770 research outputs found
Cluster Dynamical Mean Field analysis of the Mott transition
We investigate the Mott transition using a cluster extension of dynamical
mean field theory (DMFT). In the absence of frustration we find no evidence for
a finite temperature Mott transition. Instead, in a frustrated model, we
observe signatures of a finite temperature Mott critical point in agreement
with experimental studies of kappa-organics and with single site DMFT. As the
Mott transition is approached, a clear momentum dependence of the electron
lifetime develops on the Fermi surface with the formation of cold regions along
the diagonal direction of the Brillouin zone. Furthermore the variation of the
effective mass is no longer equal to the inverse of the quasi particle residue,
as in DMFT, and is reduced approaching the Mott transition.Comment: 4 page
Ni impurity induced enhancement of the pseudogap in cuprate high T_c superconductors
The influence of magnetic Ni and non-magnetic Zn impurities on the normal
state pseudogap (PG) in the c-axis optical conductivity of NdBa\{Cu(Ni,Zn)O crystals was studied by spectral
ellipsometry. We find that these impurities which strongly suppress
superconductivity have a profoundly different impact on the PG. Zn gives rise
to a gradual and inhomogeneous PG suppression while Ni strongly enhances the
PG. Our results challenge theories that relate the PG either to precursor
superconductivity or to other phases with exotic order parameters, such as flux
phase or d-density wave states, that should be suppressed by potential
scattering. The apparent difference between magnetic and non-magnetic
impurities instead points towards an important role of magnetic correlations in
the PG state.Comment: 11 pages and 2 figure
Formation of metallic magnetic clusters in a Kondo-lattice metal: Evidence from an optical study
Magnetic materials are usually divided into two classes: those with localised
magnetic moments, and those with itinerant charge carriers. We present a
comprehensive experimental (spectroscopic ellipsomerty) and theoretical study
to demonstrate that these two types of magnetism do not only coexist but
complement each other in the Kondo-lattice metal, Tb2PdSi3. In this material
the itinerant charge carriers interact with large localised magnetic moments of
Tb(4f) states, forming complex magnetic lattices at low temperatures, which we
associate with self-organisation of magnetic clusters. The formation of
magnetic clusters results in low-energy optical spectral weight shifts, which
correspond to opening of the pseudogap in the conduction band of the itinerant
charge carriers and development of the low- and high-spin intersite electronic
transitions. This phenomenon, driven by self-trapping of electrons by magnetic
fluctuations, could be common in correlated metals, including besides
Kondo-lattice metals, Fe-based and cuprate superconductors.Comment: 30 pages, 6 Figure
Theory of Quasi-Particles in the Underdoped High Tc Superconducting State
The microscopic theory of superconducting (SC) state in the SU(2) slave-boson
model is developed. We show how the pseudogap and Fermi surface (FS) segments
in the normal state develop into a d-wave gap in the superconducting state.
Even though the superfluid density is of order x (the doping concentration),
the physical properties of the low lying quasiparticles are found to resemble
those in BCS theory. Thus the microscopic theory lay the foundation for our
earlier phenomenological discussion of the unusual SC properties in the
underdoped cuprates.Comment: 4 pages in RevTeX, 1 figure in eps, revised versio
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
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
Proposal for an Experiment to Test a Theory of High Temperature Superconductors
A theory for the phenomena observed in Copper-Oxide based high temperature
superconducting materials derives an elusive time-reversal and rotational
symmetry breaking order parameter for the observed pseudogap phase ending at a
quantum-critical point near the composition for the highest . An
experiment is proposed to observe such a symmetry breaking. It is shown that
Angle-resolved Photoemission yields a current density which is different for
left and right circularly polarized photons. The magnitude of the effect and
its momentum dependence is estimated. Barring the presence of domains of the
predicted phase an asymmetry of about 0.1 is predicted at low temperatures in
moderately underdoped samples.Comment: latex, 2 figure
The Unusual Superconducting State of Underdoped Cuprates
There is increasing experimental evidence that the superconducting energy gap
in the underdoped cuprates is independent of doping concentration
while the superfluid density is linear in . We show that under these
conditions, thermal excitation of the quasiparticles is very effective in
destroying the superconducting state, so that is proportional to
and part of the gap structure remains in the normal state. We then
estimate and predict it to be proportional to . We also discuss
to what extent the assumptions that go into the quasiparticle description can
be derived in the U(1) and SU(2) formulations of the t-J model.Comment: 4 pages RevTe
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
Fermi Surface Evolution, Pseudo Gap and Stagger Gauge Field Fluctuation in Underdoped Cuprates
In the context of t-J model we show that in underdoped regime,beside the
usual long wave length gauge field fluctuation, an additional low energy
fluctuation, staggered gauge field fluctuation plays a crucial role in the
evolution of Fermi surface(FS) as well as the line shape of spectral function
for the cuprates. By including the staggered gauge field fluctuation we
calculate the spectral function of the electrons by RPA(random phase
approximation). The line shape of the spectral function near is very
broad in underdoped case and is quite sharp in overdoped case. For the spectral
function near , the quasiparticle peaks are always very sharp
in both underdoped and overdoped case. The temperature dependence of the
spectral function is also discussed in our present calculation. These results
fit well with the recent ARPES experiments. We also calculate the FS crossover
from a small four segment like FS to a large continuous FS. The reason of such
kind of FS crossover is ascribed to the staggered gauge field fluctuation which
is strong in underdoped regime and becomes much weaker in overdoped regime. The
pseudo gap extracted from the ARPES data can be also interpreted by the
calculation.Comment: 4 pages,6 eps figures include
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