105 research outputs found
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
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
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
Effect of external magnetic field on electron spin dephasing induced by hyperfine interaction in quantum dots
We investigate the influence of an external magnetic field on spin phase
relaxation of single electrons in semiconductor quantum dots induced by the
hyperfine interaction. The basic decay mechanism is attributed to the
dispersion of local effective nuclear fields over the ensemble of quantum dots.
The characteristics of electron spin dephasing is analyzed by taking an average
over the nuclear spin distribution. We find that the dephasing rate can be
estimated as a spin precession frequency caused primarily by the mean value of
the local nuclear magnetic field. Furthermore, it is shown that the hyperfine
interaction does not fully depolarize electron spin. The loss of initial spin
polarization during the dephasing process depends strongly on the external
magnetic field, leading to the possibility of effective suppression of this
mechanism.Comment: 10 pages, 2 figure
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.Comment: 8 pages, 3 figure
Spin decay and quantum parallelism
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.Comment: 6 pages, 4 figures included, version to appear in Phys. Rev.
Hole spin relaxation in semiconductor quantum dots
Hole spin relaxation time due to the hole-acoustic phonon scattering in GaAs
quantum dots confined in quantum wells along (001) and (111) directions is
studied after the exact diagonalization of Luttinger Hamiltonian. Different
effects such as strain, magnetic field, quantum dot diameter, quantum well
width and the temperature on the spin relaxation time are investigated
thoroughly. Many features which are quite different from the electron spin
relaxation in quantum dots and quantum wells are presented with the underlying
physics elaborated.Comment: 10 pages, 10 figure
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.Comment: 11 pages, 11 figures, revtex4 styl
Spin relaxation at the singlet-triplet crossing in a quantum dot
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 tabl
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