26,878 research outputs found
Superconducting gap symmetry of Ba0.6K0.4Fe2As2 studied by angle-resolved photoemission spectroscopy
We have performed high-resolution angle-resolved photoemission spectroscopy
on the optimally-doped BaKFeAs compound and determined
the accurate momentum dependence of the superconducting (SC) gap in four
Fermi-surface sheets including a newly discovered outer electron pocket at the
M point. The SC gap on this pocket is nearly isotropic and its magnitude is
comparable ( 11 meV) to that of the inner electron and hole
pockets (12 meV), although it is substantially larger than that of the
outer hole pocket (6 meV). The Fermi-surface dependence of the SC gap
value is basically consistent with () = coscos
formula expected for the extended s-wave symmetry. The observed finite
deviation from the simple formula suggests the importance of multi-orbital
effects.Comment: 4 pages, 3 figures, 1 tabl
Broken time-reversal symmetry in Josephson junction involving two-band superconductors
A novel time-reversal symmetry breaking state is found theoretically in the
Josephson junction between the two-gap superconductor and the conventional
s-wave superconductor. This occurs due to the frustration between the three
order parameters analogous to the two antiferromagnetically coupled XY-spins
put under a magnetic field. This leads to the interface states with the
energies inside the superconducting gap. Possible experimental observations of
this state with broken time-reversal symmetry are discussed.Comment: 9 pages, 1 figur
Scale-free download network for publications
The scale-free power-law behavior of the statistics of the download frequency
of publications has been, for the first time, reported. The data of the
download frequency of publications are taken from a well-constructed web page
in the field of economic physics (http://www.unifr.ch/econophysics/). The
Zipf-law analysis and the Tsallis entropy method were used to fit the download
frequency. It was found that the power-law exponent of rank-ordered frequency
distribution is which is consistent with the
power-law exponent for the cumulated frequency
distributions. Preferential attachment model of Barabasi and Albert network has
been used to explain the download network.Comment: 3 pages, 2 figure
Evolution of the Fermi surface with carrier concentration in Bi_2Sr_2CaCu_2O_{8+\delta}
We show, by use of angle-resolved photoemission spectroscopy, that underdoped
Bi_2Sr_2CaCu_2O_{8+\delta} appears to have a large Fermi surface centered at
(\pi,\pi), even for samples with a T_c as low as 15 K. No clear evidence of a
Fermi surface pocket around (\pi/2,\pi/2) has been found. These conclusions are
based on a determination of the minimum gap locus in the pseudogap regime T_c <
T < T^*, which is found to coincide with the locus of gapless excitations in
momentum space (Fermi surface) determined above T^*. These results suggest that
the pseudogap is more likely of precursor pairing rather than magnetic origin.Comment: 4 pages, revtex, 4 postscript color figure
Electronic excitations in BiSrCaCuO : Fermi surface, dispersion, and absence of bilayer splitting
From a detailed study, including polarization dependence, of the normal state
angle-resolved photoemission spectra for BiSrCaCuO, we find
only one CuO band related feature. All other spectral features can be
ascribed either to umklapps from the superlattice or to ``shadow bands''. Even
though the dispersion of the peaks looks like band theory, the lineshape is
anomalously broad and no evidence is found for bilayer splitting. We argue that
the ``dip feature'' in the spectrum below arises not from bilayer
splitting, but rather from many body effects.Comment: 4 pages, revtex, 3 uuencoded postscript figure
Coherent quasiparticle weight and its connection to high-T_c superconductivity from angle-resolved photoemission
In conventional superconductors, the pairing energy gap (\Delta) and
superconducting phase coherence go hand-in-hand. As the temperature is lowered,
both the energy gap and phase coherence appear at the transition temperature
T_c. In contrast, in underdoped high-T_c superconductors (HTSCs), a pseudogap
appears at a much higher temperature T^*, smoothly evolving into the
superconducting gap at T_c. Phase coherence on the other hand is only
established at T_c, signaled by the appearance of a sharp quasiparticle (QP)
peak in the excitation spectrum. Another important difference between the two
types of superconductors is in the ratio of 2\Delta / T_c=R. In BCS theory,
R~3.5, is constant. In the HTSCs this ratio varies widely, continuing to
increase in the underdoped region, where the gap increases while T_c decreases.
Here we report that in HTSCs it is the ratio z_A\Delta_m/T_c which is
approximately constant, where \Delta_m is the maximum value of the d-wave gap,
and z_A is the weight of the coherent excitations in the spectral function.
This is highly unusual, since in nearly all phase transitions, T_c is
determined by an energy scale alone. We further show that in the
low-temperature limit, z_{\it A} increases monotonically with increasing doping
x. The growth is linear, i.e. z_A(x)\propto x, in the underdoped to optimally
doped regimes, and slows down in overdoped samples. The reduction of z_A with
increasing temperature resembles that of the c-axis superfluid density.Comment: 11 pages, 5 figures, revised versio
The elliptic quantum algebra and its vertex operators
We construct a realization of the elliptic quantum algebra
for any given level in terms of free boson fields and
their twisted partners. It can be considered as the elliptic deformation of the
Wakimoto realization of the quantum affine algebra . We also
construct a family of screening currents, which commute with the currents of
up to total q-differences. And we give explicit twisted
expressions for the type and the type vertex operators of
by twisting the known results of the type vertex
operators of the quantum affine algebra and the new results
of the type vertex operators of we obtained in this
paper.Comment: 28 page
- …