Search

We consider a two-dimensional structure with spin-orbit-related splitting of the electron (hole) spectrum and calculate the edge spin density which appears due to the intrinsic mechanism of spin-orbit interaction in the presence of a charge current through the structure. We concentrate on the quasiballistic case when a mean-free path, being much smaller than the sample size, is larger than the spin precession length determined by the value of the spin-orbit splitting. We show that regardless of the presence or absence of the bulk spin current, the main source of the edge spin density is the boundary scattering itself. The character of the edge spin density depends on the smoothness of the bulk impurity potential. We have calculated the edge spin density profile for an arbitrary smoothness of the scattering potential in the bulk, and discussed relation to the existing experiments for two-dimensional holes.

Phonon-assisted spin-flip transitions between the Zeeman sublevels are considered for two different mesoscopic systems: GaAs quantum dots (localized states) and the 2D electron system in the quantum Hall—regime for filling factor 1 (delocalized states). The phonon-assisted spin-flip rates for all spin–orbit-related mechanisms are evaluated. The role of the localization of the electron states in the effectiveness of the spin-lattice relaxation and the different role of the Coulomb interaction for these two systems are discussed. It is shown, for example, that spin-lattice relaxation for the electron localized in a quantum dot is much less effective than for the free electron. Besides, the spin-flip rates due to several other mechanisms not related to the spin–orbit interaction are estimated for the dot electron.

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 both with and without a magnetic field. It is generally observed that the corresponding spin-flip rates are by several orders of magnitude lower than those for the spin-flip processes for free 2D electrons (i.e. in the case when there is no confinement in the plane). We have shown that it is due to the zero-dimensional character of the problem (electron wave functions are localized in all directions) which 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 on the heterointerface.

We consider theoretically the Coulomb drag effect in a wide interval of magnetic field H and temperature Δ ≪ℏ ω c , T ≪ E F , where Δ is the width of the Landau level due to disorder and ω c is the cyclotron frequency. The quantization of the electron spectrum leads to rich parametric dependences of transresistance on temperature and magnetic field. In particular, new small energy scales (for example, Δ ≪ T ) play the role of characteristic cut-off energies (instead of the temperature at H =0) which leads to a linear temperature dependence of transresistance even in a weak magnetic field. We show that even at low temperatures T >ℏ ω c a new mechanism of the Coulomb drag which is due to resonant tunneling of the magnetoplasmons between the layers occurs in the system.

We investigate the ground state of 2D electron gas in Quantum Hall regime at the filling factor slightly deviating from unity, that can be viewed as a sparse lattice of skyrmions. We have found that in the low density limit skyrmions are bound in pairs, those forming the actual lattice. We have shown that at further density increase the lattice undergoes a quantum phase transition, an analogue of superconducting phase transition in Josephson junction arrays.

The spin magnetization of a system of fermions confined to two dimensions, and subject to a strong magnetic field, has been predicted to exhibit a first-order phase transition as a function of its Zeeman energy. We present strong experimental evidence that such a paramagnetic to ferromagnetic phase transition occurs at Landau level filling n ­ 4 in the two-dimensional hole gas. We demonstrate that our data are not explained by perturbative effects such as Landau level anticrossing, and show that exchange coefficients measured at odd filling factors are consistent with the predicted size of the gap at n ­ 4. [S0031-9007(97)04681-4]

We propose a nanoscale realization of a phonon laser utilizing phonon-assisted spin flips in quantum dots to amplify sound. Owing to a long spin relaxation time, the device can be operated in a strong pumping regime, in which the population inversion is close to its maximal value allowed under Fermi statistics. In this regime, the threshold for stimulated emission is unaffected by spontaneous spin flips. Considering a nanowire with quantum dots defined along its length, we show that a further improvement arises from confining the phonons to one dimension, and thus reducing the number of phonon modes available for spontaneous emission. Our work calls for the development of nanowire-based, high-finesse phonon resonators. © 2013 American Physical Society., A. K. acknowledges financial support from the SPINMET Project (FP7-PEOPLE-2009-IRSES). The work was supported by the ONR, DOE-BES DESC0004890, and NSF-ECCS. X. Hu also acknowledges support by the U.S. ARO and NSF-PIF. The work of V. N. G. was supported by the Spanish Ministry of Economy and Competitiveness under Project No. FIS2011-28851-C02-02.

