Kondo lattices are one of the classic models of strongly correlated systems where despite a long history, a full understanding of the excitation spectra is still not available. Here we propose that recent progress in engineering heterostructures can be leveraged to gain insight into and even tune this spectra. We use a strong Kondo coupling expansion to study spin-1 excitations of a Kondo lattice in both one and two dimensions to see whether paramagnons in a Kondo insulator fractionalize into spin-1/2 excitations. We show that while paramagnons are stable in the strong Kondo coupling limit, the presence of sufficient proximity-induced superconducting pairing can favor fractionalization. Our results can be checked using a neutron scattering study of Kondo-lattice heterostructures, and they can pave the way toward engineering strongly correlated electronic systems.

The Weyl semiconductor is a type of semiconductor material that exhibits unique electronic properties. Tellurium (Te) is a recently discovered quantum material that exhibits quasilinear electronic dispersion, as opposed to the linear dispersion commonly observed in conventional Weyl semimetals. Its electrical- and magnetotransport properties, especially in Te crystals with different Fermi energy, have not been thoroughly investigated in prior studies. Here we successfully grew a series of Te crystals with varied Fermi energy through different temperature-cooling rates in the self-flux method, whose hole concentration can be adjusted from ${10}^{15}\mathrm{c}{\mathrm{m}}^{-3}$ to ${10}^{16}\mathrm{c}{\mathrm{m}}^{-3}$. In the case of low hole-concentration (${10}^{15} \mathrm{c}{\mathrm{m}}^{-3}$) Te crystals, temperature-dependent resistance shows abstract semiconductor-metal-semiconductor transition behavior with temperature varied. However, in the case of high carrier concentration, it demonstrates a transition from a semiconductor to a metallic state at a temperature $T$ ($T\sim 50\mathrm{K}$). Remarkably, the magnetoresistance (MR), under paralleled electric ($E$) and magnetic ($B$) fields ($E\parallel B$), evolves from dominant weak-antilocalization behavior in low hole-concentration samples to coexistence of weak-localization and chiral-anomaly effect in high hole-concentration ones. The dependences of coefficient $C_W$ of chiral anomaly in Te crystals with different Fermi energy on misaligned angle θ between $E$ and $B$, and $T$, were systematically analyzed. The band structure of Te and the presence of an electronic band tail resulting from high hole-concentration doping were verified using angle-resolved photoemission spectroscopy. This study quantitatively discusses the complex magnetotransport evolution of Te crystals with varying Fermi energy. The analysis is based on the impurity band and Berry curvature of Weyl points within this system. The research conducted contributes to the advancement of knowledge regarding the transport properties exhibited by Weyl semiconductors.

The 1538-nm ${}^4{\mathrm{I}}_{15/2} \to {}^4{\mathrm{I}}_{13/2}$ transition of ${\mathrm{Er}}^{3+}$ has an unusual hybrid electric-magnetic dipole character, and signatures of the hybrid moment can be expected in coherent transient measurements. Here, we investigate the effect of the hybrid moment in both sites of ${\mathrm{Er}}^{3+}$:${\mathrm{Y}}_2{\mathrm{SiO}}_5$ on Stark-modulated photon echo measurements, showing that it results in a reduction of visibility of the modulated signal as well as phase and polarization shifts. We interpret these effects using a simple optical Bloch equation model, showing that site 1 has a strongly mixed moment and site 2 is predominantly magnetic dipole. We discuss the implications of the hybrid moment for quantum information applications of quantum memories. We also use the echo measurements to extract the Stark shift of the optical transition along three orthogonal directions, finding values between 10.50 and $11.93\mathrm{kHz}{\mathrm{V}}^{-1}\mathrm{cm}$ for site 1 and 1.61 and $15.35\mathrm{kHz}{\mathrm{V}}^{-1}\mathrm{cm}$ for site 2, dependent on the direction of the electric field. We observe a modification of the Zeeman shift by the electric field in both sites and discuss how this may be used to electrically control ${\mathrm{Er}}^{3+}$ spin qubits.

