We reinterpret the chiral $U\left(N\right)$ Wess-Zumino-Witten (WZW) model at level $k>1$ in $(1+1)$ dimensions as an interacting chiral metal in two space dimensions. In this reinterpretation, spatial translations along one of the spatial dimensions in the two-dimensional chiral metal arise from a generator of the $U\left(N\right)$ symmetry of the WZW model. The WZW model at $k=1$ is equivalent to Balents and Fisher's free chiral metal [L. Balents and M. P. A. Fisher, Phys. Rev. Lett. 76, 2782 (1996)]. Here, the $U\left(N\right)$ symmetry corresponds to the IR symmetry of a chiral Fermi gas with (half of a) Fermi surface, with $N$ equal to the number of points on the Fermi surface. We argue that exactly solvable interacting generalizations occur for levels $k>1$. Importantly, these interacting chiral metals maintain the $U\left(N\right)$ symmetry of the free system. We calculate two-point correlation functions of the single-particle fermion operator, the $U\left(1\right)$ number density, and current operators in these theories for general $k$. We find that interactions ($k>1$) produce $1/N$ corrections to scaling of the single-particle fermion operator as $N\to \infty$ and renormalize the amplitudes of the density and current two-point functions. This construction illustrates the ersatz Fermi liquid proposal of Else et al. [Phys. Rev. X 11, 021005 (2021)].

We present an ab initio description of optical and shallow-core x-ray absorption spectroscopies in a unified formalism based on the pseudopotential plane-wave method at the level of the Bethe-Salpeter equation (BSE) within Green's functions theory. We show that norm-conserving pseudopotentials are reliable and accurate not only for valence but also for semicore electron excitations. To validate our approach, we compare BSE absorption spectra obtained with two different codes: the pseudopotential-based code EXC and the all-electron full-potential code Exciting. We take corundum $\alpha \text{−}{\text{Al}}_2{\text{O}}_3$ as an example, being a prototypical material that presents strong electron-hole interactions for both valence and core electron excitations. We analyze in detail the optical absorption spectrum as well as the Al ${\mathrm{L}}_1$ and ${\mathrm{L}}_{2,3}$ edges in terms of anisotropy, crystal local fields, interference, and excitonic effects. We perform a thorough inspection of the origin and localization of the lowest-energy excitons, and conclude highlighting the purely electronic character off the pre-edge of ${\mathrm{L}}_1$ and the dichroic nature of the optical and ${\mathrm{L}}_{23}$ spectra.

Quantized responses are important tools for understanding and characterizing the universal features of topological phases of matter. In this work, we show that three-dimensional ($3\mathrm{D}$) insulators with $C_n$ rotation symmetry along a fixed axis can possess a mixed geometry-charge response, in which disclination lines of the $3\mathrm{D}$ lattice carry electric polarization. These disclinations bind a fractional charge at the gapped surfaces of the insulator because of the surface-charge-polarization theorem. This response is accompanied by a dual response that binds fractional angular momentum to magnetic monopoles in the bulk (analogously to the Witten effect) and to magnetic fluxes on gapped surfaces. We show that these responses are described by a $3\mathrm{D}$ topological response term that couples the lattice curvature to the electromagnetic field strength. Additional mirror or particle-hole symmetry quantizes the mixed geometry-charge responses and defines a new class of rotation-invariant topological crystalline insulators (rTCIs). Notably, the surface charge bound to disclinations of the rTCIs is half the minimal amount that can occur in purely two-dimensional insulators. We construct lattice models of these rTCIs and numerically verify that they exhibit mixed geometry-charge responses. We also demonstrate that the particle-hole symmetric rTCI supports anomalous surface topological order and that the mirror symmetric rTCI can be smoothly deformed into a higher order octopole insulator with quantized corner charges. Additionally, we construct symmetry indicators for diagnosing the mirror symmetric rTCIs.