Quantum dots in GaAs heterostructures display a complex inner structure in a large magnetic field. The distribution of electrons over different Landau levels results in the formation of alternating compressible and incompressible regions. We investigate theoretically the situation when only a few electrons are confined to the uppermost Landau level. We show that these extra electrons form a Wigner molecule. The magnetic field controls the number of electrons in the molecule, and this creates the possibility of manipulating few-electron objects. Moreover, in this way one can manipulate fractional charges. We discuss briefly the stability of the molecule in the presence of a fluctuating donor potential and the slow charge dynamics, which is intrinsic for the system under consideration.

We consider a mesoscopic {\it ballistic} structure with Rashba spin-orbit splitting of the electron spectrum. The ballistic region is attached to the leads with a voltage applied between them. We calculate the edge spin density which appears in the presence of a charge current through the structure due to the difference in populations of electrons coming from different leads. Combined effect of the boundary scattering and spin precession leads to oscillations of the edge polarization with the envelope function decaying as a power law of the distance from the boundary. The problem is solved with the use of scattering states. The simplicity of the method allows to gain an insight into the underlaying physics. We clarify the role of the unitarity of scattering for the problem of edge spin accumulation. In case of a straight boundary it leads to exact cancellation of all long-wave oscillations of the spin density. As a result, only the Friedel-like spin density oscillations with the momentum 2k_F survive. However, this appears to be rather exceptional case. In general, the smooth spin oscillations with the spin precession length recover, as it happens, e.g., for the wiggly boundary. We demonstrate also, that there is no relation between the spin current in the bulk, which is zero in the considered case, and the edge spin accumulation., Latex, 6 pages, 2 figs

The theory of transitions between spin-split edge channels in the quantum-Hall-effect regime has been developed. The spin-orbital interaction is considered as a mechanism of spin flip. It is shown that the mechanism is capable of accounting for the experimental data.

Electron tunnelling in a transverse magnetic field is studied taking into account the electron-phonon interaction. When scattered on a phonon, the electron shifts the oscillator centre in the tunnelling direction, which decreases the field-related magnetic barrier and increases the probability of tunnelling considerably. For tunnelling over large distances, the most effective processes involve multiple scattering by phonons. An expression has been found for the resistivity of the Miller-Abrahams network element for hopping conductivity with regard to magnetic-barrier suppression upon scattering by phonons. The exponential temperature and voltage dependences of the tunnel junction conductance in a magnetic field parallel to a dielectric interlayer have been obtained. The contribution associated with the tunnelling processes involving scattering by two phonons to the relaxation of edge-state populations in the quantum Hall effect regime has been found.

The conductivity of a two-dimensional degenerate electron (2DEG) gas has been calculated in a microscopically inhomogeneous magnetic field of Abrikosov's vortices. The mean free path of electrons is assumed to be much larger than the vortex diameter 2 lambda (in this case the vortices play the role of additional asymmetric scatterers). It is shown that the Hall constant for a degenerate electron gas contains the Hall factor defined by the value of parameter kF lambda (kF is the Fermi wavevector of the electron). The Hall resistance is determined by the mean value of magnetic field in the 2DEG plane in the kF lambda >>1 limit only. The case of a multiquantum vortex has also been investigated (the number of flux quanta in the vortex is gamma >>1). In the case of classical small-angle scattering (kF lambda >> gamma ) the Hall resistance is determined by the mean value of the magnetic field. The Hall resistance decreases monotonically if scattering is of the classical, but not small-angle, type and is saturated in the region of quantum scattering. It is shown that the presence of vortices leads to finite resistance (even in the absence of other scatterers). The transport scattering time of electrons on vortices is calculated.

We study spin relaxation in a two-electron quantum dot in the vicinity of the singlet-triplet crossing. The spin relaxation occurs due to a combined effect of the spin-orbit, Zeeman, and electron-phonon interactions. The singlet-triplet relaxation rates exhibit strong variations as a function of the singlet-triplet splitting. We show that the Coulomb interaction between the electrons has two competing effects on the singlet-triplet spin relaxation. One effect is to enhance the relative strength of spin-orbit coupling in the quantum dot, resulting in larger spin-orbit splittings and thus in a stronger coupling of spin to charge. The other effect is to make the charge density profiles of the singlet and triplet look similar to each other, thus diminishing the ability of charge environments to discriminate between singlet and triplet states. We thus find essentially different channels of singlet-triplet relaxation for the case of strong and weak Coulomb interaction. Finally, for the linear in momentum Dresselhaus and Rashba spin-orbit interactions, we calculate the singlet-triplet relaxation rates to leading order in the spin-orbit interaction, and find that they are proportional to the second power of the Zeeman energy, in agreement with recent experiments on triplet-to-singlet relaxation in quantum dots., Comment: 29 pages, 14 figures, 1 table

