Valleytronics using two-dimensional materials opens unprecedented opportunities for information processing using a valley polarizer as a basic building block. Various methodologies, such as strain engineering, the inclusion of line defects, and the application of static magnetic fields, have been widely explored for creating valley polarization. However, these methods suffer from low transmission or lack of polarization directionality. To overcome the above limitations, we propose an all-electrical valley polarizer using zigzag edge graphene nanoribbons in a multiterminal device geometry. The proposed device can be gate-tuned to operate along two independent regimes: (i) a terminal-specific valley filter that utilizes band-structure engineering, and (ii) a parity-specific valley filter that exploits the parity selection rule in zigzag edge graphene. We show that the device exhibits intriguing physics in the multimode regime of operation affecting the valley polarization; we investigate various factors influencing polarization in wide-device geometries, such as optical analogs of graphene Dirac fermions, angle-selective transmission via $p-n$ junctions, and the localization of edge states. Furthermore, we evaluate the performance of the proposed device structures in the presence of Anderson short-range disorder at the edges and the bulk, and we find it to be resilient to edge disorder even for a higher disorder strength. The device geometry is optimized to achieve maximum valley polarization, thereby paving the way for a physics-based tunable valleytronic device.

In this study, we investigate the effect of random point disorder on the surface states of a topological insulator with out-of-plane magnetization. We consider the disorder within a high-order Born approximation. The Born series converges to the one branch of the self-consistent Born approximation (SCBA) solution at low disorder. As the disorder strength increases, the Born series converges to another SCBA solution with the finite density of states within the magnetization-induced gap. Further increase of the disorder strength leads to a divergence of the Born series, showing the limits of the applicability of the Born approximation. We find that the convergence properties of this Born series are closely related to the properties of the logistic map, which is known as a prototypical model of chaos. We also calculate the longitudinal and Hall conductivities within the Kubo formulas at zero temperature with the vertex corrections for the velocity operator. Vertex corrections are important for describing transport properties in the strong disorder regime. In the case of a strong disorder, the longitudinal conductivity is weakly dependent on the disorder strength, while the Hall conductivity decreases with increasing disorder.

Using atomistic, million-atom screened pseudopotential theory together with configuration interaction, as well as atomically resolved structures based on experimental characterization, we perform numerical calculations on self-assembled $\mathrm{GaAs}/{\mathrm{Al}}_x{\mathrm{Ga}}_{1-x}\mathrm{As}$(111) quantum dots that we compare with our experimental data. We show that random alloy disorder in the barrier can cause a symmetry breaking at the single-particle level (distortions of wave functions and lifting of degeneracies) which translates into the appearance of a nonzero exciton fine structure splitting (FSS) at the many-body level. Nevertheless, our results indicate that varying the concentration of aluminum in the random alloyed barrier allows simultaneous tuning of the exciton fine structure splitting and emission wavelength without altering its radiative lifetime $\tau \approx 200\text{ps}$. Additionally, the optical properties of these quantum dots are predicted to be very robust against both symmetric and asymmetric shape elongation (with $\text{FSS}\le 2.2\mu \text{eV}$), rendering postselection less essential under well-controlled growth conditions. On the other hand, the growth on miscut substrates introduces a structural anisotropy along the quantization axis to which the system is very sensitive: the FSS ranges between 5 and $50\mu \text{eV}$ while the radiative lifetime of the transition is increased up to $\tau =400\text{ps}$. The numerical results for the FSS are in perfect agreement with our experimental measurements which give $\text{FSS}=10\pm 9\mu \text{eV}$ for $2^{\circ }$ miscut angle at $x=0.15$.

Domain walls are ubiquitous in materials that undergo phase transitions driven by spontaneous symmetry breaking. Domain walls in ferroics and multiferroics have received tremendous attention recently due to their emergent properties distinct from their domain counterparts—for example, their high mobility and controllability, as well as their potential applications in nanoelectronics. However, it is extremely challenging to detect, visualize, and study the ferrorotational (FR) domain walls because the FR order, in contrast to ferromagnetism and ferroelectricity, is invariant under both the spatial-inversion and the time-reversal operations and, thus, hardly couple with conventional experimental probes. Here, a FR candidate $\mathrm{NiTi}{\mathrm{O}}_3$ is investigated by ultrasensitive electric quadrupole (EQ) second-harmonic generation rotational anisotropy (SHG RA) to probe the point symmetries of the two degenerate FR domain states, showing their relation by the vertical mirror operations that are broken below the FR critical temperature. We then visualize the real-space FR domains by scanning EQ SHG microscopy, and further, resolve the FR domain walls by revealing a suppressed SHG intensity at the domain walls. By taking local EQ SHG RA measurements, we show the restoration of the mirror symmetry at FR domain walls and prove their unconventional nonpolar nature. Our findings not only provide a comprehensive insight into FR domain walls, but also demonstrate a unique and powerful tool for future studies on domain walls of unconventional ferroics both of which pave the way towards future manipulations and applications of FR domain walls.

