42,440 research outputs found
Tunneling between single and multi-centered black hole configurations
We find a gravitational instanton that connects an initial state
corresponding to a single-centered extremal Reissner-Nordstrom (ERN) black hole
configuration, to a final state corresponding to a multi-centered
configuration. This instanton is interpreted as describing quantum tunneling
between the two different black hole solutions. We evaluate the Euclidean
action for this instanton and find that the amplitude for the tunneling process
is equal to half the difference in entropy between the initial and final
configurations.Comment: 8 pages, 4 figures. v4: final version accepted for publication in
Phys. Rev.
Improved determination of color-singlet nonrelativistic QCD matrix elements for S-wave charmonium
We present a new computation of S-wave color-singlet nonrelativistic QCD
matrix elements for the J/psi and the eta_c. We compute the matrix elements of
leading order in the heavy-quark velocity v and the matrix elements of relative
order v^2. Our computation is based on the electromagnetic decay rates of the
J/psi and the eta_c and on a potential model that employs the Cornell
potential. We include relativistic corrections to the electromagnetic decay
rates, resumming a class of corrections to all orders in v, and find that they
significantly increase the values of the matrix elements of leading order in v.
This increase could have important implications for theoretical predictions for
a number of quarkonium decay and production processes. The values that we find
for the matrix elements of relative order v^2 are somewhat smaller than the
values that one obtains from estimates that are based on the velocity-scaling
rules of nonrelativistic QCD.Comment: 31 pages, minor corrections, version published in Phys. Rev.
Erratum: Dynamics and scaling in a quantum spin chain material with bond randomness
Follow-up neutron measurements, performed on a sample much larger than the
one used in the original study, show that in the energy range 0.5-45 meV the
magnetic excitations in BaCu2SiGeO7 are indistinguishable from those in
conventional (disorder-free) quantum S=1/2 chains. Scrutinizing the previous
data, we found that the analysis was affected by a poorly identified structured
background and an additional technical mistake in the data reduction.Comment: This is a complete withdrawal of the original paper, also published
as in Phys. Rev. Lett 93, 077206 (2004). One page, one figur
Do Large-Scale Inhomogeneities Explain Away Dark Energy?
Recently, new arguments (astro-ph/0501152, hep-th/0503117) for how
corrections from super-Hubble modes can explain the present-day acceleration of
the universe have appeared in the literature. However, in this letter, we argue
that, to second order in spatial gradients, these corrections only amount to a
renormalization of local spatial curvature, and thus cannot account for the
negative deceleration. Moreover, cosmological observations already put severe
bounds on such corrections, at the level of a few percent, while in the context
of inflationary models, these corrections are typically limited to ~ 10^{-5}.
Currently there is no general constraint on the possible correction from higher
order gradient terms, but we argue that such corrections are even more
constrained in the context of inflationary models.Comment: 4 Pages, no figures. Minor modifications, added reference
Thermoelectric and thermal rectification properties of quantum dot junctions
The electrical conductance, thermal conductance, thermal power and figure of
merit (ZT) of semiconductor quantum dots (QDs) embedded into an insulator
matrix connected with metallic electrodes are theoretically investigated in the
Coulomb blockade regime. The multilevel Anderson model is used to simulate the
multiple QDs junction system. The charge and heat currents in the sequential
tunneling process are calculated by the Keldysh Green function technique. In
the linear response regime the ZT values are still very impressive in the small
tunneling rates case, although the effect of electron Coulomb interaction on ZT
is significant. In the nonlinear response regime, we have demonstrated that the
thermal rectification behavior can be observed for the coupled QDs system,
where the very strong asymmetrical coupling between the dots and electrodes,
large energy level separation between dots and strong interdot Coulomb
interactions are required.Comment: 8 page and 14 figure
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