We investigate the emission directionality of electronic intraband (intersubband) transitions in stacked coupled quantum dots. Using a well-established eight-band $\mathit{k}·\mathit{p}$ method, we demonstrate that the minor contributions from the valence band mixing into the conduction band govern the polarization and emission directionality of electronic $p$-to-$s$-type intraband transitions. Despite that the contribution from the central-cell part to the momentum matrix element is dominant, we find the contribution from the matrix elements among the envelope functions cannot be neglected. With the help of an artificial cuboidal quantum dot, we show that the vertical emission from intraband transitions can be tuned via the emitter's vertical aspect ratio. Subsequently, we show that these results can be transferred to more realistic geometries of quantum dots and quantum dot stacks and demonstrate that the vertical emission can be enhanced from $23\%$ to $46\%$ by increasing the emitter's vertical aspect ratio to the isotropic case with an aspect ratio of 1.0. Therefore, the stacking of a few quantum dots (approximately four for the investigated structures) is already sufficient to improve the vertical radiation significantly. Additionally, we discuss the impact of the number of stacked quantum dots as well as the effect of the interdot coupling strength on the radiation properties.

We propose that the hybridization between two sets of Rashba bands can lead to the unconventional Rashba band structures where the two Fermi circles from different bands own in-plane helical spin textures with the same chiralities, and possess group velocities with the same directions. Through the first-principles calculations, we predict that monolayer ${\mathrm{OsBi}}_2$ hosts such simple and pure unconventional Rashba bands near Fermi energy. Under the weak spin injection, we show that the two Fermi circles from the unconventional Rashba bands both give the positive contributions to the spin-to-charge conversion and thus induce the giant inverse Rashba-Edelstein Effect with large conversion efficiency, which is very different from the conventional Rashba-Edelstein Effect. Our studies not only provide a promising material of monolayer ${\mathrm{OsBi}}_2$ to possess unconventional Rashba bands, but also demonstrate its potential application to achieve highly efficient spin-to-charge conversion in spintronics.

We report measurements of the shot noise of atomic contacts of the ferromagnets cobalt and gadolinium and the strong paramagnets palladium, platinum, and iridium. The contacts are fabricated using the mechanically controlled break-junction technique at 4 K. Single-atom contacts of Co and Gd with conductance smaller than the conductance quantum show reduced noise compared to the expectation for the spin-degenerate single-channel case. We discuss by how far this observation gives evidence for spin-polarized transport. Pd, Pt, and Ir reveal noise levels which are above, but close to the threshold to the spin-degenerate single-channel situation. Since for such multivalent metals more than one channel is expected to contribute to the transport, our findings do not exclude spin-polarized transport for these elements. We observe an anticorrelation between the minimum noise and the bulk Stoner parameter of these elements, supporting that spin polarization might be reflected in the shot-noise signal in strong paramagnets.

In this work, we theoretically study transverse magnetic focusing in a two-dimensional electron gas with strong Rashba spin-orbit interaction when proximitized along its edge with a superconducting contact. The presence of superconducting correlations leads to the emergence of chiral Andreev edge states which—within this weak magnetic field regime—may be pictured as states following semiclassical skipping orbits with an alternating electron-hole nature. The spin-orbit-induced splitting of the Fermi surface causes these carriers to move along cyclotron orbits with different radii, allowing for their spatial spin separation. When Andreev reflection takes place at the superconducting lead, scattered carriers flip both their charge and spin, generating distinguishable features in the transport properties of the device. In particular, we report a notable enhancement of the separation between the spin-split focal points, which scales linearly with the number of Andreev scattering events at the anomalous terminal. We support our results by calculating conductance maps to arbitrary points in the sample that provide a complete image of the ballistic electron-hole cyclotron paths.

The multipolar Kondo problem, wherein the quantum impurity carries higher-rank multipolar moments, has seen recent theoretical and experimental interest due to proposals of novel non-Fermi liquid states and the availability of a variety of material platforms. The multipolar nature of local moments, in conjunction with constraining crystal field symmetries, leads to a vast array of possible interactions and resulting non-Fermi liquid ground states. Previous works on Kondo physics have typically focused on impurities that have two degenerate internal states. In this work, inspired by recent experiments on the tetragonal material ${\mathrm{YbRu}}_2{\mathrm{Ge}}_2$, which has been shown to exhibit a local moment with a quasi-fourfold degenerate ground state, we consider the Kondo effect for such a quasi-quartet multipolar impurity. In the tetragonal crystal field environment, the local moment supports dipolar, quadrupolar, and octupolar moments, which interact with conduction electrons in entangled spin and orbital states. Using renormalization group analysis, we uncover a number of emergent quantum ground states characterized by nontrivial fixed points. It is shown that these previously unidentified fixed points are described by truncated SU(4) Kondo models, where only some of the SU(4) generators (representing the impurity degrees of freedom) are coupled to conduction electrons. Such novel nontrivial fixed points are unique to the quasi-quartet multipolar impurity, reinforcing the idea that an unexplored rich diversity of phenomena may be produced by multipolar quantum impurity systems.