We study thermal radiation from a system of two coupled photonic cavities including the nonlinear Kerr effect, using numerical simulations and analytical methods. The general system, which can be implemented as two cavities in a waveguide, is asymmetrically driven with a monochromatic pumping from a single side. We describe the eigenmodes in the linear and nonlinear regime as well as the transmission of the coupled system. These results are then employed to understand the thermal radiation exhibited from both sides as each part of the system is coupled to a bath at a different temperature. Interestingly, the radiation spectrum is complicated, and can present up to four peaks due to the rich nonlinear coupling features. Furthermore, in certain regimes these spectra can drastically change upon variation of the bath temperatures. In addition, interference between the emitted and reflected radiation can lead to dips in the thermal radiation. Moreover, the system can exhibit self-pulsing, leading to comblike spectra for thermal radiation with peaks of very large amplitude. Our proposed analytical model for thermal radiation in the stable regime fits very well with the numerical results and describes a general class of devices.

We study the Kitaev model with direction-dependent interactions to investigate how the flux configuration and/or the anisotropy in the exchanges affect the Majorana excitations. It is found that the triangular flux configurations with unit length $q$ induce the quantum spin liquid states with gapped Majorana excitations. Systematic numerical calculations demonstrate that $(q-1)(q-2)/2$ gapped states bounded by gapless states are realized in the vicinity of the isotropic limit. The induced gapped quantum spin liquid states are not adiabatically connected to the gapped one realized in the large anisotropic limit. Therefore, these two states are distinct from each other. The nature of the gapped states can be explained by the fact that the hybridization between the gapless points, which originates from the superlattice potential due to the flux configuration, opens the gap in the Majorana excitations.

Using an incoherent quasielastic neutron scattering (QENS) technique, we investigate ${\mathrm{H}}^{-}$ dynamics in a series of oxyhydrides $\mathrm{La}{\mathrm{H}}_{3-2x}{\mathrm{O}}_x$ that exhibit characteristic high ${\mathrm{H}}^{-}$ conductivity. In the end member $\mathrm{La}{\mathrm{H}}_3$ ($x$ = 0), two kinds of ${\mathrm{H}}^{-}$ dynamics are identified: the jump diffusion and the localized motion. The jump length in the jump diffusion mode increases with increasing $T$. The localized motion is identified as a jump between the two inequivalent sites. These dynamics are corroborated by our molecular dynamical simulations. Our QENS data suggest that similar ${\mathrm{H}}^{-}$ dynamics occurs also in oxyhydrides $\mathrm{La}{\mathrm{H}}_{3-2x}{\mathrm{O}}_x$ ($x\ne$ 0), whose ${\mathrm{H}}^{-}$ concentration dependence is consistent with the previous measurement of ionic conductivity. We also discuss the possibility that $\mathrm{La}{\mathrm{H}}_{3-2x}{\mathrm{O}}_x$ is an example of ${\mathrm{H}}^{-}$ ion conductors governed by the concerted migration mechanism. The identified ${\mathrm{H}}^{-}$ dynamics is key to understanding the anomalous hydrogen concentration dependence of the diffusion coefficient in lanthanum hydrides, which has been a longstanding mystery in this compound.

Spin-orbitronic devices can integrate memory and logic by exploiting spin-charge interconversion (SCI), which is optimized by design and materials selection. In these devices, interfaces are crucial elements as they can prohibit or promote spin flow in a device as well as possess spin-orbit coupling resulting in interfacial SCI. Therefore, investigation of interfaces in spin-orbitronic devices is important. Here, we study the origin of SCI in a Py/Cu/W lateral spin valve and quantify its efficiency. An exhaustive characterization of the interface between Cu and W electrodes uncovers the presence of an oxidized layer ($\mathrm{W}{\mathrm{O}}_{\mathrm{x}}$). We determine that the SCI occurs at the Cu/$\mathrm{W}{\mathrm{O}}_{\mathrm{x}}$ interface with a temperature-independent interfacial spin-loss conductance of $G_{\parallel }\approx 20\times {10}^{13}{\mathrm{\Omega }}^{-1}{\text{m}}^{-2}$ and an interfacial spin-charge conductivity ${\sigma }_{\mathrm{SC}}=-1600{\mathrm{\Omega }}^{-1}\mathrm{c}{\mathrm{m}}^{-1}$ at 10 K ($-800{\mathrm{\Omega }}^{-1}\mathrm{c}{\mathrm{m}}^{-1}$ at 300 K). This corresponds to an efficiency given by the inverse Edelstein length ${\lambda }_{\mathrm{IEE}}=-0.8\mathrm{nm}$ at 10 K ($-0.4\mathrm{nm}$ at 300 K), which is remarkably larger than in metal/metal and metal/oxide interfaces and bulk heavy metals. The large SCI efficiency at such an oxidized interface makes it a promising candidate for the magnetic readout in MESO logic devices.

