We present atomistic computations within an empirical pseudopotential framework for the electron $s$-shell ground state $g$ tensor of InGaAs quantum dots (QDs) embedded to host matrices that grant electronic confinement. A large structural set consisting of geometry, size, and molar fraction variations is worked out which also includes a few representative uniform strain cases. The tensor components are observed to display insignificant discrepancies even for the highly anisotropic shapes. The family of $g$-factor curves associated with these parameter combinations coalesces to a single universal one when plotted as a function of the gap energy, thus confirming a recent assertion reached under much restrictive conditions. Our work extends its validity to alloy QDs with various shapes and finite confinement that allows for penetration to the host matrix, placing it on a more realistic basis. Accordingly, the electrons in InGaAs QDs having $s$-shell transition energies close to 1.13 eV will be least susceptible to magnetic field. We also show that low indium concentration offers limited $g$-factor tunability under shape or confinement variations. These findings can be taken into consideration in the fabrication and the use of InGaAs QDs with $g$-near-zero or other targeted $g$ values for spintronic or electron spin resonance-based direct quantum logic applications.

Irradiation of strong light on a material leads to numerous nonlinear effects that are essential to understand the physics of excited states of the system and optoelectronics. Here, we study the nonlinear thermoelectric effects due to the electric and thermal fields applied on a noncentrosymmetric system. The phenomenon arises on the Fermi surface with the transitions of electrons from the valence to conduction bands. We derive the formalism to investigate these effects and find that the nonlinearity in these effects, namely, nonlinear Seebeck and nonlinear Peltier effects, depends on the ratio of the nonlinear to linear conductivities. The theory is tested for a hexagonally warped and gapped topological insulator. Results show enhancement in the longitudinal and Hall effects on increasing the warping strength but show opposite behavior with the surface gap.

Collective behavior is one of the most intriguing aspects of the hydrodynamic approach to electronic transport. Here we provide a consistent, unified calculation of the dispersion relations of the hydrodynamic collective modes in graphene. Taking into account viscous effects, we show that the hydrodynamic sound mode in graphene becomes overdamped at sufficiently large momentum scales. Extending the linearized theory beyond the hydrodynamic regime, we connect the diffusive hydrodynamic charge density fluctuations with plasmons.

We report on a combined far- and near-field study of surface plasmon polaritons on freestanding perforated gold films. The samples were fabricated by focused ion beam milling of a periodic hole array into a carbon membrane followed by thermal evaporation of gold and plasma ashing of the carbon film. Optical transmission spectra showed a series of characteristic features, which can be attributed to the excitation of surface plasmon modes via the periodic nanohole array. The corresponding near-field distributions were mapped by electron energy loss spectroscopy. In addition to the optically bright surface plasmon modes, we observed in the near-field a dark plasmon mode which is absent in the normal incidence far-field spectra. Our experimental results are in good agreement with numerical computations based on a discontinuous Galerkin time-domain method.

We study two-dimensional (2D) electron systems in the hydrodynamic regime. We show that a geometrical Berry curvature modifies the effective Navier-Stokes equation for viscous electron flow in topological materials. For small electric fields, the Hall current becomes negligible compared to the viscous longitudinal current. In this regime, we highlight an unconventional Poiseuille flow with an asymmetric profile and a deviation of the maximum of the current from the center of the system. In a 2D infinite geometry, the Berry curvature leads to current whirlpools and an asymmetry of the potential profile. This phenomenon can be probed by measuring the asymmetric nonlocal resistance profile.

We numerically examine the depinning and sliding dynamics of a Wigner crystal in the presence of quenched disorder and a magnetic field. In the disorder-free limit, the Wigner crystal Hall angle is independent of crystal velocity, but when disorder is present, we find that the Hall angle starts near zero at the depinning threshold and increases linearly with increasing drive before reaching a saturation close to the disorder free value at the highest drives. The drive dependence is the result of a side jump effect produced when the charges move over pinning sites. The magnitude of the side jump is reduced at the higher velocities. The drive dependent Hall angle is robust for a wide range of disorder parameters and should be a generic feature of classical charges driven in the presence of quenched disorder and a magnetic field.