We have measured the Landau-level spin-splitting of two-dimensional electrons in the composite InAs/InGaAs channels of two InAs/InGaAs/InAlAs heterostructures with different alloy compositions by magnetotransport and THz magneto-photoconductivity in magnetic fields up to 10 T. The structures differ importantly in the mobility of the channel, the electron density and the composition of the barriers. The magnitudes of the experimental g-factors for B along the quantization axis and their anisotropies are larger by at least a factor of 2 than the corresponding calculated single particle values. The angular dependence of many-body exchange contributions and the effects of broadening of Landau–level densities of states are necessary for understanding this behaviour. We find evidence for a marked decrease of the exchange contribution at low perpendicular magnetic fields in the higher mobility sample from coincidence measurements, but no indications of such behaviour in the lower mobility sample.

We have described electron spin dynamics in the presence of the spin-orbit interaction and disorder using the spin-density matrix method. Exact solution is obtained for an arbitrary 2D spin-orbit Hamiltonian and arbitrary smoothness of the disorder potential. Spin current depends explicitely on the disorder properties, namely the smoothness of the disorder potential, even in the ballistic limit when broadening by scattering is much smaller than the spin-orbit related splitting of the energy spectrum. In this sense universal intrinsic spin current does not exist., Comment: LaTeX, 6 pages, 1 fig., Eqs.22,28 are corrected

We have described the electron spin dynamics in the presence of the spin-orbit interaction and disorder using the spin-density matrix method. We showed that in the Born approximation in the scattering amplitude the spin current is zero for an arbitrary ratio of the spin-orbit splitting and the scattering rate. Various types of the disorder potential are studied. We argue that the bulk spin current has always an {\it extrinsic} nature and depends explicitely on scattering by impurities since it appears only beyond the Born approximation in the scattering amplitude., 5 pages, RevTex, 1 Fig

We study spin relaxation and decoherence in a GaAs quantum dot due to spin-orbit interaction. We derive an effective Hamiltonian which couples the electron spin to phonons or any other fluctuation of the dot potential. We show that the spin decoherence time $T_2$ is as large as the spin relaxation time $T_1$, under realistic conditions. For the Dresselhaus and Rashba spin-orbit couplings, we find that, in leading order, the effective magnetic field can have only fluctuations transverse to the applied magnetic field. As a result, $T_2=2T_1$ for arbitrarily large Zeeman splittings, in contrast to the naively expected case $T_2\ll T_1$. We show that the spin decay is drastically suppressed for certain magnetic field directions and values of the Rashba coupling constant. Finally, for the spin coupling to acoustic phonons, we show that $T_2=2T_1$ for all spin-orbit mechanisms in leading order in the electron-phonon interaction., 5 pages, 1 figure

We have reconsidered the problem of spin injection across ferromagnet/non-magnetic-semiconductor (FM/NMS) and dilute-magnetic-semiconductor/non-magnetic-semiconductor interfaces, for structures with \textit{finite} magnetic layers (FM or DMS). By using appropriate physical boundary conditions, we find expressions for the resistances of these structures which are in general different from previous results in the literature. When the magnetoresistance of the contacts is negligible, we find that the spin-accumulation effect alone cannot account for the $d$ dependence observed in recent magnetoresistance data. In a limited parameter range, our formulas predict a strong $d$ dependence arising from the magnetic contacts in systems where their magnetoresistances are sizable., 6 pages, 3 eps figs. (extended version- new title + two new figures added)

We review and summarize recent theoretical and experimental work on electron spin dynamics in quantum dots and related nanostructures due to hyperfine interaction with surrounding nuclear spins. This topic is of particular interest with respect to several proposals for quantum information processing in solid state systems. Specifically, we investigate the hyperfine interaction of an electron spin confined in a quantum dot in an s-type conduction band with the nuclear spins in the dot. This interaction is proportional to the square modulus of the electron wave function at the location of each nucleus leading to an inhomogeneous coupling, i.e. nuclei in different locations are coupled with different strength. In the case of an initially fully polarized nuclear spin system an exact analytical solution for the spin dynamics can be found. For not completely polarized nuclei, approximation-free results can only be obtained numerically in sufficiently small systems. We compare these exact results with findings from several approximation strategies., Comment: 26 pages, 9 figures. Topical Review to appear in J. Phys.: Condens. Matter

