115 research outputs found
Nuclear Spin Relaxation for Higher Spin
We study the relaxation of a spin I that is weakly coupled to a quantum
mechanical environment. Starting from the microscopic description, we derive a
system of coupled relaxation equations within the adiabatic approximation.
These are valid for arbitrary I and also for a general stationary
non--equilibrium state of the environment. In the case of equilibrium, the
stationary solution of the equations becomes the correct Boltzmannian
equilibrium distribution for given spin I. The relaxation towards the
stationary solution is characterized by a set of relaxation times, the longest
of which can be shorter, by a factor of up to 2I, than the relaxation time in
the corresponding Bloch equations calculated in the standard perturbative way.Comment: 4 pages, Latex, 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
Electron spin coherence in semiconductors: Considerations for a spin-based solid state quantum computer architecture
We theoretically consider coherence times for spins in two quantum computer
architectures, where the qubit is the spin of an electron bound to a P donor
impurity in Si or within a GaAs quantum dot. We show that low temperature
decoherence is dominated by spin-spin interactions, through spectral diffusion
and dipolar flip-flop mechanisms. These contributions lead to 1-100 s
calculated spin coherence times for a wide range of parameters, much higher
than former estimates based on measurements.Comment: Role of the dipolar interaction clarified; Included discussion on the
approximations employed in the spectral diffusion calculation. Final version
to appear in Phys. Rev.
Topological defects and Goldstone excitations in domain walls between ferromagnetic quantum Hall effect liquids
It is shown that the low-energy spectrum of a ferromagnetic quantum Hall
effect liquid in a system with a multi-domain structure generated by an
inhomogeneous bare Zeeman splitting is formed by excitations
localized at the walls between domains. For a step-like , the
domain wall spectrum includes a spin-wave with a linear dispersion and a small
gap due to spin-orbit coupling, and a low-energy topological defects. The
latter are charged and may dominate in the transport under conditions that the
percolation through the network of domain walls is provided.Comment: 4 pages, 1 fi
A New Type of Electron Nuclear-Spin Interaction from Resistively Detected NMR in the Fractional Quantum Hall Effect Regime
Two dimensional electron gases in narrow GaAs quantum wells show huge
longitudinal resistance (HLR) values at certain fractional filling factors.
Applying an RF field with frequencies corresponding to the nuclear spin
splittings of {69}Ga, {71}Ga and {75}As leads to a substantial decreases of the
HLR establishing a novel type of resistively detected NMR. These resonances are
split into four sub lines each. Neither the number of sub lines nor the size of
the splitting can be explained by established interaction mechanisms.Comment: 4 pages, 3 figure
Nuclear spin relaxation probed by a single quantum dot
We present measurements on nuclear spin relaxation probed by a single quantum
dot in a high-mobility electron gas. Current passing through the dot leads to a
spin transfer from the electronic to the nuclear spin system. Applying electron
spin resonance the transfer mechanism can directly be tuned. Additionally, the
dependence of nuclear spin relaxation on the dot gate voltage is observed. We
find electron-nuclear relaxation times of the order of 10 minutes
Nuclear Spin Qubit Dephasing Time in the Integer Quantum Hall Effect Regime
We report the first theoretical estimate of the nuclear-spin dephasing time
T_2 owing to the spin interaction with the two-dimensional electron gas, when
the latter is in the integer quantum Hall state, in a two-dimensional
heterojunction or quantum well at low temperature and in large applied magnetic
field. We establish that the leading mechanism of dephasing is due to the
impurity potentials that influence the dynamics of the spin via virtual
magnetic spin-exciton scattering. Implications of our results for
implementation of nuclear spins as quantum bits (qubits) for quantum computing
are discussed.Comment: 19 pages in plain Te
Strong, Ultra-narrow Peaks of Longitudinal and Hall Resistances in the Regime of Breakdown of the Quantum Hall Effect
With unusually slow and high-resolution sweeps of magnetic field, strong,
ultra-narrow (width down to ) resistance peaks are observed in
the regime of breakdown of the quantum Hall effect. The peaks are dependent on
the directions and even the history of magnetic field sweeps, indicating the
involvement of a very slow physical process. Such a process and the sharp peaks
are, however, not predicted by existing theories. We also find a clear
connection between the resistance peaks and nuclear spin polarization.Comment: 5 pages with 3 figures. To appear in PR
Dynamic nuclear polarization at the edge of a two-dimensional electron gas
We have used gated GaAs/AlGaAs heterostructures to explore nonlinear
transport between spin-resolved Landau level (LL) edge states over a submicron
region of two-dimensional electron gas (2DEG). The current I flowing from one
edge state to the other as a function of the voltage V between them shows
diode-like behavior---a rapid increase in I above a well-defined threshold V_t
under forward bias, and a slower increase in I under reverse bias. In these
measurements, a pronounced influence of a current-induced nuclear spin
polarization on the spin splitting is observed, and supported by a series of
NMR experiments. We conclude that the hyperfine interaction plays an important
role in determining the electronic properties at the edge of a 2DEG.Comment: 8 pages RevTeX, 7 figures (GIF); submitted to Phys. Rev.
Triplet-Singlet Spin Relaxation via Nuclei in a Double Quantum Dot
The spin of a confined electron, when oriented originally in some direction,
will lose memory of that orientation after some time. Physical mechanisms
leading to this relaxation of spin memory typically involve either coupling of
the electron spin to its orbital motion or to nuclear spins. Relaxation of
confined electron spin has been previously measured only for Zeeman or exchange
split spin states, where spin-orbit effects dominate relaxation, while spin
flips due to nuclei have been observed in optical spectroscopy studies. Using
an isolated GaAs double quantum dot defined by electrostatic gates and direct
time domain measurements, we investigate in detail spin relaxation for
arbitrary splitting of spin states. Results demonstrate that electron spin
flips are dominated by nuclear interactions and are slowed by several orders of
magnitude when a magnetic field of a few millitesla is applied. These results
have significant implications for spin-based information processing
- …