Cyclotron resonance (CR) is considered one of the fundamental phenomena in conducting systems. In this paper, we study CR in a gated two-dimensional (2D) electron system (ES). Namely, we analyze the absorption of electromagnetic radiation incident normal to the gated 2DES, where a standard dielectric substrate separates the 2D electron sheet and the metallic steering electrode (“gate”) the whole system is placed in the perpendicular magnetic field. Our analysis reveals the redshift of the absorption peak frequency compared to the electron cyclotron frequency. The redshift appears in the low-frequency regime, when the resonant frequency is much less than the frequency of Fabry-Perot modes in the natural resonator “2D electron sheet – substrate – gate.” Moreover, we find this shift to be dependent on the electron density of 2DES. Therefore, it can be controlled by varying the gate voltage. We predict that the shift can be large in realistic gated or back-gated 2DESs. The obtained controllability of CR in gated 2DES opens the door for exploring new physics and applications of this phenomenon.

We report a systematic study on the intrinsic spin Hall conductivity (ISHC) of bilayer ${\mathrm{PtTe}}_2$ and explore the connection between the stacking order and ISHC. We find that by changing the stacking mode, ISHC can be manipulated from positive to negative values. Such strong stacking-dependent ISHC originates from the interlayer coupling, in which Te atoms in the upper and lower layers can form either van der Waals or covalentlike quasibonding depending on the stacking modes. Thus ISHC can be effectively tuned by changing the stacking order. These results not only allow us to establish fundamental understanding of ISHC in bilayer ${\mathrm{PtTe}}_2$ dependent on the stacking mode but also provide guidelines for the application of bilayer ${\mathrm{PtTe}}_2$ in next-generation spintronic devices.

We analyze the thermoelectric cooling properties of a Corbino device in the quantum Hall regime on the basis of experimental data of electrical conductance. We focus on the cooling power and the coefficient of performance within and beyond linear response. Thermovoltage measurements in this device reported by Real et al., [Phys. Rev. Appl. 14, 034019 (2020)] indicated that the transport takes place in the diffusive regime, without signatures of effects due to the electron-phonon interaction in a wide range of temperatures and filling factors. In this regime, the heat and charge currents by electrons can be described by a single transmission function. We infer this function from experimental data of conductance measurements and we calculate the cooling power and the coefficient of performance for a wide range of filling factors and temperatures, as functions of the thermal and electrical biases. We predict an interesting cooling performance in several parameter regimes.

${\mathrm{Mg}}_3{\mathrm{Bi}}_2$ has been proved to be a semimetal with topological surface states (referred to as a topological semimetal, TSM) in the presence of spin-orbit coupling (SOC), and predicted to be a type-II nodal line semimetal (NSM) in the absence of SOC. It is possible to tune the effective SOC in ${\mathrm{Mg}}_3{\mathrm{Bi}}_2$ by substituting the heavy Bi atom with lighter atoms, such as Sb or As, which results in topological phase transitions. Here in our work we investigate the evolution of the transport properties and band structures of ${\mathrm{Mg}}_3{\mathrm{Bi}}_2$ by doping different concentrations of Sb through molecular beam epitaxy. Our results demonstrate the existence of phase transitions of TSM to trivial semimetal (SM) and SM to trivial insulator, with Sb concentration $x$ at $x_1\approx 0.44–0.64$ and $x_2\approx 1.39$, respectively. Further theoretical calculations give values of $x_1\approx 0.523$ and $x_2\approx 1.10$, which agrees well with our experimental results. And all these results are concluded into a ${\text{Mg}}_3{\text{Bi}}_{2-x}{\text{Sb}}_x$ phase diagram. Our work will stimulate more explorations to the possibilities of the realization of type-II NSM in ${\mathrm{Mg}}_3{\mathrm{Bi}}_2$ by various elements substitution.

