Non-Hermiticity from nonreciprocal hoppings has been shown recently to demonstrate the non-Hermitian skin effect (NHSE) under open boundary conditions (OBCs). Here we study the interplay of this effect and the Anderson localization (AL) in a nonreciprocal quasiperiodic lattice, dubbed nonreciprocal Aubry-André model, and a rescaled transition point is exactly proved. The nonreciprocity can induce not only NHSEs but also the asymmetry in localized states, characterized by two Lyapunov exponents. Meanwhile, this transition is also topological, in the sense of a winding number associated with complex eigenenergies under periodic boundary conditions (PBCs), establishing a bulk-bulk correspondence. This interplay can be realized straightforwardly by an electrical circuit with only linear passive RLC components instead of elusive nonreciprocal ones, showing the transport of a continuous wave undergoes a transition between insulating and amplifying. This paradigmatic scheme can be immediately accessed in experiments even for more nonreciprocal models and will definitely inspire the study of interplay of NHSEs and ALs as well as more other quantum/topological phenomena in various systems.

The intrinsic and effective anisotropies, both in the liquid and solid vortex regimes, of $\mathrm{YB}{\mathrm{a}}_2\mathrm{C}{\mathrm{u}}_3{\mathrm{O}}_7$ (YBCO) pristine and nanocomposite thin films have been investigated. Angular resistivity measurements under varying fields and temperatures were performed to characterize the intrinsic vs effective anisotropy of samples in the regime of long-range vortex displacements. The effective anisotropy ${\gamma }_{\mathrm{eff}}$ was determined from the scaling of the irreversibility line, applying the Blatter approach developed for uniaxial anisotropic superconductors. Resistive measurements in flux flow were utilized, in addition to $H_{c2}$ measurements in ultrahigh-fields, to determine the intrinsic anisotropy mass γ, enabling the study of a large number of samples with varied nanoparticle compositions. In order to access the intrinsic anisotropy in the vortex solid phase, complex impedance measurements at high microwave frequencies were performed, allowing us to access the flux-flow intrinsic anisotropy in the regime of very short vortex oscillations within the pinning potential wells. Results show that while the effective anisotropy ${\gamma }_{\mathrm{eff}}$ decays as the nanoparticles-induced nanostrain in the YBCO films increases, the intrinsic anisotropy γ (determined both in dc and at microwave frequency) remains unaltered.

We consider a fluctuating superconductor in the vicinity of the transition temperature, $T_c$. The fluctuation shear viscosity is calculated. In two dimensions, the leading correction to viscosity is negative and scales as $\delta \eta \left(T\right)\propto ln(T-T_c)$. Critical hydrodynamics of the fluctuating superconductor involves two fluids: a fluid of fluctuating pairs and a quasiparticle fluid of single-electron excitations. The pair viscosity (Aslamazov-Larkin) term is shown to be zero. The (density of states) correction to viscosity of single-electron excitations is negative, which is due to fluctuating pairing that results in a reduction of electron density. Scattering of electrons off of the fluctuations gives rise to an enhanced quasiparticle scattering and another (Maki-Thomson) negative correction to viscosity. Our results suggest that fluctuating superconductors provide a promising platform to investigate low-viscosity electronic media and may potentially host fermionic/electronic turbulence. Some experimental probes of two-fluid critical hydrodynamics are proposed such as time-of-flight measurement of turbulent energy cascades in critical cold atom superfluids and magnetic dynamos in three-dimensional fluctuating superconductors.

This paper presents a study of dc photocurrents in biased insulators to the third order in the electric field. We find three photocurrents which are characterized by physical divergences of the third-order free-electron polarization susceptibility. In the absence of momentum relaxation and saturation effects, these dc photocurrents grow as $t^n(n=2,1,0)$ with illumination time. The photocurrents are dubbed jerk, third-order injection, and third-order shift current, respectively, and are generalizations of the second-order injection and shift currents of the bulk photovoltaic effect (BPVE). We also revisit the theory of the second-order bulk photovoltaic effect and include Fermi surface contributions which are important in metals. Finally, we show that injection, shift, and jerk currents admit simple physical interpretations in terms of semiclassical wave packet dynamics in electric fields. Experimental signatures and extensions to higher-order susceptibilities are also discussed.

