arXiv
(422.153 recursos)
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Mostrando recursos 1 - 20 de 175
1.
Two-Dimensional t-J Model at Low Electron Density - Hellberg, C. Stephen; Manousakis, Efstratios
The phase diagram of the two-dimensional t-J model is determined accurately
at low electron density by a combination of analytic and numerical techniques.
The ground state exhibits three phases in the limit of zero electron density:
an unpaired state at small J/t, a gas of s-wave pairs for 2 < J/t < 3.4367, and
a phase separated state at larger interaction strengths. Bound states of larger
clusters are never realized, and the instabilities present at small densities
are discussed.
2.
Symmetries and Mean-Field Phases of the Extended Hubbard Model - Eriksson, A. B.; Einarsson, T.; Ostlund, S.
The two-dimensional extended Hubbard model that includes a nearest- neighbor
Heisenberg interaction is studied using a mean-field theory where
quasiparticles are defined by an U(8) group of canonical transformations. The
theory is a generalization of the ordinary BCS theory, and Balian and
Werthamer's theory of He-3 that permits both broken gauge, spin and sublattice
symmetry. This allows us to investigate superconductivity, antiferromagnetic
order, charge density waves and, by twisting the spin quantization axis, spiral
antiferromagnetic order in the same theory. Our results for positive Hubbard U
and Heisenberg exchange J suggest that antiferromagnetic ordering dominates
close to half filling, while spiral states and d-wave superconductivity compete
when doping is introduced....
3.
Phenomenological Models for the Gap Anisotropy of Bi-2212 as Measured by
ARPES - Norman, M. R.; Randeria, M.; Ding, H.; Campuzano, J. C.
Recently, high resolution angle-resolved photoemission spectroscopy has been
used to determine the detailed momentum dependence of the superconducting gap
in the high temperature superconductor Bi-2212. In this paper, we first
describe tight binding fits to the normal state dispersion and superlattice
modulation effects. We then discuss various theoretical models in light of the
gap measurements. We find that the simplest model which fits the data is the
anisotropic s-wave gap $\cos(k_x)\cos(k_y)$, which within a one-band BCS frame-
work suggests the importance of next near neighbor Cu-Cu interactions. Various
alternative interpretations of the observed gap are also discussed, along with
the implications for microscopic theories of high temperature superconductors.
4.
Theory of Electronic Raman Scattering in Disordered $d-$wave
Superconductors - Devereaux, T. P.
A theory for the effect of impurity scattering on electronic Raman scattering
in unconventional superconductors is presented. The impurity dependence of the
the spectra can be used to to distinguish between conventional (anisotropic
$s-$wave) or unconventional ($d$-wave) energy gaps, and excellent agreement
with the data on three cuprate superconductors is shown for $d_{x^{2}-y^{2}}$
pairing. The full frequency dependence of the spectra can be reproduced by
including inelastic scattering due to spin fluctuations and/or Coulomb
scattering on a partially nested Fermi surface.
5.
Phenomenological BCS theory of the high-$T_c$ cuprates - Fehrenbacher, R.; Norman, M. R.
A BCS model characterized by a phenomenological pair potential with on-site
($V_0$), nearest ($V_1$), and next nearest ($V_2$) neighbour coupling
constants, and an empirical quasiparticle dispersion taken from angle-resolved
photoemission spectra is considered. The model can consistently explain the
experimental data concerning the pair state of the hole doped cuprates. Three
ingredients are required to make the interpretation possible: the existence of
flat bands, a very small effective on-site repulsion, and a slightly dominating
effective nnn attraction $V_2$ of the order of 60-80meV with a ratio $V_2/V_1
\approx 1.5$.
6.
Gapless Time-Reversal Symmetry Breaking Superconductivity - Tikofsky, A. M.; Bailey, D. B.
We consider a layered superconductor with a complex order parameter whose phase
switches sign from one layer to the next. This system is shown to exhibit
gapless superconductivity for sufficiently large interlayer pairing or
interlayer hopping. In addition, this description is consistent with
experiments finding signals of time-reversal symmetry breaking in
high-temperature superconductors only at the surface and not in the sample
bulk.
7.
Linear Response of Thin Superconductors in Perpendicular Magnetic
Fields: An Asymptotic Analysis - Dorsey, Alan T.
The linear response of a thin superconducting strip subjected to an applied
perpendicular time-dependent magnetic field is treated analytically using the
method of matched asymptotic expansions. The calculation of the induced current
density is divided into two parts: an ``outer'' problem, in the middle of the
strip, which can be solved using conformal mapping; and an ``inner'' problem
near each of the two edges, which can be solved using the Wiener-Hopf method.
The inner and outer solutions are matched together to produce a solution which
is uniformly valid across the entire strip, in the limit that the effective
screening length $\lambda_{\rm eff}$ is small compared to the strip...