Since the discovery of cuprate superconductors, the interlayer tunneling (ILT) and Lawrence-Doniach (LD) models, which connect CuO planes by Josephson coupling, were considered the leading theoretical proposals for these materials. However, measurements of the interlayer magnetic penetration depth ${\lambda }_c$ yielded larger values than required by the ILT model. After the discovery of planar stripes and incommensurate charge ordering, it was also possible to consider Josephson coupling between these mesoscopic charge domains or blocks. We show that the average intralayer is larger than the interlayer coupling and is comparable with the condensation energy, leading to a superconducting transition by long-range phase order. Another consequence is that the ratio ${[{\lambda }_c/{\lambda }_{ab}]}^2$ is related to the resistivity ratio ${\rho }_c/{\rho }_{ab}$ near the superconducting transition temperature, in agreement with several measurements.

We report neutron scattering measurements on ${\mathrm{YbMnSb}}_2$ which shed light on the nature of the magnetic moments and their interaction with Dirac fermions. Using half-polarized neutron diffraction we measured the field-induced magnetization distribution in the paramagnetic phase and found that the magnetic moments are well localized on the Mn atoms. Using triple-axis neutron scattering we measured the magnon spectrum throughout the Brillouin zone in the antiferromagnetically ordered phase, and we determined the dominant exchange interactions from linear spin-wave theory. The analysis shows that the interlayer exchange is five times larger than in several related compounds containing Bi instead of Sb. We argue that the coupling between the Mn local magnetic moments and the topological band states is more important in ${\mathrm{YbMnSb}}_2$ than in the Bi compounds.

While the phonon hydrodynamic regime has recently been highlighted experimentally in graphite films, the understanding and modeling of heat transport along their basal plane remain elusive. From first-principles-based modeling, we predict a significant influence of the surface roughness on basal-plane thermal conductivity due to the collective phonon drift. The occurrence condition of the phonon Knudsen minimum is also shown to strongly depend on the surface roughness. We summarize the basal-plane heat transport along graphite thin films into coherent and incoherent regimes and also present a speculation for the recently observed anomalous thickness-dependent thermal conductivity. Our work provides guidance for the experimental explorations of phonon hydrodynamics in graphitic micro- and nanostructures in the future.

We study a single two-dimensional Dirac fermion at finite density, subject to a quenched random magnetic field. At low energies and sufficiently weak disorder, the theory maps onto an infinite collection of 1D chiral fermions (associated to each point on the Fermi surface) coupled by a random vector potential. This low-energy theory exhibits an exactly solvable random fixed line, along which we directly compute various disorder-averaged observables without the need for the usual replica, supersymmetry, or Keldysh techniques. We find the longitudinal dc conductivity in the collisionless $\hslash \omega /k_{B}T\to \infty$ limit to be nonuniversal and to vary continuously along the fixed line.

The quantum hydrodynamic model (QHDM) has become a versatile and efficient tool for plasmonics at nanometer and even subnanometer length scales, but the theory is principally applicable only to simple metals. For the most common plasmonic materials, i.e., noble metals, QHDM has not been duly justified to describe their optical responses. In particular, when noble metal structures interface with dielectric materials, electron spillover plays an important role. Although a background optical permittivity is usually invoked in Ampère's law to encompass the $d$-band electrons' effect on optical responses in a direct manner, their indirect effects, e.g., acting via conduction electrons, still remain elusive. Here we identify these indirect effects and develop a near-field potential $U_{\mathrm{aff}}$ refined QHDM (U-QHDM), in which the $d$-band electrons' effects on the conduction electrons are recognized and the electrostatic properties of dielectric are incorporated. We report the detailed procedures of calibrating U-QHDM. Material-independent model parameters are fitted with density functional theory (DFT) calculations for simple metals. Last, the calibrated U-QHDM is benchmarked against DFT and time-dependent DFT within stabilized jellium model for Ag, Au, and Al nanostructures interfacing with dielectrics. We expect U-QHDM will be a valuable asset for the rapidly developing field of extreme nanophotonics.

We introduce an efficient method to simulate the dynamics of an interacting quantum impurity coupled to noninteracting fermionic reservoirs. Viewing the impurity as an open quantum system, we describe the reservoirs by their Feynman-Vernon influence functionals (IFs). The IFs are represented as matrix-product states in the temporal domain, which enables an efficient computation of the dynamics for arbitrary interactions. We apply our method to study quantum quenches and transport in an Anderson impurity model, including highly nonequilibrium setups, and find a favorable performance compared to state-of-the-art methods. The computational resources required for an accurate computation of the dynamics scale polynomially with evolution time, indicating that a broad class of out-of-equilibrium quantum impurity problems are efficiently solvable. This approach will provide additional insights into the dynamical properties of mesoscopic devices and correlated materials.