arXiv subjects

Yuli V. Nazarov

Publications and source records attributed to Yuli V. Nazarov.

At least 91 records · Page 5Linked to original sources

arXiv2004

Evolution of a localized electron spin in a nuclear spin environment

Motivated by recent interest in the role of the hyperfine interaction in quantum dots we study the dynamics of a localized electron spin coupled to many nuclei. An important feature of the model is that the coupling to an individual nuclear spin depends on its position in the quantum dot. We introduce a semi-classical description of the system valid in the limit of a large number of nuclei and analyze the resulting classical dynamics. Contrary to a natural assumption, the correlation functions of electron spin with an arbitrary initial condition show no decay in time. Rather, they exhibit complicated undamped oscillations. This may be attributed to the fact that the system has many integrals of motion and is close to an integrable one. The ensemble averaged correlation functions do exhibit a slow decay (1/ln(t)) for t -> \infty.

cond-mat.mes-hall↗

arXiv2003

Full Counting Statistics of Spin Currents

We discuss how to detect fluctuating spin currents and derive full counting statistics of electron spin transfers. It is interesting to consider several detectors in series that simultaneously monitor different components of the spins transferred. We have found that in general the statistics of the measurement outcomes cannot be explained with the projection postulate and essentially depends on the quantum dynamics of the detectors.

cond-mat↗

arXiv2003

Coherent oscillations of current due to nuclear spins

We propose a mechanism for very slow coherent oscillations of current and nuclear spins in a quantum dot system, that may qualitatively explain some recent experimental observations. We concentrate on an experimentally relevant double dot setup where hyperfine interaction lifts the spin blockade. We study the dependence of the magnitude and period of the oscillations on magnetic field and anisotropy.

cond-mat.mes-hall↗

arXiv2003

Full counting statistics in electric circuits

We review several ways of defining the statistics of charge that is transmitted through an electric circuit, the so-called "counting statistics". We first pursue two approaches that explain the origin of "negative probabilities" that have been found earlier in this context. The first analysis involves degrees of freedom of the electric circuit only. In the second approach an idealized charge detector is introduced. We study then general linear detectors and how their back action on the electric conductor affects the statistics of transferred charge. Phase-coherent conductors in the presence of an electromagnetic environment are studied within a Keldysh action approach.

cond-mat.mes-hall↗

arXiv2003

Multiterminal counting statistics

The review is given of the calculational schemes that allows for easy evaluation of full current statistics (FCS) in multi-terminal mesoscopic systems. First, the scattering approach by Levitov {\it et.al} to FCS is outlined. Then the multi-terminal FCS of the non-interacting electrons is considered. We show, that this theory appears to be a circuit theory of $2\times 2$ matrices associated with Keldysh Green functions. Further on the FCS in the opposite situation of mesoscopic systems placed in a strong Coulomb blockade limit is discussed. We prove that the theory of FCS in this case turns out to be an elegant extension of the master equation approach. We illustrate both methods by applying them to the various three-terminal circuits. We study the FCS of electron transfer in the three-terminal chaotic quantum dot and compare it with the statistics of charge transfer in the Coulomb blockade island with three leads attached. We demonstrate that Coulomb interaction suppresses the relative probabilities of big current fluctuations. Finally, for the generic case of single resonance level the equivalence of scattering and master equation approach to FCS is established.

cond-mat.mes-hall↗

arXiv2002

Multi-parameter scaling of the Kondo effect in quantum dots with an even number of electrons

We address a recent theoretical discrepancy concerning the Kondo effect in quantum dots with an even number of electrons where spin-singlet and -triplet states are nearly degenerate. We show that the discrepancy arises from the fact that the Kondo scaling involves many parameters, which makes the results depend on concrete microscopic models. We illustrate this by the scaling calculations of the Kondo temperature, $T_K$, as a function of the energy difference between the singlet and triplet states $Δ$. $T_K(Δ)$ decreases with increasing $Δ$, showing a crossover from a power law with a universal exponent to that with a nonuniversal exponent. The crossover depends on the initial parameters of the model.

cond-mat.mes-hall↗

arXiv2002

Hyperfine-mediated transitions between a Zeeman split doublet in GaAs quantum dots: The role of the internal field

We consider the hyperfine-mediated transition rate between Zeeman split spin states of the lowest orbital level in a GaAs quantum dot. We separate the hyperfine Hamiltonian into a part which is diagonal in the orbital states and another one which mixes different orbitals. The diagonal part gives rise to an effective (internal) magnetic field which, in addition to an external magnetic field, determines the Zeeman splitting. Spin-flip transitions in the dots are induced by the orbital mixing part accompanied by an emission of a phonon. We evaluate the rate for different regimes of applied magnetic field and temperature. The rates we find are bigger that the spin-orbit related rates provided the external magnetic field is sufficiently low.

cond-mat.mes-hall↗

arXiv2001

Spin-flip transitions between Zeeman sublevels in semiconductor quantum dots

We have studied spin-flip transitions between Zeeman sublevels in GaAs electron quantum dots. Several different mechanisms which originate from spin-orbit coupling are shown to be responsible for such processes. It is shown that spin-lattice relaxation for the electron localized in a quantum dot is much less effective than for the free electron. The spin-flip rates due to several other mechanisms not related to the spin-orbit interaction are also estimated.

cond-mat.mes-hall↗

arXiv2001

Nucleus-mediated spin-flip transitions in GaAs quantum dots

Spin-flip rates in GaAs quantum dots can be quite slow, thus opening up the possibilities to manipulate spin states in the dots. We present here estimations of inelastic spin-flip rates mediated by hyperfine interaction with nuclei. Under general assumptions the nucleus mediated rate is proportional to the phonon relaxation rate for the corresponding non-spin-flip transitions. The rate can be accelerated in the vicinity of a singlet-triplet excited states crossing. The small proportionality coefficient depends inversely on the number of nuclei in the quantum dot. We compare our results with known mechanisms of spin-flip in $GaAs$ quantum dot.

