13,534 research outputs found
Theory of nuclear induced spectral diffusion: Spin decoherence of phosphorus donors in Si and GaAs quantum dots
We propose a model for spectral diffusion of localized spins in
semiconductors due to the dipolar fluctuations of lattice nuclear spins. Each
nuclear spin flip-flop is assumed to be independent, the rate for this process
being calculated by a method of moments. Our calculated spin decoherence time
ms for donor electron spins in Si:P is a factor of two longer than
spin echo decay measurements. For P nuclear spins we show that spectral
diffusion is well into the motional narrowing regime. The calculation for GaAs
quantum dots gives s depending on the quantum dot size. Our
theory indicates that nuclear induced spectral diffusion should not be a
serious problem in developing spin-based semiconductor quantum computer
architectures.Comment: 15 pages, 9 figures. Accepted for publication in Phys. Rev.
Magnetic Structure of Rapidly Rotating FK Comae-Type Coronae
We present a three-dimensional simulation of the corona of an FK Com-type
rapidly rotating G giant using a magnetohydrodynamic model that was originally
developed for the solar corona in order to capture the more realistic,
non-potential coronal structure. We drive the simulation with surface maps for
the radial magnetic field obtained from a stellar dynamo model of the FK Com
system. This enables us to obtain the coronal structure for different field
topologies representing different periods of time. We find that the corona of
such an FK Com-like star, including the large scale coronal loops, is dominated
by a strong toroidal component of the magnetic field. This is a result of part
of the field being dragged by the radial outflow, while the other part remains
attached to the rapidly rotating stellar surface. This tangling of the magnetic
field,in addition to a reduction in the radial flow component, leads to a
flattening of the gas density profile with distance in the inner part of the
corona. The three-dimensional simulation provides a global view of the coronal
structure. Some aspects of the results, such as the toroidal wrapping of the
magnetic field, should also be applicable to coronae on fast rotators in
general, which our study shows can be considerably different from the
well-studied and well-observed solar corona. Studying the global structure of
such coronae should also lead to a better understanding of their related
stellar processes, such as flares and coronal mass ejections, and in
particular, should lead to an improved understanding of mass and angular
momentum loss from such systems.Comment: Accepted to ApJ, 10 pages, 6 figure
- …