The wave-vector- and frequency-dependent spin susceptibility of an interacting disordered two-dimensional electron gas at frequencies near the spin resonance has been considered. It is assumed that the external magnetic field is moderately strong ${\tau }^{-1}\ll {\omega }_c\ll {\epsilon }_F$ and the Coulomb interaction is weak $(e^2/\kappa \lambda )/{\omega }_c\ll 1$ ($\lambda$ stands for the magnetic length). The spin susceptibility has been calculated within the ladder diagram approximation. A way of dealing with the degeneracy of Landau levels has been proposed and a transport-type equation free from the degeneracy indexes has been derived. It has been shown that the resonant part of the spin susceptibility has the form $\widehat{\chi }(\omega ,\mathbf{q})\sim {[\omega -{\omega }_{\mathrm{res}}-(d^{\prime }+i{d}^{\prime \prime }){\mathbf{q}}^2]}^{-1}$, where in the hydrodynamic limit ${\omega }_s\tau \ll 1$ (${\omega }_s$ stands for the Larmour frequency $g{\mu }_{B}H$) and to first order in $(e^2/\kappa \lambda )/{\omega }_c$ the imaginary part, which stems from the diffusion of the spin magnetization due to the stochastic scattering on impurities, has a conventional form, $d^{\prime \prime }=\frac{{\epsilon }_F\tau /m}{{\left({\omega }_c\tau \right)}^2}$, while the real part, which characterizes the coherent motion of the spin excitations through the crystal, is $d{}^{\prime }\sim M^{-1}\left({\omega }_s\tau \right)$, where $M\sim {(\lambda e^2/\kappa )}^{-1}$ is the effective mass of the spin excitation in the clean system. Thus, at ${\omega }_s\tau \ll 1$, the disorder strongly impedes the coherent motion of the excitations though it does not forbid it completely. The resonant frequency is unaffected by the Coulomb interaction, ${\omega }_{\mathrm{res}}={\omega }_s$, just as in the clean case. The results may be applied to experiments on the spin-flip Raman scattering, where the finite wave-vector susceptibility can be directly measured, as well to the electromagnetic absorption studying by means of a grating superimposed on the structure.

We performed noise measurements in a two-dimensional electron gas to investigate the nonequilibrium quantum Hall effect (QHE) state. While excess noise is perfectly suppressed around the zero-biased QHE state reflecting the dissipationless electron transport of the QHE state, considerable finite excess noise is observed in the breakdown regime of the QHE. The noise temperature deduced from the excess noise is found to be of the same order as the energy gap between the highest occupied Landau level and the lowest empty one. Moreover, unexpected finite excess noise is observed at a finite source-drain bias voltage smaller than the onset voltage of the QHE breakdown, which indicates finite dissipation in the QHE state and may be related to the prebreakdown of the QHE.

We investigate two schemes for pumping spin adiabatically from a ferromagnet through an interacting quantum dot into a normal lead that exploit the possibility to vary in time the ferromagnet's magnetization, either its amplitude or its direction. For this purpose, we extend a diagrammatic real-time technique for pumping to situations in which the leads' properties are time dependent. In the first scheme, the time-dependent magnetization amplitude is combined with a time-dependent level position of the quantum dot to establish both a charge and a spin current. The second scheme uses a uniform rotation of the ferromagnet's magnetization direction to generate a pure spin current without a charge current. We discuss the influence of an interaction-induced exchange field on the pumping characteristics.

In materials with strong electron-electron interactions, photoexcitation can trigger a cascade in which multiple particle-hole excitations are generated. Here we analyze the cascade of impact-excitation processes in graphene in which many hot carriers are generated by a single absorbed photon. We show that the number of generated carriers has a strong dependence on doping (gate tunability). Linear scaling with photon energy is predicted for the number of pairs and for the duration of the cascade. These dependencies, along with a sharply peaked angular distribution of excited carriers, provide clear experimental signatures of hot carrier multiplication.

