554 research outputs found
Ground state properties and dynamics of the bilayer t-J model
We present an exact diagonalization study of bilayer clusters of t-J model.
Our results indicate a crossover between two markedly different regimes which
occurs when the ratio J_perp/J between inter-layer and intra-layer exchange
constants increases: for small J_perp/J the data suggest the development of 3D
antiferromagnetic correlations without appreciable degradation of the
intra-layer spin order and the d_(x2-y2) hole pairs within the planes persist.
For larger values of J_perp/J local singlets along the inter-layer bonds
dominate, leading to an almost complete suppression of the intra-layer spin
correlation and the breaking of the intra-layer pairs. The ground state with
two holes in this regime has s-like symmetry. The data suggest that the
crossover may occur for values of J_perp/J as small as 0.2. We present data for
static spin correlations, spin gap, and electron momentum distribution and
spectral function of the `inter-layer RVB state' realized for large J_perp/J.
The latter deviates from the single layer ground state, making it an
implausible candidate for modelling high-temperature superconductors.Comment: Revtex-file, 6 PRB pages, figures appended as uu-encoded postscript.
Hardcopies of figures (or the entire manuscript) can be obtained by e-mailing
to: [email protected]
An upper limit on the evolution of carbon monoxide from Comet Kohoutek
The rate of evolution of CO from Comet Kohoutex is presented. The observations were made approximately two months after perihelion. The rate of evolution of CO at that time was apparently less than that of CH3CN observed before perihelion
The Superconducting Instabilities of the non half-filled Hubbard Model in Two Dimensions
The problem of weakly correlated electrons on a square lattice is formulated
in terms of one-loop renormalization group. Starting from the action for the
entire Brillouin zone (and not with a low-energy effective action) we reduce
successively the cutoff about the Fermi surface and follow the
renormalization of the coupling as a function of three energy-momenta. We
calculate the intrinsic scale where the renormalization group flow
crosses over from the regime () where the electron-electron
(e-e) and electron-hole (e-h) terms are equally important to the regime
() where only the e-e term plays a role. In the low energy
regime only the pairing interaction is marginally relevant, containing
contributions from all renormalization group steps of the regime . After diagonalization of , we identify its most
attractive eigenvalue . At low filling,
corresponds to the representation ( symmetry), while near half
filling the strongest attraction occurs in the representation
( symmetry). In the direction of the van Hove singularities, the
order parameter shows peaks with increasing strength as one approaches half
filling. Using the form of pairing and the structure of the renormalization
group equations in the low energy regime, we give our interpretation of ARPES
experiments trying to determine the symmetry of the order parameter in the
Bi2212 high- compound.Comment: 24 pages (RevTeX) + 11 figures (the tex file appeared incomplete
Intrinsic Josephson Effect in the Layered Two-dimensional t-J Model
The intrinsic Josephson effect in the high-Tc superconductors is studied
using the layered two-dimensional t-J model. The d.c.Josephson current which
flows perpendicular to the t-J planes is obtained within the mean-field
approximation and the Gutzwiller approximation. We find that the Josephson
current has its maximum near the optimum doping region as a function of the
doping rate.Comment: 4 pages, 3 figure
Effect of spatial variations of superconducting gap on suppression of the transition temperature by impurities
We calculate correction to the critical temperature of a dirty
superconductor, which results from the local variations of the gap function
near impurity sites. This correction is of the order of T_c/E_F and becomes
important for short-coherence length superconductors. It generally reduces a
pair-breaking effect. In s-wave superconductors small amounts of nonmagnetic
impurities can increase the transition temperature.Comment: 5 pages, ReVTE
Gap Renormalization in Dirty Anisotropic Superconductors: Implications for the Order Parameter of the Cuprates
We contrast the effects of non-magnetic impurities on the properties of
superconductors having a \dw\ order parameter, and a highly anisotropic s-wave
(ASW) gap with the same nodal structure. The non-vanishing, impurity induced,
off-diagonal self-energy in the ASW state is shown to gap out the low energy
excitations present in the clean system, leading to a qualitatively different
impurity response of the single particle density of states compared to the \dw\
state. We discuss how this behaviour can be employed to distinguish one state
from the other by an analysis of high-resolution angle-resolved photoemission
spectra.Comment: 12 pages, uuencoded Postscrip
Josephson tunneling in high- superconductors
This article describes the Josephson tunneling from time-reversal
symmetry-breaking states and compares it with that from time-reversal invariant
states for both twinned and untwinned crystals and for both -axis and
basal-plane currents, in a model for orthorhombic YBCO. A macroscopic
invariance group describing the superconducting state of a twinned crystal is
introduced and shown to provide a useful framework for the discussion of the
results for twinned crystals. In addition, a ring geometry, which allows
-wave and -wave superconductivity in a tetragonal
superconductor to be distinguished on the basis of symmetry arguments only, is
proposed and analyzed. Finally, an appendix gives details of the experimental
Josephson tunneling evidence for a superconducting state of orthorhombic
symmetry in YBCO.Comment: Latex File, 18 pages, 6 Postscript figures, submitted to Phys. Rev.
Induction of non-d-wave order-parameter components by currents in d-wave superconductors
It is shown, within the framework of the Ginzburg-Landau theory for a
superconductor with d_{x^2-y^2} symmetry, that the passing of a supercurrent
through the sample results, in general, in the induction of order-parameter
components of distinct symmetry. The induction of s-wave and
d_{xy(x^2-y^2)-wave components are considered in detail. It is shown that in
both cases the order parameter remains gapless; however, the structure of the
lines of nodes and the lobes of the order parameter are modified in distinct
ways, and the magnitudes of these modifications differ in their dependence on
the (a-b plane) current direction. The magnitude of the induced s-wave
component is estimated using the results of the calculations of Ren et al.
[Phys. Rev. Lett. 74, 3680 (1995)], which are based on a microscopic approach.Comment: 15 pages, includes 2 figures. To appear in Phys. Rev.
Quasiparticle transport and localization in high-T_c superconductors
We present a theory of the effects of impurity scattering in d_{x^2-y^2}
superconductors and their quantum disordered counterparts, based on a
non-linear sigma model formulation. We show the existence, in a
quasi-two-dimensional system, of a novel spin-metal phase with a non-zero spin
diffusion constant at zero temperature. With decreasing inter-layer coupling,
the system undergoes a quantum phase transition (in a new universality class)
to a localized spin-insulator. Experimental implications for spin and thermal
transport in the high-temperature superconductors are discussed.Comment: 4 pages, 1 figur
Tilt Grain-Boundary Effects in S- and D-Wave Superconductors
We calculate the s- and d-wave superconductor order parameter in the vicinity
of a tilt grain boundary. We do this self-consistently within the Bogoliubov de
Gennes equations, using a realistic microscopic model of the grain boundary. We
present the first self-consistent calculations of supercurrent flows in such
boundaries, obtaining the current-phase characteristics of grain boundaries in
both s-wave and d-wave superconductors
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