The Fe-doped III-V ferromagnetic semiconductor (FMS) (In,Fe)Sb is a promising material for spintronic device applications because of the $n$-type carrier conduction and the ferromagnetism with high Curie temperature $\left(T_{\mathrm{C}}>300\mathrm{K}\right)$. To clarify the mechanism of the $\text{high}\text{−}{T}_{\mathrm{C}}$ ferromagnetism, we have investigated the electronic structure and magnetic properties of an $\left({\mathrm{In}}_{0.94},{\mathrm{Fe}}_{0.06}\right)\mathrm{Sb}$ thin film by performing x-ray absorption spectroscopy (XAS) and x-ray magnetic circular dichroism (XMCD) measurements at the Fe $L_{2,3}$ edges. The magnetic-field $\left({\mu }_{0}H\right)$ dependence of the XMCD spectra reveals that there are ferromagneticlike Fe and paramagneticlike Fe components in the (In,Fe)Sb thin film. The XAS and XMCD spectra of the ferromagneticlike and paramagneticlike Fe components resemble those of other Fe-doped FMSs and extrinsic oxides, respectively. The finite value of the ratio between the orbital and spin magnetic moments estimated by applying the XMCD sum rules indicates that the valence state of the Fe ions substituting for the In sites in (In,Fe)Sb is not purely ionic ${\mathrm{Fe}}^{3+}$, but intermediate between ${\mathrm{Fe}}^{3+}$ and ${\mathrm{Fe}}^{2+}$. The qualitative correspondence between the ${\mu }_{0}H$ dependence of the visible-light magnetic circular dichroism intensity and that of the XMCD intensity demonstrates that the Zeeman splitting of the InSb band is proportional to the net magnetization of the doped Fe. These results suggest that the ferromagnetism of (In,Fe)Sb originates from the Fe $3d$ orbitals hybridized with the host InSb bands.

We investigate the finite-temperature magnetic order at the edges of hexagonal CrN nanoribbons by using density functional theory combined with the density matrix renormalization group (DMRG) method. Moreover, the spin-dependent transport in nanoribbons is calculated within the semiclassical Boltzmann transport theory. We find that the zigzag edges have lower energy with respect to armchair edges. The zigzag edges of CrN nanoribbon show half-metallic electronic character, which is the same as for the two-dimensional (2D) monolayer. The localized electronic states on the zigzag edges reduce the electronic band gap energy for spin-down electrons. The ab initio electronic results are mapped into an effective 1D Heisenberg spin model up to the next-nearest-neighbor exchange interaction term. For zigzag ribbons, the nearest-neighbor and next-nearest-neighbor magnetic exchange are around 10–12 and $-2$ to 0 $\text{meV}/\text{Cr}\text{atom}$, respectively. The finite spin correlation length in 1D nanoribbons drops sharply to zero with temperature. The absence of long-range spin correlations at the edges is a practical drawback for future room temperature 2D spintronic devices. The maximally localized Wannier functions are used for band interpolation and spin-dependent transport calculations by using the semiclassical Boltzmann equation. We show that the zigzag edges of CrN are a perfect spin filter under both electron and hole doping.

Non-Hermitian descriptions often model open or driven systems away from the equilibrium. Nonetheless, in equilibrium electronic systems, a non-Hermitian nature of an effective Hamiltonian manifests itself as unconventional observables such as a bulk Fermi arc and skin effects. We theoretically reveal that spin-dependent quasiparticle lifetimes, which signify the non-Hermiticity of an effective model in the equilibrium, induce the anomalous Hall effect, namely, the Hall effect without an external magnetic field. We first examine the effect of nonmagnetic and magnetic impurities and obtain a non-Hermitian effective model. Then, we calculate the Kubo formula from the microscopic model to ascertain a non-Hermitian interpretation of the longitudinal and Hall conductivities. Our results elucidate the vital role of the non-Hermitian equilibrium nature in the quantum transport phenomena.

Dual-unitary circuits are a class of locally interacting quantum many-body systems displaying unitary dynamics also when the roles of space and time are exchanged. These systems have recently emerged as a remarkable framework where certain features of many-body quantum chaos can be studied exactly. In particular, they admit a class of “solvable” initial states for which, in the thermodynamic limit, one can access the full nonequilibrium dynamics. This reveals a surprising property: when a dual-unitary circuit is prepared in a solvable state the quantum entanglement between two complementary spatial regions grows at the maximal speed allowed by the local structure of the evolution. Here we investigate the fate of this property when the system is prepared in a generic pair-product state. We show that in this case, the entanglement increment during a time step is submaximal for finite times, however, it approaches the maximal value in the infinite-time limit. This statement is proven rigorously for dual-unitary circuits generating high enough entanglement, while it is argued to hold for the entire class.