We show that strong lateral photon confinement in a vertical Fabry-Pérot microcavity can be achieved without sacrificing the quality factor $Q$ or the need for pillar structures. A submicrometer Gaussian-shaped defect inserted between two distributed Bragg reflectors confines the photons on resonance in a small modal volume $V$ on the order of ${(\lambda /n)}^3$ while maintaining a $Q$ up to ${10}^5$. This design minimizes light scattering induced by lateral dielectric discontinuities, and therefore our simulations demonstrate that the $Q/V$ is increased by nearly two orders of magnitude relative to cavities with an embedded mesa defect. We further discuss possible fabrication strategies for such devices, which are well suited for the study of cavity quantum electrodynamics with either organic or inorganic quantum emitters, and hold promise for the implementation of scalable quantum photonic networks.

Excitonic parameters of ZnO have been determined from 5 to 300 K using the combination of time-integrated four-wave mixing, continuous wave reflectivity, autocorrelation of reflectivity at the femtosecond scale, and photoluminescence spectroscopy. These techniques allow an accurate determination of both inhomogeneous and homogeneous linewidths, oscillator strength, and other parameters characteristic for excitonic resonances. It turns out that, due to impurity-exciton scattering, the homogeneous damping of A and B excitons (0.55 and 1.35 meV, respectively) predominates over inhomogeneous one in high-quality ZnO at low temperature. Our data also prove that the strong exciton-phonon coupling enhances the excitonic broadening at room temperature up to 47 meV. This approach, which combines various sophisticated spectroscopy experiments, can be employed for an accurate determination of excitonic parameters in other semiconductor compounds and in any system where excitons are confined.

The orbital ordering transition in the metallic perovskite PbRuO${}_3$ is suppressed from 90 K at ambient pressure towards zero temperature at 50 kbar, where non-Fermi liquid resistivity with a temperature exponent $n$ $=$ 1.6 is observed. This evidences a possible quantum critical point brought about by orbital fluctuations, rather than spin fluctuations as observed in Sr${}_3$Ru${}_2$O${}_7$ and heavy fermion conductors. An anomalous increase of resistivity is observed at pressures above ∼100 kbar, and a transition to a more resistive, possibly semiconducting, phase is observed at 300 kbar and ambient temperature.

We revise the tight-binding approach to strained or curved graphene in the presence of external probes such as photoemission or scanning tunneling microscopy experiments. We show that extra terms arise in the continuum limit of the tight-binding Hamiltonian which cannot be accounted for by changes in the hopping parameters due to lattice deformations, encoded in the parameter $\beta$. These material-independent extra couplings are of the same order of magnitude as the standard ones and have a geometric origin. They include corrections to the position-dependent Fermi velocity and to a new vector field. We show that the new vector field does not couple to electrons like a standard gauge field and that no $\beta$-independent pseudomagnetic fields exist in strained graphene.

We provide evidence for the unique effect of hydrogen on the transport properties of the mid-infrared alloy In(AsN). High electron concentrations and mobilities are simultaneously achieved in hydrogenated In(AsN), and Shubnikov-de Haas oscillations are observed up to near room temperature. These results can be accounted for by the formation of N-H donor complexes with energy levels well above the Fermi energy, far from resonance with the conduction electrons, thus resulting in weak electron scattering even at high donor concentrations. Similar effects should be found in other narrow band gap dilute nitride alloys.

We present a detailed theoretical framework for the calculation of thermopower, $S$, in two-dimensional electron gases confined in (001) AlAs quantum wells (QWs) taking into account the band anisotropy. Particular emphasis is given on the phonon-drag contribution $S^g$ that is related to the momentum exchange between acoustic phonons and electrons in the presence of a weak in-plane temperature gradient via the electron-phonon coupling. Our model is based on the semiclassical Boltzmann formalism and is a generalization of previous models that are applicable only for an isotropic energy spectrum. We find that the electron anisotropy in AlAs affects strongly the electron-acoustic phonon coupling and $S^g$. Considerable enhancement of $S^g$ is reported in relation to isotropic GaAs QWs. More interestingly, we find that strong anisotropy in phonon-drag thermopower occurs by tuning the valley occupancy. Namely, the predicted $S_{xx}^g$ and $S_{yy}^g$ can differ by over a factor of 5 while a giant magnitude for $S^g$ can be achieved in a particular direction exceeding 10 mV/K at $T=5$ K. The diffusion thermopower is also calculated for comparison.