${\mathrm{Cu}}_4$ triplet clusters form the relevant spin entities for the formation of long-range magnetic order in the cluster Mott insulator ${\mathrm{Cu}}_2{\mathrm{OSeO}}_3$. Using time-resolved Raman spectroscopy, we probed photoinduced spin and lattice dynamics in this cluster magnet. Multiple picosecond-decade spin-lattice relaxation dynamics is observed, evidencing a separation of the order parameter dynamics into disordering of long-range and cluster-internal order on the tens and 100s on the picosecond timescale, respectively. We argue that this separation originates in vastly different interaction strengths between low- and high-energy spin cluster excitations and acoustic phonons. Our results exemplify the double-order parameter dynamics intrinsic to the novel quantum material class of cluster Mott insulators.

Due to strong spin-orbit coupling, charge carriers in three-dimensionsal quadratic-band-touching Luttinger semimetals have nontrivial wave functions characterized by a pseudospin of 3/2. We compute the dielectric permittivity of such semimetals at finite doping, within the random phase approximation. Because of interband coupling, the dielectric screening shows a reduced plasma frequency and an increased spectral weight at small wave vectors compared to a regular quadratic band. This weakens the effective Coulomb potential, modifying the single-particle self-energy for which we present both analytical and numerical results. At a low carrier density, the quasiparticle properties of this model strongly deviate from that of a single quadratic band. We compare our findings with experimental results for $\alpha$-Sn, HgSe, HgTe, YPtBi, and ${\mathrm{Pr}}_2{\mathrm{Ir}}_2{\mathrm{O}}_7$.

Bosonization is one of the most significant frameworks to analyze fermionic systems. In this work, we propose a new bosonization of Dirac fermion coupled with $U\left(1\right)$ background gauge field. Our new bosonization is consistent with gauge invariance, global chiral anomaly matching and fermion-boson operator correspondence, either of which is not satisfied by previously developed bosonizations. The bilinear Dirac-mass term condensation paradox and its generalized form are resolved by our bosonization. This new bosonization approach to interacting systems also correctly captures the conformal characters of a significant class of critical lattice models, such as conformal dimensions and the conformal anomaly of XXZ spin chain with twisted boundary condition. Our work clarifies the equivalence of fermionic flux insertion and bosonic background charge insertion for two-dimensional conformal field theory.

We study the many-body ground states of $\mathrm{SU}\left(N\right)$ symmetric hardcore bosons on the topological flat-band model by using controlled numerical calculations. By introducing strong intracomponent and intercomponent interactions, we demonstrate that a hierarchy of bosonic $\mathrm{SU}\left(N\right)$ fractional quantum Hall (FQH) states emerges at fractional filling factors $\nu =N/(MN+1)$ (odd $M=3$). In order to characterize this series of FQH states, we figure the effective $\mathbf{K}$ matrix from the inverse of the Chern number matrix. The topological characterization of the $\mathbf{K}$ matrix also reveals quantized drag Hall responses and fractional charge pumping that could be detected in future experiments. In addition, we address the general one-to-one correspondence to the spinless FQH states in topological flat bands with Chern number $C=N$ at fillings $\tilde{\nu }=1/(MC+1)$.

Quantum dots based on the graphene stripes show unconventional optical properties in the THz frequency range. The graphene quantum dot (GQD) is made of an electrically gated stripe with zigzag edges. Inside the active region (AR), which is enclosed between the source and drain electrodes, there are two sharp energy ($\pm$) levels, whose separation $2\mathrm{\Delta }$ is controlled with Stark effect by applying the lateral dc electric field. Such edge states determine the unique nature of elementary excitations, chiral fermions, that are responsible for the nonlinear optical response revealing a potential for many applications. They are, e.g., the frequency multiplication and self-focusing of two-dimensional solitons. Furthermore, when injection of the nonequilibrium electrons causes an inverse population of the levels localized in AR, the subsequent recombination of electrons and holes leads to a coherent emission of the THz waves.

