287 research outputs found
Spherical functions on the de Sitter group
Matrix elements and spherical functions of irreducible representations of the
de Sitter group are studied on the various homogeneous spaces of this group. It
is shown that a universal covering of the de Sitter group gives rise to
quaternion Euler angles. An explicit form of Casimir and Laplace-Beltrami
operators on the homogeneous spaces is given. Different expressions of the
matrix elements and spherical functions are given in terms of multiple
hypergeometric functions both for finite-dimensional and unitary
representations of the principal series of the de Sitter group.Comment: 40 page
Effect of strain-induced electronic topological transitions on the superconducting properties of LaSrCuO thin films
We propose a Ginzburg-Landau phenomenological model for the dependence of the
critical temperature on microscopic strain in tetragonal high-Tc cuprates. Such
a model is in agreement with the experimental results for LSCO under epitaxial
strain, as well as with the hydrostatic pressure dependence of Tc in most
cuprates. In particular, a nonmonotonic dependence of Tc on hydrostatic
pressure, as well as on in-plane or apical microstrain, is derived. From a
microscopic point of view, such results can be understood as due to the
proximity to an electronic topological transition (ETT). In the case of LSCO,
we argue that such an ETT can be driven by a strain-induced modification of the
band structure, at constant hole content, at variance with a doping-induced
ETT, as is usually assumed.Comment: EPJB, to be publishe
Strong vortex-antivortex fluctuations in the type II superconducting film
The small size vortex-antivortex pairs proliferation in type II
superconducting film is considered for the wide interval of temperatures below
Tc. The corresponding contribution to free energy is calculated. It is shown
that these fluctuations give the main contribution to the heat capacity of the
film both at low temperatures and in the vicinity of transition
UCN anomalous losses and the UCN capture cross-section on material defects
Experimental data shows anomalously large Ultra Cold Neutrons (UCN)
reflection losses and that the process of UCN reflection is not completely
coherent. UCN anomalous losses under reflection cannot be explained in the
context of neutron optics calculations. UCN losses by means of incoherent
scattering on material defects are considered and cross-section values
calculated. The UCN capture cross-section on material defects is enhanced by a
factor of 10^4 due to localization of UCN around defects. This phenomenon can
explain anomalous losses of UCN.Comment: 13 pages, 4 figure
Giant Nernst Effect due to Fluctuating Cooper Pairs in Superconductors
A theory of the fluctuation-induced Nernst effect is developed for arbitrary
magnetic fields and temperatures beyond the upper critical field line in a
two-dimensional superconductor. First, we derive a simple phenomenological
formula for the Nernst coefficient, which naturally explains the giant Nernst
signal due to fluctuating Cooper pairs. The latter is shown to be large even
far from the transition and may exceed by orders of magnitude the Fermi liquid
terms. We also present a complete microscopic calculation (which includes
quantum fluctuations) of the Nernst coefficient and give its asymptotic
dependencies in various regions on the phase diagram. It is argued that the
magnitude and the behavior of the Nernst signal observed experimentally in
disordered superconducting films can be well-understood on the basis of the
superconducting fluctuation theory.Comment: 4 pages, 3 figure
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