We report the non-Markovian nature of heavy hole-heavy hole and heavy hole-light hole exciton scattering in a GaAs single quantum well. The homogeneous width depends on the exciton population, which is controlled by the delay time of phase-locked pulses. This effect is explained on the basis of excitation-induced dephasing in Markovian approximation. For short times, the homogeneous width decreases and deviates from the Markovian behavior, due to the non-Markovian behavior of the exciton-exciton scattering. We analyze the homogeneous width by using a weakly interacting boson model including the non-Markovian effect. The correlation time of the exciton-exciton scattering is estimated as 0.43 ps.

We study theoretically magnetoresistance (MR) of graphene with different types of disorder. For short-range disorder, the key parameter determining magnetotransport properties—a product of the cyclotron frequency and scattering time—depends in graphene not only on magnetic field $H$ but also on the electron energy $\varepsilon$. As a result, a strong, square root in $H$, MR arises already within the Drude-Boltzmann approach. The MR is particularly pronounced near the Dirac point. Furthermore, for the same reason, “quantum” (separated Landau levels) and “classical” (overlapping Landau levels) regimes may coexist in the same sample at fixed $H.$ We calculate the conductivity tensor within the self-consistent Born approximation for the case of relatively high temperature, when Shubnikov-de Haas oscillations are suppressed by thermal averaging. We predict a square-root MR both at very low and at very high $H:$ $[{\varrho }_{xx}\left(H\right)-{\varrho }_{xx}\left(0\right)]/{\varrho }_{xx}\left(0\right)\approx C\sqrt{H},$ where $C$ is a temperature-dependent factor, different in the low- and strong-field limits and containing both “quantum” and “classical” contributions. We also find a nonmonotonic dependence of the Hall coefficient both on magnetic field and on the electron concentration. In the case of screened charged impurities, we predict a strong temperature-independent MR near the Dirac point. Furthermore, we discuss the competition between disorder- and collision-dominated mechanisms of the MR. In particular, we find that the square-root MR is always established for graphene with charged impurities in a generic gated setup at low temperature.

We report first-principles calculations that clarify atomic structures and coverage of the metallic phases of In overlayers on Si(111) surfaces. Calculated energy bands and scanning tunneling microscopy images along with the obtained energetics of various phases reveal that the two metallic phases with the $\sqrt{7}\times \sqrt{3}$ periodicity observed experimentally are single and double In overlayers, as opposed to prevailing assignments.

Recent studies of transition metal alloys with compositions that place them near their limits of elastic stability [e.g., near the body-centered-cubic (BCC) to hexagonal-close-packed (HCP) transition] suggest interesting behavior for the dislocation cores. Specifically, the dislocation core size is predicted to diverge as the stability limit is approached. Here a simple analysis rooted in elasticity theory and the computation of ideal strength is used to analyze this divergence. This analysis indicates that dislocation core radii should diverge as the elastic limits of stability are approached in the BCC, HCP, and face-centered-cubic (FCC) structures. Moreover, external stresses and dislocation-induced stresses also increase the core radii. Density functional theory based total-energy calculations are combined with anisotropic elasticity theory to compute numerical estimates of dislocation core radii.

We construct a three-color Potts-like model for the graphene zigzag edge reconstructed with Stone-Wales carbon rings in order to study its thermal equilibrium properties. We consider two cases which have different ground states: the edge with nonpassivated dangling carbon bonds and the edge fully passivated with hydrogen. We study the concentration of defects perturbing the ground-state configuration as a function of the temperature. The defect concentration is found to be exponentially dependent on the effective parameters that describe the model at all temperatures. Moreover, we analytically compute the domain-size distribution of the defective domains and conclude that it does not have fat tails. In an appendix, we show how the exchange parameters of the model can be estimated using density functional theory results. Such equilibrium mechanisms place a lower bound on the concentration of defects in zigzag edges since the formation of such defects is due to nonequilibrium kinetic mechanisms.

