12,095 research outputs found
Small-mass effects in heavy-to-light form factors
We present the heavy-to-light form factors with two different non-vanishing
masses at next-to-next-to-leading order and study its expansion in the small
mass. The leading term of this small-mass expansion leads to a factorized
expression for the form factor. The presence of a second mass results in a new
feature, in that the soft contribution develops a factorization anomaly. This
cancels with the corresponding anomaly in the collinear contribution. With the
generalized factorization presented here, it is possible to obtain the leading
small-mass terms for processes with large masses, such as muon-electron
scattering, from the corresponding massless amplitude and the soft
contribution.Comment: 20 pages, 4 figures, 1 ancillary file, published versio
Controlled exchange interaction for quantum logic operations with spin qubits in coupled quantum dots
A two-electron system confined in two coupled semiconductor quantum dots is
investigated as a candidate for performing quantum logic operations on spin
qubits. We study different processes of swapping the electron spins by
controlled switching on/off the exchange interaction. The resulting spin swap
corresponds to an elementary operation in quantum information processing. We
perform a direct time evolution simulations of the time-dependent Schroedinger
equation. Our results show that -- in order to obtain the full interchange of
spins -- the exchange interaction should change smoothly in time. The presence
of jumps and spikes in the corresponding time characteristics leads to a
considerable increase of the spin swap time. We propose several mechanisms to
modify the exchange interaction by changing the confinement potential profile
and discuss their advantages and disadvantages
No particular action needed? A necessary condition analysis of gestation activities and firm emergence
A subtraction scheme for massive QED
We present an extension of the FKS subtraction scheme beyond next-to-leading
order to deal with soft singularities in fully differential calculations within
QED with massive fermions. After a detailed discussion of the
next-to-next-to-leading order case, we show how to extend the scheme to even
higher orders in perturbation theory. As an application we discuss the
computation of the next-to-next-to-leading order QED corrections to the muon
decay and present differential results with full electron mass dependence.Comment: 25 pages, 6 figures, published versio
Theory of Spin Hall conductivity in n-doped GaAs
We develop a theory of extrinsic spin currents in semiconductors, resulting
from spin-orbit coupling at charged scatterers, which leads to skew scattering
and side jump contributions to the spin Hall conductance. Applying the theory
to bulk n-GaAs, without any free parameters, we find spin currents that are in
reasonable agreement with recent experiments by Kato et al. [Science 306, 1910
(2004)].Comment: 5 pages, 1 figur
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