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In a previous article [M. S. Grbi\ifmmode \acute{c}\else \'{c}\fi{} et al., Phys. Rev. B 83, 144508 (2011)] we reported data on the high-temperature superconductor ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}\ensuremath{\delta}}$ collected by a contactless microwave absorption technique, which provided evidence for superconducting correlations over a limited temperature range (10--20 K above ${T}_{c}$). The paraconductivity signal was determined by subtraction of zero-magnetic-field data from 16-T data. In the preceding Comment [D. S\'o\~nora et al., preceding Comment, Phys. Rev. B 102, 176501 (2020)] D. S\'o\~nora et al. argue that a 16-T magnetic field is not enough to quench all superconducting fluctuations in the microwave spectral range above ${T}_{c}$. As is obvious from the experimental data presented here, as well as from the data shown in the original paper, such conclusions disagree with our experimental results. Moreover, our initial experimental findings recently received independent robust confirmation from several different experimental techniques.

We extend the description of the nonequilibrium Gross-Pitaevskii equation for microcavity polaritons by including not only the kinetics of the nonresonantly pumped exciton reservoir but also that of the collective excitations---called bogolons---above the condensate. Quasiequilibrium Hartree-Fock-Bogoliubov-Popov self-energies are used to describe the condensate and the bogolons. With nonequilibrium Green functions, the quantum Boltzmann equation of the bogolons in a frame moving with the condensate, together with the coupled Gross-Pitaevskii equation can be derived. The resulting scattering rates contain vertex corrections in form of symmetrized products of the $u$ and $v$ pairing functions of all involved scattering states. Corresponding results have been derived for ultracold atoms by A. Griffin, T. Nikuni, and E. Zaremba [Bose-Condensed Gases at Finite Temperatures, 1st ed. (Cambridge University Press, Cambridge, New York, 2009)] Furthermore, we show that the results of the quantum kinetics also determine the luminescence spectrum including the ghost branch. The kinetics is solved numerically, the resulting condensate, the bogolon distribution, and the luminescence are calculated temporally and spatially resolved after the switch-on of a $\mathit{cw}$ nonresonant pump beam with a Gaussian profile. It is studied in detail to which extent the bogolons approach a local thermal equilibrium.

Semiconductor quantum dots are potential building blocks for scalable qubit architectures. Efficient control over the exchange interaction and the possibility of coherently manipulating electron states are essential ingredients towards this goal. We studied experimentally the shuttling of electrons trapped in serial quantum dot arrays isolated from the reservoirs. The isolation hereby enables a high degree of control over the tunnel couplings between the quantum dots, while electrons can be transferred through the array by gate voltage variations. Model calculations are compared with our experimental results for double, triple, and quadruple quantum dot arrays. We are able to identify all transitions observed in our experiments, including cotunneling transitions between distant quantum dots. The shuttling of individual electrons between quantum dots along chosen paths is demonstrated.

We investigate the properties of the magnetocapacitance and dissipation factor of epitaxial graphene Hall bars with different electrode configurations to gain insight into the underlying physical mechanisms. The dependence of magnetocapacitance and dissipation factor on the magnetic field shows how the screening ability of the two-dimensional electron gas (2DEG) changes at the transition from the nonquantized to the quantized state. Both magnetocapacitance and dissipation factor exhibit a characteristic and correlated voltage dependence, which is attributed to the alternating contraction and expansion of the nonscreening 2DEG regions due to the alternating local electric field. Two regimes with seemingly different voltage dependencies are explained as the limiting cases of weak and strong electric fields of the same general voltage dependence. Electric fields in the plane of the 2DEG are found to cause about three orders of magnitude more ac dissipation than perpendicular electric fields. This strong directionality is attributed to the fact that the electrons are mobile in the plane of the 2DEG but are confined in the third dimension. In the quantized state, not only the screening edge of the 2DEG but also compressible puddles embedded in the bulk cause ac dissipation, as follows from the measured frequency dependence. Finally, characteristic parameters like the width of the screening edge, the threshold voltage, and the charging time of the compressible puddles are determined.

