3,475 research outputs found
Casimir Densities for a Massive Fermionic Quantum Field in a Global Monopole Background with Spherical Boundary
We investigate the vacuum expectation value of the energy-momentum tensor
associated with a massive fermionic field obeying the MIT bag boundary
condition on a spherical shell in the global monopole spacetime. The asymptotic
behavior of the vacuum densities is investigated near the sphere center and
surface, and at large distances from the sphere. In the limit of strong
gravitational field corresponding to small values of the parameter describing
the solid angle deficit in global monopole geometry, the sphere-induced
expectation values are exponentially suppressed.Comment: 8 pages, 4 figures, 6th Alexander Friedmann International Seminar on
Gravitation and Cosmolog
Theory of the Fermi Arcs, the Pseudogap, and the Anisotropy in k-space of Cuprate Superconductors
The appearance of the Fermi arcs or gapless regions at the nodes of the Fermi
surface just above the critical temperature is described through
self-consistent calculations in an electronic disordered medium. We develop a
model for cuprate superconductors based on an array of Josephson junctions
formed by grains of inhomogeneous electronic density derived from a phase
separation transition. This approach provides physical insights to the most
important properties of these materials like the pseudogap phase as forming by
the onset of local (intragrain) superconducting amplitudes and the zero
resistivity critical temperature due to phase coherence activated by
Josephson coupling. The formation of the Fermi arcs and the dichotomy in
k-space follows from the direction dependence of the junctions tunneling
current on the d-wave symmetry on the planes. We show that this
semi-phenomenological approach reproduces also the main future of the cuprates
phase diagram.Comment: 5 pages 7 fig
Electronic Phase Separation Transition as the Origin of the Superconductivity and the Pseudogap Phase of Cuprates
We propose a new phase of matter, an electronic phase separation transition
that starts near the upper pseudogap and segregates the holes into high and low
density domains. The Cahn-Hilliard approach is used to follow quantitatively
this second order transition. The resulting grain boundary potential confines
the charge in domains and favors the development of intragrain superconducting
amplitudes. The zero resistivity transition arises only when the intergrain
Josephson coupling is of the order of the thermal energy and phase
locking among the superconducting grains takes place. We show that this
approach explains the pseudogap and superconducting phases in a natural way and
reproduces some recent scanning tunneling microscopy dataComment: 4 pages and 5 eps fig
Chiral String in a Curved Space: Gravitational Self-Action
We analyze the effective action describing the linearised gravitational
self-action for a classical superconducting string in a curved spacetime. It is
shown that the divergent part of the effective action is equal to zero for the
both Nambu-Goto and chiral superconducting string.Comment: 5 pages, LaTe
Effects of electron-phonon coupling range on the polaron formation
The polaron features due to electron-phonon interactions with different
coupling ranges are investigated by adopting a variational approach. The
ground-state energy, the spectral weight, the average kinetic energy, the mean
number of phonons, and the electron-lattice correlation function are discussed
for the system with coupling to local and nearest neighbor lattice
displacements comparing the results with the long range case. For large values
of the coupling with nearest neighbor sites, most physical quantities show a
strong resemblance with those obtained for the long range electron-phonon
interaction. Moreover, for intermediate values of interaction strength, the
correlation function between electron and nearest neighbor lattice
displacements is characterized by an upturn as function of the electron-phonon
coupling constant.Comment: 5 pages and 4 figure
Induced vacuum currents in anti-de Sitter space with toral dimensions
We investigate the Hadamard function and the vacuum expectation value of the
current density for a charged massive scalar field on a slice of anti-de Sitter
(AdS) space described in Poincar\'{e} coordinates with toroidally compact
dimensions. Along compact dimensions periodicity conditions are imposed on the
field with general phases. Moreover, the presence of a constant gauge field is
assumed. The latter gives rise to Aharonov-Bohm-like effects on the vacuum
currents. The current density along compact dimensions is a periodic function
of the gauge field flux with the period equal to the flux quantum. It vanishes
on the AdS boundary and, near the horizon, to the leading order, it is
conformally related to the corresponding quantity in Minkowski bulk for a
massless field. For large values of the length of the compact dimension
compared with the AdS curvature radius, the vacuum current decays as power-law
for both massless and massive fields. This behavior is essentially different
from from the corresponding one in Minkowski background, where the currents for
a massive field are suppressed exponentially.Comment: 15 pages, 4 figure
Measurements and analysis of the upper critical field on an underdoped and overdoped compounds
The upper critical field is one of the many non conventional
properties of high- cuprates. It is possible that the
anomalies are due to the presence of inhomogeneities in the local charge
carrier density of the planes. In order to study this point, we
have prepared good quality samples of polycrystalline
using the wet-chemical method, which has demonstrated to produce samples with a
better cation distribution. In particular, we have studied the temperature
dependence of the second critical field, , through the magnetization
measurements on two samples with opposite average carrier concentration
() and nearly the same critical temperature, namely
(underdoped) and (overdoped). The results close to do not
follow the usual Ginzburg-Landau theory and are interpreted by a theory which
takes into account the influence of the inhomogeneities.Comment: Published versio
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