3,325 research outputs found
Internal structure of preformed Cooper pairs
In order to obtain information about the internal structure of the preformed
pairs in the pseudogap state of high superconductors, we calculate the
propagator of a singlet pair with center of mass coordinate , and
relative distance , by solving the Bethe-Salpeter equation,
representing the sum over repeated two-particle scattering events due to a
distance dependent attraction. We define then a ``pair structure function''
that depends on the internal distance
between the partners and on the momentum of the pair.
We calculate this function both for a local potential and wave symmetry of
the order parameter and for a separable potential and wave symmetry of the
order parameter. The influence of the center of mass momentum, strenght of the
interaction, temperature, density of particles and of the pseudogap in the
one-electron spectrum is studied for both cases.Comment: 12 pages, REVTeX4, 8 EPS figure
Riemann-Hurwitz formula for Morita-Mumford classes and surface symmetries
Let a finite group G act on a compact Riemann surface C in a faithful and orientation preserving way. Then we describe the Morita-Mumford classes en (Ca) E H2n (G;Z) of the homotopy quotient (or the Borel construction) Ca of the action in terms of fixed-point data. This fixed-point formula is deduced from a higher analogue of the classical Riemann-Hurwitz formula based on computations of Miller [Mi] and Morita [Mo]
Study of the in-plane magnetic penetration depth in the cuprate superconductor Ca_2-xNa_xCuO_2Cl_2: role of the apical sites
A study of the in-plane magnetic penetration depth \lambda_ab in a series of
the cuprate superconductors Ca_2-xNa_xCuO_2Cl_2 (Na-CCOC) with Na content
x=0.11, 0.12, 0.15, 0.18, and 0.19 is reported. The zero temperature values of
\lambda_ab(0) were obtained by means of the muon-spin rotation technique, as
well as from measurements of the intrinsic susceptibility \chi^int(0) by using
the procedure developed by Kanigel et al. [Phys.Rev.B 71, 224511 (2005)].
\lambda_ab at T=0K was found to increase with decreasing doping from
\lambda_ab(0)=316(19)nm for the x=0.19 sample to \lambda_ab(0)=430(26)nm for
the x=0.11 one. From a comparison of the present Na-CCOC data with those of
Bi2201 and La214 cuprate superconductors it is concluded that substitution of
the apical oxygen by chlorine decreases the coupling between the
superconducting CuO_2 planes, leading to an enhancement of the two-dimensional
properties of Na-CCOC.Comment: 8 pages, 7 figure
Inequivalent representations of commutator or anticommutator rings of field operators and their applications
Hamiltonian of a system in quantum field theory can give rise to infinitely
many partition functions which correspond to infinitely many inequivalent
representations of the canonical commutator or anticommutator rings of field
operators. This implies that the system can theoretically exist in infinitely
many Gibbs states. The system resides in the Gibbs state which corresponds to
its minimal Helmholtz free energy at a given range of the thermodynamic
variables. Individual inequivalent representations are associated with
different thermodynamic phases of the system. The BCS Hamiltonian of
superconductivity is chosen to be an explicit example for the demonstration of
the important role of inequivalent representations in practical applications.
Its analysis from the inequivalent representations' point of view has led to a
recognition of a novel type of the superconducting phase transition.Comment: 25 pages, 6 figure
Metallic mean-field stripes, incommensurability and chemical potential in cuprates
We perform a systematic slave-boson mean-field analysis of the three-band
model for cuprates with first-principle parameters. Contrary to widespread
believe based on earlier mean-field computations low doping stripes have a
linear density close to 1/2 added hole per lattice constant. We find a
dimensional crossover from 1D to 2D at doping followed by a breaking
of particle-hole symmetry around doping 1/8 as doping increases. Our results
explain in a simple way the behavior of the chemical potential, the magnetic
incommensurability, and transport experiments as a function of doping. Bond
centered and site-centered stripes become degenerate for small overdoping.Comment: submitted to PR
Superconducting transition temperatures and coherence length in non s-wave pairing materials correlated with spin-fluctuation mediated interaction
Following earlier work on electron or hole liquids flowing through assemblies
with magnetic fluctuations, we have recently exposed a marked correlation of
the superconducting temperature Tc, for non s-wave pairing materials, with
coherence length xi and effective mass m*. The very recent study of Abanov et
al. [Europhys. Lett. 54, 488 (2001)] and the prior investigation of Monthoux
and Lonzarich [Phys. Rev. B 59, 14598 (1999)] have each focussed on the concept
of a spin-fluctuation temperature T_sf, which again is intimately related to
Tc. For the d-wave pairing via antiferromagnetic spin fluctuations in the
cuprates, these studies are brought into close contact with our own work, and
the result is that k_B T_sf ~ hbar^2 / m* xi^2. This demonstrates that xi is
also determined by such antiferromagnetic spin-fluctuation mediated pair
interaction. The coherence length in units of the lattice spacing is then
essentially given in the cuprates as the square root of the ratio of two
characteristic energies, namely: the kinetic energy of localization of a charge
carrier of mass m* in a specified magnetic correlation length to the hopping
energy. The quasi-2D ruthenate Sr_2RuO_4, with Tc ~ 1.3 K, has p-wave
spin-triplet pairing and so is also briefly discussed here.Comment: Accepted for publication in Phys. Rev.
Multiwavelength observation from radio through very-high-energy Gamma-ray of OJ 287 during the 12-year cycle flare in 2007
We performed simultaneous multiwavelength observations of OJ 287 with the
Nobeyama Millimeter Array for radio, the KANATA telescope and the KVA telescope
for optical, the Suzaku satellite for X-ray and the MAGIC telescope for very
high energy (VHE) gamma-ray in 2007. The observations were conducted for a
quiescent state in April and in a flaring state in November-December. We
clearly observed increase of fluxes from radio to X-ray bands during the
flaring state while MAGIC could not detect significant VHE gamma-ray emission
from the source. We could derive an upper limit (95% confidence level) of 1.7%
of the Crab Nebula flux above 150 GeV from about 41.2 hours of the MAGIC
observation. A simple SSC model suggests that the observed flaring activity
could be caused by evolutions in the distribution of the electron population
rather than changes of the magnetic field strength or Doppler beaming factor in
the jet.Comment: Contribution to the 31st ICRC, Lodz, Poland, July 200
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