2,681 research outputs found
Electronic structure of strongly correlated d-wave superconductors
We study the electronic structure of a strongly correlated d-wave
superconducting state. Combining a renormalized mean field theory with direct
calculation of matrix elements, we obtain explicit analytical results for the
nodal Fermi velocity, v_F, the Fermi wave vector, k_F, and the momentum
distribution, n_k, as a function of hole doping in a Gutzwiller projected
d-wave superconductor. We calculate the energy dispersion, E_k, and spectral
weight of the Gutzwiller-Bogoliubov quasiparticles, and find that the spectral
weight associated with the quasiparticle excitation at the antinodal point
shows a non monotonic behavior as a function of doping. Results are compared to
angle resolved photoemission spectroscopy (ARPES) of the high temperature
superconductors.Comment: final version, comparison to experiments added, 4+ pages, 4 figure
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
Evidence for a Bulk Complex Order-Parameter in Y0.9Ca0.1Ba2Cu3O7-delta Thin Films
We have measured the penetration depth of overdoped Y0.9Ca0.1Ba2Cu3O7-delta
(Ca-YBCO) thin films using two different methods. The change of the penetration
depth as a function of temperature has been measured using the parallel plate
resonator (PPR), while its absolute value was obtained from a quasi-optical
transmission measurements. Both sets of measurements are compatible with an
order parameter of the form: Delta*dx2-y2+i*delta*dxy, with Delta=14.5 +- 1.5
meV and delta=1.8 meV, indicating a finite gap at low temperature. Below 15 K
the drop of the scattering rate of uncondensed carriers becomes steeper in
contrast to a flattening observed for optimally doped YBCO films. This decrease
supports our results on the penetration depth temperature dependence. The
findings are in agreement with tunneling measurements on similar Ca-YBCO thin
films.Comment: 11 pages, 4 figure
Magnetic Proximity Effect in Perovskite Superconductor/Ferromagnet Multilayers
superconducting/ferromagnetic
(SC/FM) multilayers have been studied by neutron reflectometry. Evidence for a
characteristic difference between the structural and magnetic depth profiles is
obtained from the occurrence of a structurally forbidden Bragg peak in the FM
state. The comparison with simulated reflectivity curves allows us to identify
two possible magnetization profiles: a sizable magnetic moment within the SC
layer antiparallel to the one in the FM layer (inverse proximity effect), or a
``dead'' region in the FM layer with zero net magnetic moment. The former
scenario is supported by an anomalous SC-induced enhancement of the
off-specular reflection, which testifies to a strong mutual interaction of SC
and FM order parameters.Comment: 4 pages, 2 figures, submitted to PR
Anderson transitions in three-dimensional disordered systems with randomly varying magnetic flux
The Anderson transition in three dimensions in a randomly varying magnetic
flux is investigated in detail by means of the transfer matrix method with high
accuracy. Both, systems with and without an additional random scalar potential
are considered. We find a critical exponent of with random
scalar potential. Without it, is smaller but increases with the system
size and extrapolates within the error bars to a value close to the above. The
present results support the conventional classification of universality classes
due to symmetry.Comment: 4 pages, 2 figures, to appear in Phys. Rev.
Correlation between the Josephson coupling energy and the condensation energy in bilayer cuprate superconductors
We review some previous studies concerning the intra-bilayer Josephson
plasmons and present new ellipsometric data of the c-axis infrared response of
almost optimally doped Bi_{2}Sr_{2}CaCu_{2}O_{8}. The c-axis conductivity of
this compound exhibits the same kind of anomalies as that of underdoped
YBa_{2}Cu_{3}O_{7-delta}. We analyze these anomalies in detail and show that
they can be explained within a model involving the intra-bilayer Josephson
effect and variations of the electric field inside the unit cell. The Josephson
coupling energies of different bilayer compounds obtained from the optical data
are compared with the condensation energies and it is shown that there is a
reasonable agreement between the values of the two quantities. We argue that
the Josephson coupling energy, as determined by the frequency of the
intra-bilayer Josephson plasmon, represents a reasonable estimate of the change
of the effective c-axis kinetic energy upon entering the superconducting state.