We study the time evolution of a single spin coupled inhomogeneously to a spin environment. Such a system is realized by a single electron spin bound in a semiconductor nanostructure and interacting with surrounding nuclear spins. We find striking dependencies on the type of the initial state of the nuclear spin system. Simple product states show a profoundly different behavior than randomly correlated states whose time evolution provides an illustrative example of quantum parallelism and entanglement in a decoherence phenomenon., 6 pages, 4 figures included, version to appear in Phys. Rev. B

We study the decoherence of a single electron spin in an isolated quantum dot induced by hyperfine interaction with nuclei for times smaller than the nuclear spin relaxation time. The decay is caused by the spatial variation of the electron envelope wave function within the dot, leading to a non-uniform hyperfine coupling $A$. We show that the usual treatment of the problem based on the Markovian approximation is impossible because the correlation time for the nuclear magnetic field seen by the electron spin is itself determined by the flip-flop processes. The decay of the electron spin correlation function is not exponential but rather power (inverse logarithm) law-like. For polarized nuclei we find an exact solution and show that the precession amplitude and the decay behavior can be tuned by the magnetic field. The decay time is given by $\hbar N/A$, where $N$ is the number of nuclei inside the dot. The amplitude of precession, reached as a result of the decay, is finite. We show that there is a striking difference between the decoherence time for a single dot and the dephasing time for an ensemble of dots., Comment: Revtex, 11 pages, 5 figures

W. Apel and Yu.A. Bychkov have recently considered the spin relaxation in a 2D quantum Hall system for the filling factor close to unity [PRL v.82, 3324 (1999)]. The authors considered only one spin flip mechanism (direct spin-phonon coupling) among several possible spin-orbit related ones and came to the conclusion that the spin relaxation time due to this mechanism is quite short: around $10^{-10}$ s at B=10 T (for GaAs). This time is much shorter than the typical time ($10^{-5}$ s) obtained earlier by D. Frenkel while considering the spin relaxation of 2D electrons in a quantizing magnetic field without the Coulomb interaction and for the same spin-phonon coupling. I show that the authors' conclusion about the value of the spin-flip time is wrong and have deduced the correct time which is by several orders of magnitude longer. I also discuss the admixture mechanism of the spin-orbit interaction., Comment: 1 page

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., RevTex, 7 pages (extended journal version, PRB, in press)

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., Comment: 6 pages, RevTex

We investigated theoretically the Coulomb drag effect in coupled 2D electron gases in a wide interval of magnetic field and temperature $ 1/\tau \ll \omega_c \ll E_F/\hbar$, $T \ll E_F$, $\tau$ being intralayer scattering time, $\omega_c$ 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., Comment: ZIP archive,10 pages, 3 ps figures, submitted to PRB

We consider the influence of the electron-electron interaction on the nonlinearity of the current-voltage characteristic of the tunnel junction at low bias (diffusive anomaly) in the presence of the classical magnetic field. We present the theory of a new phenomenon which manifests itself in the strong anisotropy of a diffusive anomaly for different orientations of the magnetic field with respect to the interface of the tunnel junction. The nonlinear differential tunneling conductance has a universal magnetic field dependence, so that only the magnetic field component perpendicular to the interface is involved. In particular, when the magnetic field is parallel to the interface, the I-V characteristic does not depend on the value of the magnetic field., 12 pages, LaTeX format, 2 figures (available from the authors), accepted for publication by PRB

We consider ballistic transport of composite fermions in a realistic microconstriction. An unambiguous signature of such transport is revealed: the linear conductance increases with increasing magnetic field near filling factor 1/2, and then drops abruptly in a narrow magnetic field interval.

We present exact solution of an electrostatic problem in terms of the potential distribution and density profile of an incompletely depleted two-dimensional electron gas in a heterostructure covered by a half-infinite gate. Using this solution, we calculate configurations of compressible and incompressible (insulating) strips that are formed in the presence of a quantizing magnetic field. We discuss the application of our results to transport experiments on the inter-Landau-level and spin diodes, as well as edge-magnetoplasmon propagation.

We investigate experimentally the ``mean free path'' of electrons injected selectively into the outer spin-polarized edge channel of the lowest Landau level. A simple model allows us to determine the characteristic length between two spin-flip scattering events directly from the measured resistance plateaus. Typical spin-flip equilibration lengths between 100 \ensuremath{\mu}m and 1 mm are found. The size of the observed equilibration lengths is interpreted in terms of the spin-orbit interaction, which also leads to a local g-factor enhancement at the edge.