The Anderson impurity model (AIM) has long served as a cornerstone in the study of correlated electron systems. While numerical renormalization group (RG) offers great flexibility for metallic reservoirs, it becomes impossible in an unbiased way when a spectral gap $\mathrm{\Delta }$ opens up in the tunneling density of states. The only known exception is provided by the superconducting bath. In this paper, we lift these limitations by developing a numerical RG procedure that employs a discretization of the gapped tunneling densities of states into patches that accumulate at the gap edges. This reveals an unusual double scaling, which is a shared behavior between the superconducting and the scalar gapped AIMs. Moreover, it requires a special iterative diagonalization procedure with an alternating scheme for discarding states only every second iteration. The discretization and the diagonalization scheme form together what we refer to as the log-gap numerical RG. It is successfully applied to the superconducting and to the scalar gapped AIM. Consequently, it reveals that both models belong to the same RG equivalence class, which manifests physically in common singlet-doublet quantum phase transitions accompanied by in-gap bound states of given parities. While superconducting AIM is mainly used for benchmarking the log-gap numerical RG, we also rigorously confirm the phenomenon of in-gap states escaping into the continuum. The gapped AIM is then tackled in an exact numerical RG approach and confirms quantitatively the assertions based on models with auxiliary metallic leads, but reveals that the other common approach used, for example, in the work of Moca and Roman [Phys. Rev. B 81, 235106 (2010)] is of strictly approximate nature.

We extend the natural orbital impurity solver [Y. Lu, M. Höppner, O. Gunnarsson, and M. W. Haverkort, Phys. Rev. B 90, 085102 (2014)] to finite temperatures and apply it to calculate spectral and transport properties of correlated electrons within the dynamical mean-field theory. First, we benchmark our method against the exact diagonalization result for small clusters, finding that the natural orbital scheme works well not only for zero temperature but for low finite temperatures. The method yields smooth and sufficiently accurate spectra, which agree well with the results of the numerical renormalization group. Using the smooth spectra, we calculate the electric conductivity and Seebeck coefficient for the two-dimensional Hubbard model at low temperatures which are within the scope of many experiments and practical applications. These results demonstrate the usefulness of the natural orbital framework for obtaining the real frequency information of correlated electron systems.

The hybridization of different topological phases is attractive because it can facilitate the simultaneous utilization of their respective advantages. Three-dimensional (3D) higher-order topological insulators host gapless hinge states or corner states, while Weyl semimetals have novel properties such as chiral anomaly. However, so far, the general method to induce 3D composite topological phases with coexisting Weyl semimetals and higher-order topological insulators is still lacking. Here, we propose a scheme to induce this kind of composite topological phase by periodic driving and demonstrate that such a phase is the intermediate phase between anomalous Floquet higher-order topological insulators and Floquet first-order Weyl semimetals. Our results may open a possibility to realize different phases without static analogs.

Non-Hermitian physics has added ingredients to topological physics, leading to the rising frontier of non-Hermitian topological phases. In this paper, we investigate Chern insulator phases emerging from non-Hermitian kagome models with nonreciprocal and pure imaginary next-nearest-neighbor hoppings. In the presence or absence of $C_3$ rotation symmetry, hybrid topological-skin effects are explored through the identification of distinct corner skin modes in different energy regions within two band gaps. By employing the dynamical analysis, the underlying physics is revealed from the non-Hermitian skin effects associated with the chiral edge states, leading to diverse non-Hermitian bulk-boundary responses. The simplicity of these kagome models and their rich emergent topological phenomena suggest that they are appealing candidates for studying non-Hermitian topological phases. We further discuss the possible realizations of these models in non-Hermitian metamaterials.

We theoretically predict the existence of a nonzero magnetochiral anisotropy-induced nonlinear Hall effect or a second harmonic Hall voltage transverse to an applied current in spin-orbit coupled Rashba conductors in the presence of an in-plane magnetic field $\mathbf{B}$. This is distinct from the Berry curvature dipole (BCD)-induced nonlinear Hall effect in systems with a nontrivial band structure because the former requires broken time-reversal symmetry while the BCD-induced effect is nonzero even in time-reversal symmetric systems. This result is independent of the existence of other types of interactions, such as hexagonal warping or cubic Dresselhaus interactions. We find that for $\mathbf{E}\parallel \mathbf{B}$, the magnitude of the nonlinear Hall current flowing in a direction perpendicular to the applied electric field is exactly $1/3$ of the magnitude of the nonlinear rectification current responsible for magnetoresistance parallel to the applied electric field obtained in the $\mathbf{E}\perp \mathbf{B}$ configuration.

