1,278 research outputs found
Remnant Fermi Surfaces in Photoemission
Recent experiments have introduced a new concept for analyzing the
photoemission spectra of correlated electrons -- the remnant Fermi surface
(rFs), which can be measured even in systems which lack a conventional Fermi
surface. Here, we analyze the rFs in a number of interacting electron models,
and find that the results fall into two classes. For systems with pairing
instabilities, the rFs is an accurate replica of the true Fermi surface. In the
presence of nesting instabilities, the rFs is a map of the resulting
superlattice Brillouin zone. The results suggest that the gap in Ca_2CuO_2Cl_2
is of nesting origin.Comment: 4 pages LaTex, 3 ps figure
A competing order scenario of two-gap behavior in hole doped cuprates
Angle-dependent studies of the gap function provide evidence for the
coexistence of two distinct gaps in hole doped cuprates, where the gap near the
nodal direction scales with the superconducting transition temperature ,
while that in the antinodal direction scales with the pseudogap temperature. We
present model calculations which show that most of the characteristic features
observed in the recent angle-resolved photoemission spectroscopy (ARPES) as
well as scanning tunneling microscopy (STM) two-gap studies are consistent with
a scenario in which the pseudogap has a non-superconducting origin in a
competing phase. Our analysis indicates that, near optimal doping,
superconductivity can quench the competing order at low temperatures, and that
some of the key differences observed between the STM and ARPES results can give
insight into the superlattice symmetry of the competing order.Comment: 9 pages, 7 fig
Theory of non-Fermi liquid and pairing in electron-doped cuprates
We apply the spin-fermion model to study the normal state and pairing
instability in electron-doped cuprates near the antiferromagnetic QCP. Peculiar
frequency dependencies of the normal state properties are shown to emerge from
the self-consistent equations on the fermionic and bosonic self-energies, and
are in agreement with experimentally observed ones. We argue that the pairing
instability is in the channel, as in hole-doped cuprates, but
theoretical is much lower than in the hole-doped case. For the same
hopping integrals and the interaction strength as in hole-doped materials, we
obtain K at the end point of the antiferromagnetic phase. We argue
that a strong reduction of in electron-doped cuprates compared to
hole-doped ones is due to critical role of the Fermi surface curvature for
electron-doped materials. The -pairing gap
is strongly non-monotonic along the Fermi surface.
The position of the gap maxima, however, does not coincide with the hot spots,
as the non-monotonic gap persists even at doping when the hot
spots merge on the Brillouin zone diagonals.Comment: 16 page
Pinned Balseiro-Falicov Model of Tunneling and Photoemission in the Cuprates
The smooth evolution of the tunneling gap of Bi_2Sr_2CaCu_2O_8 with doping
from a pseudogap state in the underdoped cuprates to a superconducting state at
optimal and overdoping, has been interpreted as evidence that the pseudogap
must be due to precursor pairing. We suggest an alternative explanation, that
the smoothness reflects a hidden SO(N) symmetry near the (pi,0) points of the
Brillouin zone (with N = 3, 4, 5, or 6). Because of this symmetry, the
pseudogap could actually be due to any of a number of nesting instabilities,
including charge or spin density waves or more exotic phases. We present a
detailed analysis of this competition for one particular model: the pinned
Balseiro-Falicov model of competing charge density wave and (s-wave)
superconductivity. We show that most of the anomalous features of both
tunneling and photoemission follow naturally from the model, including the
smooth crossover, the general shape of the pseudogap phase diagram, the
shrinking Fermi surface of the pseudogap phase, and the asymmetry of the
tunneling gap away from optimal doping. Below T_c, the sharp peak at Delta_1
and the dip seen in the tunneling and photoemission near 2Delta_1 cannot be
described in detail by this model, but we suggest a simple generalization to
account for inhomogeneity, which does provide an adequate description. We show
that it should be possible, with a combination of photoemission and tunneling,
to demonstrate the extent of pinning of the Fermi level to the Van Hove
singularity. A preliminary analysis of the data suggests pinning in the
underdoped, but not in the overdoped regime.Comment: 18 pages LaTeX, 26 ps. figure
Renormalization of f-levels away from the Fermi energy in electron excitation spectroscopies: Density functional results of NdCeCuO
Relaxation energies for photoemission, when an occupied electronic state is
excited, and for inverse photoemission, when an empty state is filled, are
calculated within the density functional theory with application to
NdCeCuO. The associated relaxation energies are obtained by
computing differences in total energies between the ground state and an excited
state in which one hole or electron is added into the system. The relaxation
energies of f-electrons are found to be of the order of several eV's,
indicating that f-bands will appear substantially away from the Fermi energy
() in their spectroscopic images, even if these bands lie near . Our
analysis explains why it would be difficult to observe f electrons at the
even in the absence of strong electronic correlations.Comment: 6 pages, 1 figure, 1 tabl
Stripes, Pseudogaps, and Van Hove Nesting in the Three-band tJ Model