8.
Raman spectrum and charge fluctuations in the copper-oxide
superconductors - Niksic, H.; Tutis, E.; Barisic, S.
The effect of the charge fluctuations on the electronic spectrum and the Raman
spectrum of high temperature superconductors is examined within the slave boson
approach. Instead of using the saddle point approximation for slave bosons, we
confine ourselves to the non-crossing approximation (NCA) in summing the
diagrams for the Green functions, thus obtaining the renormalized hole spectrum
and its lifetime on equal footing. The electronic Raman spectrum is calculated,
showing the characteristic featureless behaviour up to the frequency of the
order of renormalized $\Delta_{pd}$ parameter. The dependence on the
polarization of the incident and the scattered light agrees with experiments.
9.
The effect of large $U_d$ on the Raman spectrum in the copper-oxide
superconductors - Niksic, H.; Tutis, E.; Barisic, S.
The effect of the charge fluctuations on the electronic and Raman spectrum of
high temperature superconductors is examined, using the slave boson approach to
the large Coulomb repulsion $U_d$ on the copper site. Instead of the saddle
point approximation $\srv{b}\ne 0$ for the slave boson, characteristic for
various $N\ide\infty$ approaches, we confine ourselves to the non-crossing
approximation (NCA) diagrams for the Green functions. In this way the effects
of charge fluctuations and of the constraint of no double occupancy on the
copper site on the shape of the electronic spectrum are studied primarily,
while the slave boson diagrams responsible for copper-copper spin correlations
are intentionally not included. The novel...
10.
Neutron scattering and superconducting order parameter in YBa2Cu3O7 - Mazin, I. I.; Yakovenko, Victor M.
We discuss the origin of the neutron scattering peak at 41 meV observed in
YBa$_2$Cu$_3$O$_7$ below $T_c$. The peak may occur due to spin-flip electron
excitations across the superconducting gap which are enhanced by the
antiferromagnetic interaction between Cu spins. In this picture, the experiment
is most naturally explained if the superconducting order parameter has $s$-wave
symmetry and opposite signs in the bonding and antibonding electron bands
formed within a Cu$_2$O$_4$ bilayer.
11.
The Nonlinear Meissner Effect in Unconventional Superconductors - Xu, D.; Yip, S. K.; Sauls, J. A.
We examine the long-wavelength current response in anisotropic superconductors
and show how the field-dependence of the Meissner penetration length can be
used to detect the structure of the order parameter. Nodes in the excitation
gap lead to a nonlinear current-velocity constitutive equation at low
temperatures which is distinct for each symmetry class of the order parameter.
The effective Meissner penetration length is linear in $H$ and exhibits a
characteristic anisotropy for fields in the $ab$-plane that is determined by
the positions of the nodes in momentum space. The nonlinear current-velocity
relation also leads to an intrinsic magnetic torque for in-plane fields that
are not parallel to a nodal or...
12.
Crossover from BCS to Composite Boson (Local Pair) Superconductivity in
Quasi-2D systems - Gorbar, E. V.; Loktev, V. M.; Sharapov, S. G.
The crossover from cooperative Cooper pairing to independent bound state
(composite bosons) formation and condensation in quasi-2D systems is studied.
It is shown that at low carrier density the critical superconducting
temperature is equal to the temperature of Bose-condensation of ideal quasi-2D
Bose-gas with heavy dynamical mass, meanwhile at high densities the BCS result
remains valid. The evident nonmonotoneous behaviour of the critical temperature
as function of the coupling constant (the energy of the two particle bound
state) is a qualitative difference of quasi-2D crossover from 3D one.
13.
Critical Exponents of the Superconducting Phase Transition - Kiometzis, Michael; Kleinert, Hagen; Schakel, Adriaan M. J.
We study the critical exponents of the superconducting phase transition in the
context of renormalization group theory starting from a dual formulation of the
Ginzburg-Landau theory. The dual formulation describes a loop gas of Abrikosov
flux tubes which proliferate when the critical temperature is approached from
below. In contrast to the Ginzburg-Landau theory, it has a spontaneously broken
global symmetry and possesses an infrared stable fixed point. The exponents
coincide with those of a superfluid with reversed temperature axis.
14.
Quantum critical phenomena in ladders of Josephson junctions in a
magnetic field - Granato, Enzo
A model of a ladder of Josephson junctions in a magnetic field is considered.
The topological features of the zero-temperature phase diagram, as a function
of charging energy and small deviations from commensurability of the vortex
lattice, are strongly dependent on the field. In addition to
superconductor-insulator transitions, commensurate-incommensurate transitions
and depinning by quantum fluctuations of the vortex-lattice are also possible.
the critical behavior of the superconductor-insulator transition at f = 1/2 is
found to depend on the ratio between interchain and intrachain Josephson
couplings and is in the universality of the classical XY-Ising model.