We show that $^{63}\mathrm{Cu}$-NMR spectra place strong constraints on both the nature and the concentration of oxygen defects in ortho-II ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{y}$. Systematic deviation from ideal ortho-II order is revealed by the presence of inequivalent Cu sites in either full or empty chains. The results can be explained by two kinds of defects: oxygen clustering into additional chains, or fragments thereof, most likely present at all concentrations $(6.4lyl6.6)$, and oxygen vacancies randomly distributed in the full chains for $yl6.50$ only. Furthermore, the remarkable reproducibility of the spectra in different samples with optimal ortho-II order $(y\ensuremath{\simeq}6.55)$ shows that chain-oxygen disorder, known to limit electronic coherence, is ineluctable because it is inherent to these compounds.

vortex-antivortex pairs, but beyond that, yield an image of the superfluid flow patterns linked with the structured condensate density. The fast superfluid flow driven by the spatially changing phase with velocities of the order of several mμ/ps is found to be often supersonic. In order to test dynamically the stability of the spontaneously formed flow patterns under external perturbations, we apply an additional resonant Laguerre-Gauss beam with angular momentum. This beam causes complex response of the phase patterns. This response is shown to be basically an oscillation or rotation of the vortex-antivortex pair depending on the strength of the extra beam. The rotation is induced via a ring of vortices induced by the Laguerre-Gaus beam. The main result of these studies is the extraordinary stability of the vortex-antivortex pair even under strongly perturbing external fields.

We report on a photoinduced transient state of ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{2}$O${}_{6+\ensuremath{\delta}}$ in which transport perpendicular to the Cu-O planes becomes highly coherent. This effect is achieved by excitation with mid-infrared optical pulses, tuned to the resonant frequency of apical oxygen vibrations, which modulate both lattice and electronic properties. Below the superconducting transition temperature ${T}_{c}$, the equilibrium signatures of superconducting interlayer coupling are enhanced. Most strikingly, the optical excitation induces a new reflectivity edge at higher frequency than the equilibrium Josephson plasma resonance, with a concomitant enhancement of the low-frequency imaginary conductivity ${\ensuremath{\sigma}}_{2}(\ensuremath{\omega})$. Above ${T}_{c}$, the incoherent equilibrium conductivity becomes highly coherent, with the appearance of a reflectivity edge and a positive ${\ensuremath{\sigma}}_{2}(\ensuremath{\omega})$ that increases with decreasing frequency. These features are observed up to room temperature in ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{2}$O${}_{6.45}$ and ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{2}$O${}_{6.5.}$ The data above ${T}_{c}$ can be fitted by hypothesizing that the light establishes a transient superconducting state over only a fraction of the solid, with a lifetime of a few picoseconds. Non-superconducting transport could also explain these observations, although one would have to assume transient carrier mobilities near 10${}^{4}$ cm${}^{2}$/V sec at 100 K, with a density of charge carriers similar to the below-${T}_{c}$ superfluid density. Our results are indicative of highly unconventional nonequilibrium physics and open new prospects for optical control of complex solids.

The space and time dependent nonequilibrium Keldysh-Green functions are employed to derive the scattering rates between the condensed microcavity polaritons described by a Gross-Pitaevskii equation and an uncondensed higher lying exciton reservoir. Slowly varying center coordinates and rapidly varying relative coordinates are assumed. For particle-particle and particle-phonon interactions the scattering rates which provide gain to the condensate are calculated explicitly. These processes result in scattering rates which are quadratic and linear in the density of reservoir excitons, respectively. The resulting quantum Boltzmann equation for the reservoir is simplified by assuming local thermal equilibrium to rate equations for the exciton density and their temperature. Using the microscopically calculated (not phenomenologically chosen) transition amplitudes for CdTe microcavity polaritons we demonstrate that our model is able to describe the spontaneous pattern formation for a ring-shaped nonresonant excitation as seen in recent experiments

For a nonequilibrium electron system in semiconductors the time-dependent screened effective interaction due to longitudinal optical phonon and Coulomb scattering is derived. The polarization is described in terms of a time-dependent random-phase approximation. The nonequilibrium electron propagators are expressed in terms of the single-time density matrix by means of the genaralized Kadanoff-Baym approximation. We illustrate the theory for a femtosecond-pulse-excited semiconductor by calculating the resulting two-time-dependent interaction. Using an incomplete Fourier transformation we study the buildup of the phonon-plasmon double resonance in time as a function of the excited carrier density.

The correct formulation of the two-time self-consistent Coulomb quantum kinetics with screening requires a polarization loop obeying the Ward identity stemming from the charge conservation. Otherwise an infrared divergence occurs. For a self-consistent self-energy without vertex part, a polarization loop which includes a vertex part calculated in the ladder approximation, is shown to satisfy the required Ward identity.

The coherent transients generated by femtosecond interband photoexcitation of a semiconductor in crossed electric and magnetic fields are calculated. The scattering with LO phonons is considered and the complex interplay between excitation and dephasing is analyzed. While below the LO-phonon threshold the signal is not effectively damped, above the threshold damping takes place on a picosecond time scale, in qualitative agreement with corresponding experiments.

