1,111 research outputs found
Berry phase and adiabaticity of a spin diffusing in a non-uniform magnetic field
An electron spin moving adiabatically in a strong, spatially non-uniform
magnetic field accumulates a geometric phase or Berry phase, which might be
observable as a conductance oscillation in a mesoscopic ring. Two contradicting
theories exist for how strong the magnetic field should be to ensure
adiabaticity if the motion is diffusive. To resolve this controversy, we study
the effect of a non-uniform magnetic field on the spin polarization and on the
weak-localization effect. The diffusion equation for the Cooperon is solved
exactly. Adiabaticity requires that the spin-precession time is short compared
to the elastic scattering time - it is not sufficient that it is short compared
to the diffusion time around the ring. This strong condition severely
complicates the experimental observation.Comment: 16 pages REVTEX, including 3 figure
Direct Measurement of the Spin-Orbit Interaction in a Two-Electron InAs Nanowire Quantum Dot
We demonstrate control of the electron number down to the last electron in
tunable few-electron quantum dots defined in catalytically grown InAs
nanowires. Using low temperature transport spectroscopy in the Coulomb blockade
regime we propose a simple method to directly determine the magnitude of the
spin-orbit interaction in a two-electron artificial atom with strong spin-orbit
coupling. Due to a large effective g-factor |g*|=8+/-1 the transition from
singlet S to triplet T+ groundstate with increasing magnetic field is dominated
by the Zeeman energy rather than by orbital effects. We find that the
spin-orbit coupling mixes the T+ and S states and thus induces an avoided
crossing with magnitude =0.25+/-0.05 meV. This allows us to
calculate the spin-orbit length 127 nm in such systems
using a simple model.Comment: 21 pages, 7 figures, including supplementary note
Singlet-triplet decoherence due to nuclear spins in a double quantum dot
We have evaluated hyperfine-induced electron spin dynamics for two electrons
confined to a double quantum dot. Our quantum solution accounts for decay of a
singlet-triplet correlator even in the presence of a fully static nuclear spin
system, with no ensemble averaging over initial conditions. In contrast to an
earlier semiclassical calculation, which neglects the exchange interaction, we
find that the singlet-triplet correlator shows a long-time saturation value
that differs from 1/2, even in the presence of a strong magnetic field.
Furthermore, we find that the form of the long-time decay undergoes a
transition from a rapid Gaussian to a slow power law () when
the exchange interaction becomes nonzero and the singlet-triplet correlator
acquires a phase shift given by a universal (parameter independent) value of
at long times. The oscillation frequency and time-dependent phase
shift of the singlet-triplet correlator can be used to perform a precision
measurement of the exchange interaction and Overhauser field fluctuations in an
experimentally accessible system. We also address the effect of orbital
dephasing on singlet-triplet decoherence, and find that there is an optimal
operating point where orbital dephasing becomes negligible.Comment: 12 pages, 4 figure
Nuclear spin state narrowing via gate--controlled Rabi oscillations in a double quantum dot
We study spin dynamics for two electrons confined to a double quantum dot
under the influence of an oscillating exchange interaction. This leads to
driven Rabi oscillations between the --state and the
--state of the two--electron system. The width of the
Rabi resonance is proportional to the amplitude of the oscillating exchange. A
measurement of the Rabi resonance allows one to narrow the distribution of
nuclear spin states and thereby to prolong the spin decoherence time. Further,
we study decoherence of the two-electron states due to the hyperfine
interaction and give requirements on the parameters of the system in order to
initialize in the --state and to perform a
operation with unit fidelity.Comment: v1:9 pages, 1 figure; v2: 13 pages, 2 figures, added section on
measurement, to appear in Phys. Rev.
Quantum versus classical hyperfine-induced dynamics in a quantum dot
In this article we analyze spin dynamics for electrons confined to
semiconductor quantum dots due to the contact hyperfine interaction. We compare
mean-field (classical) evolution of an electron spin in the presence of a
nuclear field with the exact quantum evolution for the special case of uniform
hyperfine coupling constants. We find that (in this special case) the
zero-magnetic-field dynamics due to the mean-field approximation and quantum
evolution are similar. However, in a finite magnetic field, the quantum and
classical solutions agree only up to a certain time scale t<\tau_c, after which
they differ markedly.Comment: 6 pages, 1 figure, accepted for publication in the Journal of Applied
Physics (ICPS06 conference proceedings); v2: updated references, final
published versio
Low Temperature Anomaly in Mesoscopic Kondo Wires
We report the observation of an anomalous magnetoresistance in extremely
dilute quasi-one-dimensional AuFe wires at low temperatures, along with a
hysteretic background at low fields. The Kondo resistivity does not show the
unitarity limit down to the lowest temperature, implying uncompensated spin
states. We suggest that the anomalous magnetoresistance may be understood as
the interference correction from the accumulation of geometric phase in the
conduction electron wave function around the localized impurity spin.Comment: Four pages, five figure
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