We develop the many-body theory of dipolar exciton-polaritons in an optical microcavity in crossed out-of-plane electric and in-plane magnetic fields. Even for relatively weak fields, we reveal the existence of two minima in the bare lower-polariton dispersion, which give rise to the tunable transition between the polariton Bose-Einstein condensate and that of excitons, produced by the competition between these minima. We predict that such dipolar condensate exhibits a roton-maxon character of the excitation spectrum, never before observed for polaritons. We show that upon the transition between the two condensation regimes, the weak correlations in the polariton gas give way to the intermediate interparticle correlations characteristic for excitons, and that the transition is accompanied by a sharp quenching of photoluminescence as the lifetime is increased by several orders of magnitude. While in the polariton regime, the luminescence peak corresponding to the condensate is shifted to a nonzero angle. The angular dependence of the two-photon decay time in the Hanbury Brown and Twiss experiment is calculated and used as a tool to evidence the formation of the macroscopically coherent state. Our proposal opens opportunities towards manipulating the superfluid properties and extended-range dipole-dipole correlations of exciton-polariton condensates.

Optically driven electron spins coupled in quantum dots to nuclear spins show a prepulse signal (revival amplitude) after having been brought to a specific nonequilibrium stationary state by long periodic sequences of pulses. The effect bears similarity to the spin Hahn echo in nuclear magnetic resonance. The size of this revival amplitude depends on the external magnetic field in a specific way due to the varying commensurability of the nuclear Larmor precession period with the time $T_{\text{rep}}$ between two consecutive pulses. In theoretical simulations, sharp dips occur at fields when an integer number of precessions fits in $T_{\text{rep}}$; this feature could be used to identify nuclear isotopes spectroscopically. But these sharp and characteristic dips have not (yet) been detected in experiment. We study whether the nuclear quadrupolar interaction is the reason for this discrepancy because it perturbs the nuclear precessions. But our simulations show that the absolute width of the dips and their relative depth are not changed by quadrupolar interactions. Only the absolute depth decreases. We conclude that quadrupolar interaction alone cannot be the reason for the absence of the characteristic dips in experiment.

Periodic driving and Floquet engineering have emerged as invaluable tools for controlling and uncovering novel phenomena in quantum systems. In this study, we adopt these methods to manipulate nonequilibrium processes within electronic-vibronic open quantum systems. Through resonance mechanisms and by focusing on the limit-cycle dynamics and quantum thermodynamic properties, we illustrate the intricate interplay between the driving field and vibronic states and its overall influence on the electronic system. Specifically, we observe an effective decoupling of the electronic system from the periodic driving at specific frequencies, a phenomenon that is mediated by the vibrational mode interaction. Additionally, we engineer the driving field to obtain a partial removal of the Franck-Condon blockade. These insights hold promise for efficient charge current control. Our results are obtained from numerically exact calculations of the hierarchical equations of motion and further analyzed by a time-periodic master-equation approach.

We consider non-Hermitian energy band theory in two-dimensional systems, and study eigenenergy braids on slices in the two-dimensional Brillouin zone. We show the consequences of reciprocity and geometric symmetry on such eigenenergy braids. The point-gap topology of the energy bands can be found from the projection of the eigenenergy braid onto the complex energy plane. We show that the conjugacy class transition in the eigenenergy braid results in changes in the number of bands in a complete point-gap loop. This transition occurs at exceptional points. We numerically demonstrate these concepts using two-dimensional reciprocal and nonreciprocal photonic crystals.

We have theoretically investigated the spin- and valley-dependent transport in the bilayer silicene magnetic superlattices consisting of alternatively ferromagnetic and nonmagnetic domain. The calculation with a four-band scattering matrix method demonstrates that due to the spin- and valley-resolved band splitting in the field-tunable ferromagnetic domains, perfect spin and valley polarization can be engineered for the symmetric superlattice only in its parallel magnetization configuration, while for the asymmetric superlattice, it can be engineered in both the parallel and antiparallel magnetization configurations. For both types of superlattice, a strong contrast in the configuration-dependent conductance makes a field-controllable, yet colossal tunneling magnetoresistance accessible. These features indicate that a bilayer silicene magnetic superlattice might be a potential choice for silicene-based spin-valleytronic applications.

We propose an analytical formulation for the scanning gate microscopy (SGM) response to local tips with arbitrary strength in two terminal nanostructures. The real space resolved conductance is expressed in terms of the unperturbed quantities underlying the scattering problem. Providing a nondynamical approach for obtaining the SGM maps, the proposed expression enables for a significant reduction in the computational cost of SGM response calculations. This feature is particularly advantageous for deep learning-based approaches which have been recently proposed for accessing local properties and disorder landscapes from conductance measurements. This opens up new possibilities for the SGM technique and holds exciting prospects for quantum transport. Further, the formula's versatility extends beyond this specific application, offering a straightforward and computationally efficient method for obtaining the SGM response in a more general context.

