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We study the origin of the step-like shoulder on the high energy side of the low temperature photoluminescence spectrum of heavily $p$-doped GaAs. We show experimentally that it is controlled by the Fermi-Dirac distribution of the holes and by the energy distribution of the photoexcited electrons showing a sharp step-like dependence. This step is attributed to the percolation threshold in the conduction band separating localized from delocalized electron states. A comprehensive set of optical techniques based on spin orientation of electrons, namely the Hanle effect, time- and polarization-resolved photoluminescence, as well as transient pump-probe Faraday rotation are used for these studies. We identify two different electron ensembles with substantially different spin lifetimes of 20 and 280~ps, limited by the lifetime of the electrons. Their spin relaxation times are longer than 2~ns. The relative contribution of short- and long-lived photoexcited electrons to the emission spectrum changes abruptly at the high-energy photoluminescence step-like tail. For energies above the percolation threshold the electron states are empty due to fast energy relaxation, while for lower energies the relaxation is suppressed and the majority of photoelectrons populate these states., Comment: 6 figs

We demonstrate the optical orientation and alignment of excitons in a two-dimensional layer of CsPbI$_3$ perovskite nanocrystals prepared by colloidal synthesis and measure the anisotropic exchange splitting of exciton levels in the nanocrystals. From the experimental data at low temperature (2K), we obtain the average value of anisotropic splitting of bright exciton states of the order of 120{\mu}eV. Our calculations demonstrate that there is a significant contribution to the splitting due to the nanocrystal shape anisotropy for all inorganic perovskite nanocrystrals., Comment: 10 pages

The dynamic polarization of nuclear spins by photoexcited electrons is studied in a high quality GaAs/AlGaAs quantum well. We find a surprisingly high efficiency of the spin transfer from the electrons to the nuclei as reflected by a maximum nuclear field of 0.9 T in a tilted external magnetic field of 1 T strength only. This high efficiency is due to a low leakage of spin out of the polarized nuclear system, because mechanisms of spin relaxation other than the hyperfine interaction are strongly suppressed, leading to a long nuclear relaxation time of up to 1000 s. A key ingredient to that end is the low impurity concentration inside the heterostructure, while the electrostatic potential from charged impurities in the surrounding barriers becomes screened through illumination by which the spin relaxation time is increased compared to keeping the system in the dark. This finding indicates a strategy for obtaining high nuclear spin polarization as required for long-lasting carrier spin coherence., Comment: 6 pages, 4 figures

Nuclear spin relaxation is studied in n-GaAs thick layers and microcavity samples with different electron densities. We reveal that both in metallic samples where electrons are free and mobile, and in insulating samples, where electrons are localized, nuclear spin relaxation is strongly enhanced at low magnetic field. The origin of this effect could reside in the quadrupole interaction between nuclei and fluctuating electron charges, that has been proposed to drive nuclear spin dynamics at low magnetic fields in the insulating samples. The characteristic values of these magnetic fields are given by dipole-dipole interaction between nuclei in bulk samples, and are greatly enhanced in microcavities, presumably due to additional strain, inherent to micro and nanostructures., Comment: 11 pages, 4 figures

We show that the spin-lattice relaxation in n-type insulating GaAs is dramatically accelerated at low magnetic fields. The origin of this effect, that cannot be explained in terms of well-known diffusion-limited hyperfine relaxation, is found in the quadrupole relaxation, induced by fluctuating donor charges. Therefore, quadrupole relaxation, that governs low field nuclear spin relaxation in semiconductor quantum dots, but was so far supposed to be harmless to bulk nuclei spins in the absence of optical pumping can be studied and harnessed in much simpler model environment of n-GaAs bulk crystal., Comment: 5 pages, 4 figures

The optical spin orientation effect in a GaAs/(Ga,Al)As quantum well containing a high-mobility 2D electron gas was found to be due to spin-polarized minority carriers, the holes. The observed oscillations of both the intensity and polarization of the photoluminescence in a magnetic field are well described in a model whose main elements are resonant absorption of the exciting light by the Landau levels and mixing of the heavy- and light-hole subbands. After subtraction of these effects, the observed influence of magnetic fields on the spin polarization can be well interpreted by a standard approach of the optical orientation method. The spin relaxation of holes is controlled by the Dyakonov-Perel' mechanism. Deceleration of the spin relaxation by the magnetic field occurs through the Ivchenko mechanism - due to the cyclotron motion of holes. Mobility of holes was found to be two orders of magnitude smaller than that of electrons, being determined by the scattering of holes upon the electron gas., Comment: 20 pages, 6 figures

