796 research outputs found
The coherent {\it d}-wave superconducting gap in underdoped LaSrCuO as studied by angle-resolved photoemission
We present angle-resolved photoemission spectroscopy (ARPES) data on
moderately underdoped LaSrCuO at temperatures below and
above the superconducting transition temperature. Unlike previous studies of
this material, we observe sharp spectral peaks along the entire underlying
Fermi surface in the superconducting state. These peaks trace out an energy gap
that follows a simple {\it d}-wave form, with a maximum superconducting gap of
14 meV. Our results are consistent with a single gap picture for the cuprates.
Furthermore our data on the even more underdoped sample
LaSrCuO also show sharp spectral peaks, even at the
antinode, with a maximum superconducting gap of 26 meV.Comment: Accepted by Phys. Rev. Let
Hot Spots on the Fermi Surface of Bi2212: Stripes versus Superstructure
In a recent paper Saini et al. have reported evidence for a pseudogap around
(pi,0) at room temperature in the optimally doped superconductor Bi2212. This
result is in contradiction with previous ARPES measurements. Furthermore they
observed at certain points on the Fermi surface hot spots of high spectral
intensity which they relate to the existence of stripes in the CuO planes. They
also claim to have identified a new electronic band along Gamma-M1 whose one
dimensional character provides further evidence for stripes. We demonstrate in
this Comment that all the measured features can be simply understood by
correctly considering the superstructure (umklapp) and shadow bands which occur
in Bi2212.Comment: 1 page, revtex, 1 encapsulated postscript figure (color
Electronic structure of strongly correlated d-wave superconductors
We study the electronic structure of a strongly correlated d-wave
superconducting state. Combining a renormalized mean field theory with direct
calculation of matrix elements, we obtain explicit analytical results for the
nodal Fermi velocity, v_F, the Fermi wave vector, k_F, and the momentum
distribution, n_k, as a function of hole doping in a Gutzwiller projected
d-wave superconductor. We calculate the energy dispersion, E_k, and spectral
weight of the Gutzwiller-Bogoliubov quasiparticles, and find that the spectral
weight associated with the quasiparticle excitation at the antinodal point
shows a non monotonic behavior as a function of doping. Results are compared to
angle resolved photoemission spectroscopy (ARPES) of the high temperature
superconductors.Comment: final version, comparison to experiments added, 4+ pages, 4 figure
Doped carrier formulation of the t-J model: the projection constraint and the effective Kondo-Heisenberg lattice representation
We show that the recently proposed doped carrier Hamiltonian formulation of
the t-J model should be complemented with the constraint that projects out the
unphysical states. With this new important ingredient, the previously used and
seemingly different spin-fermion representations of the t-J model are shown to
be gauge related to each other. This new constraint can be treated in a
controlled way close to half-filling suggesting that the doped carrier
representation provides an appropriate theoretical framework to address the t-J
model in this region. This constraint also suggests that the t-J model can be
mapped onto a Kondo-Heisenberg lattice model. Such a mapping highlights
important physical similarities between the quasi two-dimensional heavy
fermions and the high-T superconductors. Finally we discuss the physical
implications of our model representation relating in particular the small
versus large Fermi surface crossover to the closure of the lattice spin gap.Comment: corrected and enlarged versio
High Tc Superconductors -- A Variational Theory of the Superconducting State
We use a variational approach to gain insight into the strongly correlated
d-wave superconducting state of the high Tc cuprates at T=0. We show that
strong correlations lead to qualitatively different trends in pairing and phase
coherence: the pairing scale decreases monotonically with hole doping while the
SC order parameter shows a non-monotonic dome. We obtain detailed results for
the doping-dependence of a large number of experimentally observable
quantities, including the chemical potential, coherence length, momentum
distribution, nodal quasiparticle weight and dispersion, incoherent features in
photoemission spectra, optical spectral weight and superfluid density. Most of
our results are in remarkable quantitative agreement with existing data and
some of our predictions, first reported in Phys. Rev. Lett. {\bf 87}, 217002
(2001), have been recently verified.Comment: (Minor revisions, 1 figure added, version to appear in PRB) 23 RevTeX
pages, 11 eps figs, long version of cond-mat/0101121, contains detailed
comparisons with experiments, analytical insights, technical aspects of the
calculation, and comparison with slave boson MF
Evolution of the Fermi surface with carrier concentration in Bi_2Sr_2CaCu_2O_{8+\delta}
We show, by use of angle-resolved photoemission spectroscopy, that underdoped
Bi_2Sr_2CaCu_2O_{8+\delta} appears to have a large Fermi surface centered at
(\pi,\pi), even for samples with a T_c as low as 15 K. No clear evidence of a
Fermi surface pocket around (\pi/2,\pi/2) has been found. These conclusions are
based on a determination of the minimum gap locus in the pseudogap regime T_c <
T < T^*, which is found to coincide with the locus of gapless excitations in
momentum space (Fermi surface) determined above T^*. These results suggest that
the pseudogap is more likely of precursor pairing rather than magnetic origin.Comment: 4 pages, revtex, 4 postscript color figure
Photoelectron Escape Depth and Inelastic Secondaries in High Temperature Superconductors
We calculate the photoelectron escape depth in the high temperature
superconductor Bi2212 by use of electron energy-loss spectroscopy data. We find
that the escape depth is only 3 Ang. for photon energies typically used in
angle resolved photoemission measurements. We then use this to estimate the
number of inelastic secondaries, and find this to be quite small near the Fermi
energy. This implies that the large background seen near the Fermi energy in
photoemission measurements is of some other origin.Comment: 2 pages, revtex, 3 encapsulated postscript figure
Bulk and surface low-energy excitations in YBa2Cu3O7-d studied by high-resolution angle-resolved photoemission spectroscopy
We have performed high-resolution angle-resolved photoemission spectroscopy
on YBa2Cu3O7-delta (Y123; delta = 0.06; Tc = 92 K). By accurately determining
the Fermi surface and energy band dispersion, we solve long-standing
controversial issues as to the anomalous electronic states of Y-based high-Tc
cuprates. We unambiguously identified surface-bilayer-derived bonding and
antibonding bands, together with their bulk counterparts. The surface bands are
highly overdoped (hole concentration x = 0.29), showing no evidence for the gap
opening or the dispersion anomaly in the antinodal region, while the bulk bands
show a clear dx2-y2-wave superconducting gap and the Bogoliubov
quasiparticle-like behavior with a characteristic energy scale of 50-60 meV
indicative of a strong electron-boson coupling in the superconducting state.
All these results suggest that the metallic and superconducting states coexist
at the adjacent bilayer of Y123 surface.Comment: Accepted for publication in Phys. Rev.
Tachyon warm inflationary universe models
Warm inflationary universe models in a tachyon field theory are studied.
General conditions required for these models to be realizable are derived and
discussed. We describe scalar perturbations (in the longitudinal gauge) and
tensor perturbations for these scenarios. We develop our models for a constant
dissipation parameter in one case and one dependent on in the
other case. We have been successful in describing such of inflationary universe
models. We use recent astronomical observations for constraining the parameters
appearing in our model. Also, our results are compared with their analogous
found in the cool inflationary case.Comment: 21 pages, Accepted by JCA
- …