With infrared spectroscopy, we studied the bulk electronic properties of the topological antiferromagnet $\mathrm{Mn}{\mathrm{Bi}}_2{\mathrm{Te}}_4$ with $T_N\simeq 25\mathrm{K}$. With the support of band-structure calculations, we assign the intra- and interband excitations and determine the band gap of $E_g\approx 0.17$ eV. We also obtain evidence for two types of conduction bands with light and very heavy carriers. The multiband free-carrier response gives rise to an unusually strong increase of the combined plasma frequency, ${\omega }_{\mathrm{pl}}$, below 300 K. The band reconstruction below $T_N$ yields an additional increase of ${\omega }_{\mathrm{pl}}$ and a splitting of the transition between the two conduction bands by about 54 meV. Our study thus reveals a complex and strongly temperature-dependent multiband low-energy response that has important implications for the study of the surface states and device applications.

We introduce an analytical model for ideal organic laser diodes based on the argument that their intrinsic active layers necessitate operation in the bipolar space charge-limited current regime. Expressions for the threshold current and voltage agree well with drift-diffusion modeling of complete p-i-n devices and an analytical bound is established for laser operation in the presence of annihilation and excited-state absorption losses. These results establish a foundation for the development of organic laser diode technology.

The Berry phase generally plays a key role in characterizing the topological phase for quantum and classic systems. However, topological edge states in a two-dimensional acoustic system are generally characterized by nonzero Berry curvature. Here we achieve the topological phase transition characterized by zero Berry curvature, based on the phononic crystal composed of 8-metaatomic composite metamolecules. We demonstrate that the phase transition can be induced through either modulating the intervals between the adjacent metaatoms or changing their sizes. The nontrivial Zak phase ensures the existence of the acoustic topological edge states, confining sound waves along the boundaries of finite sonic crystal rather than transmitting along the interface between the two structures possessing opposite topological phases. Furthermore, the multiple topological phase transitions are simply realized thanks to the two limbic metaatoms introduced. Although the eigenfrequencies of topological edge states, which appear in the partial band gap, overlap with the bulk states, we still numerically observe the topological edge states selectively through modulating the wavenumber in real space. The proposed acoustic topological insulator may have potential applications in acoustic waveguiding and isolating.

Parity-time ($\mathcal{PT}$)-symmetric or, more generally, non-Hermitian systems have opened a new area for unconventional management of waves, with significant applications, especially in optics. However, fewer proposals are found in acoustics, possibly due to the lack of a simple mechanism for coherent gain. In this paper, we propose a composite non-Hermitian system in acoustics consisting of assemblies of $\mathcal{PT}$-symmetric Helmholtz resonator (HR) dipoles. Like meta-atoms are used as building elements in metamaterials, we propose $\mathcal{PT}$-symmetric dipoles to design non-Hermitian systems intended to engineer complicated directivity fields. We theoretically analyze, numerically confirm, and experimentally show the symmetry breaking in a two-dimensional space of non-Hermitian dipoles consisting of a pair of Helmholtz resonators with different levels of gain and loss. In particular, we explore, as an application, a metastructure to concentrate the sound pressure inside the circular array formed by $\mathcal{PT}$-symmetric dipoles. The proposed HR dipoles may be a convenient composite element for smart control of sound.

We report on first-principles calculations of spin-transfer torque (STT) in epitaxial magnetic tunnel junctions (MTJs) based on ferrimagnetic tetragonal ${\mathrm{Mn}}_3\mathrm{Ga}$ electrodes, both as analyzer in a Fe/MgO stack, and also in an analogous stack with a second ${\mathrm{Mn}}_3\mathrm{Ga}$ electrode (instead of Fe) as polarizer. Solving the ballistic transport problem ($\mathrm{NEGF}+\mathrm{DFT}$) for the nonequilibrium spin density in a scattering region extended to over 7.6 nm into the ${\mathrm{Mn}}_3\mathrm{Ga}$ electrode, we find long-range spatial oscillations of the STT decaying on a length scale of a few tens of angstroms, both in the linear response regime and for finite bias. The oscillatory behavior of the STT in ${\mathrm{Mn}}_3\mathrm{Ga}$ is robust against variations in the stack geometry (e.g., the barrier thickness and the interface spacing) and the applied bias voltage, which may affect the phase and the amplitude of the spacial oscillation, but the high (carrier) frequency mode is only responsive to variations in the longitudinal lattice constant of ${\mathrm{Mn}}_3\mathrm{Ga}$ (for fixed in-plane geometry) without being commensurate with the lattice. Our interpretation of the long-range STT oscillations is based on the bulk electronic structure of ${\mathrm{Mn}}_3\mathrm{Ga}$, taking also into account the spin-filtering properties of the MgO barrier. Comparison to a fully ${\mathrm{Mn}}_3\mathrm{Ga}$-based stack shows similar STT oscillations, but a significant enhancement of both the TMR effect at the Fermi level and the STT at the interface, due to resonant tunneling for the mirror-symmetric junction with thinner barrier (three monoatomic layers). From the calculated energy dependence of the spin-polarized transmissions at 0 V, we anticipate asymmetric or symmetric TMR as a function of the applied bias voltage for the Fe-based and the all-${\mathrm{Mn}}_3\mathrm{Ga}$ stacks, respectively, which also both exhibit a sign change below $\pm 1\mathrm{V}$. In the latter, symmetric, case we expect a TMR peak at zero, which is larger for the thinner barriers because of a spin-polarized resonant tunneling contribution.