The ability to modify and tune the spin-wave dispersion is one of the most important requirements for engineering of magnonic networks. In this study, we demonstrate the promise of synthetic thin-film hybrids composed of an antiferromagnetic insulator (AF) and a normal (N) or superconducting (S) metal for tuning and modifying the spin-wave dispersion in antiferromagnetic insulators. The key ingredient is the uncompensated magnetic moment at the AF/S(N) interface, which induces an effective exchange field in the adjacent metal via the interface exchange interaction. The exchange field spin polarizes quasiparticles in the metal and induces spinful triplet Cooper pairs screening the magnon. The quasiparticle and Cooper pair polarization renormalizes the magnon dispersion. The renormalization results in the splitting of the otherwise degenerate AF magnon modes with no need to apply a magnetic field. It is also proposed that measurements of the renormalized dispersion relations can provide the amplitude of the effective exchange field induced by the AF in the adjacent metal.

In this paper, we study the eigenstate properties of a quantum Hopfield model by the exact diagonalization method. The local permutational symmetry in this model organizes the spins into clusters, which can each be considered a large quantum spin interacting with others. It is shown that such a quantum Hopfield model, even though without dissipation, is interesting in its own right as an example of quantum frustrated magnetism and quantum spin glass. It exhibits three distinct phases: a low-energy spin-glass phase at a low transverse field, a thermal paramagnetic phase at a high transverse field, and a nonthermal high-energy paramagnetic phase. The dynamics of the revival probability starting from a memory pattern in such a closed quantum many-body model has also been studied.

We investigate the interplay between non-Hermiticity and finite temperature in the context of a mixed-state dynamical quantum phase transition (MSDQPT). We consider a $p$-wave superconductor model, encompassing complex hopping and non-Hermiticity, that can lead to gapless phases in addition to gapped phases, to examine the MSDQPT and winding number via the intraphase quench. We find that the MSDQPT is always present irrespective of the gap structure of the underlying phase; however, the profile of Fisher zeros changes between the above phases. Such occurrences of MSDQPT are in contrast to the zero-temperature case in which a DQPT does not take place for the gapped phase. Surprisingly, the half-integer jumps in winding number at zero temperature are washed away for finite temperature in the gapless phase. We study the evolution of the minimum time required by the system to experience MSDQPT with the inverse temperature such that gapped and gapless phases can be differentiated. Our study indicates that the minimum time shows monotonic (nonmonotonic) behavior for the gapped (gapless) phase.

We systematically investigated the low-temperature transport properties of two series of NbN epitaxial films. The first series of films with thickness ranging from $\sim 2.0$ to $\sim 4.0\mathrm{nm}$ are two-dimensional (2D) with respect to superconductivity, while another series with thickness around $\sim 20\mathrm{nm}$ is quasi-three-dimensional (quasi-3D). Those 2D NbN films undergo a superconductor-insulator transition (SIT) with decreasing film thickness, and the critical sheet resistance for the SIT is close to the quantum resistance of Cooper pairs $h/4e^2$ (6.45 $\mathrm{k}\mathrm{\Omega }$). Besides the Berezinski-Koterlitz-Thouless transition, a magnetic-field-driven SIT is observed in those 2D superconducting films ($2.6\mathrm{nm}\lesssim t\lesssim 4.0\mathrm{nm}$). The field-driven quantum metal state does not appear in these 2D superconducting films. However, it is found that both in the 2D and quasi-3D superconducting films the low-temperature magnetoresistance isotherms do not cross at a very narrow region (generally treated as a single point) but cross at a well-distinguished wide region. The dynamical critical exponent obtained by analyzing these magnetoresistance isotherms is divergent as the quantum critical point is approached. The divergence of the dynamical critical exponent near the critical point is consistent with the quantum Griffiths singularity behavior. Our results suggest that the quantum Griffiths singularity can also occur in a 2D superconductor with SIT or a quasi-3D superconductor. The origin of the infinite-randomness point of the quantum phase transition is attributed to the emergence of superconducting rare regions due to the quench disorder in low temperatures.