15.
GINZBURG-LANDAU THEORY OF VORTICES IN $d$-WAVE SUPERCONDUCTORS - Berlinsky, A. J.; Fetter, A. L.; Franz, M.; Kallin, C.; Soininen, P. I.
Ginzburg-Landau theory is used to study the properties of single vortices and
of the Abrikosov vortex lattice in a $d_{x^2-y^2}$ superconductor. For a single
vortex, the $s$-wave order parameter has the expected four-lobe structure in a
ring around the core and falls off like $1/r^2$ at large distances. The
topological structure of the $s$-wave order parameter consists of one
counter-rotating unit vortex, centered at the core, surrounded by four
symmetrically placed positive unit vortices. The Abrikosov lattice is shown to
have a triangular structure close to $T_c$ and an oblique structure at lower
temperatures. Comparison is made to recent neutron scattering data.
16.
Superconductivity in the two dimensional Hubbard Model. - Beenen, J.; Edwards, D. M.
Quasiparticle bands of the two-dimensional Hubbard model are calculated using
the Roth two-pole approximation to the one particle Green's function. Excellent
agreement is obtained with recent Monte Carlo calculations, including an
anomalous volume of the Fermi surface near half-filling, which can possibly be
explained in terms of a breakdown of Fermi liquid theory. The calculated bands
are very flat around the (pi,0) points of the Brillouin zone in agreement with
photoemission measurements of cuprate superconductors. With doping there is a
shift in spectral weight from the upper band to the lower band. The Roth method
is extended to deal with superconductivity within a four-pole approximation
allowing electron-hole mixing. It...
17.
Coupling between planes and chains in YBa2Cu3O7 : a possible solution
for the order parameter controversy - Combescot, R.; Leyronas, X.
We propose to explain the contradictory experimental evidence about the
symmetry of the order parameter in $YBa_{2}Cu_{3}O_{7} $ by taking into account
the coupling between planes and chains. This leads to an anticrossing of the
plane and chain band. We include an attractive pairing interaction within the
planes and a repulsive one between planes and chains, leading to opposite signs
for the order parameter on planes and chains, and to nodes of the gap because
of the anticrossing. Our model blends s-wave and d-wave features, and provides
a natural explanation for all the contradictory experiments
18.
INTERLAYER COUPLING AND THE METAL-INSULATOR TRANSITION IN Pr-SUBSTITUTED
Bi(2)Sr(2)CaCu(2)O(8+y) - Quitmann, C.; Beschoten, B.; Kelley, R. J.; Guentherodt, G.; Onellion, M.
Substitution of rare-earth ions for Ca in Bi2Sr2CaCu2O8+y is known to cause a
metal-insulator transition. Using resonant photoemission we study how this
chemical substitution affects the electronic structure of the material. For the
partial Cu-density of states at E_F and in the region of the valence band we
observe no significant difference between a pure superconducting sample and an
insulating sample with 60% Pr for Ca. This suggests that the states responsible
for superconductivity are predomi- nately O-states. The partial Pr-4f density
of states was extracted utilizing the Super- Koster-Kronig Pr 4d-4f resonance.
It consists of a single peak at 1.36eV binding energy. The peak shows a
strongly assymetric...
19.
"Chain scenario" for Josephson tunneling with pi-shift in YBa2Cu3O7 - Mazin, I. I.; Golubov, A. A.; Zaikin, A. D.
We point out that all current Josephson-junction experiments probing directly
the symmetry of the superconducting state in YBa2Cu3O7, can be interpreted in
terms of the bilayer antiferromagnetic spin fluctuation model, which renders
the superconducting state with the order parameters of extended $s$ symmetry,
but with the opposite signs in the bonding and antibonding Cu-O plane bands.
The essential part of our interpretation includes the Cu-O chain band which
would have the order parameter of the same sign as antibonding plane band. We
show that in this case net Josephson currents along and perpendicular to the
chains have the phase shift equal to pi.
20.
Peak Effect in Superconductors: Melting of Larkin Domains - Ling, Xinsheng; Tang, Chao; Bhattacharya, S.; Chaikin, Paul M.
Motivated by the recent observations of the peak effect in high-$T_c$ YBCO
superconductors, we reexamine the origin of this unusual phenomenon. We show
that the mechanism based on the $k$-dependence (nonlocality) of the
vortex-lattice tilt modulus $C_{44}({\bf k})$ cannot account for the essential
feature of the peak effect. We propose a scenario in which the peak effect is
related to the melting of Larkin domains. In our model, the rise of critical
current with increasing temperature is a result of a crossover from the Larkin
pinning length to the length scale set by thermally excited free dislocations.
1.