The optical activity refers to the rotation of the vibration plane of linearly polarized light traversing certain substances. Here we report an analogous phenomenon for spin waves (SWs) in cylindrical nanowires, whose azimuthal distribution undergoes a continuous rotation as traveling along the wire. This effect, in line with the Fresnel model of the optical activity, is attributed to different propagating speeds of two circular states (left and right handed) composing the SW modes. The SW circular birefringence observed here originates from the overall curvilinear shape of the waveguide. Moreover, the fact that an external field is able to alter the rotatory power of the nanowires is reminiscent of the Faraday magneto-optical effect.

The analysis of the magnetic structure factor obtained after imaging a field-demagnetized artificial square ice reveals qualitative deviations from what would predict the six-vertex model. More specifically, additional features appear for certain $q$ vectors in the experimental magnetic structure factor that are absent in the theoretical one. Using Monte Carlo simulations, we demonstrate that these features originate from dipolarlike, farther neighbor couplings. Our results thus show that the long-range magnetostatic interactions coupling the nanomagnets are not totally washed out in a field-demagnetized artificial square ice and cannot be neglected as they impact the magnetic correlations within or at the vicinity of the ice manifold. Artificial square ice systems are then expected to ultimately order, provided that the ground state can be approached.

Contrary to the theoretical predictions of one-parameter scaling theory that all waves in two-dimensional disordered materials are localized, Anderson localization is observed only for sufficiently high frequencies in an isotropically jammed two-dimensional disordered granular packing of photoelastic disks. More specifically, we have performed an experiment in analyzing the level statistics of normal mode vibrations. We find that the level-distance distribution obeys Gaussian-orthogonal-ensemble (GOE) statistics of the Wigner-Dyson classification in the low-frequency (boson-peak) and intermediate-frequency regimes, whereas in the high-frequency regime Poisson statistics are observed. This means that at low and intermediate frequencies we have delocalized modes, and only at very high frequencies do localized modes exist. Evaluating the system-size dependence of the delocalization-localization crossover frequency, we obtain evidence of a true transition with a mobility edge at 80% of the Debye frequency and a value of the critical correlation-length exponent $\nu \sim$ 1.66 that is similar to that of the three-dimensional electronic Anderson model. We argue that for force-constant disorder one-parameter scaling might not be applicable.

The effect of inertial spin dynamics is compared between ferromagnetic, antiferromagnetic, and ferrimagnetic systems. The linear response to an oscillating external magnetic field is calculated within the framework of the inertial Landau-Lifshitz-Gilbert equation using analytical theory and computer simulations. Precession and nutation resonance peaks are identified, and it is demonstrated that the precession frequencies are reduced by the spin inertia, while the lifetime of the excitations is enhanced. The interplay between precession and nutation is found to be the most prominent in antiferromagnets, where the timescale of the exchange-driven sublattice dynamics is comparable to inertial relaxation times. Consequently, antiferromagnetic resonance techniques should be better suited for the search for intrinsic inertial spin dynamics on ultrafast timescales than ferromagnetic resonance.