72,700 research outputs found
Non-markovian dynamics of double quantum dot charge qubit with static bias
The dynamics of charge qubit in double quantum dot coupled to phonons is
investigated theoretically. The static bias is considered. By means of the
perturbation approach based on unitary transformations, the dynamical tunneling
current is obtained explicitly. The biased system displays broken symmetry and
a significantly larger coherence-incoherence transition critical point . We also analyzed the decoherence induced by piezoelectric coupling
phonons in detail. The results show that reducing the coupling between system
and bath make coherence frequency increase and coherence time prolong. To
maintain quantum coherence, applying static bias also is a good means.Comment: 13 pages, 5 figure
Decay Modes of the Hoyle State in
Recent experimental results give an upper limit less than 0.043\% (95\% C.L.)
to the direct decay of the Hoyle state into 3 respect to the sequential
decay into {Be}+. We performed one and two-dimensional tunneling
calculations to estimate such a ratio and found it to be more than one order of
magnitude smaller than experiment depending on the range of the nuclear force.
This is within high statistics experimental capabilities. Our results can also
be tested by measuring the decay modes of high excitation energy states of
C where the ratio of direct to sequential decay might reach 10\% at
(C)=10.3 MeV. The link between a Bose Einstein Condensate (BEC) and
the direct decay of the Hoyle state is also addressed. We discuss a
hypothetical `Efimov state' at (C)=7.458 MeV, which would mainly
{\it sequentially} decay with 3 of {\it equal energies}: a
counterintuitive result of tunneling. Such a state, if it would exist, is at
least 8 orders of magnitude less probable than the Hoyle's, thus below the
sensitivity of recent and past experiments.Comment: 6 pages, 2 figures, accepted by Phys. Lett.
Superluminal Caustics of Close, Rapidly-Rotating Binary Microlenses
The two outer triangular caustics (regions of infinite magnification) of a
close binary microlens move much faster than the components of the binary
themselves, and can even exceed the speed of light. When , where
is the caustic speed, the usual formalism for calculating the lens
magnification breaks down. We develop a new formalism that makes use of the
gravitational analog of the Li\'enard-Wiechert potential. We find that as the
binary speeds up, the caustics undergo several related changes: First, their
position in space drifts. Second, they rotate about their own axes so that they
no longer have a cusp facing the binary center of mass. Third, they grow larger
and dramatically so for . Fourth, they grow weaker roughly in
proportion to their increasing size. Superluminal caustic-crossing events are
probably not uncommon, but they are difficult to observe.Comment: 12 pages, 7 ps figures, submitted to Ap
A Dispersive Analysis on the and Resonances in Processes
We estimate the di-photon coupling of , and
resonances in a coupled channel dispersive approach. The di-photon
coupling is also reinvestigated using a single channel matrix for
scattering with better analyticity property, and it is found to be
significantly smaller than that of a state. Especially we also
estimate the di-photon coupling of the third sheet pole located near
threshold, denoted as .
It is argued that this third sheet pole may be originated from a coupled
channel Breit-Wigner description of the resonance.Comment: 24 pages and 13 eps figures. A nuerical bug in previous version is
fixed. Some results changed. References and new figures added. Version to
appear in Phys. Rev.
Vanishing Gamow-Teller Transition Rate for A=14 and the Nucleon-Nucleon Interaction in the Medium
The problem of the near vanishing of the Gamow-Teller transition () in
the A=14 system between the lowest and states is
revisited. The model space is extended from the valence space to the
valence space plus all 2 excitations. The question is addressed
as to what features of the effective nucleon-nucleon interaction in the medium
are required to obtain the vanishing strength in this extended space. It
turns out that a combination of a realistic strength of the tensor force
combined with a spin-orbit interaction which is enhanced as compared to the
free interaction yields a vanishing strength. Such an interaction can be
derived from a microscopic meson exchange potential if the enhancement of the
small component of the Dirac spinors for the nucleons is taken into account.Comment: RevTex file, 7 pages, four postscript figures. submitted to Phys.
Rev. C as a brief repor
Quantum dynamics of a qubit coupled with structured bath
The dynamics of an unbiased spin-boson model with Lorentzian spectral density
is investigated theoretically in terms of the perturbation theory based on a
unitary transformation. The non-equilibrium correlation function and
susceptibility are calculated for both the
off-resonance case and the on-resonance case
. The approach is checked by the Shiba's relation and the
sum rule. Besides, the coherent-incoherent transition point can be
determined, which has not been demonstrated for the structured bath by previous
authors up to our knowledge.Comment: 25 pages, 11 figure
Phase diagram and optical conductivity of the one-dimensional spinless Holstein model
The effects of quantum lattice fluctuations on the Peierls transition and the
optical conductivity in the one-dimensional Holstein model of spinless fermions
have been studied by developing an analytical approach, based on the unitary
transformation method. We show that when the electron-phonon coupling constant
decreases to a finite critical value the Peierls dimerization is destroyed by
the quantum lattice fluctuations. The dimerization gap is much more reduced by
the quantum lattice fluctuations than the phonon order parameter. The
calculated optical conductivity does not have the inverse-square-root
singularity but have a peak above the gap edge and there exists a significant
tail below the peak. The peak of optical-conductivity spectrum is not directly
corresponding to the dimerized gap. Our results of the phase diagram and the
spectral-weight function agree with those of the density matrix renormalization
group and the exact diagonalization methods.Comment: 9 pages, 4 figures include
- …