Hybrid structures synthesized from different materials have attracted considerable attention because they may allow not only combination of the functionalities of the individual constituents but also mutual control of their properties. To obtain such a control an interaction between the components needs to be established. For coupling the magnetic properties, an exchange interaction has to be implemented which typically depends on wave function overlap and is therefore short-ranged, so that it may be compromised across the interface. Here we study a hybrid structure consisting of a ferromagnetic Co-layer and a semiconducting CdTe quantum well, separated by a thin (Cd,Mg)Te barrier. In contrast to the expected p-d exchange that decreases exponentially with the wave function overlap of quantum well holes and magnetic Co atoms, we find a long-ranged, robust coupling that does not vary with barrier width up to more than 10 nm. We suggest that the resulting spin polarization of the holes is induced by an effective p-d exchange that is mediated by elliptically polarized phonons., Comment: Main text: 21 pages, 5 figures. Supplementing material: 12 pages, 3 figures

We develop a superoperator coupled cluster method for nonequilibrium open many-body quantum systems described by the Lindblad master equation. The method is universal and applicable to systems of interacting fermions, bosons or their mixtures. We present a general theory and consider its application to the problem of quantum transport through the system with electron-phonon correlations. The results are assessed against the perturbation theory and nonequilibrium configuration interaction theory calculations., Comment: Minor revision according to referees, one new figure

We present a new approach to treat correlations in nonequilibrium quantum many-particle system. The method is based on ideas of configuration interaction theory of exact nonperturbative ground state electronic structure calculations. We use superoperator techniques in Liouville-Fock space and represent the nonequilibrium density matrix as a linear combination of all possible nonequilibrium quasiparticle excitations built on the appropriate reference state. As an example we consider the electron transport through the system with electron-phonon interaction. The concept of embedding (buffer zones between the reservoirs and the correlated quantum system) is used to derive an exact master equation for the reduced density matrix. Using approximate (truncated) expansion of the trial density matrix we obtain the linear system of equations for two-quasiparticle amplitudes. Then we compute the steady-state current and compare the result with other approaches. The current conserving property of the method is proved.

We present an escape rate theory for current-induced chemical reactions. We use Keldysh nonequilibrium Green's functions to derive a Langevin equation for the reaction coordinate. Due to the out of equilibrium electronic degrees of freedom, the friction, noise, and effective temperature in the Langevin equation depend locally on the reaction coordinate. As an example, we consider the dissociation of diatomic molecules induced by the electronic current from a scanning tunnelling microscope tip. In the resonant tunnelling regime, the molecular dissociation involves two processes which are intricately interconnected: a modification of the potential energy barrier and heating of the molecule. The decrease of the molecular barrier (i.e. the current induced catalytic reduction of the barrier) accompanied by the appearance of the effective, reaction-coordinate-dependent temperature is an alternative mechanism for current-induced chemical reactions, which is distinctly different from the usual paradigm of pumping vibrational degrees of freedom., Comment: Accepted by JCP. Minor revision according to referees, one new figure

We use a superoperator representation of the quantum kinetic equation to develop nonequilibrium perturbation theory for an inelastic electron current through a quantum dot. We derive a Lindblad-type kinetic equation for an embedded quantum dot (i.e. a quantum dot connected to Lindblad dissipators through a buffer zone). The kinetic equation is converted to non-Hermitian field theory in Liouville-Fock space. The general nonequilibrium many-body perturbation theory is developed and applied to the quantum dot with electron-vibronic and electron-electron interactions. Our perturbation theory becomes equivalent to a Keldysh nonequilibrium Green's function perturbative treatment provided that the buffer zone is large enough to alleviate the problems associated with approximations of the Lindblad kinetic equation., Comment: 22 pages, 5 figures

We consider a single molecule circuit embedded into solvent. The Born dielectric solvation model is combined with Keldysh nonequilibrium Green's functions to describe the electron transport properties of the system. Depending on the dielectric constant, the solvent induces multiple nonequilibrium steady states with corresponding hysteresis in molecular current-voltage characteristics as well as negative differential resistance. We identify the physical range of solvent and molecular parameters where the effects are present. The position of the negative differential resistance peak can be controlled by the dielectric constant of the solvent., Comment: 5 pages, 5 figures

We present a method to perform stability analysis of nonequilibrium fixed points appearing in self-consistent electron transport calculations. The nonequilibrium fixed points are given by the self-consistent solution of stationary, nonlinear kinetic equation for single-particle density matrix. We obtain the stability matrix by linearizing the kinetic equation around the fixed points and analyze the real part of its spectrum to assess the asymptotic time behavior of the fixed points. We derive expressions for the stability matrices within Hartree-Fock and linear response adiabatic time-dependent density functional theory. The stability analysis of multiple fixed points is performed within the nonequilibrium Hartree-Fock approximation for the electron transport through a molecule with a spin-degenerate single level with local Coulomb interaction.