A continuous reorientation and an abrupt flip to a canted structure in the magnetization of stripe domains are observed under the influence of an electric field in an yttrium-iron-garnet (YIG)/lead-zirconate-titanate (PZT) bilayer. Magneto-optic techniques have been utilized for the observation of the domain structure and the magnetization flip. It is found that electrically generated mechanical stress in PZT induces an uniaxial anisotropy field in YIG, which is large enough to initially cause a gradual change in the domain magnetization and then a transition from out-of-plane orientation to a canted state for a threshold electric field. Additional evidence for the spin flip has been obtained from data on the modulation of intensity of linearly polarized light due to diffraction by the stripe domains. A comprehensive theory for the voltage-induced magnetization flip is discussed and compared with the data. The magnetic transitions and the theory discussed here are of interests for electric-field-controlled magneto-optic and spintronic devices.

We theoretically investigate spin dynamics and $L_3$-edge resonant inelastic x-ray scattering (RIXS) of chromium with commensurate spin-density wave (SDW) order, based on a multiband Hubbard model composed of 3$d$ and 4$s$ orbitals. Obtaining the ground state with the SDW mean-field approximation, we calculate the dynamical transverse and longitudinal spin susceptibility by using random-phase approximation. We find that a collective spin-wave excitation seen in inelastic neutron scattering hardly damps up to $\sim$0.6 eV. Above the energy, the excitation overlaps individual particle-hole excitations as expected, leading to broad spectral weight. On the other hand, the collective spin-wave excitation in RIXS spectra has a tendency to be masked by large spectral weight coming from particle-hole excitations with various orbital channels. This is in contrast with inelastic neutron scattering, where only selected diagonal orbital channels contribute to the spectral weight. However, it may be possible to detect the spin-wave excitation in RIXS experiments in the future if resolution is high enough.

A theory of a ground-state switching in an array of axially magnetized cylindrical magnetic dots arranged in a square lattice is developed. An array can be switched into a quasiregular chessboard-antiferromagnetic state by the application of a short pulse of external in-plane magnetic field having a sufficiently long trailing front. The statistical properties of an array magnetization in its final (after switching) state are determined at the linear stage of growth of unstable collective spin-wave modes of the array under the action of a time-dependent magnetic field, and depend critically on the rate of the field decrease: the slower this decrease, the more regular is the final magnetization state. An analytical procedure is presented that allows one to relate the statistical properties of the final demagnetized state of the array and the linewidth of the array's microwave absorption to the parameters of the external switching pulse. The comparison of the developed analytic theory with the results of numerical simulations is presented and demonstrates good agreement between analytical and numerical results.

A comprehensive study of the temperature ($T$) and magnetic field ($H$) dependence of magnetic entropy change ($\Delta S_M$) for different materials exhibiting inverse magnetocaloric effect (IMCE) is reported. We show that $\Delta S_M$ follows a power-law dependence of $H$ ($\Delta S_M$ ∼ $H^n$; $n$ is an exponent) for these compounds. In contrast to conventional magnetocaloric effect (CMCE), $n$ is independent of $H$ and $T$ in the case of IMCE. As a result, a universal master curve can be constructed to describe $\Delta S_M\left(T\right)$ of the IMCE systems for different $H$ without rescaling the temperature axis. This is completely different from that reported for CMCE, where the rescaling of the temperature axis with the introduction of at least one reference temperature is needed for constructing a universal curve. The different universal behavior of IMCE is attributed to the constant value of $n$ in any field and temperature, which is a generic feature of IMCE systems irrespective of their magnetic state and nature of phase transition. From the proposed phenomenological universal curve, one can extrapolate the magnetocaloric properties of IMCE systems in any temperature and magnetic field range, which would be helpful in designing controlled active magnetic refrigeration devices.