Using the time-dependent Lanczos method, we study the nonequilibrium dynamics of the one-dimensional ionic-mass imbalanced Hubbard chain driven by a quantum quench of the on-site Coulomb interaction, where the system is prepared in the ground state of the Hamiltonian with a different Hubbard interaction. The exact diagonalization method (Lanczos algorithm) is adopted to study the zero temperature phase diagram in equilibrium, which is shown to be in good agreement with previous studies using density matrix renormalization group (DMRG). We then study the nonequilibrium quench dynamics of the spin and charge order parameters by fixing the initial and final Coulomb interactions while changing the quenching time protocols. Our study shows that the time evolution of the charge and spin order parameters strongly depend on the quenching time protocols. In particular, the effective temperature of the system will decrease monotonically as the quenching time is increased. By taking the final Coulomb interaction strength to be in the strong coupling regime, we find that the oscillation frequency of the charge order parameter increases monotonically with the Coulomb interaction. By contrast, the frequency of the spin order parameter decreases monotonically with increasing Coulomb interaction. We explain this result using an effective spin model and a two-site Hubbard model in the strong coupling limit. Finally, we take the final Coulomb interaction strength to be in the weak coupling regime and find that the oscillation frequency of both the charge and spin order parameters increases monotonically with decreasing Coulomb interaction. Our study suggests strategies to engineer the relaxation behavior of interacting quantum many-particle systems.

Motivated by the resistive switchings in transition-metal oxides (TMOs) induced by a voltage bias, we study the far-from-equilibrium dynamics of an electric-field-driven strongly correlated model featuring a first-order insulator-to-metal transition at equilibrium, namely the dimer-Hubbard model. We use a nonequilibrium implementation of the dynamical cluster approximation to access the steady-state spectral and transport properties. We show that the electric field can drive both metal-to-insulator and insulator-to-metal transitions. While they proceed by quite distinct mechanisms, specifically simple heating of the metal versus nonequilibrium effects in the correlated charge gap, we show that both of these nonequilibrium transitions can be unified in a single framework once the excitations are accounted for in terms of an effective temperature. This conceptual advance brings together the two sides of the long-lasting debate over the origins of the electrically driven resistive switching in TMOs.

In this work, we report the fully relativistic (FR) first-principles quantum transport simulation of noncollinear spin transfer and spin Hall current in the device structure. In this method, the noncollinear FR exact muffin-tin orbital method is combined with Keldysh's nonequilibrium Green's function approach and mean-field theory to account for the multiple disorder scattering. We adopt the Bargmann-Wigner polarization operator to define the appropriate FR spin current so that the current-induced spin transfer, in the noncollinear magnetic device or due to the spin Hall effect, can be studied from first principles. As applications, we calculate the spin transfer torque in noncollinear spin valves Co/Cu/FM/Cu (FM = Co, ${\mathrm{Ni}}_{0.8}{\mathrm{Fe}}_{0.2}$) and spin Hall angles in various ${\mathrm{Pt}}_{1-x}{Y}_x$ [$Y$ = vacancy (Va), Au, Ag, Pd] alloys. We find that our FR results agree well with previous theoretical simulations and experimental measurements. Moreover, it is found that the applied finite bias can significantly enhance the spin Hall angle in ${\mathrm{Pt}}_{1-x}{\mathrm{Va}}_x$, and PtAg alloy presents a much higher spin Hall angle than that of PtAu and PtPd alloys. Our implementation of the FR method provides an important first-principles tool for studying various nonequilibrium spin phenomena and the associated relativistic effects in realistic device structures with atomic disorders, including both current-induced spin transfer and spin-orbit torques.

The hydrodynamic behavior of charged carriers leads to nonlinear phenomena such as solitary waves and shocks, among others. As an application, such waves might be exploited on plasmonic devices either for modulation or signal propagation along graphene waveguides. We study the nature of nonlinear perturbations following an approach similar to Sagdeev potential analysis and also by performing the reductive perturbation method on the hydrodynamic description of graphene electrons, taking into consideration the effect of Bohm quantum potential and odd viscosity. Thus, deriving a dissipative Kadomtsev-Petviashvili-Burgers (KPB) equation for the bidimensional flow as well as its unidimensional limit in the form of Korteweg–de Vries–Burgers (KdVB). The stability analysis of these equations unveils the existence of unstable modes that can be excited and launched through graphene plasmonic devices.