Slave boson calculations have been carried out in the three-band tJ model for
the high-T_c cuprates, with the inclusion of coupling to oxygen breathing mode
phonons. Phonon-induced Van Hove nesting leads to a phase separation between a
hole-doped domain and a (magnetic) domain near half filling, with long-range
Coulomb forces limiting the separation to a nanoscopic scale. Strong
correlation effects pin the Fermi level close to, but not precisely at the Van
Hove singularity (VHS), which can enhance the tendency to phase separation. The
resulting dispersions have been calculated, both in the uniform phases and in
the phase separated regime. In the latter case, distinctly different
dispersions are found for large, random domains and for regular (static)
striped arrays, and a hypothetical form is presented for dynamic striped
arrays. The doping dependence of the latter is found to provide an excellent
description of photoemission and thermodynamic experiments on pseudogap
formation in underdoped cuprates. In particular, the multiplicity of observed
gaps is explained as a combination of flux phase plus charge density wave (CDW)
gaps along with a superconducting gap. The largest gap is associated with VHS
nesting. The apparent smooth evolution of this gap with doping masks a
crossover from CDW-like effects near optimal doping to magnetic effects (flux
phase) near half filling. A crossover from large Fermi surface to hole pockets
with increased underdoping is found. In the weakly overdoped regime, the CDW
undergoes a quantum phase transition (), which could be obscured
by phase separation.Comment: 15 pages, Latex, 18 PS figures Corrects a sign error: major changes,
esp. in Sect. 3, Figs 1-4,6 replace
Phase Separation Models for Cuprate Stripe Arrays
An electronic phase separation model provides a natural explanation for a
large variety of experimental results in the cuprates, including evidence for
both stripes and larger domains, and a termination of the phase separation in
the slightly overdoped regime, when the average hole density equals that on the
charged stripes. Several models are presented for charged stripes, showing how
density waves, superconductivity, and strong correlations compete with quantum
size effects (QSEs) in narrow stripes. The energy bands associated with the
charged stripes develop in the middle of the Mott gap, and the splitting of
these bands can be understood by considering the QSE on a single ladder.Comment: significant revisions: includes island phase, 16 eps figures, revte
Systematic Low-Energy Effective Field Theory for Electron-Doped Antiferromagnets
In contrast to hole-doped systems which have hole pockets centered at , in lightly electron-doped antiferromagnets
the charged quasiparticles reside in momentum space pockets centered at
or . This has important consequences for
the corresponding low-energy effective field theory of magnons and electrons
which is constructed in this paper. In particular, in contrast to the
hole-doped case, the magnon-mediated forces between two electrons depend on the
total momentum of the pair. For the one-magnon exchange
potential between two electrons at distance is proportional to ,
while in the hole case it has a dependence. The effective theory
predicts that spiral phases are absent in electron-doped antiferromagnets.Comment: 25 pages, 7 figure
Anisotropic softening of collective charge modes in the vicinity of critical doping in a doped Mott insulator
Momentum resolved inelastic resonant x-ray scattering is used to map the
evolution of charge excitations over a large range of energies, momenta and
doping levels in the electron doped Mott insulator class
NdCeCuO. As the doping induced AFM-SC
(antiferromagnetic-superconducting) transition is approached, we observe an
anisotropic softening of collective charge modes over a large energy scale
along the Gamma to (\pi,\pi)-direction, whereas the modes exhibit broadening
( 1 eV) with relatively little softening along Gamma to (\pi,0) with
respect to the parent Mott insulator (x=0). Our study indicates a systematic
collapse of the gap consistent with the scenario that the system dopes
uniformly with electrons even though the softening of the modes involves an
unusually large energy scale.Comment: 5 pages + 5 Figure
Dispersion of Ordered Stripe Phases in the Cuprates
A phase separation model is presented for the stripe phase of the cuprates,
which allows the doping dependence of the photoemission spectra to be
calculated. The idealized limit of a well-ordered array of magnetic and charged
stripes is analyzed, including effects of long-range Coulomb repulsion.
Remarkably, down to the limit of two-cell wide stripes, the dispersion can be
interpreted as essentially a superposition of the two end-phase dispersions,
with superposed minigaps associated with the lattice periodicity. The largest
minigap falls near the Fermi level; it can be enhanced by proximity to a (bulk)
Van Hove singularity. The calculated spectra are dominated by two features --
this charge stripe minigap plus the magnetic stripe Hubbard gap. There is a
strong correlation between these two features and the experimental
photoemission results of a two-peak dispersion in LaSrCuO, and
the peak-dip-hump spectra in BiSrCaCuO. The
differences are suggestive of the role of increasing stripe fluctuations. The
1/8 anomaly is associated with a quantum critical point, here expressed as a
percolation-like crossover. A model is proposed for the limiting minority
magnetic phase as an isolated two-leg ladder.Comment: 24 pages, 26 PS figure
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