We develop a theory of the kinetics of the Bose-Einstein condensation of bosonic excitons interacting with a thermal bath of acoustic phonons. We emphasize several delicate aspects of the condensation kinetics within the framework of rate equations. We give detailed proofs about the existence and uniqueness of the solution with a condensate and illustrate details within exactly or almost exactly solvable models. In particular, we predict an exciton condensation time in ${\mathrm{Cu}}_{2}\mathrm{O}$ which is shorter than the lifetime of paraexcitons.

A system of coherently excited weakly interacting excitons is investigated with respect to its coherent and incoherent dynamics. Both the mean-field terms and the scattering terms are calculated on a microscopic level including the contribution of the anomalous functions. In this framework, the transition from coherent to incoherent excitons is studied. At high excitation densities near the critical density for a Bose-Einstein condensation a pronounced slowing down of the dephasing is obtained.

The effects of hydrogen on Ni~100! submonolayer homoepitaxy have been investigated by kinetic Monto Carlo ~KMC! simulations of the Ni growth process with and without the presence of a small hydrogen impurity. The two-dimensional KMC simulations are based upon rate constants for a set of uncorrelated Ni site-to-site hopping mobilities with and without the presence of hydrogen atoms which are found to act as a catalyst. The rate constant activation energies are determined by classical-potential total-energy calculations with semiclassical zero-point energy corrections for the hydrogen atom. This set of KMC simulations extends preliminary work on the activation energies which were reported along with their connection to available experiments in Ref. 1. We find that fast diffusion of H atoms occurs on the flat Ni~100! surface and the presence of these highly mobile H atoms is found to have significant effects on the mobility of lone Ni adatoms, and therefore also on the Ni islands which form during the epitaxial growth. The H atoms increase the mobility of lone Ni adatoms across the flat Ni~100! terrace by stabilizing the transition state for site-to-site hopping of the Ni adatom. While the effects of the H atom on the Ni mobility are primarily catalytic, the kinetically determined Ni island morphologies differ substantially as a function of H atom concentration over time periods which are long on the deposition time scale, and therefore the morphology differences can become frozen in place. Even quite low H atom coverage, by acting to increase lone Ni atom mobility, tends to decrease Ni island density which results in a corresponding increase in the average Ni island size. This suggests that the H atoms are acting as an antisurfactant in the Ni~100! homoepitaxy at low H coverage. @S0163-1829~99!06339-0#

We apply the non-Markovian quantum kinetics of carrier relaxation scattering due to carrier-carrier and carrier-LO-phonon interactions to an electron-hole gas in a semiconductor excited resonantly by coherent femtosecond laser pulses. Our theory employs the full two-time-dependent random-phase approximation (RPA) screened Coulomb potential that evolves self-consistently from a bare potential to the well-known dynamically screened RPA potential for long times. The dependence on the delay time of the differential transmission spectra for a thin layer of GaAs at 15 K is calculated and measured for a 150-fs pump pulse tuned to optical transitions between the light- and heavy-hole valence bands and the conduction band, while a delayed 30-fs probe pulse is tuned to transitions between the spin-orbit split-off valence band to conduction band. The calculated spectra explain the measured ones in a broad range of delay times and for various pump frequencies and intensities qualitatively rather well.

We investigate the femtosecond kinetics of an optically excited two-dimensional magnetoplasma at intermediate and high densities under a strong magnetic field perpendicular to the quantum well (QW). We assume an additional weak lateral confinement which lifts the degeneracy of the Landau levels partially. We calculate the femtosecond dephasing and relaxation kinetics of the laser pulse excited magnetoplasma due to bare Coulomb potential scattering, because screening is of minor importance under these conditions. In particular, the time-resolved and time-integrated four-wave-mixing (FWM) signals are calculated by taking into account three Landau subbands in both the valance and the conduction band assuming an electron-hole symmetry. The FWM signals exhibit quantum beats mainly with twice the cyclotron frequency. Contrary to general expectations, we find no pronounced slowing down of the dephasing with increasing magnetic field. On the contrary, one obtains a decreasing dephasing time because of the increase of the Coulomb matrix elements and the number of states in a given Landau subband. In the situation when the loss of scattering channels exceeds these increasing effects, one gets a slight increase at the dephasing time. However, details of the strongly modulated scattering kinetics depend sensitively on the detuning, the plasma density, and the spectral pulse width relative to the cyclotron frequency.

