21 research outputs found
Gated combo nanodevice for sequential operations on single electron spin
An idea for a nanodevice in which an arbitrary sequence of three basic
quantum single qubit gates - negation, Hadamard and phase shift - can be
performed on a single electron spin. The spin state is manipulated using the
spin-orbit coupling and the electron trajectory is controlled by the electron
wave function self-focusing mechanism due to the electron interaction with the
charge induced on metal gates. We present results of simulations based on
iterative solution of the time dependent Schr\"odinger equation in which the
subsequent operations on the electron spin can be followed and controlled.
Description of the moving electron wave packet requires evaluation of the
electric field within the entire nanodevice in each time step
Spin rotations induced by electron running on closed trajectories in gated semiconductor nanodevices
A design for a quantum gate performing transformations of a single electron
spin is presented. The spin rotations are performed by the electron going
around the closed loops in a gated semiconductor device. We demonstrate the
operation of NOT, phase-flip and Hadamard quantum gates, i.e. the single-qubit
gates which are most commonly used in the algorithms. The proposed devices
employ the self-focusing effect for the electron wave packet interacting with
the electron gas on the electrodes and the Rashba spin-orbit coupling. Due to
the self-focusing effect the electron moves in a compact wave packet. The
spin-orbit coupling translates the spatial motion of the electron into the
rotations of the spin. The device does not require microwave radiation and
operates using low constant voltages. It is therefore suitable for selective
single-spin rotations in larger registers.Comment: submitte
Effect of electrical bias on spin transport across a magnetic domain wall
We present a theory of the current-voltage characteristics of a magnetic
domain wall between two highly spin-polarized materials, which takes into
account the effect of the electrical bias on the spin-flip probability of an
electron crossing the wall. We show that increasing the voltage reduces the
spin-flip rate, and is therefore equivalent to reducing the width of the domain
wall. As an application, we show that this effect widens the temperature window
in which the operation of a unipolar spin diode is nearly ideal.Comment: 11 pages, 3 figure
Decoherence of localized spins interacting via RKKY interaction
We theoretically study decoherence of two localized spins interacting via the
RKKY interaction in one-, two-, and three-dimensional electron gas. We derive
the kinetic equation for the reduced density matrix of the localized spins and
show that energy relaxation caused by singlet-triplet transition is suppressed
when the RKKY interaction is ferromagnetic. We also estimate the decoherence
time of the system consisting of two quantum dots embedded in a two dimensional
electron gas.Comment: 4pages, 2figure
Resonant harmonic generation and collective spin rotations in electrically driven quantum dots
Spin rotations induced by an AC electric field in a two-electron double
quantum dot are studied by an exact numerical solution of the time dependent
Schroedinger equation in the context of recent electric dipole spin resonance
experiments based on the Pauli blockade. We demonstrate that the splitting of
the main resonance line by the spin exchange coupling is accompanied by the
appearance of fractional resonances and that both these effects are triggered
by interdot tunnel coupling. We find that the AC driven system generates
residual but distinct harmonics of the driving frequency which are amplified
when tuned to the main transition frequency. The mechanism is universal for
electron systems in electrically driven potentials and works also in the
absence of electron-electron interaction or spin-orbit coupling.Comment: Corrected version accepted for PR
Spin Exciton in quantum dot with spin orbit coupling in high magnetic field
Coulomb interactions of few () electrons confined in a disk shaped
quantum dot, with a large magnetic field applied in the z-direction
(orthogonal to the dot), produce a fully spin polarized ground state. We
numerically study the splitting of the levels corresponding to the multiplet of
total spin (each labeled by a different total angular momentum )
in presence of an electric field parallel to , coupled to by a
Rashba term. We find that the first excited state is a spin exciton with a
reversed spin at the origin. This is reminiscent of the Quantum Hall
Ferromagnet at filling one which has the skyrmion-like state as its first
excited state. The spin exciton level can be tuned with the electric field and
infrared radiation can provide energy and angular momentum to excite it.Comment: 9 pages, 9 figures. submitted to Phys.Rev.