cond-mat.mes-hall↗

arXiv2001

Mean Field Theory of the Kondo Effect in Quantum Dots with an Even Number of Electrons

We investigate the enhancement of the Kondo effect in quantum dots with an even number of electrons, using a scaling method and a mean field theory. We evaluate the Kondo temperature $T_K$ as a function of the energy difference between spin-singlet and triplet states in the dot, $Δ$, and the Zeeman splitting, $E_Z$. If the Zeeman splitting is small, $E_Z \ll T_K$, the competition between the singlet and triplet states enhances the Kondo effect. $T_K$ reaches its maximum around $Δ=0$ and decreases with $Δ$ obeying a power law. If the Zeeman splitting is strong, $E_Z \gg T_K$, the Kondo effect originates from the degeneracy between the singlet state and one of the components of the triplet state at $-Δ\sim E_Z$. We show that $T_K$ exhibits another power-law dependence on $E_Z$. The mean field theory provides a unified picture to illustrate the crossover between these regimes. The enhancement of the Kondo effect can be understood in terms of the overlap between the Kondo resonant states created around the Fermi level. These resonant states provide the unitary limit of the conductance $G\sim 2e^2/h$.

cond-mat.mes-hall↗

arXiv2001

Spin-Flip Transistor

The recently developed semiclassical theory for magnetoelectronic circuits is applied to a transistor-like device consisting of a normal metal island and three magnetic terminals. The electric current between source and drain can be controlled by the magnetization of a ``base'' reservoir up to distances of the order of the spin-flip diffusion length.

cond-mat.mes-hall↗

arXiv2000

Semiclassical Concepts in Magnetoelectronics

Semiclassical theories of electron and spin transport in metallic magnetic structures are reviewed with emphasis on the role of disorder and electronic band structures in the current perpendicular to the interface plane (CPP) transport configuration.

cond-mat.mes-hall↗

arXiv2000

Enhancement of Kondo effect in quantum dots with an even number of electrons

We investigate the Kondo effect in a quantum dot with almost degenerate spin-singlet and triplet states for an even number of electrons. We show that the Kondo temperature as a function of the energy difference between the states Delta reaches its maximum around Delta=0 and decreases with increasing Delta. The Kondo effect is thus enhanced by competition between singlet and triplet states. Our results explain recent experimental findings. We evaluate the linear conductance in the perturbative regime.

cond-mat.mes-hall↗

arXiv1999

Universalities of weak localization

We present here a novel approach to evaluation of the weak localization correction (WLC) to transport properties of a mesoscopic conductor. It is based on an extension of Keldysh technique and allows one to evaluate the full counting statistics of the current in the conductor. In our opinion, it provides a fresh look on the theory of weak localization.

cond-mat.mes-hall↗

arXiv1999

Spin relaxation in semiconductor quantum dots

We have studied the physical processes responsible for the spin -flip in GaAs quantum dots. We have calculated the rates for different mechanisms which are related to spin-orbit coupling and cause a spin-flip during the inelastic relaxation of the electron in the dot both with and without a magnetic field. We have shown that the zero-dimensional character of the problem when electron wave functions are localized in all directions leads to freezing out of the most effective spin-flip mechanisms related to the absence of the inversion centers in the elementary crystal cell and at the heterointerface and, as a result, to unusually low spin-flip rates.

cond-mat.mes-hall↗

arXiv1999

Classical Quantization of Local Hall Conductivity in 2D Ballistic Systems

We study the linear Hall response of 2D ballistic system on inhomogeneous magnetic field. We establish that in classical limit the Hall conductivity response on local magnetic field is quantized in units of $α_H \equiv \frac{e^3}{2 π^2 \hbar c}$. The quantized value depends on the point where the field is applied, this dependence being irregular in chaotic billiards. The phenomenon allows for direct tracing of special electron trajectories that belong to fractal repellor. We discuss how quantum effects smooth the quantization.

cond-mat.mes-hall↗

arXiv1998

Novel Circuit Theory of Andreev Reflection

We review here a novel circuit theory of superconductivity. The existed circuit theory of Andreev reflection has been revised to account for decoherence between electrons and holes and twofold nature of the distribution function. The description of arbitrary connectors has been elaborated. In this way one can cope with the most of the factors that limited applicability of the old circuit theory. We give a simple example and discuss numerical implementation of the theory.

cond-mat.mes-hall↗

arXiv1998

Coulomb drag in intermediate magnetic fields

We investigated theoretically the Coulomb drag effect in coupled 2D electron gases in a wide interval of magnetic field and temperature $ 1/τ\ll ω_c \ll E_F/\hbar$, $T \ll E_F$, $τ$ being intralayer scattering time, $ω_c$ being the cyclotron frequency. We show that the quantization of the electron spectrum leads to rich parametric dependences of drag transresistance on temperature and magnetic field. This is in contrast to usual resistance. New small energy scales are found to cut typical excitation energies to values lower than temperature. This may lead to a linear temperature dependence of transresistance even in a relatively weak magnetic field and can explain some recent experimental data. We present a novel mechanism of Coulomb drag when the current in the active layer causes a magnetoplasmon wind and the magnetoplasmons are absorbed by the electrons of the passive layer providing a momentum transfer. We derived general relations that describe the drag as a result of resonant tunneling of magnetoplasmons.

cond-mat.mes-hall↗