The elastic and thermal properties of graphene were investigated by illuminating graphen bubbles with a laser spot. Tempertures above $1000{}^{\circ }\mathrm{C}$ were obtained in large ($>10\mu \mathrm{m}$) graphene bubbles. The formation of standing optical waves lead to laser heating depending on the height of the garphene bubble, which results in Raman band oscillations when scanning the laser spot across the bubble. The profile of the bubble under laser illumination can be deduced from the Raman G band oscillations. A distinct swelling at the center of the bubble is observed which is attributed to the strong softening of graphene above $1000{}^{\circ }\mathrm{C}$. From the size and height of the swelling it is deduced that the elastic modulus is reduced by at least $40\%$ at $1000{}^{\circ }\mathrm{C}$. On solving the heat equation for the heat dissipation through the graphene only, analytical expressions are obtained for the isotherms on the bubble for both symmetrical and asymmetrical positions of the laser spot. It was found that a large fraction of the absorbed heat dissipates through the gas in the bubble using the finite volume method. Analytical expressions for the temperature distribution in the bubble are deduced from the numerical results. Heat conduction through the gas in the bubble influences the temperature distribution and needs to be taken into account in the heat dissipation in graphene bubbles.

We study the collective excitation modes of the Chern insulator states in magic-angle twisted bilayer graphene aligned with hexagonal boron nitride (TBG/BN) at odd integer fillings ($\nu$) of the flat bands. For the $1\times 1$ commensurate double moiré superlattices in TBG/BN at three twist angles (${\theta }^{\prime }$) between BN and graphene, self-consistent Hartree-Fock (HF) calculations show that the electron-electron interaction and the broken $C_{2z}$ symmetry lead to the Chern-insulator ground states with valley-spin flavor polarized HF bands at odd $\nu$. In the active-band approximation, the HF bands in the same flavor of TBG/BN are much more separated than those of the pristine TBG with TBG/BN having a larger intraflavor band gap so that the energies of the lowest intraflavor exciton modes of TBG/BN computed within the time-dependent HF method are much higher than those of TBG and reach about 20 meV and the exciton wave functions of TBG/BN become less localized than those of TBG. The interflavor valley-wave modes in TBG/BN have excitation energies higher than 2.5 meV, which is also much larger than that of TBG, while the spin-wave modes all have zero excitation gap. In contrast to TBG with particle-hole symmetric excitation modes for positive and negative $\nu$, the excitation spectrums and gaps of TBG/BN at positive $\nu$ are rather different from those at negative $\nu$. The quantitative behavior of the excitation spectrum of TBG/BN also varies with ${\theta }^{\prime }$. Full HF calculations demonstrate that more HF bands besides the two central bands can have rather large contributions from the single-particle flat-band states; then the lowest exciton modes that determine the optical properties of the Chern insulator states in TBG/BN are generally the ones between the remote and flatlike bands, while the valley-wave modes have similar energies as those in the active-band approximation.

We investigate the external pressure dependence of the crystal structure, Yb valence, and local magnetization in ${\mathrm{YbMn}}_6{\mathrm{Ge}}_{6-x}{\mathrm{Sn}}_x$ using x-ray diffraction, x-ray absorption near edge spectroscopy (XANES), and x-ray magnetic circular dichroism (XMCD). The application of physical pressure results in a weakly anisotropic unit cell volume contraction without modifying the crystal structure. The external pressure allows crossing the Yb magnetic instability, which occurs for strongly hybridized Yb (valence $\approx 2.76$), with a concomitant slope change in the valence-pressure curve. In the Ge-rich compounds, Yb local magnetism vanishes at very high pressure ($P \ge 39.8$ GPa) likely because of the simultaneous collapse of the Mn magnetism. The results underline similarities between external and chemical pressure increase, which favors trivalent and magnetic Yb. Some differences are observed in the Sn-rich compounds highlighting potential local pressure and/or electronic effects.

We calculated the diffusion coefficient of hydrogen (H) on metal surfaces. In our method, the potential energy surface obtained from the ab initio calculations is diagonalized to calculate the wave function of H, which is used to obtain the free energy surface. We combined this free energy surface with transition state theory to obtain the diffusion coefficient. This combination of the free energy surface and transition state theory allowed us to include quantum effects and entropy effects in the transition state theory without stochastic simulations. In addition, by defining diffusion sited from the free energy surface rather than the potential energy surface, the shape and size of the diffusion sites became temperature dependent. The calculated diffusion coefficients were compared with previous studies. Although the present method reproduced the results at and above the crossover temperatures, it was found that more sophisticated quantum mechanical schemes are required at cryogenic temperatures.