Thermally Activated Resonant Magnetization Tunneling in Molecular Magnets: Mn_12Ac and others
The dynamical theory of thermally activated resonant magnetization tunneling
in uniaxially anisotropic magnetic molecules such as Mn_12Ac (S=10) is
developed.The observed slow dynamics of the system is described by master
equations for the populations of spin levels.The latter are obtained by the
adiabatic elimination of fast degrees of freedom from the density matrix
equation with the help of the perturbation theory developed earlier for the
tunneling level splitting [D. A. Garanin, J. Phys. A, 24, L61 (1991)]. There
exists a temperature range (thermally activated tunneling) where the escape
rate follows the Arrhenius law, but has a nonmonotonic dependence on the bias
field due to tunneling at the top of the barrier. At lower temperatures this
regime crosses over to the non-Arrhenius law (thermally assisted tunneling).
The transition between the two regimes can be first or second order, depending
on the transverse field, which can be tested in experiments. In both regimes
the resonant maxima of the rate occur when spin levels in the two potential
wells match at certain field values. In the thermally activated regime at low
dissipation each resonance has a multitower self-similar structure with
progressively narrowing peaks mounting on top of each other.Comment: 18 pages, 8 figure
Quantum-Classical Transition of the Escape Rate of a Uniaxial Spin System in an Arbitrarily Directed Field
The escape rate \Gamma of the large-spin model described by the Hamiltonian H
= -DS_z^2 - H_zS_z - H_xS_x is investigated with the help of the mapping onto a
particle moving in a double-well potential U(x). The transition-state method
yields in the moderate-damping case as a Boltzmann average of the
quantum transition probabilities. We have shown that the transition from the
classical to quantum regimes with lowering temperature is of the first order
(d\Gamma/dT discontinuous at the transition temperature T_0) for h_x below the
phase boundary line h_x=h_{xc}(h_z), where h_{x,z}\equiv H_{x,z}/(2SD), and of
the second order above this line. In the unbiased case (H_z=0) the result is
h_{xc}(0)=1/4, i.e., one fourth of the metastability boundary h_{xm}=1, at
which the barrier disappears. In the strongly biased limit \delta\equiv 1-h_z
<< 1, one has h_{xc} \cong (2/3)^{3/4}(\sqrt{3}-\sqrt{2})\delta^{3/2}\cong
0.2345 \delta^{3/2}, which is about one half of the boundary value h_{xm} \cong
(2\delta/3)^{3/2} \cong 0.5443 \delta^{3/2}.The latter case is relevant for
experiments on small magnetic particles, where the barrier should be lowered to
achieve measurable quantum escape rates.Comment: 17 PR pages, 16 figures; published versio
Fabry-Perot interference and spin filtering in carbon nanotubes
We study the two-terminal transport properties of a metallic single-walled
carbon nanotube with good contacts to electrodes, which have recently been
shown [W. Liang et al, Nature 441, 665-669 (2001)] to conduct ballistically
with weak backscattering occurring mainly at the two contacts. The measured
conductance, as a function of bias and gate voltages, shows an oscillating
pattern of quantum interference. We show how such patterns can be understood
and calculated, taking into account Luttinger liquid effects resulting from
strong Coulomb interactions in the nanotube. We treat back-scattering in the
contacts perturbatively and use the Keldysh formalism to treat non-equilibrium
effects due to the non-zero bias voltage. Going beyond current experiments, we
include the effects of possible ferromagnetic polarization of the leads to
describe spin transport in carbon nanotubes. We thereby describe both
incoherent spin injection and coherent resonant spin transport between the two
leads. Spin currents can be produced in both ways, but only the latter allow
this spin current to be controlled using an external gate. In all cases, the
spin currents, charge currents, and magnetization of the nanotube exhibit
components varying quasiperiodically with bias voltage, approximately as a
superposition of periodic interference oscillations of spin- and
charge-carrying ``quasiparticles'' in the nanotube, each with its own period.
The amplitude of the higher-period signal is largest in single-mode quantum
wires, and is somewhat suppressed in metallic nanotubes due to their sub-band
degeneracy.Comment: 12 pages, 6 figure