid plasmonic mode. The main reasons to consider graphene in this study are the achievement of high field confinement and the graphene two carrier transport channel near its charge neutrality point. We have observed a nonlinear behavior of the hybrid plasmonic mode near the graphene charge neutrality point because of the existence of the two carrier model. This nonlinear behavior reflects the intrinsic nanoelectronic characteristics of graphene. The proposed nanoelectronic device presents an ultraconfined tunable hybrid hotspot, the mode area of which is ${10}^5$ times smaller than the diffraction limited spot area. Moreover, this area is three orders of magnitude smaller than the modal area of the similar metal-based hybrid nanostructure. Such a nanoscale 3D hotspot with plasmonic-photonic character has been reported here. Furthermore, a hybrid structure based on bilayer twisted graphene was numerically investigated which renders ultrahigh confinement and ultralong propagation distance. The proposed nanodevice can be applied to design compact integrated circuits, optical neural networks, and high resolution sensors.

Materials in low dimensions show interesting physics and excellent potential in practical applications. The atomic-level control of two-dimensional (2D) behavior in a traditional three-dimensional (3D) system is quite hard, and the ${\left({\mathrm{La}}_{0.7}{\mathrm{Sr}}_{0.3}\mathrm{Mn}{\mathrm{O}}_3\right)}_n/{\left(\mathrm{LaAl}{\mathrm{O}}_3\right)}_n$ (LMSO/LAO) superlattice on a (001) $\mathrm{SrTi}{\mathrm{O}}_3$ substrate was investigated to explore its quasi-2D behavior. The number of magnetic phase transitions has a close relationship with the number of unit cells ($n$): only one phase transition for $n=2$ and four phase transitions for $n=8$. It was found that the local structure of each ${\mathrm{La}}_{0.7}{\mathrm{Sr}}_{0.3}\mathrm{Mn}{\mathrm{O}}_3$ atomic layer is responsible for the multiple phase transitions: the gradual change of the tetragonal ratio and the in-plane tensile strain-induced $3d$ electronic orbital occupancy control the magnetic properties of 2D-LSMO atomic layers. For the outmost 2D-LSMO atomic layer contacting $\mathrm{LaAl}{\mathrm{O}}_3$ directly, interface-related factors reduce the intensity of magnetic interaction significantly. With the increase of $n$, the properties around the LSMO/LAO interface changes, which demonstrates that the superlattice configuration and the lattice-mismatch strain could be employed to explore the quasi-2D behavior of a traditional 3D system.

Bismuth-activated light emissions have been much reported in ternary alkaline earth $d^{10}$ metal oxides, and the self-activation nature of these hosts makes it possible to achieve efficient luminescence via energy transfer between the host and the activator ions. A detailed analysis of the potential intrinsic defects accounting for the intrinsic emissions is still absent, and the identifications of bismuth-related transitions remain unclear or controversial. Given these facts, first-principles calculations using hybrid functionals are employed to systematically study the bismuth-doped $(\mathrm{Sr}/\mathrm{Ca}){\mathrm{Sb}}_2{\mathrm{O}}_6$, ${(\mathrm{Sr}/\mathrm{Ca})}_2{\mathrm{Sb}}_2{\mathrm{O}}_7$, $(\mathrm{Sr}/\mathrm{Ca}){\mathrm{SnO}}_3$, and $(\mathrm{Sr}/\mathrm{Ca}){\mathrm{Ga}}_2{\mathrm{O}}_4$, which contain $d^{10}$ metals belonging to VA, IVA, and IIIA groups, respectively. Primary intrinsic defects and self-trapped holes in these hosts are revealed and the roles played by them on intrinsic emissions are evaluated. Bismuth ions are identified as trivalent dopants and their ${}^{3}P_{0,1}\to {}^{1}S_0$ (referred to as $A$-band) transition is assigned to account for the ultraviolet emission observed in bismuth-doped $(\mathrm{Sr}/\mathrm{Ca}){\mathrm{Sb}}_2{\mathrm{O}}_6$, while the visible emission observed in $(\mathrm{Sr}/\mathrm{Ca}){\mathrm{Sb}}_2{\mathrm{O}}_6$:Bi is attributed to ${\mathrm{Bi}}^{3+}$ dopants in the minute amount of ${(\mathrm{Sr}/\mathrm{Ca})}_2{\mathrm{Sb}}_2{\mathrm{O}}_7$ impurities. In ${\mathrm{Sr}}_2{\mathrm{Sb}}_2{\mathrm{O}}_7$ and ${\mathrm{Ca}}_2{\mathrm{Sb}}_2{\mathrm{O}}_7$ with bismuth, the excitation bands are identified as ${}^{1}S_0\to {}^{3}P_{0,1}$ transition of ${\mathrm{Bi}}^{3+}$, and the observed emission bands are attributed to charge transfer transition (${\mathrm{Bi}}^{2+}+h_{\mathrm{VBM}}^{+}\to {\mathrm{Bi}}^{3+}$) and $A$-band transition (${}^{3}P_{0,1}\to {}^{1}S_0$), respectively. Furthermore, the bismuth-related transitions are also well identified in $(\mathrm{Sr}/\mathrm{Ca}){\mathrm{SnO}}_3$ and $(\mathrm{Sr}/\mathrm{Ca}){\mathrm{Ga}}_2{\mathrm{O}}_4$ and the large Stokes shifts of the $A$-band transition in $(\mathrm{Sr}/\mathrm{Ca}){\mathrm{Ga}}_2{\mathrm{O}}_4$ are revealed and analyzed. The insights gained in this work extend our understanding of the properties of intrinsic defects and the luminescence mechanisms of insulating bismuth-doped ternary oxides.

We use ab initio simulations based on density functional theory to calculate the electrical and thermal conductivity of solid iron in face-centered cubic and hexagonal phases at high pressures and temperatures up to Earth's core conditions. Both our electrical and thermal conductivities increase systematically with density and reasonably follow the Wiedemann-Franz law, in particular at low temperatures. A trend towards density-independent thermal conductivity observed in recent experiments is not supported by our calculations.

We present a systematic investigation of the magnetoresponses of the Weyl points (WPs) with a topological charge of $n=2,3$, and 4, and with both linear and quadratic energy tilt. The linear tilt always tends to squeeze the Landau levels (LLs) of both conduction and valence bands of all the WPs, and eventually leads to LL collapse in the type-II phase. However, the quadratic energy tilt has more complex influences on the LLs of the unconventional WPs. For charge-$n$ $(n=2,4)$ WP, the influence of the quadratic tilt on the LLs of conduction and valence bands are opposite, i.e., if the LLs of conduction (valence) bands are squeezed, then that of the valence (conduction) bands are broadened and the squeezed LL spectrum will be collapsed in the type-III phase. But, the LL collapse generally cannot be found in the type-III charge-3 WP. Moreover, for charge-$n$ $(n=2,3)$ WP, the quadratic tilt breaks the degeneracy of the chiral LLs regardless of the direction of the magnetic field, leading to additional optical transitions and magneto-optical conductivity peaks at low frequencies. Interestingly, the four chiral LLs in charge-4 WP are always not degenerate. Hence there inevitably exist magneto-optical conductivity peaks at low frequencies for charge-4 WP. Since the density of state of the LL spectrum is very large, one can expect that the low-frequency magneto-optical responses in unconventional WPs would be significant and may be used for developing efficient terahertz photodetectors.