266 research outputs found
Unitarity of scattering and edge spin accumulation
We consider a 2D ballistic and quasi-ballistic structures with
spin-orbit-related 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 arises in the presence of a charge current through the
structure. We solve the problem with the use of the method of scattering states
and clarify the important role of the unitarity of scattering. In the case of a
straight boundary it leads to exact cancellation of long-wavelength
oscillations of the spin density. In general, however, the smooth spin
oscillations with the spin precession length may arise, as it happens, e.g.,
for the wiggly boundary. Moreover, we show that the result crucially depends on
the form of the spin-orbit Hamiltonian.Comment: 5 pages, 3 figures. arXiv admin note: substantial text overlap with
arXiv:1007.113
Giant edge spin accumulation in a symmetric quantum well with two subbands
We have studied the edge spin accumulation in a high mobility two-dimensional
electron gas formed in a symmetric well with two subbands. This study is
strongly motivated by the recent experiment of Hernandez et al. [Phys. Rev. B
{\bf 88}, 161305(R) (2013)] who demonstrated the spin accumulation near the
edges of a bilayer symmetric GaAs structure in contrast to no effect in a
single-layer configuration. The intrinsic mechanism of the spin-orbit
interaction we consider arises from the coupling between two subband states of
opposite parities. We obtain a parametrically large magnitude of the edge spin
density for the two-subband sample as compared to the usual single-subband
structure. We show that the presence of a gap in the system, i.e., the energy
separation between the two subband bottoms, changes drastically the
picture of the edge spin accumulation. Thus one can easily proceed from the
regime of weak spin accumulation to the regime of strong one by varying the
Fermi energy (electron density) and/or . We estimate that by changing
the gap from zero up to K, the magnitude of the effect
changes by three orders of magnitude. This opens up the possibility for the
design of new spintronic devices.Comment: 6 pages, 2 figures, expanded text and added Supplementary Materia
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.Comment: RevTex, 7 pages (extended journal version, PRB, in press
Comment on "Spin relaxation in quantum Hall systems"
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 s at B=10 T (for GaAs). This time is much shorter than
the typical time ( 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 pag
Electron spin evolution induced by interaction with nuclei in a quantum dot
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 . 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 ,
where 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 figure
Hyperfine interaction in a quantum dot: Non-Markovian electron spin dynamics
We have performed a systematic calculation for the non-Markovian dynamics of
a localized electron spin interacting with an environment of nuclear spins via
the Fermi contact hyperfine interaction. This work applies to an electron in
the s -type orbital ground state of a quantum dot or bound to a donor impurity,
and is valid for arbitrary polarization p of the nuclear spin system, and
arbitrary nuclear spin I in high magnetic fields. In the limit of p=1 and
I=1/2, the Born approximation of our perturbative theory recovers the exact
electron spin dynamics. We have found the form of the generalized master
equation (GME) for the longitudinal and transverse components of the electron
spin to all orders in the electron spin--nuclear spin flip-flop terms. Our
perturbative expansion is regular, unlike standard time-dependent perturbation
theory, and can be carried-out to higher orders. We show this explicitly with a
fourth-order calculation of the longitudinal spin dynamics. In zero magnetic
field, the fraction of the electron spin that decays is bounded by the
smallness parameter \delta=1/p^{2}N, where N is the number of nuclear spins
within the extent of the electron wave function. However, the form of the decay
can only be determined in a high magnetic field, much larger than the maximum
Overhauser field. In general the electron spin shows rich dynamics, described
by a sum of contributions with non-exponential decay, exponential decay, and
undamped oscillations. There is an abrupt crossover in the electron spin
asymptotics at a critical dimensionality and shape of the electron envelope
wave function. We propose a scheme that could be used to measure the
non-Markovian dynamics using a standard spin-echo technique, even when the
fraction that undergoes non-Markovian dynamics is small.Comment: 22 pages, 8 figure
Spin-dependent Andreev reflection tunneling through a quantum dot with intradot spin-flip scattering
We study Andreev reflection (AR) tunneling through a quantum dot (QD)
connected to a ferromagnet and a superconductor, in which the intradot
spin-flip interaction is included. By using the nonequibrium-Green-function
method, the formula of the linear AR conductance is derived at zero
temperature. It is found that competition between the intradot spin-flip
scattering and the tunneling coupling to the leads dominantes resonant
behaviours of the AR conductance versus the gate voltage.A weak spin-flip
scattering leads to a single peak resonance.However, with the spin-flip
scattering strength increasing, the AR conductance will develop into a double
peak resonannce implying a novel structure in the tunneling spectrum of the AR
conductance. Besides, the effect of the spin-dependent tunneling couplings, the
matching of Fermi velocity, and the spin polarization of the ferromagnet on the
AR conductance is eximined in detail.Comment: 14 pages, 4 figure
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