The interplay between magnetism and the topological electronic structure provides a large freedom for designing strong anomalous Hall effect (AHE) materials. A nodal line from band inversion is a typical band structure for generating strong AHE. On the other band, in most collinear antiferromagnets (AFMs), the integral of the Berry curvatures on the Brillouin zone is forced to zero by the joint $TO$ symmetry, where $T$ and $O$ are the time reversal and a space group operation, respectively. Even with inverted band structures, such AFM cannot have an AHE. Therefore, the AFM nodal line band structures constructed by spin degenerated bands do not receive much attention in AHE materials. In this work, we illustrate that such a band structure provides a promising starting point for generating strong local and net Berry curvature and, hence, strong intrinsic AHE. In specific AFM compounds of $A{\mathrm{MnBi}}_2$ ($A=\mathrm{Ca}$ and Yb) with an inverted band structure, we observed a strong AHE induced by a weak spin canting from temperature or doping. The anomalous Hall conductivity continues to grow with the canting angle owing to the nodal line in the band structure. Since such spin canting can be adjusted experimentally via doping, it provides another effective strategy to generate and manipulate a strong AHE.

An extended Bose-Hubbard model (EBHM) with three- and four-body constraints can be feasible in cold atoms in an optical lattice. A rich phase structure including various symmetry-protected topological (SPT) phases is obtained numerically with suitable parameter settings and particle filling. The SPT phase is characterized by the Berry phase as a local topological order parameter and the structure of the entanglement spectrum (ES). Based on the presence of various topological phases, separated by gapless phase boundaries, the EBHM exhibits various bosonic topological pumps, which are constructed by connecting the different SPT phases without gap closing. The bulk topological pumps exhibit the plateau transitions characterized by many-body Chern numbers. For the system with boundary, the center of mass (CoM) under grand canonical ensemble elucidates the contributions of multiple edge states and reveals the topology of the system. We demonstrate that the interacting bosonic pumps obey the bulk-edge correspondence.

Surface lattice resonance (SLR) is the collective excitation of nanoparticle resonances arising from the hybridization between localized surface plasmons (LSPs) and propagating Rayleigh anomalies (RAs). When comparing with the corresponding LSPs, SLRs exhibit a much higher quality factor. In fact, as the quality factor depends on the constituting resonances and their hybridization, how one can parametrize it in an analytic form is an important issue. We have studied the SLRs arising from two-dimensional Au monopartite nanoparticle arrays by angle- and polarization-resolved reflectivity spectroscopy, temporal coupled mode theory (CMT), and finite-difference time-domain simulation. The scattering matrix of the SLRs is formulated, revealing the importance of the spectral detuning and the interaction strengths between the LSP and the RAs in governing the quality factor. We then extend the CMT approach to study bipartite arrays where a nanoparticle dimer is employed and find the coupling between two LSPs plays a major role in further boosting the quality factor. Specifically, the coupling takes part in controlling the detuning factor as well as determining whether the coupled bright or dark mode is hybridized with the RAs. The dark mode hybridization can strongly enhance the quality factor, which is otherwise not possible in the monopartite counterparts.

Thermoelectric phenomena in photoexcited graphene have been the topic of several theoretical and experimental studies because of their potential usefulness in optoelectronic applications. However, available theoretical descriptions of the thermoelectric effect in terms of the Seebeck coefficient do not take into account the role of the photoexcited electron density. In this work, we adopt the concept of effective Seebeck coefficient [G. D. Mahan, J. Appl. Phys. 87, 7326 (2000)] and extend it to the case of a photoexcited two-dimensional (2D) electron gas. We calculate the effective Seebeck coefficient for photoexcited graphene, we compare it to the commonly used “phenomenological” Seebeck coefficient, and we show how it depends on the photoexcited electron density and temperature. Our results are necessary inputs for any quantitative microscopic theory of thermoelectric effects in graphene and related 2D materials.

