10,254 research outputs found
Diffusivity in one-dimensional generalized Mott variable-range hopping models
We consider random walks in a random environment which are generalized
versions of well-known effective models for Mott variable-range hopping. We
study the homogenized diffusion constant of the random walk in the
one-dimensional case. We prove various estimates on the low-temperature
behavior which confirm and extend previous work by physicists.Comment: Published in at http://dx.doi.org/10.1214/08-AAP583 the Annals of
Applied Probability (http://www.imstat.org/aap/) by the Institute of
Mathematical Statistics (http://www.imstat.org
Two tone response of radiofrequency signals using the voltage output of a Superconducting Quantum Interference Filter
In the presence of weak time harmonic electromagnetic fields, Superconducting
Quantum Interference Filters (SQIFs) show the typical behavior of non linear
mixers. The SQIFs are manufactured from high-T_c grain boundary Josephson
junctions and operated in active microcooler. The dependence of dc voltage
output V_dc vs. static external magnetic field B is non-periodic and consists
of a well pronounced unique dip at zero field, with marginal side modulations
at higher fields. We have successfully exploited the parabolic shape of the
voltage dip around B=0 to mix quadratically two external time harmonic
rf-signals, at frequencies f_1 and f_2 below the Josephson frequency f_J, and
detect the corresponding mixing signal at f_1-f_2. When the mixing takes place
on the SQIF current-voltage characteristics the component at 2f_2 - f_1 is
present. The experiments suggest potential applications of a SQIF as a
non-linear mixing device, capable to operate at frequencies from dc to few GHz
with a large dynamic range.Comment: 10 pages, 3 Figures, submitted to J. Supercond. (as proceeding of the
HTSHFF Symposium, June 2006, Cardiff
Relaxation time of -reversal chains and other chromosome shuffles
We prove tight bounds on the relaxation time of the so-called -reversal
chain, which was introduced by R. Durrett as a stochastic model for the
evolution of chromosome chains. The process is described as follows. We have
distinct letters on the vertices of the -cycle ( mod
); at each step, a connected subset of the graph is chosen uniformly at
random among all those of length at most , and the current permutation is
shuffled by reversing the order of the letters over that subset. We show that
the relaxation time , defined as the inverse of the spectral gap of
the associated Markov generator, satisfies . Our results can be interpreted as strong evidence for a
conjecture of R. Durrett predicting a similar behavior for the mixing time of
the chain.Comment: Published at http://dx.doi.org/10.1214/105051606000000295 in the
Annals of Applied Probability (http://www.imstat.org/aap/) by the Institute
of Mathematical Statistics (http://www.imstat.org
Invariance principle for Mott variable range hopping and other walks on point processes
We consider a random walk on a homogeneous Poisson point process with energy
marks. The jump rates decay exponentially in the A-power of the jump length and
depend on the energy marks via a Boltzmann--like factor. The case A=1
corresponds to the phonon-induced Mott variable range hopping in disordered
solids in the regime of strong Anderson localization. We prove that for almost
every realization of the marked process, the diffusively rescaled random walk,
with arbitrary start point, converges to a Brownian motion whose diffusion
matrix is positive definite, and independent of the environment. Finally, we
extend the above result to other point processes including diluted lattices.Comment: 47 pages, minor corrections, submitte
Recurrence and transience for long-range reversible random walks on a random point process
We consider reversible random walks in random environment obtained from
symmetric long--range jump rates on a random point process. We prove almost
sure transience and recurrence results under suitable assumptions on the point
process and the jump rate function. For recurrent models we obtain almost sure
estimates on effective resistances in finite boxes. For transient models we
construct explicit fluxes with finite energy on the associated electrical
network.Comment: 34 page
RR LYRAE VARIABLE STARS: PULSATIONAL CONSTRAINTS RELEVANT TO THE OOSTERHOFF CONTROVERSY
A solution to the old Oosterhoff controversy is proposed on the basis of a
new theoretical pulsational scenario concerning RR Lyrae cluster variables
(Bono and coworkers). We show that the observed constancy of the lowest
pulsation period in both Oosterhoff type I (OoI) and Oosterhoff type II (OoII)
prototypes (M3, M15) can be easily reproduced only by assuming the canonical
evolutionary horizontal-branch luminosity levels of these Galactic globular
clusters and therefore by rejecting the Sandage period shift effect (SPSE).Comment: postscript file of 7 pages and 2 figures; one non postcript figure is
available upon request; for any problem please write to
[email protected]
The seesaw portal in testable models of neutrino masses
A Standard Model extension with two Majorana neutrinos can explain the
measured neutrino masses and mixings, and also account for the
matter-antimatter asymmetry in a region of parameter space that could be
testable in future experiments. The testability of the model relies to some
extent on its minimality. In this paper we address the possibility that the
model might be extended by extra generic new physics which we parametrize in
terms of a low-energy effective theory. We consider the effects of the
operators of the lowest dimensionality, , and evaluate the upper bounds on
the coefficients so that the predictions of the minimal model are robust. One
of the operators gives a new production mechanism for the heavy neutrinos at
LHC via higgs decays. The higgs can decay to a pair of such neutrinos that,
being long-lived, leave a powerful signal of two displaced vertices. We
estimate the LHC reach to this process.Comment: 19 pages, 11 figure
A Note on Wetting Transition for Gradient Fields
We prove existence of a wetting transition for two types of gradient fields:
1) Continuous SOS models in any dimension and 2) Massless Gaussian model in two
dimensions. Combined with a recent result showing the absence of such a
transition for Gaussian models above two dimensions by Bolthausen et al, this
shows in particular that absolute-value and quadratic interactions can give
rise to completely different behaviors.Comment: 6 pages, latex2
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