54 research outputs found
One-electron spectral functions of the attractive Hubbard model at intermediate coupling
We calculate the one-electron spectral function of the attractive-U Hubbard
model in two dimensions. We work in the intermediate coupling and low density
regime and evaluate analytically the self-energy. The results are obtained in a
framework based on the self-consistent T-matrix approximation. We also
calculate the chemical potential of the bound pairs as a function of
temperature. On the basis of this calculation we analyze the low-temperature
resistivity and specific heat in the normal state of this system. We compare
our results with recent beautiful tunneling experiments in the underdoped
regime of HTSC-materials.Comment: 2 pages, LT22 Conference paper, phbauth and elsart style files
include
Internal structure of fluctuating Cooper pairs
Abstract.: In order to obtain information about the internal structure of fluctuating Cooper pairs in the pseudogap state and below the transition temperature of high Tc superconductors, we solve the Bethe-Salpeter equation for the two-electron propagator in order to calculate a "pair structure functionâ that depends on the internal distance between the partners and on the center of mass momentum P of the pair. We use an attractive Hubbard model with a local potential for s-wave and a separable potential for d-wave symmetry. The amplitude of gP for small Ï depends on temperature, chemical potential and interaction symmetry, but the Ï dependence itself is rather insensitive to the interaction strength. Asymptotically gP decreases as an inverse power of Ï for weak coupling, but exponentially when a pseudogap develops for stronger interaction. Some possibilities of observing the pair structure experimentally are mentione
Screening effects in superconductors
The partition function of the Hubbard model with local attraction and long
range Coulomb repulsion between electrons is written as a functional integral
with an action involving a pairing field and a local potential
. After integration over and over fluctuations in , the
final form of involves a Josephson coupling between the local phases of
and a "kinetic energy" term, representing the screened Coulomb
interaction between charge fluctuations. The competition between Josephson
coupling and charging energy allows to understand the relation between
and composition in high- materials, in particular superlattices, alloys
and bulk systems of low doping.Comment: 4 pages, revtex, no figures, submitted to Physica B (Proceedings of
SCES '96 International Conference, held in Zurich from 19th to 21st of
August
Pair fluctuation induced pseudogap in the normal phase of the two-dimensional attractive Hubbard model at weak coupling
One-particle spectral properties in the normal phase of the two-dimensional
attractive Hubbard model are investigated in the weak coupling regime using the
non-selfconsistent T-matrix approximation. The corresponding equations are
evaluated numerically directly on the real frequency axis. For temperatures
sufficiently close to the superconducting transition temperature a pseudogap in
the one-particle spectral function is observed, which can be assigned to the
increasing importance of pair fluctuations.Comment: 22 pages, 13 figure
Untapped seasonal storage potential in Swiss hydropower schemes
We have analysed all Swiss hydropower reservoirs with a capacity above 1 million m3: â 63 reservoirs > 1 hm3 â 40 reservoirs > 10 hm3 â 29 reservoirs > 30 hm3 â 36 reservoirs > 60 hm3 These 168 dams represent 8778 GWhof storage capacity, which is over 99% of the total Swiss hydropower storage capacity. The results show that a significant number of dams could increase storage in a significant way: â 48 dams could capture existing excess natural inflows if their current storage capacity were to be increased. â 7 dams are sub-optimally used, and could catch additional inflows using existing infrastructure
Persistence of pseudogap formation in quasi-2D systems with arbitrary carrier density
The existence of a pseudogap above the critical temperature has been widely
used to explain the anomalous behaviour of the normal state of high-temperature
superconductors. In two dimensions the existence of a pseudogap phase has
already been demonstrated in a simple model. It can now be shown that the
pseudogap phase persists even for the more realistic case where coherent
interlayer tunneling is taken into account. The effective anisotropy is
surprisingly large and even increases with increasing carrier density.Comment: 17 pages, LaTeX, 1 EMTeX figure; extended versio
Magnetic-field dependence of dynamical vortex response in two-dimensional Josephson junction arrays and superconducting films
The dynamical vortex response of a two-dimensional array of the resistively
shunted Josephson junctions in a perpendicular magnetic field is inferred from
simulations. It is found that, as the magnetic field is increased at a fixed
temperature, the response crosses over from normal to anomalous, and that this
crossover can be characterized by a single dimensionless parameter. It is
described how this crossover should be reflected in measurements of the complex
impedance for Josephson junction arrays and superconducting films.Comment: 4 pages including 5 figures in two columns, final versio
Charge Transport in the Dense Two-Dimensional Coulomb Gas
The dynamics of a globally neutral system of diffusing Coulomb charges in two
dimensions, driven by an applied electric field, is studied in a wide
temperature range around the Berezinskii-Kosterlitz-Thouless transition. I
argue that the commonly accepted ``free particle drift'' mechanism of charge
transport in this system is limited to relatively low particle densities. For
higher densities, I propose a modified picture involving collective ``partner
transfer'' between bound pairs. The new picture provides a natural explanation
for recent experimental and numerical findings which deviate from standard
theory. It also clarifies the origin of dynamical scaling in this context.Comment: 4 pages, RevTeX, 2 eps figures included; some typos corrected, final
version to be published in Phys. Rev. Let
Pseudogap phase formation in the crossover from Bose-Einstein condensation to BCS superconductivity
A phase diagram for a 2D metal with variable carrier density has been
derived. It consists of a normal phase, where the order parameter is absent; a
so-called ``abnormal normal'' phase where this parameter is also absent but the
mean number of composite bosons (bound pairs) exceeds the mean number of free
fermions; a pseudogap phase where the absolute value of the order parameter
gradually increases but its phase is a random value, and finally a
superconducting (here Berezinskii-Kosterlitz-Thouless) phase. The
characteristic transition temperatures between these phases are found. The
chemical potential and paramagnetic susceptibility behavior as functions of the
fermion density and the temperature are also studied. An attempt is made to
qualitatively compare the resulting phase diagram with the features of
underdoped high- superconducting compounds above their critical
temperature.Comment: 26 pages, revtex, 5 EMTeX figures; more discussion and references
added; to be published in JET
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