The effects of hydrogen in Ni(100) submonolayer homoepitaxy have been investigated by classical-potential total-energy calculations with semiclassical hydrogen zero-point-energy corrections. The fast diffusion of H on a flat Ni(100) surface is found to persist even when the surface is decorated with various intrinsic defects such as Ni adatoms, islands, and steps created during epitaxial growth. On the other hand, the presence of H significantly enhances the mobility of both Ni adatoms and Ni islands, resulting in a decrease in the Ni island density and a corresponding increase in the average island size. These results suggest that hydrogen will function as an antisurfactant in Ni(100) homoepitaxy. Connections to available experiments are made. {copyright} {ital 1997} {ital The American Physical Society}

We fabricated submicrometer-diameter double-barrier diodes from four wafers with different barrier-thickness asymmetry. All samples exhibit staircaselike features in the current-voltage characteristic at the current threshold due to single-electron tunneling. Our study focuses on the properties of the first current step which arises from tunneling through the energetically lowest discrete electron state within the double-barrier region. The analysis of the bias position of the step allows a spatial spectroscopy of the vertical position of the lowest discrete level in the double-barrier region. The magnitude of the step is in excellent agreement with theory for all barrier-thickness asymmetries whereas the broadening of the step edge exceeds the lifetime-related width of the discrete state by one order of magnitude.

Using 63Cu NMR, we establish that the enhancement of spin order by a magnetic field H in YBa2Cu3O6.45 arises from a competition with superconductivity because the effect occurs for H perpendicular, but not parallel, to the CuO2 planes, and it persists up to field values comparable to Hc2. We also find that the spin-freezing has a glassy nature and that the frozen state onsets at a temperature which is independent of the magnitude of H. These results, together with the presence of a competing charge-ordering instability at nearby doping levels, are strikingly parallel to those previously obtained in La-214. This suggests a universal interpretation of magnetic field effects in underdoped cuprates where the enhancement of spin order by the field may not be the primary phenomenon but rather a byproduct of the competition between superconductivity and charge order. Low-energy spin fluctuations are manifested up to relatively high temperatures where they partially mask the signature of the pseudogap in 1/T1 data of planar Cu sites.

${\mathrm{In}}_{0.53}$${\mathrm{Ga}}_{0.47}$As/InP quantum wires have been investigated by cw high-excitation luminescence spectroscopy. Quantum-wire states up to the edge of the InP conduction band have been populated in the dense neutral electron-hole plasma. Up to three lateral subband transitions are clearly observed in the luminescence spectra. By using calculated line shapes, the temperature and the density of the $e\ensuremath{-}h$ plasma as well as the band-gap reduction of the different lateral subbands have been determined. We observe a clear dependence of the renormalization on the subband index, which is traced to the density dependence of the exchange energy.

The few-electron ground states of a strongly asymmetric nanometer-scale ${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$As-GaAs double-barrier quantum dot are probed by single-electron tunneling spectroscopy. Magnetotunneling measurements reveal transitions of spin and orbital angular momentum of the two- and three-electron quantum-dot ground states, respectively. The experimental data are compared with exact calculations of the few-electron ground-state energies. Magnetic-field-induced modulations of the tunneling rate in the many-electron regime indicate correlations in the involved states.

We analyze the decay of the first- and second-order correlations, ${g}^{(1)}(t)$ and ${g}^{(2)}(t)$, using a Langevin equation of the order parameter for polaritons in a microcavity, pumped nonresonantly with a noise-free laser diode. We consider a coupling of the condensate to the excited states by two types of scattering mechanism both derived from the polariton-polariton interaction: (a) one particle is scattered from an excited state to the ground state, while another excited-state particle is scattered to a higher-energy state, and (b) two ground-state particles are scattered into low-lying excited states with opposite momenta. This nonresonant scattering rate increases with the condensate population. We calculate from the Langevin equation for the order parameter the temporal decay of ${g}^{(1)}(t)$ and the linewidth $\ensuremath{\kappa}$ analytically. Our results make close contacts with the linewidth enhancement factor well known from semiconductor lasers. A semiclassical evaluation for the polariton kinetics yields a first-order correlation function, which is in good agreement with experimental results and with the results of Schwendimann and Quattropani, although our formulation is more complete because it contains also the dispersive effects of both types of scattering processes and uses no fitting parameters. Our analysis also provides an understanding of the rather different pump dependencies of the linewidth reported in the literature. The calculated second-order correlation function ${g}^{(2)}(0)$ is shown to stay for larger pump values as observed on a plateau above the coherent limit again due to the nonresonant scattering processes.

For a two-dimensional electron gas in the quantum Hall regime the time dependences of the induced charge and the current through the sample are investigated. The fastest electrodynamical process in the system is the propagation of a wave packet along the edge with the corresponding charge being held by the edge channel structure. The width of the charge strip is determined and the dispersion relation is obtained.