We identify an effect of chirality in the electrical conduction along magnetic vortices in a Weyl superconductor. The conductance depends on whether the magnetic field is parallel or antiparallel to the vector in the Brillouin zone that separates Weyl points of opposite chirality.

We consider the problem of finite resistance $R$ in superconducting films with geometry of a strip of width $W$ near zero temperature. The resistance is generated by vortex configurations of the phase field. In the first type of process, quantum phase slip, the vortex world line in $2+1$ dimensional space-time is spacelike (i.e., the superconducting phase winds in time and space). In the second type, vortex tunneling, the world line is timelike (i.e., the phase winds in the two spatial directions) and connects opposite edges of the film. For moderately disordered samples, processes of the second type favor a train of vortices, each of which tunnels only across a fraction of the sample. Optimization with respect to the number of vortices yields a tunneling distance of the order of the coherence length $\xi$, and the train of vortices becomes equivalent to a quantum phase slip. Based on this theory, we find the resistance $lnR\sim -gW/\xi$, where $g$ is the dimensionless normal-state conductance. Incorporation of quantum fluctuations indicates a quantum phase transition to an insulating state for $g\lesssim 1$.

The unconventional multigap superconductivity in elemental Pb were reported previously by the surface sensitive tunneling experiments as well as predicted by several theory works. To obtain bulk evidence for such multiple gap behavior, the thermodynamic critical-field $B_{\mathrm{c}}$ was measured along three different crystallographic directions ([100], [110], and [111]) in a high-quality Pb single crystal by means of muon spin rotation/relaxation. No difference in temperature evolution of $B_{\mathrm{c}}$ for all three directions was detected. The average reduced gap $\alpha =\mathrm{\Delta }/k_{\mathrm{B}}{T}_{\mathrm{c}}=2.312\left(3\right)$ ($\mathrm{\Delta }$ is the zero-temperature gap value, and $T_{\mathrm{c}}$ is the transition temperature) was further obtained by employing the phenomenological $\alpha$ model. Our results imply that the elemental Pb is an isotropic superconductor with a single energy gap.

Motivated by the recent discovery of metallic kagome lattice materials, ${\mathrm{AV}}_3{\mathrm{Sb}}_5$ (A = K, Rb, Cs), we investigate the ground state of the half-filled kagome lattice Hubbard model by employing the density-matrix renormalization group method. We identify a metal-insulator transition around $U\sim U_{c1}$ and four distinct phases as a function of $U/t$ on narrower cylinders, including a metallic phase at $U<U_{c1}$, two insulating intermediate phases (a translational symmetry breaking phase at $U_{c1}<U<U_{c2}$ and a quantum spin liquid phase at $U_{c2}<U<U_{c3}$), and the kagome antiferromagnetic phase at $U>U_{c3}$. We confirm that the translational symmetry breaking phase is robust for wider cylinders, while the quantum spin liquid phase is smoothly connected to the kagome antiferromagnetic phase with increasing the system width. Moreover, our numerical observations indicate a continuous metal-insulator transition at $U_{c1}$ whose nature is consistent with Slater's transition scenario. The magnetic phase transition between two insulating intermediate phases at $U_{c2}$ is first order. Our findings may provide insights into exotic kagome lattice materials.

${\mathrm{Ca}}_3{\mathrm{Ru}}_2{\mathrm{O}}_7$ is a correlated and spin-orbit coupled system with an extraordinary anisotropy. It is both interesting and unique largely because this material exhibits conflicting phenomena that are often utterly inconsistent with traditional precedents, particularly, the quantum oscillations in the nonmetallic state and colossal magnetoresistivity achieved by avoiding a fully spin-polarized state. This work focuses on the relationship between the lattice and transport properties along each crystalline axis and reveals that application of magnetic field, $H$, along different crystalline axes readily $\mathit{stretches}$ or $\mathit{shrinks}$ the lattice in a uniaxial manner, resulting in distinct electronic states. Furthermore, application of modest pressure drastically amplifies the $\mathit{anisotropic}$ magnetoelastic effect, leading to either an occurrence of a robust metallic state at $H\left|\right|\mathrm{hard}$ axis or a reentrance of the nonmetallic state at $H\left|\right|\mathrm{easy}$ axis. ${\mathrm{Ca}}_3{\mathrm{Ru}}_2{\mathrm{O}}_7$ presents a rare lattice-dependent magnetotransport mechanism, in which the extraordinary lattice flexibility enables an exquisite control of the electronic state via magnetically stretching or shrinking the crystalline axes, and the spin polarization plays an unconventional role unfavorable for maximizing conductivity. At the heart of the intriguing physics is the anisotropic magnetostriction that leads to exotic states.

