1,180 research outputs found
Photoemission Evidence for a Remnant Fermi Surface and d-Wave-Like Dispersion in Insulating Ca2CuO2Cl2
An angle resolved photoemission study on Ca2CuO2Cl2, a parent compound of
high Tc superconductors is reported. Analysis of the electron occupation
probability, n(k) from the spectra shows a steep drop in spectral intensity
across a contour that is close to the Fermi surface predicted by the band
calculation. This analysis reveals a Fermi surface remnant even though
Ca2CuO2Cl2 is a Mott insulator. The lowest energy peak exhibits a dispersion
with approximately the |cos(kxa)-cos(kya)| form along this remnant Fermi
surface. Together with the data from Dy doped Bi2Sr2CaCu2O(8 + delta) these
results suggest that this d-wave like dispersion of the insulator is the
underlying reason for the pseudo gap in the underdoped regime.Comment: 9 pages, including 7 figures. Published in Science, one figure
correcte
Theory of Electron Differentiation, Flat Dispersion and Pseudogap Phenomena
Aspects of electron critical differentiation are clarified in the proximity
of the Mott insulator. The flattening of the quasiparticle dispersion appears
around momenta and on square lattices and determines the
criticality of the metal-insulator transition with the suppressed coherence in
that momentum region of quasiparticles. Such coherence suppression at the same
time causes an instability to the superconducting state if a proper incoherent
process is retained. The d-wave pairing interaction is generated from such
retained processes without disturbance from the coherent single-particle
excitations. Pseudogap phenomena widely observed in the underdoped cuprates are
then naturally understood from the mode-mode coupling of d-wave
superconducting(dSC) fluctuations with antiferromagnetic ones. When we assume
the existence of a strong d-wave pairing force repulsively competing with
antiferromagnetic(AFM) fluctuations under the formation of flat and damped
single-particle dispersion, we reproduce basic properties of the pseudogap seen
in the magnetic resonance, neutron scattering, angle resolved photoemission and
tunneling measurements in the cuprates.Comment: 9 pages including 2 figures, to appear in J. Phys. Chem. Solid
Pseudogap and photoemission spectra in the attractive Hubbard model
Angle-resolved photoemission spectra are calculated microscopically for the
two-dimensional attractive Hubbard model. A system of self-consistent T-matrix
equations are solved numerically in the real-time domain. The single-particle
spectral function has a two-peak structure resulting from the presense of bound
states. The spectral function is suppressed at the chemical potential, leading
to a pseudogap-like behavior. At high temperatures and densities the pseudogap
diminishes and finally disappears; these findings are similar to experimental
observations for the cuprates.Comment: 5 pages, 4 figures, published versio
Abrupt Onset of Second Energy Gap at Superconducting Transition of Underdoped Bi2212
The superconducting gap - an energy scale tied to the superconducting
phenomena-opens on the Fermi surface at the superconducting transition
temperature (TC) in conventional BCS superconductors. Quite differently, in
underdoped high-TC superconducting cuprates, a pseudogap, whose relation to the
superconducting gap remains a mystery, develops well above TC. Whether the
pseudogap is a distinct phenomenon or the incoherent continuation of the
superconducting gap above TC is one of the central questions in high-TC
research. While some experimental evidence suggests they are distinct, this
issue is still under intense debate. A crucial piece of evidence to firmly
establish this two-gap picture is still missing: a direct and unambiguous
observation of a single-particle gap tied to the superconducting transition as
function of temperature. Here we report the discovery of such an energy gap in
underdoped Bi2212 in the momentum space region overlooked in previous
measurements. Near the diagonal of Cu-O bond direction (nodal direction), we
found a gap which opens at TC and exhibits a canonical (BCS-like) temperature
dependence accompanied by the appearance of the so-called Bogoliubov
quasiparticles, a classical signature of superconductivity. This is in sharp
contrast to the pseudogap near the Cu-O bond direction (antinodal region)
measured in earlier experiments. The emerging two-gap phenomenon points to a
picture of richer quantum configurations in high temperature superconductors.Comment: 16 pages, 4 figures, authors' version Corrected typos in the abstrac
Landau mapping and Fermi liquid parameters of the 2D t-J model
We study the momentum distribution function n(k) in the 2D t-J model on small
clusters by exact diagonalization. We show that n(k) can be decomposed
systematically into two components with Bosonic and Fermionic doping
dependence. The Bosonic component originates from the incoherent motion of
holes and has no significance for the low energy physics. For the Fermionic
component we exlicitely perform the one-to-one Landau mapping between the low
lying eigenstates of the t-J model clusters and those of an equivalent system
of spin-1/2 quasiparticles. This mapping allows to extract the quasiparticle
dispersion, statistics, and Landau parameters. The results show conclusively
that the 2D t-J model for small doping is a Fermi liquid with a `small' Fermi
surface and a moderately strong attractive interaction between the
quasiparticles.Comment: Revtex file, 5 pages with 5 embedded eps-files, hardcopies of figures
(or the entire manuscript) can be obtained by e-mail request to:
[email protected]
High-Resolution Photoemission Study of MgB2
We have performed high-resolution photoemission spectroscopy on MgB2 and
observed opening of a superconducting gap with a narrow coherent peak. We found
that the superconducting gap is s-like with the gap value of 4.5 meV at 15 K.
