159 research outputs found
Evolution of Magnetic and Superconducting Fluctuations with Doping of High-Tc Superconductors (An electronic Raman scattering study)
For YBa_2Cu_3O_{6+\delta} and Bi_2Sr_2CaCu_2O_8 superconductors, electronic
Raman scattering from high- and low-energy excitations has been studied in
relation to the hole doping level, temperature, and energy of the incident
photons. For underdoped superconductors, it is concluded that short range
antiferromagnetic (AF) correlations persist with hole doping and doped single
holes are incoherent in the AF environment. Above the superconducting (SC)
transition temperature T_c the system exhibits a sharp Raman resonance of B_1g
symmetry and about 75 meV energy and a pseudogap for electron-hole excitations
below 75 meV, a manifestation of a partially coherent state forming from doped
incoherent quasi-particles. The occupancy of the coherent state increases with
cooling until phase ordering at T_c produces a global SC state.Comment: 5 pages, 4 EPS figures; SNS'97 Proceedings to appear in J. Phys.
Chem. Solid
Statistical and Dynamical Study of Disease Propagation in a Small World Network
We study numerically statistical properties and dynamical disease propagation
using a percolation model on a one dimensional small world network. The
parameters chosen correspond to a realistic network of school age children. We
found that percolation threshold decreases as a power law as the short cut
fluctuations increase. We found also the number of infected sites grows
exponentially with time and its rate depends logarithmically on the density of
susceptibles. This behavior provides an interesting way to estimate the
serology for a given population from the measurement of the disease growing
rate during an epidemic phase. We have also examined the case in which the
infection probability of nearest neighbors is different from that of short
cuts. We found a double diffusion behavior with a slower diffusion between the
characteristic times.Comment: 12 pages LaTex, 10 eps figures, Phys.Rev.E Vol. 64, 056115 (2001
No Far-Infrared-Spectroscopic Gap in Clean and Dirty High-T Superconductors
We report far infrared transmission measurements on single crystal samples
derived from BiSrCaCuO. The impurity scattering rate of
the samples was varied by electron-beam irradiation, 50MeV O ion
irradiation, heat treatment in vacuum, and Y doping. Although substantial
changes in the infrared spectra were produced, in no case was a feature
observed that could be associated with the superconducting energy gap. These
results all but rule out ``clean limit'' explanations for the absence of the
spectroscopic gap in this material, and provide evidence that the
superconductivity in BiSrCaCuO is gapless.Comment: 4 pages and 3 postscript figures attached. REVTEX v3.0. Accepted for
publication in Phys. Rev. Lett. IRDIRT
Vanishing Loss Effect on the Effective ac Conductivity behavior for 2D Composite Metal-Dielectric Films At The Percolation Threshold
We study the imaginary part of the effective conductivity as well as its
distribution probability for vanishing losses in 2D composites. This
investigation showed that the effective medium theory provides only
informations about the average conductivity, while its fluctuations which
correspond to the field energy in this limit are neglected by this theory.Comment: 6 pages, 2 figures, submitted to Phys.Rev.
C-axis Raman spectra of a normal plane-chain bilayer cuprate and the pseudogap
We investigate the Raman spectra in the geometry where both incident and
scattered photon polarizations are parallel to the -direction, for a
plane-chain bilayer coupled via a single-particle tunneling . The
Raman vertex is derived in the tight-binding limit and in the absence of
Coulomb screening, the Raman intensity can be separated into intraband
() and interband () transitions. In the
small- limit, the interband part dominates and a pseudogap will appear
as it does in the conductivity. Coulomb interactions bring in a two-particle
coupling and result in the breakdown of intra- and interband separation.
Nevertheless, when is small, the Coulomb screening () has little effect on the intensity to which the unscreened
interband transitions contribute most. In general, the total Raman spectra are
strongly dependent on the magnitude of .Comment: 23 pages, 6 figures, submitted to Phys. Rev.