It is further explained that this is not the case for the estimate based on the
use of the simplest ``tight-binding'' sum rule. We discuss possible
interpretations of the remarkable agreement between the Josephson coupling
energies and the condensation energies. The most plausible interpretation is
that the interlayer tunneling of the Cooper pairs provides the dominant
contribution to the condensation energy of the bilayer compounds; in other
words that the condensation energy of these compounds can be accounted for by
the interlayer tunneling theory. We suggest an extension of this theory, which
may also explain the high values of T_{c} in the single layer compounds
Tl_{2}Ba_{2}CuO_{6} and HgBa_{2}CuO_{4}, and we make several experimentally
verifiable predictions.Comment: 16 pages (including Tables) and 7 figures; accepted for publication
in Physical Review
Prevalence and occurrence of zoonotic bacterial pathogens in surface waters determined by quantitative PCR
The prevalence and concentrations of Campylobacter jejuni, Salmonella spp. and enterohaemorrhagic E. coli (EHEC) were investigated in surface waters in Brisbane, Australia using quantitative PCR (qPCR) based methodologies. Water samples were collected from Brisbane City Botanic Gardens (CBG) Pond, and two urban tidal creeks (i.e., Oxley Creek and Blunder Creek). Of the 32 water samples collected, 8 (25%), 1 (3%), 9 (28%), 14 (44%), and 15 (47%) were positive for C. jejuni mapA, Salmonella invA, EHEC O157 LPS, EHEC VT1, and EHEC VT2 genes, respectively. The presence/absence of the potential pathogens did not correlate with either E. coli or enterococci concentrations as determined by binary logistic regression. In conclusion, the high prevalence, and concentrations of potential zoonotic pathogens along with the concentrations of one or more fecal indicators in surface water samples indicate a poor level of microbial quality of surface water, and could represent a significant health risk to users. The results from the current study would provide valuable information to the water quality managers in terms of minimizing the risk from pathogens in surface waters
Green's function approach to the magnetic properties of the kagome antiferromagnet
The Heisenberg antiferromagnet is studied on the kagom\'e lattice by
using a Green's function method based on an appropriate decoupling of the
equations of motion. Thermodynamic properties as well as spin-spin correlation
functions are obtained and characterize this system as a two-dimensional
quantum spin liquid. Spin-spin correlation functions decay exponentially with
distance down to low temperature and the calculated missing entropy at T=0 is
found to be . Within the present scheme, the specific heat exhibits
a single peak structure and a dependence at low temperature.Comment: 6 (two-column revtex4) pages, 5 ps figures. Submitted to Phys. Rev.
Absence of Meissner State and Robust Ferromagnetism in the Superconducting State of UCoGe: Possible Evidence of Spontaneous Vortex State
We report ac magnetic susceptibility and dc magnetization measurements on the
superconducting ferromagnet UCoGe (with superconducting and Curie temperatures
of ~K and ~K, respectively).
In the normal, ferromagnetic state (), the
magnetization curve exhibits a hysteresis loop similar to that of a regular
itinerant ferromagnet. Upon lowering the temperature below , the
spontaneous magnetization is unchanged, but the hysteresis is markedly
enhanced. Even deeply inside the superconducting state, ferromagnetism is not
completely shielded, and there is no Meissner region, a magnetic field region
of (a lower critical field). From these results, we suggest
that UCoGe is the first material in which ferromagnetism robustly survives in
the superconducting state and a spontaneous vortex state without the Meissner
state is realized.Comment: 5 pages, 4 figures, to be published in J. Phys. Soc. Jp
Effect of spatial variations of superconducting gap on suppression of the transition temperature by impurities
We calculate correction to the critical temperature of a dirty
superconductor, which results from the local variations of the gap function
near impurity sites. This correction is of the order of T_c/E_F and becomes
important for short-coherence length superconductors. It generally reduces a
pair-breaking effect. In s-wave superconductors small amounts of nonmagnetic
impurities can increase the transition temperature.Comment: 5 pages, ReVTE
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