In this paper, first-principles calculations provide structural characterization of three low-index Mg surfaces—Mg(0001), $\mathrm{Mg}(10\overline{1}0$), and $\mathrm{Mg}(11\overline{2}0$)—and their respective surface core-level shifts (SCLSs). Inspired by the close similarities between Be and Mg surfaces, we also explore the reconstruction of $\mathrm{Mg}(11\overline{2}0$). Through the calculation of surface energies and the use of the angular-component decomposed density of states, we show that reconstructions are likely to occur at the $\mathrm{Mg}(11\overline{2}0$) surface, similarly to what was found earlier for $\mathrm{Be}(11\overline{2}0$). Indeed, the surface energy of some of the explored reconstructions is slightly lower than that of the unreconstructed surface. In addition, because of lattice symmetry, the morphology of the unreconstructed surface ($11\overline{2}0$) results in a steplike zig-zag chain packing, with topmost chains supporting a resonant, quasi-one-dimensional (1D), partially filled electronic state. As the presence of partially filled quasi-1D bands is a necessary condition for Peierls-like dimerization, we verify that the undimerized surface chain remains stable with respect to it. Some of the reconstructions, namely, the $2\times 1$ and $3\times 1$ added row reconstructions, induce a stronger relaxation of the topmost chains, increasing the coupling with lower layers and thus significantly damping the quasi-1D character of this state. The original approach followed offers a common and general framework to identify quasi-1D bands—even in the case of resonant electronic surface states—and to meaningfully compare calculated and measured SCLSs even in the presence of multicomponent peak contributions.

Reflection characteristics of surface plasmon polariton (SPP) by a rectangular ridge on the silver surface were examined theoretically and experimentally. The amplitude and phase shift of the SPP reflection by the rectangular ridge were calculated by SPP scattering theory using Green's tensor method and expressed in terms of ridge width and SPP energy. It was found that this calculation result can be reproduced well by a simple model expressed by superposition of reflections by the two steps constituting the ridge. A one-dimensional cavity composed of a pair of rectangular ridges was fabricated, and the characteristics of the SPP standing wave mode (cavity mode) was clarified for the first time using the scanning transmission electron microscope (STEM) - cathodoluminescence (CL) method and was used to evaluate the theoretical calculations of the reflection coefficient. The cavity mode energies are derived from the angle-resolved spectral (ARS) pattern, and the phase shift of the SPP reflection can be deduced through the cavity condition. We successfully explained the resonance energy of the cavity mode and the spatial distribution of the SPP standing wave by using the SPP reflection coefficient of the rectangular ridge derived by the theory.

We explore a source-free local spin density approximation (LSDA)$+U$ functional within density functional theory for its capabilities in describing noncollinear spin textures; capturing the noncollinear magnetic ground state of the spinel vanadates $A{\mathrm{V}}_2{\mathrm{O}}_4$ ($A=\text{Mn}$, Fe, and Co) remains an outstanding challenge for state-of-the-art ab initio methods. We demonstrate that both the noncollinear spin texture, as well as the magnitude of local moments, are captured, provided the source term (i.e., magnetic monopole term) is removed from the exchange-correlation magnetic field ${\mathbf{B}}_{\text{XC}}$. This suggests that for treatment of strongly correlated magnetic materials within the $\mathrm{LSDA}+U$ method the subtraction of the unphysical magnetic monopole term from the ${\mathbf{B}}_{\text{XC}}$ is essential.

To elucidate the energy transfer and reexcitation processes in Eu-doped GaN layers that are used in recently developed, highly efficient red light-emitting diodes, a systematic series of photoluminescence and time-resolved photoluminescence (TR-PL) measurements was performed. Critical insights on how “slow” Eu processes (∼µs) can compete against fast semiconductor processes (∼ps) are revealed using TR-PL with a high temporal resolution, as it is found that the initial energy transfer from GaN to the $\mathrm{E}{\mathrm{u}}^{3+}$ ions takes place rapidly, on a timescale of <100 ps. Below band-gap resonant excitation was used to identify the states into which the energy transfer occurs. For the most efficient Eu defect complexes, this transfer dominantly occurs directly into the ${}^{5}D_0$ state of $\mathrm{E}{\mathrm{u}}^{3+}$. Less efficient complexes also exhibit transfer into the ${}^{5}D_2$ state, the emission of which can be detected using photoluminescence at low temperature, indicating the importance of the excitation pathway on device efficiency. Under high excitation intensity, reexcitation can also occur, leading to a redistribution of population into the ${}^{5}D_2$, ${}^{5}D_1$, or ${}^{5}D_0$ states.