Raman study of carrier-overdoping effects on the gap in high-Tc superconducting cuprates
Raman scattering in the heavily overdoped (Y,Ca)Ba_2Cu_3O_{7-d} (T_c = 65 K)
and Bi_2Sr_2CaCu_2O_{8+d} (T_c = 55 K) crystals has been investigated. For the
both crystals, the electronic pair-breaking peaks in the A_{1g} and B_{1g}
polarizations were largely shifted to the low energies close to a half of
2Delta_0, Delta_0 being the maximum gap. It strongly suggests s-wave mixing
into the d-wave superconducting order parameter and the consequent
manifestation of the Coulomb screening effect in the B_{1g}-channel. Gradual
mixing of s-wave component with overdoping is not due to the change of crystal
structure symmetry but a generic feature in all high-T_c superconducting
cuprates.Comment: 5 pages, 4 figures, to be published in Phys. Rev. B, Rapid
communicaito
C-axis electronic Raman scattering in Bi_2Sr_2CaCu_2O_{8+\delta}
We report a c-axis-polarized electronic Raman scattering study of
Bi_2Sr_2CaCu_2O_{8+\delta} single crystals. In the normal state, a resonant
electronic continuum extends to 1.5 eV and gains significant intensity as the
incoming photon energy increases. In the superconducting state, a coherence
2\Delta peak appears around 50 meV, with a suppression of the scattering
intensity at frequencies below the peak position. The peak energy, which is
higher than that seen with in-plane polarizations, signifies distinctly
different dynamics of quasiparticle excitations created with out-of-plane
polarization.Comment: 12 pages, REVTEX, 3 postscript figure
Multiple Andreev Reflection and Giant Excess Noise in Diffusive Superconductor/Normal-Metal/Superconductor Junctions
We have studied superconductor/normal metal/superconductor (SNS) junctions
consisting of short Au or Cu wires between Nb or Al banks. The Nb based
junctions display inherent electron heating effects induced by the high thermal
resistance of the NS boundaries. The Al based junctions show in addition
subharmonic gap structures in the differential conductance dI/dV and a
pronounced peak in the excess noise at very low voltages V. We suggest that the
noise peak is caused by fluctuations of the supercurrent at the onset of
Josephson coupling between the superconducting banks. At intermediate
temperatures where the supercurrent is suppressed a noise contribution ~1/V
remains, which may be interpreted as shot noise originating from large multiple
charges.Comment: 7 pages, 7 figures, extended versio
Relation between the superconducting gap energy and the two-magnon Raman peak energy in Bi2Sr2Ca{1-x}YxCu2O{8+\delta}
The relation between the electronic excitation and the magnetic excitation
for the superconductivity in Bi2Sr2Ca{1-x}YxCu2O{8+\delta} was investigated by
wide-energy Raman spectroscopy. In the underdoping region the B1g scattering
intensity is depleted below the two-magnon peak energy due to the "hot spots"
effects. The depleted region decreases according to the decrease of the
two-magnon peak energy, as the carrier concentration ncreases. This two-magnon
peak energy also determines the B1g superconducting gap energy as
from under to overdoping hole concentration.Comment: 10 pages, 4 figure
Doping dependence of the superconducting gap in Bi2Sr2CaCu2O{8 + delta}
Bi2Sr2CaCu2O{8 + \delta} crystals with varying hole concentrations (0.12 < p
< 0.23) were studied to investigate the effects of doping on the symmetry and
magnitude of the superconducting gap. Electronic Raman scattering experiments
that sample regions of the Fermi surface near the diagonal (B_{2g}) and
principal axes (B_{1g}) of the Brillouin Zone have been utilized. The frequency
dependence of the Raman response function at low energies is found to be linear
for B_{2g} and cubic for B_{1g} (T< T_c). The latter observations have led us
to conclude that the doping dependence of the superconducting gap is consistent
with d_{x^2-y^2} symmetry, for slightly underdoped and overdoped crystals.
Studies of the pair-breaking peak found in the B_{1g} spectra demonstrate that
the magnitude of the maximum gap decreases monotonically with increasing hole
doping, for p > 0.12. Based on the magnitude of the B_{1g} renormalization, it
is found that the number of quasiparticles participating in pairing increases
monotonically with increased doping. On the other hand, the B_{2g} spectra show
a weak "pair-breaking peak" that follows a parabolic-like dependence on hole
concentration, for 0.12 < p < 0.23.Comment: 9 pages REvTex document including 8 eps figures; new table II;
changes to Fig. 5 and tex
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