760 research outputs found
Transient Magnetic and Doppler Features Related to the White-light Flares in NOAA 10486
Rapidly moving transient features have been detected in magnetic and Doppler
images of super-active region NOAA 10486 during the X17/4B flare of 28 October
2003 and the X10/2B flare of 29 October 2003. Both these flares were extremely
energetic white-light events. The transient features appeared during impulsive
phases of the flares and moved with speeds ranging from 30 to 50 km s.
These features were located near the previously reported compact acoustic
\cite{Donea05} and seismic sources \cite{Zharkova07}. We examine the origin of
these features and their relationship with various aspects of the flares, {\it
viz.}, hard X-ray emission sources and flare kernels observed at different
layers - (i) photosphere (white-light continuum), (ii) chromosphere (H
6563\AA), (iii) temperature minimum region (UV 1600\AA), and (iv) transition
region (UV 284\AA).Comment: 26 pages, 13 figures, 2 tables, accepted for publication in Solar
Physic
S and D Wave Mixing in High Superconductors
For a tight binding model with nearest neighbour attraction and a small
orthorhombic distortion, we find a phase diagram for the gap at zero
temperature which includes three distinct regions as a function of filling. In
the first, the gap is a mixture of mainly -wave with a smaller extended
-wave part. This is followed by a region in which there is a rapid increase
in the -wave part accompanied by a rapid increase in relative phase between
and from 0 to . Finally, there is a region of dominant with a
mixture of and zero phase. In the mixed region with a finite phase, the
-wave part of the gap can show a sudden increase with decreasing temperature
accompanied with a rapid increase in phase which shows many of the
characteristics measured in the angular resolved photoemission experiments of
Ma {\em et al.} in Comment: 12 pages, RevTeX 3.0, 3 PostScript figures uuencoded and compresse
Evolution of the pairing pseudogap in the spectral function with interplane anisotropy
We study the pairing pseudogap in the spectral function as a function of
interplane coupling. The analytical expressions for the self-energy in the
critical regime are obtained for any degree of anisotropy. The frequency
dependence of the self-energy is found to be qualitatively different in two and
three dimensions, and the crossover from two to three dimensional behavior is
discussed. In particular, by considering the anisotropy of the Fermi velocity
and gap along the Fermi surface, we can qualitatively explain recent
photoemission experiments on high temperature superconductors concerning the
temperature dependent Fermi arcs seen in the pseudogap phase.Comment: 20 pages, revtex, 5 encapsulated postscript figures include
Mixed symmetry superconductivity in two-dimensional Fermi liquids
We consider a 2D isotropic Fermi liquid with attraction in both and
channels and examine the possibility of a superconducting state with mixed
and symmetry of the gap function. We show that both in the weak coupling
limit and at strong coupling, a mixed symmetry state is realized in a
certain range of interaction. Phase transitions between the mixed and the pure
symmetry states are second order. We also show that there is no stable mixed
symmetry state at any coupling.Comment: 3 figures attached in uuencoded gzipped file
Phase transition from a to superconductor
We study the phase transition from a to
superconductor using the tight-binding model of two-dimensional cuprates. As
the temperature is lowered past the critical temperature , first a superconducting phase is created. With further reduction of
temperature, the phase is created at temperature
. We study the temperature dependencies of the order parameter,
specific heat and spin susceptibility in these mixed-angular-momentum states on
square lattice and on a lattice with orthorhombic distortion. The
above-mentioned phase transitions are identified by two jumps in specific heat
at and .Comment: Latex file, 5 pages, 6 postscript figures, Accepted in Physical
Review
Pair Phase Fluctuations and the Pseudogap
The single-particle density of states and the tunneling conductance are
studied for a two-dimensional BCS-like Hamiltonian with a d_{x^2-y^2}-gap and
phase fluctuations. The latter are treated by a classical Monte Carlo
simulation of an XY model. Comparison of our results with recent scanning
tunneling spectra of Bi-based high-T_c cuprates supports the idea that the
pseudogap behavior observed in these experiments can be understood as arising
from phase fluctuations of a d_{x^2-y^2} pairing gap whose amplitude forms on
an energy scale set by T_c^{MF} well above the actual superconducting
transition.Comment: 5 pages, 6 eps-figure
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
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
Dispersion of a single hole in the t-J model
The dispersion of a single hole in the t-J model obtained by the exact result
of 32 sites and the results obtained by self-consistent Born approximation and
the Green function Monte Carlo method can be simply derived by a mean-field
theory with d-RVB and antiferromagnetic order parameters. In addition, it
offers a simple explanation for the difference observed between those results.
The presence of the extended van Hove region at (pi,0) is a consequence of the
d-RVB pairing independenct of the antiferromagnetic order. Results including t'
and t" are also presented and explained consistently in a similar way.Comment: LaTex file, 5 pages with 5 embedded eps figure
Inherent Inhomogeneities in Tunneling Spectra of BSCCO Crystals in the Superconducting State
Scanning Tunneling Spectroscopy on cleaved BSCCO(2212) single crystals reveal
inhomogeneities on length-scales of 30 . While most of the surface
yields spectra consistent with a d-wave superconductor, small regions show a
doubly gapped structure with both gaps lacking coherence peaks and the larger
gap having a size typical of the respective pseudo-gap for the same sample.Comment: 4 pages, 4 figure
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