The temperature dependence (15 - 40 K) of gap value follows well the BCS form,
suggesting that 2Delta/kBTc at T=0 is about 3. No pseudogap behavior is
observed in the normal state. The present results strongly suggest that MgB2 is
categorized into a phonon-mediated BCS superconductor in the weak-coupling
regime.Comment: 3 pages, 3 figures, accepted in Physical Review Letter
Detecting fractions of electrons in the high- cuprates
We propose several tests of the idea that the electron is fractionalized in
the underdoped and undoped cuprates. These include the ac Josephson effect, and
tunneling into small superconducting grains in the Coulomb blockade regime. In
both cases, we argue that the results are qualitatively modified from the
conventional ones if the insulating tunnel barrier is fractionalized. These
experiments directly detect the possible existence of the chargon - a charge
spinless boson - in the insulator. The effects described in this paper
provide a means to probing whether the undoped cuprate (despite it's magnetism)
is fractionalized. Thus, the experiments discussed here are complementary to
the flux-trapping experiment we proposed in our earlier work(cond-mat/0006481).Comment: 7 pages, 5 figure
Transport properties in the d-density wave state: Wiedemann-Franz law
We study the Wiedemann-Franz (WF) law in the d-density wave (DDW) model. Even
though the opening of the DDW gap profoundly modifies the electronic
density of states and makes it dependent on energy, the value of the WF ratio
at zero temperature (T=0) remains unchanged. However, neither electrical nor
thermal conductivity display universal behavior. For finite temperature, with T
greater than the value of the impurity scattering rate at zero frequency
i.e. , the usual WF ratio is obtained only in
the weak scattering limit. For strong scattering there are large violations of
the WF law.Comment: 1 figur
A Theory of the Pseudogap State of the Cuprates
The phase diagram for a general model for Cuprates is derived in a mean-field
approximation. A phase violating time-reversal without breaking translational
symmetry is possible when both the ionic interactions and the local repulsions
are large compared to the energy difference between the Cu and O
single-particle levels. It ends at a quantum critical point as the hole or
electron doping is increased. Such a phase is necessarily accompanied by
singular forward scattering such that, in the stable phase, the density of
states at the chemical potential, projected to a particular point group
symmetry of the lattice is zero producing thereby an anisotropic gap in the
single-particle spectrum. It is suggested that this phase occupies the
"pseudogap" region of the phase diagram of the cuprates. The temperature
dependence of the single-particle spectra, the density of states, the specific
heat and the magnetic susceptibility are calculated with rather remarkable
correspondence with the experimental results. The importance of further direct
experimental verification of such a phase in resolving the principal issues in
the theory of the Cuprate phenomena is pointed out. To this end, some
predictions are provided.Comment: 41 pages, 8 figure
Intrinsic tunneling spectra of Bi_2(Sr_{2-x}La_x)CuO_6
We have measured intrinsic-tunneling spectra of a single CuO-layer La-doped
Bi_2Sr_{2-x}La_xCuO_{6+\delta} (Bi2201-La_x). Despite a difference of a factor
of three in the optimal superconducting critical temperatures for
Bi2201-La_{0.4} and Bi2212 (32 and 95 K, respectively) and different spectral
energy scales, we find that the pseudogap vanishes at a similar characteristic
temperature T*\approx 230-300K for both compounds. We find also that in
Bi2201-La_x, PG humps are seen as sharp peaks and, in fact, even dominate the
intrinsic spectra.Comment: Submitted to Phys. Rev. Let
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