1,057 research outputs found
Fermi pockets and correlation effects in underdoped YBa2Cu3O6.5
The detection of quantum oscillations in the electrical resistivity of
YBa2Cu3O6.5 provides direct evidence for the existence of Fermi surface pockets
in an underdoped cuprate. We present a theoretical study of the electronic
structure of YBa2Cu3O7-d (YBCO) aiming at establishing the nature of these
Fermi pockets, i.e. CuO2 plane versus CuO chain or BaO. We argue that electron
correlation effects, such as orbital-dependent band distortions and highly
anisotropic self-energy corrections, must be taken into account in order to
properly interpret the quantum oscillation experiments.Comment: A high-resolution version can be found at
http://www.physics.ubc.ca/~quantmat/ARPES/PUBLICATIONS/Articles/YBCO_OrthoII_LDA.pd
A spectral function tour of electron-phonon coupling outside the Migdal limit
We simulate spectral functions for electron-phonon coupling in a filled band
system - far from the asymptotic limit often assumed where the phonon energy is
very small compared to the Fermi energy in a parabolic band and the Migdal
theorem predicting 1+lambda quasiparticle renormalizations is valid. These
spectral functions are examined over a wide range of parameter space through
techniques often used in angle-resolved photoemission spectroscopy (ARPES).
Analyzing over 1200 simulations we consider variations of the microscopic
coupling strength, phonon energy and dimensionality for two models: a
momentum-independent Holstein model, and momentum-dependent coupling to a
breathing mode phonon. In this limit we find that any `effective coupling',
lambda_eff, inferred from the quasiparticle renormalizations differs from the
microscopic dimensionless coupling characterizing these Hamiltonians, lambda,
and could drastically either over- or under-estimate it depending on the
particular parameters and model. In contrast, we show that perturbation theory
retains good predictive power for low coupling and small momenta, and that the
momentum-dependence of the self-energy can be revealed via the relationship
between velocity renormalization and quasiparticle strength. Additionally we
find that (although not strictly valid) it is often possible to infer the
self-energy and bare electronic structure through a self-consistent
Kramers-Kronig bare-band fitting; and also that through lineshape alone, when
Lorentzian, it is possible to reliably extract the shape of the imaginary part
of a momentum-dependent self-energy without reference to the bare-band.Comment: 15 pages, 11 figures. High resolution available here:
http://www.physics.ubc.ca/~quantmat/ARPES/PUBLICATIONS/Articles/sf_tour.pd
Tunneling spectra of strongly coupled superconductors: Role of dimensionality
We investigate numerically the signatures of collective modes in the
tunneling spectra of superconductors. The larger strength of the signatures
observed in the high-Tc superconductors, as compared to classical low-Tc
materials, is explained by the low dimensionality of these layered compounds.
We also show that the strong-coupling structures are dips (zeros in the d2I/dV2
spectrum) in d-wave superconductors, rather than the steps (peaks in d2I/dV2)
observed in classical s-wave superconductors. Finally we question the
usefulness of effective density of states models for the analysis of tunneling
data in d-wave superconductors.Comment: 8 pages, 6 figure
Evidence for pairing above Tc from the dispersion in the pseudogap phase of cuprates
In the underdoped high temperature superconductors, instead of a complete
Fermi surface above Tc, only disconnected Fermi arcs appear, separated by
regions that still exhibit an energy gap. We show that in this pseudogap phase,
the energy-momentum relation of electronic excitations near E_F behaves like
the dispersion of a normal metal on the Fermi arcs, but like that of a
superconductor in the gapped regions. We argue that this dichotomy in the
dispersion is hard to reconcile with a competing order parameter, but is
consistent with pairing without condensation
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
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
The Fermi surface of Bi2Sr2CaCu2O8
We study the Fermi surface of Bi2Sr2CaCu2O8 (Bi2212) using angle resolved
photoemission (ARPES) with a momentum resolution of ~ 0.01 of the Brillouin
zone. We show that, contrary to recent suggestions, the Fermi surface is a
large hole barrel centered at (pi,pi), independent of the incident photon
energy.Comment: 4 pages (revtex), 4 figures (eps, 2 color
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