We analyze the non-Hermitian Haldane model where the spin-orbit interaction is made non-Hermitian. The Dirac mass becomes complex. We propose to realize it by an $LC$ circuit with operational amplifiers. A topological phase transition is found to occur at a critical point where the real part of the bulk spectrum is closed. The Chern number changes its value when the real part of the mass becomes zero. In the topological phase of a nanoribbon, two non-Hermitian chiral edges emerge connecting well-separated conduction and valence bands. The emergence of the chiral edge states is signaled by a strong enhancement in impedance. Remarkably, it is possible to observe either the left-going or right-going chiral edge by measuring the one-point impedance. Furthermore, it is also possible to distinguish them by the phase of the two-point impedance. Namely, the phase of the impedance acquires a dynamical degree of freedom in the non-Hermitian system.

We derive exact sum rules that relate the tunneling density-of-states of spinful electrons in the fractional quantum Hall regime to the spin-dependent many-body ground-state correlation energy. Because the tunneling process is spin-conserving, the two-dimensional (2D) to 2D tunneling current $I$ at a given bias voltage $V$ in a spin-polarized system is a sum of majority and minority spin contributions. The sum rules can be used to define spin-dependent gaps that we associate with peaks in 2D to 2D tunneling $I\text{−}V$ curves. We comment in the light of our sum rules on what recent tunneling experiments say about the spin-dependence of correlation energy contributions and propose new measurements that could provide more specific experimental estimates.

Revealing the spin excitations of complex quantum magnets is key to developing a minimal model that explains the underlying magnetic correlations in the ground state. We investigate the low-energy magnons in $\alpha \text{−}{\mathrm{RuCl}}_3$ by combining time-domain terahertz spectroscopy under an external magnetic field and model Hamiltonian calculations. We observe two absorption peaks around 2.0 and 2.4 meV, which we attribute to zone-center spin waves. Using linear spin-wave theory with only nearest-neighbor terms of the exchange couplings, we calculate the antiferromagnetic resonance frequencies and reveal their dependence on an external field applied parallel to the nearest-neighbor Ru-Ru bonds. We find that the magnon behavior in an applied magnetic field can be understood only by including an off-diagonal $\mathrm{\Gamma }$ exchange term to the minimal Heisenberg-Kitaev model. Such an anisotropic exchange interaction that manifests itself as a result of strong spin-orbit coupling can naturally account for the observed mixing of the modes at higher fields strengths.

We introduce non-Hermitian generalizations of Majorana zero modes (MZMs) which appear in the topological phase of a weakly dissipative Kitaev chain coupled to a Markovian bath. Notably, the presence of MZMs ensures that the steady state in the absence of decoherence events is twofold degenerate. Within a stochastic wave-function approach, the effective Hamiltonian governing the coherent nonunitary dynamics retains BDI classification of the closed limit but belongs to one of four non-Hermitian “flavors” of the tenfold way. We argue for the stability of MZMs due to a generalization of particle-hole symmetry and uncover the resulting topological phase diagram. Qualitative features of our paper generalize to two-dimensional chiral superconductors. The dissipative superconducting chain can be mapped to an Ising model in a complex transverse field, and we discuss potential signatures of the degeneracy.

We investigate the infrared optical transitions between excitons of the yellow, green, and blue series in the cuprous oxide ${\mathrm{Cu}}_2\mathrm{O}$. We show that, in many cases, the dipole approximation is inadequate and, in particular, that it breaks down in yellow-blue transitions even for moderate principal quantum numbers of $n\approx 10$. The interband matrix elements of the transition operator needed for the evaluation of the excitonic transition strengths are derived from known as well as from fitted band parameters.