We study the low-frequency admittance of a quantum Hall bar of a size much larger than the electronic coherence length. We find that this macroscopic conductor behaves as an ideal quantum conductor with vanishing longitudinal resistance and purely inductive behavior up to $f\lesssim 1\mathrm{M}\mathrm{Hz}$. Using several measurement configurations, we study the dependence of this inductance on the length of the edge channel and on the integer quantum Hall filling factor. The experimental data are well described by a scattering model for edge magnetoplasmons taking into account the effective long range Coulomb interactions within the sample. We find that the inductance's dependence on the filling factor arises predominantly from the effective quantum inertia of charge carriers induced by Coulomb interactions.

We theoretically report the emergence of $Z_4$ parafermion edge modes in a periodically driven spinful superconducting chain with a modest fermionic Hubbard interaction. These parafermion edge modes represent $\pm \pi /\left(2T\right)$ quasienergy excitations ($T$ being the driving period), which have no static counterpart and arise from the interplay between an interaction effect and periodic driving. At special parameter values, these exotic quasiparticles can be analytically and exactly derived. Strong numerical evidence of their robustness against variations in parameter values and spatial disorder is further presented. Our proposal offers a route toward realizing parafermions without fractional quantum Hall systems or complicated interactions.

We study exact zeros of Loschmidt echo and quantum speed limit time for dynamical quantum phase transition in finite size systems. Our results illustrate that exact zeros of Loschmidt echo exist even in finite size quantum systems when the postquench parameter takes some discrete values in regions with the corresponding equilibrium phase different from the initial phase. As the system size increases and tends to infinity, the discrete parameters distribute continuously in the parameter regions. We further analyze the time for the appearance of the first exact zero of Loschmidt echo which is known as the quantum speed limit time ${\tau }_{\text{QSL}}$. We demonstrate that the maximal value of ${\tau }_{\text{QSL}}$ is proportional to $L$ and approaches infinity in the thermodynamical limit, when we quench the initial noncritical state to the critical phase. We also calculate the minimal value of ${\tau }_{\text{QSL}}$ and find that its behavior is dependent on the phase of initial state.

A critical parameter in characterizing the properties of single-domain nanoparticles is their magnetic relaxation time. It must be known, for example, to estimate the anisotropy from magnetization versus temperature measurements. The time it takes for the magnetization to relax also determines the behavior of particles in various oscillating applied fields, which is critically important for their application in magnetic particle imaging and hyperthermia treatment. However, an analytic expression for this relaxation time has been generally missing. Brown's [Phys. Rev. 130, 1677 (1963)] famous result is only valid for the easy anisotropy axes of each particle in the ensemble aligned along the external field direction and overestimates the relaxation time. Despite this overestimation, this expression is most commonly used to extract magnetic nanoparticle parameters such as anisotropy energy from magnetometry data. Here, we use Brown's formalism to derive a different, simple, approximate relaxation time expression that is valid for randomly aligned easy axes. Using parameters appropriate for magnetite, we compare our results to other results in the literature and with stochastic Landau-Lifshitz-Gilbert simulations to show that our result is more accurate across a range of applied field strengths and temperatures. We note that several analytic expressions for the relaxation time do a reasonable job, as long as one uses a full calculation for the attempt time, rather than the commonly used estimate ${\tau }_0\sim 1$ ns.

We investigate the influence of magnons on the temperature dependence and the anisotropy of the spin-orbit torque (SOT). For this purpose, we use third-order perturbation theory in the framework of the Keldysh formalism to derive suitable equations to compute the magnonic SOT. We find several contributions to the magnonic SOT, which depend differently on the spin-wave stiffness $\mathcal{A}$ and the temperature $T$, with the dominating contribution scaling like $T^2/{\mathcal{A}}^2$. Based on this formalism, we compute the magnonic SOT in the ferromagnetic Rashba model. For large Rashba parameters, the magnonic SOT is strongly anisotropic, and for small quasiparticle broadening, it may become larger than the nonmagnonic SOT.