38 research outputs found
Landau Transport equations in slave-boson mean-field theory of t-J model
In this paper we generalize slave-boson mean-field theory for model to
the time-dependent regime, and derive transport equations for model, both
in the normal and superconducting states. By eliminating the boson and
constraint fields exactly in the equations of motion we obtain a set of
transport equations for fermions which have the same form as Landau transport
equations for normal Fermi liquid and Fermi liquid superconductor, respectively
with all Landau parameters explicity given. Our theory can be viewed as a
refined version of U(1) Gauge theory where all lattice effects are retained and
strong correlation effects are reflected as strong Fermi-liquid interactions in
the transport equation. Some experimental consequences are discussed.Comment: 19 page
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
D-wave superconductivity in doped Mott insulators
The effect of proximity to a Mott insulating phase on the charge transport
properties of a superconductor is determined. An action describing the low
energy physics is formulated and different scenarios for the approach to the
Mott phase are distinguished by different variation with doping of the
parameters in the action. A crucial issue is found to be the doping dependence
of the quasiparticle charge which is defined here and which controls the
temperature and field dependence of the electromagnetic response functions.
Presently available data on high-T superconductors are analysed. The
data, while neither complete nor entirely consistent, suggest that neither the
quasiparticle velocity nor the quasiparticle charge vanish as the Mott phase is
approached, in contradiction to the predictions of several widely studied
theories of lightly doped Mott insulators. Implications of the results for the
structure of vortices in high-T superconductors are determined. The
numerical coefficients in the field-dependent specific heat are given for
square and triangular vortex lattices.Comment: 12 pages. No figures. Submitted to JPCS (Proceedings of Chicago SNS
conference
Big, Fast Vortices in the d-RVB theory of High Temperature Superconductivity
The effect of proximity to a Mott insulating phase on the superflow
properties of a d-wave superconductor is studied using the slave boson-U(1)
gauge theory model. The model has two limits corresponding to superconductivity
emerging either out of a 'renormalized fermi liquid' or out of a
non-fermi-liquid regime. Three crucial physical parameters are identified: the
size of the vortex \textit{as determined from the supercurrent it induces;} the
coupling of the superflow to the quasiparticles and the 'nondissipative time
derivative' term. As the Mott phase is approached, the core size as defined
from the supercurrent diverges, the coupling between superflow and
quasiparticles vanishes, and the magnitude of the nondissipative time
derivative dramatically increases. The dissipation due to a moving vortex is
found to vary as the third power of the doping. The upper critical field and
the size of the critical regime in which paraconductivity may be observed are
estimated, and found to be controlled by the supercurrent length scale
Effective Actions and Phase Fluctuations in d-wave Superconductors
We study effective actions for order parameter fluctuations at low
temperature in layered d-wave superconductors such as the cuprates. The order
parameter lives on the bonds of a square lattice and has two amplitude and two
phase modes associated with it. The low frequency spectral weights for
amplitude and relative phase fluctuations is determined and found to be
subdominant to quasiparticle contributions. The Goldstone phase mode and its
coupling to density fluctuations in charged systems is treated in a
gauge-invariant manner. The Gaussian phase action is used to study both the
-axis Josephson plasmon and the more conventional in-plane plasmon in the
cuprates. We go beyond the Gaussian theory by deriving a coarse-grained quantum
XY model, which incorporates important cutoff effects overlooked in previous
studies. A variational analysis of this effective model shows that in the
cuprates, quantum effects of phase fluctuations are important in reducing the
zero temperature superfluid stiffness, but thermal effects are small for .Comment: Some numerical estimates corrected and figures changed. to appear in
PRB, Sept.1 (2000
Flux Phase as a Dynamic Jahn-Teller Phase: Berryonic Matter in the Cuprates?
There is considerable evidence for some form of charge ordering on the
hole-doped stripes in the cuprates, mainly associated with the low-temperature
tetragonal phase, but with some evidence for either charge density waves or a
flux phase, which is a form of dynamic charge-density wave. These three states
form a pseudospin triplet, demonstrating a close connection with the E X e
dynamic Jahn-Teller effect, suggesting that the cuprates constitute a form of
Berryonic matter. This in turn suggests a new model for the dynamic Jahn-Teller
effect as a form of flux phase. A simple model of the Cu-O bond stretching
phonons allows an estimate of electron-phonon coupling for these modes,
explaining why the half breathing mode softens so much more than the full
oxygen breathing mode. The anomalous properties of provide a coupling
(correlated hopping) which acts to stabilize density wave phases.Comment: Major Revisions: includes comparisons with specific cuprate phonon
modes, 16 eps figures, revte
Carrier relaxation, pseudogap, and superconducting gap in high-Tc cuprates: A Raman scattering study
We describe results of electronic Raman-scattering experiments in differently
doped single crystals of Y-123 and Bi-2212. The comparison of AF insulating and
metallic samples suggests that at least the low-energy part of the spectra
originates predominantly from excitations of free carriers. We therefore
propose an analysis of the data in terms of a memory function approach.
Dynamical scattering rates and mass-enhancement factors for the carriers are
obtained. In B2g symmetry the Raman data compare well to the results obtained
from ordinary and optical transport. For underdoped materials the dc scattering
rates in B1g symmetry become temperature independent and considerably larger
than in B2g symmetry. This increasing anisotropy is accompanied by a loss of
spectral weight in B2g symmetry in the range between the superconducting
transition at Tc and a characteristic temperature T* of order room temperature
which compares well with the pseudogap temperature found in other experiments.
The energy range affected by the pseudogap is doping and temperature
independent. The integrated spectral loss is approximately 25% in underdoped
samples and becomes much weaker towards higher carrier concentration. In
underdoped samples, superconductivity related features in the spectra can be
observed only in B2g symmetry. The peak frequencies scale with Tc. We do not
find a direct relation between the pseudogap and the superconducting gap.Comment: RevTeX, 21 pages, 24 gif figures. For PostScript with embedded eps
figures, see http://www.wmi.badw-muenchen.de/~opel/k2.htm