1,330 research outputs found
Andreev scattering in the asymmetric ladder with preformed bosonic pairs
We discuss the phase coherence which emanates from the ladder-like proximity
effect between a ``weak superconductor'' with preformed bosonic pairs (here, a
single-chain Luther-Emery liquid with superconducting correlations that decay
approximately as ) and a Fermi gas with unpaired fermions. Carefully
studying tunneling mechanism(s), we show that the boson-mediated Cooper pairing
between remaining unpaired electrons results in a quasi long-range
superconductivity: Superconducting correlations decay very slowly as
with . This process is reminiscent of the coupling
of fermions to preformed bosonic pairs introduced in the context of high-Tc
cuprates.Comment: 5 pages, final version (To appear in PRB Rapid Communication
Proximity to a Nearly Superconducting Quantum Critical Liquid
The coupling between superconductors and a quantum critical liquid that is
nearly superconducting provides natural interpretation for the Josephson effect
over unexpectedly long junctions, and the remarkable stripe-spacing dependence
of the critical temperature in LSCO and YBCO superconductors.Comment: four two-column pages, no figure
Application of the scattering rate sum-rule to the interplane optical conductivity of high temperature superconductors: pseudogap and bi-layer effects
We use a recently proposed model of the interplane conductivity of high
temperature superconductors to investigate the `scattering rate sum-rule'
introduced by Basov and co-workers. We present a new derivation of the
sum-rule. The quantal and thermal fluctuations of the order parameter which
have been argued to produce the observed pseudogap behavior are shown to
increase the total integrated `scattering rate' but may either increase or
decrease the `quasiparticle' contribution from frequencies greater than twice
the superconducting gap.Comment: 4 pages, 5 figures, revise
Low energy collective modes, Ginzburg-Landau theory, and pseudogap behavior in superconductors with long-range pairing interactions
We study the superconducting instability in systems with long but finite
ranged, attractive, pairing interactions. We show that such long-ranged
superconductors exhibit a new class of fluctuations in which the internal
structure of the Cooper pair wave function is soft, and thus lead to
"pseudogap" behavior in which the actual transition temperature is greatly
depressed from its mean field value. These fluctuations are {\it not} phase
fluctuations of the standard superconducting order parameter, and lead to a
highly unusual Ginzburg-Landau description. We suggest that the crossover
between the BCS limit of a short-ranged attraction and our problem is of
interest in the context of superconductivity in the underdoped cuprates.Comment: 20 pages with one embedded ps figure. Minor revisions to the text and
references. Final version to appear in PRB on Nov. 1st, 200
The types of Mott insulator
There are two classes of Mott insulators in nature, distinguished by their
responses to weak doping. With increasing chemical potential, Type I Mott
insulators undergo a first order phase transition from the undoped to the doped
phase. In the presence of long-range Coulomb interactions, this leads to an
inhomogeneous state exhibiting ``micro-phase separation.'' In contrast, in Type
II Mott insulators charges go in continuously above a critical chemical
potential. We show that if the insulating state has a broken symmetry, this
increases the likelihood that it will be Type I. There exists a close analogy
between these two types of Mott insulators and the familiar Type I and Type II
superconductors
Ground state of graphite ribbons with zigzag edges
We study the interaction effects on the ground state of nanographite ribbons
with zigzag edges. Within the mean-field approximation, we found that there are
two possible phases: the superconducting (SC) phase and the excitonic insulator
(EI). The two phases are separated by a first-order transition point. After
taking into account the low-lying fluctuations around the mean-field solutions,
the SC phase becomes a spin liquid phase with one gapless charge mode.
On the other hand, all excitations in the EI phase, especially the spin
excitations, are gapped.Comment: 6 pages, 3 figure
Direct Observation of a One Dimensional Static Spin Modulation in Insulating La1.95Sr0.05CuO4
We report the results of an extensive elastic neutron scattering study of the
incommensurate (IC) static spin correlations in La1.95Sr0.05CuO4 which is an
insulating spin glass at low temperatures. The present neutron scattering
experiments on the same x=0.05 crystal employ a narrower instrumental
Q-resolution and thereby have revealed that the crystal has only two
orthorhombic twins at low temperatures with relative populations of 2:1. We
find that, in a single twin, only two satellites are observed at (1, +/-0.064,
L)(ortho) and (0, 1+/-0.064, L)(ortho), that is, the modulation vector is only
along the orthorhombic b*-axis. This demonstrates unambiguously that
La1.95Sr0.05CuO4 has a one-dimensional static diagonal spin modulation at low
temperatures, consistent with certain stripe models. We have also reexamined
the x=0.04 crystal that previously was reported to show a single commensurate
peak. By mounting the sample in the (H, K, 0) zone, we have discovered that the
x=0.04 sample in fact has the same IC structure as the sample. The
incommensurability parameter d for x=0.04 and 0.05, where d is the distance
from (1/2, 1/2) in tetragonal reciprocal lattice units, follows the linear
relation d=x. These results demonstrate that the insulator to superconductor
transition in the under doped regime (0.05 </= x </= 0.06) in La2-xSrxCuO4 is
coincident with a transition from diagonal to collinear static stripes at low
temperatures thereby evincing the intimate coupling between the one dimensional
spin density modulation and the superconductivity.Comment: 9 pages 8 figure
Fermi Edge Singularities and Backscattering in a Weakly Interacting 1D Electron Gas
The photon-absorption edge in a weakly interacting one-dimensional electron
gas is studied, treating backscattering of conduction electrons from the core
hole exactly. Close to threshold, there is a power-law singularity in the
absorption, , with where is the forward scattering
phase shift of the core hole. In contrast to previous theories, is
finite (and universal) in the limit of weak core hole potential. In the case of
weak backscattering , the exponent in the power-law dependence of
absorption on energy crosses over to a value above an energy scale , where is a dimensionless measure of the
electron-electron interactions.Comment: 8 pages + 1 postscript figure, preprint TPI-MINN-93/40-
The Influence of Quantum Critical Fluctuations of Circulating Current Order Parameters on the Normal State Properties of Cuprates
We study a model of the quantum critical point of cuprates associated with
the "circulating current" order parameter proposed by Varma. An effective
action of the order parameter in the quantum disordered phase is derived using
functional integral method, and the physical properties of the normal state are
studied based on the action. The results derived within the ladder
approximation indicate that the system is like Fermi liquid near the quantum
critical point and in disordered regime up to minor corrections. This implies
that the suggested marginal Fermi liquid behavior induced by the circulating
current fluctuations will come in from beyond the ladder diagrams.Comment: 7pages, 1 figure included in RevTex file. To appear in Phys. Rev.
Staggered Currents in the Vortex Core
We study the electronic structure of the vortex core in the cuprates using
the U(1) slave-boson mean-field wavefunctions and their Gutzwiller projection.
We conclude that there exists local orbital antiferromagnetic order in the core
near optimal doping. We compare the results with that of BCS theory and analyze
the spatial dependence of the local tunneling density of states.Comment: 4 pages, 3 figure
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