A single Josephson junction in the phase-slip regime exhibits Bloch oscillations in the voltage when biased with a dc current $I_{\text{dc}}$. The frequency of the oscillation is given by $\pi I_{\text{dc}}/e$, with $e$ the elementary charge, linking the current to the frequency via fundamental constants of nature. If an additional ac drive is applied, the Bloch oscillations may synchronize with the external drive. This leads to the emergence of dual Shapiro steps at fixed current in the $IV$ characteristics of the device. For applications as a current standard, frequencies of the order of 10 GHz are required. These are challenging to implement experimentally without detrimental effects due to stray capacitances. Here, we propose to employ an additional Josephson junction with a dc voltage bias as an on-chip ac source due to the ac Josephson effect. We study the back action of the Bloch oscillations on the Josephson oscillations and identify a parameter regime in which it is minimized. Furthermore, we find that the back action can even be utilized to further enhance the driving signal, which can lead to increased widths of the resulting dual Shapiro steps. Finally, we show dual Shapiro steps for a set of realistic experimental parameters at finite temperatures.

We demonstrate on-chip generation of indistinguishable photons based on a nanowire quantum dot. From a growth substrate containing arrays of positioned-controlled single-dot nanowires, we select a single nanowire which is placed on a SiN waveguide fabricated on a Si-based chip. Coupling of the quantum dot emission to the SiN waveguide is via the evanescent mode in the tapered nanowire. Postselected two-photon interference visibilities using continuous-wave excitation above band and into a $p$ shell of the dot were 100% consistent with a single-photon source having negligible multiphoton emission probability. Visibilities over the entire photon wave packet, measured using pulsed excitation, were reduced by a factor of 5 when exciting quasiresonantly and by a factor of 10 for above-band excitation. The role of excitation timing jitter, spectral diffusion, and pure dephasing in limiting visibilities over the temporal extent of the photon is investigated using additional measurements of the coherence and linewidth of the emitted photons.

The extremely high pseudo magnetic field emerging in strained graphene suggests that an oscillating nanodeformation will induce a very high current even without electric bias. In this paper, we demonstrate the subterahertz (THz) dynamics of a valley current and the corresponding charge pumping with a periodically excited nanobubble. We discuss the amplitude of the pseudo electric field and investigate the dependence of the pumped valley current on the different parameters of the system. Finally, we report the signature of extra-harmonics generation in the valley current that might lead to potential modern device development operating in the nonlinear regime.

We study both analytically and numerically the interaction between bound states in the continuum (BICs) in 2D Fabry-Pérot (FP) resonators in waveguides with waveguide expansions playing the role of the “mirrors.” The derived analytical model illustratively captures recently proposed twin-BICs (TBICs) stemmed from BICs in individual mirrors as well as the well-known FP BICs related to an FP resonance. We show that the formation of TBICs is a universal phenomenon, which occurs in different types of FP resonators, and is related to the sharp dependence of the phase of mirror reflection coefficient on system parameters near the BIC point in the parameter space of individual mirror. We also show explicitly that the complicated multimode interference taking place in the mirrors resulting in the coalescence of two perfect transmission Fano antiresonances can explain the merging of corresponding FP BICs without invoking topological concepts. Moreover, the coalescence of antiresonances in an individual mirror can be continuously changed into the crossing or even the avoided crossing (repulsion) of antiresonances by an infinitesimal change of parameter. The behavior of FP BICs follows the antiresonances pretty closely and demonstrates a similar transformation (either merging or repulsion, respectively). The results of the analytical model are confirmed by numerical simulations of quantum-mechanical and optical waveguides.

Two metals at different temperatures separated by large gaps exchange heat under the form of electromagnetic radiation. When the separation distance is reduced and they approach contact (nanometer and subnanometer gaps), electrons and phonons can tunnel between the bodies, competing and eventually going beyond the flux mediated by thermal photons. In this transition regime the accurate modeling of electronic current and heat flux is of major importance. Here we show that, in order to quantitatively model this transfer, a careful description of the tunneling barrier between two metals is needed and going beyond the traditional WKB approximation is also essential. We employ analytical and numerical approaches to model the electronic potential between two semi-infinite jellium planar substrates separated by a vacuum gap in order to calculate the electronic heat flow and compare it with its radiative counterpart described by near-field radiative heat transfer. We demonstrate that the results for heat flux and electronic current density are extremely sensitive to both the shape and height of the barrier, as well as the calculation scheme for the tunneling probability, with variations up to several orders of magnitude. Using the proximity force approximation, we also provide estimates for tip-plane geometries. The present work provides realistic models to describe the electronic heat flux, in the scanning-thermal-microscopy experiments.