69 research outputs found
Dimensional Effects on Densities of States and Interactions in Nanostructures
We consider electrons in the presence of interfaces with different effective electron mass, and electromagnetic fields in the presence of a high-permittivity interface in bulk material. The equations of motion for these dimensionally hybrid systems yield analytic expressions for Greenâs functions and electromagnetic potentials that interpolate between the two-dimensional logarithmic potential at short distance, and the three-dimensional râ1 potential at large distance. This also yields results for electron densities of states which interpolate between the well-known two-dimensional and three-dimensional formulas. The transition length scales for interfaces of thickness L are found to be of order Lm/2m* for an interface in which electrons move with effective mass m*, and for a dielectric thin film with permittivity in a bulk of permittivity . We can easily test the merits of the formalism by comparing the calculated electromagnetic potential with the infinite series solutions from image charges. This confirms that the dimensionally hybrid models are excellent approximations for distances r âł L/2
Spin-dephasing anisotropy for electrons in a diffusive quasi-1D GaAs wire
We present a numerical study of dephasing of electron spin ensembles in a
diffusive quasi-one-dimensional GaAs wire due to the D'yakonov-Perel'
spin-dephasing mechanism. For widths of the wire below the spin precession
length and for equal strength of Rashba and linear Dresselhaus spin-orbit
fields a strong suppression of spin-dephasing is found. This suppression of
spin-dephasing shows a strong dependence on the wire orientation with respect
to the crystal lattice. The relevance for realistic cases is evaluated by
studying how this effect degrades for deviating strength of Rashba and linear
Dresselhaus fields, and with the inclusion of the cubic Dresselhaus term
Anomalous Rashba spin splitting in two-dimensional hole systems
It has long been assumed that the inversion asymmetry-induced Rashba spin
splitting in two-dimensional (2D) systems at zero magnetic field is
proportional to the electric field that characterizes the inversion asymmetry
of the confining potential. Here we demonstrate, both theoretically and
experimentally, that 2D heavy hole systems in accumulation layer-like single
heterostructures show the opposite behavior, i.e., a decreasing, but nonzero
electric field results in an increasing Rashba coefficient.Comment: 4 pages, 3 figure
Spin-Orbit Coupling and Time-Reversal Symmetry in Quantum Gates
We study the effect of spin-orbit coupling on quantum gates produced by
pulsing the exchange interaction between two single electron quantum dots.
Spin-orbit coupling enters as a small spin precession when electrons tunnel
between dots. For adiabatic pulses the resulting gate is described by a unitary
operator acting on the four-dimensional Hilbert space of two qubits. If the
precession axis is fixed, time-symmetric pulsing constrains the set of possible
gates to those which, when combined with single qubit rotations, can be used in
a simple CNOT construction. Deviations from time-symmetric pulsing spoil this
construction. The effect of time asymmetry is studied by numerically
integrating the Schr\"odinger equation using parameters appropriate for GaAs
quantum dots. Deviations of the implemented gate from the desired form are
shown to be proportional to dimensionless measures of both spin-orbit coupling
and time asymmetry of the pulse.Comment: 10 pages, 3 figure
Hall Coefficient in an Interacting Electron Gas
The Hall conductivity in a weak homogeneous magnetic field, , is calculated. We have shown that to leading order in
the Hall coefficient is not renormalized by the
electron-electron interaction. Our result explains the experimentally observed
stability of the Hall coefficient in a dilute electron gas not too close to the
metal-insulator transition. We avoid the currently used procedure that
introduces an artificial spatial modulation of the magnetic field. The problem
of the Hall effect is reformulated in a way such that the magnetic flux
associated with the scattering process becomes the central element of the
calculation.Comment: 23 pages, 15 figure
Spin-orbit coupling and intrinsic spin mixing in quantum dots
Spin-orbit coupling effects are studied in quantum dots in InSb, a narrow-gap
material. Competition between different Rashba and Dresselhaus terms is shown
to produce wholesale changes in the spectrum. The large (and negative)
-factor and the Rashba field produce states where spin is no longer a good
quantum number and intrinsic flips occur at moderate magnetic fields. For dots
with two electrons, a singlet-triplet mixing occurs in the ground state, with
observable signatures in intraband FIR absorption, and possible importance in
quantum computation.Comment: REVTEX4 text with 3 figures (high resolution figs available by
request). Submitted to PR
Magnetotunneling Between Two-dimensional Electron Gases in InAs-AlSb-GaSb Heterostructures
We have observed that the tunneling magnetoconductance between
two-dimensional (2D) electron gases formed at nominally identical InAs-AlSb
interfaces most often exhibits two sets of Shubnikov-de Haas oscillations with
almost the same frequency. This result is explained quantitatively with a model
of the conductance in which the 2D gases have different densities and can
tunnel between Landau levels with different quantum indices. When the epitaxial
growth conditions of the interfaces are optimized, the zero-bias
magnetoconductance shows a single set of oscillations, thus proving that the
asymmetry between the two electron gases can be eliminated.Comment: RevTeX format including 4 figures; submit for publicatio
Spin Precession and Oscillations in Mesoscopic Systems
We compare and contrast magneto-transport oscillations in the fully quantum
(single-electron coherent) and classical limits for a simple but illustrative
model. In particular, we study the induced magnetization and spin current in a
two-terminal double-barrier structure with an applied Zeeman field between the
barriers and spin disequilibrium in the contacts. Classically, the spin current
shows strong tunneling resonances due to spin precession in the region between
the two barriers. However, these oscillations are distinguishable from those in
the fully coherent case, for which a proper treatment of the electron phase is
required. We explain the differences in terms of the presence or absence of
coherent multiple wave reflections.Comment: 9 pages, 5 figure
Spin Accumulation in Quantum Wires with Strong Rashba Spin-Orbit Coupling
We present analytical and numerical results for the effect of Rashba
spin-orbit coupling on band structure, transport, and interaction effects in
quantum wires when the spin precession length is comparable to the wire width.
In contrast to the weak-coupling case, no common spin-quantization axis can be
defined for eigenstates within a single-electron band. The situation with only
the lowest spin-split subbands occupied is particularly interesting because
electrons close to Fermi points of the same chirality can have approximately
parallel spins. We discuss consequences for spin-dependent transport and
effective Tomonaga-Luttinger descriptions of interactions in the quantum wire.Comment: 4 pages, 4 figures, expanded discussion of spin accumulatio
Spin dynamics in high-mobility two-dimensional electron systems
Understanding the spin dynamics in semiconductor heterostructures is highly
important for future semiconductor spintronic devices. In high-mobility
two-dimensional electron systems (2DES), the spin lifetime strongly depends on
the initial degree of spin polarization due to the electron-electron
interaction. The Hartree-Fock (HF) term of the Coulomb interaction acts like an
effective out-of-plane magnetic field and thus reduces the spin-flip rate. By
time-resolved Faraday rotation (TRFR) techniques, we demonstrate that the spin
lifetime is increased by an order of magnitude as the initial spin polarization
degree is raised from the low-polarization limit to several percent. We perform
control experiments to decouple the excitation density in the sample from the
spin polarization degree and investigate the interplay of the internal HF field
and an external perpendicular magnetic field. The lifetime of spins oriented in
the plane of a [001]-grown 2DES is strongly anisotropic if the Rashba and
Dresselhaus spin-orbit fields are of the same order of magnitude. This
anisotropy, which stems from the interference of the Rashba and the Dresselhaus
spin-orbit fields, is highly density-dependent: as the electron density is
increased, the kubic Dresselhaus term becomes dominant and reduces the
anisotropy.Comment: 13 pages, 6 figure
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