We discuss the use of tunneling electron current to control and catalyze chemical reactions. Assuming the separation of time scales for electronic and nuclear dynamics we employ the Langevin equation for the reaction coordinate. The Langevin equation contains current-induced forces and is used to define nonequilibrium, effective potential energy surface for current-carrying molecular systems. The current-induced forces are computed via Keldysh nonequilibrium Green's functions. Once the nonequilibrium, current-depended potential energy surface is defined, the chemical reaction is modeled as an escape of a Brownian particle from the potential well. We demonstrate that the barrier between the reactant and the product states can be controlled by the bias voltage. When the molecule is asymmetrically coupled to the electrodes, the reaction can be catalyzed or stopped depending on the polarity of the tunneling current., Comment: 4 pages, 2 figures

Based on the super-fermion representation of quantum kinetic equations we develop nonequilibrium, post-Hartree-Fock many-body perturbation theory for the current through a region of interacting electrons. We apply the theory to out of equilibrium Anderson model and discuss practical implementation of the approach. Our calculations show that nonequilibrium electronic correlations may produce significant quantitative and qualitative corrections to mean-field electronic transport properties. We find that the nonequilibrium leads to enhancement of electronic correlations.

We report on optical orientation of Mn^2+ in bulk GaAs under application of weak longitudinal magnetic field (B <= 100 mT). The manganese spin polarization of 25% is directly evaluated using spin flip Raman scattering spectroscopy. The dynamical polarization of Mn^2+ occurs due to s-d exchange interaction with optically oriented conduction band electrons. Time-resolved photoluminescence uncovers nontrivial electron spin dynamics where the oriented Mn^2+ ions tend to stabilize the electron spin., Comment: 6 pages, 4 figures

We discuss the use of super-fermion formalism to represent and solve quantum kinetic equations for the electron transport problem. Starting with the Lindblad master equation for the molecule connected to two metal electrodes, we convert the problem of finding the nonequilibrium steady state to the many-body problem with non-Hermitian Liouvillian in super-Fock space. We transform the Liouvillian to the normal ordered form, introduce nonequilibrium quasiparticles by a set of canonical nonunitary transformations and develop general many-body theory for the electron transport through the interacting region. The approach is applied to the electron transport through a single level. We consider a minimal basis hydrogen atom attached to two metal leads in Coulomb blockade regime (out of equilibrium Anderson model) within the nonequilibrium Hartree-Fock approximation as an example of the system with electron interaction. Our approach agrees with exact results given by the Landauer theory for the considered models., Comment: Accepted for publication in Journal of Chemical Physics (18 pages, 5 figures)

Nuclear quadrupole interaction extends the limits imposed by hyperfine interaction on the spin coherence of the electron and nuclei in a quantum dot. The strain-induced nuclear quadrupole interaction suppresses the nuclear spin flip and makes possible the zero-field dynamic nuclear polarization in self-organized InP/InGaP quantum dots. The direction of the effective nuclear magnetic field is fixed in space, thus quenching the magnetic depolarization of the electron spin in the quantum dot. The quadrupole interaction suppresses the zero-field electron spin decoherence also for the case of non-polarized nuclei. These results provide a new vision of the role of the nuclear quadrupole interaction in nanostructures: it elongates the spin memory of the electron-nuclear system., Comment: 18 pages including 3 figures. Shortened version has been accepted for publication in Physical Review Letters

We report a giant electric field induced increase of spin orientation of excitons in n-type GaAs/AlGaAs quantum well. It correlates strongly with the formation of negatively charged excitons (trions) in the photoluminescence spectra. Under resonant excitation of neutral heavy-hole excitons, the polarization of excitons and trions increases dramatically with electrical injection of electrons within the narrow exciton-trion bias transition in the PL spectra, implying a polarization sensitivity of 200 % per Volt. This effect results from a very efficient trapping of neutral excitons by the quantum well interfacial fluctuations (so-called "natural" quantum dots) containing resident electrons., Comment: 18 pages, 4 figures