1,601 research outputs found
Singular current response from isolated impurities in d-wave superconductors
The current response of a d-wave superconductor containing a single impurity
is calculated and shown to be singular in the low-temperature limit, leading in
the case of strong scattering to a 1/T term in the penetration depth
similar to that induced by Andreev surface bound states. For a
small number of such impurities, we argue this low- upturn could be
observable in cuprate superconductors.Comment: 4 pages, 2 .eps figures. Minor changes to match the published versio
Cubic Curves, Finite Geometry and Cryptography
Some geometry on non-singular cubic curves, mainly over finite fields, is
surveyed. Such a curve has 9,3,1 or 0 points of inflexion, and cubic curves are
classified accordingly. The group structure and the possible numbers of
rational points are also surveyed. A possible strengthening of the security of
elliptic curve cryptography is proposed using a `shared secret' related to the
group law. Cubic curves are also used in a new way to construct sets of points
having various combinatorial and geometric properties that are of particular
interest in finite Desarguesian planes.Comment: This is a version of our article to appear in Acta Applicandae
Mathematicae. In this version, we have corrected a sentence in the third
paragraph. The final publication is available at springerlink.com at
http://www.springerlink.com/content/xh85647871215644
Residual absorption at zero temperature in d-wave superconductors
In a d-wave superconductor with elastic impurity scattering, not all the
available optical spectral weight goes into the condensate at zero temperature,
and this leads to residual absorption. We find that for a range of impurity
parameters in the intermediate coupling regime between Born (weak) and unitary
(strong) limit, significant oscillator strength remains which exhibits a cusp
like behavior of the real part of the optical conductivity with upward
curvature as a function of frequency, as well as a quasilinear temperature
dependence of the superfluid density. The calculations offer an explanation of
recent data on ortho-II YBaCuO which has been considered
anomalous.Comment: Accepted for publication by Phys. Rev. B 7 Pages and 4 Figure
Optical conductivity in non-equilibrium d-wave superconductors
We consider the optical conductivity of a d-wave BCS superconductor in the
presence of a non-equilibrium distribution of excess quasiparticles. Two
different simplified models used in the past for the s-wave case are considered
and results compared. In the -model of Parker the excess quasiparticles
are assumed to be in a thermal distribution at some temperature larger
than the equilibrium sample temperature. In the - model of Owen and
Scalapino a chemical potential is introduced to accommodate the excess
quasiparticles. Some of the results obtained are specific to the model, most
are qualitatively similar in both.Comment: 11 pages, 6 figures this manuscript has been accepted for publication
in abbreviated form by Physical Review
Spin fluctuations and superconductivity in a 3D tight-binding model for BaFe2As2
Despite the wealth of experimental data on the Fe-pnictide compounds of the
KFe2As2-type, K = Ba, Ca, or Sr, the main theoretical work based on
multiorbital tight-binding models has been restricted so far to the study of
the related 1111 compounds. This can be ascribed to the more three dimensional
electronic structure found by ab initio calculations for the 122 materials,
making this system less amenable to model development. In addition, the more
complicated Brillouin zone (BZ) of the body-centered tetragonal symmetry does
not allow a straightforward unfolding of the electronic band structure into an
effective 1Fe/unit cell BZ. Here we present an effective 5-orbital
tight-binding fit of the full DFT band structure for BaFeAs including the kz
dispersions. We compare the 5-orbital spin fluctuation model to one previously
studied for LaOFeAs and calculate the RPA enhanced susceptibility. Using the
fluctuation exchange approximation to determine the leading pairing
instability, we then examine the differences between a strictly two dimensional
model calculation over a single kz cut of the BZ and a completely three
dimensional approach. We find pairing states quite similar to the 1111
materials, with generic quasi-isotropic pairing on the hole sheets and nodal
states on the electron sheets at kz = 0 which however are gapped as the system
is hole doped. On the other hand, a substantial kz dependence of the order
parameter remains, with most of the pairing strength deriving from processes
near kz = pi. These states exhibit a tendency for an enhanced anisotropy on the
hole sheets and a reduced anisotropy on the electron sheets near the top of the
BZ.Comment: 12 pages, 15 figure
Surface effects in multiband superconductors. Application to MgB
Metals with many bands at the Fermi level can have different band dependent
gaps in the superconducting state. The absence of translational symmetry at an
interface can induce interband scattering and modify the superconducting
properties. We dicuss the relevance of these effects to recent experiments in
MgB
Impurity-Induced Quasiparticle Transport and Universal Limit Wiedemann-Franz Violation in d-Wave Superconductors
Due to the node structure of the gap in a d-wave superconductor, the presence
of impurities generates a finite density of quasiparticle excitations at zero
temperature. Since these impurity-induced quasiparticles are both generated and
scattered by impurities, prior calculations indicate a universal limit (\Omega
-> 0, T -> 0) where the transport coefficients obtain scattering-independent
values, depending only on the velocity anisotropy v_f/v_2. We improve upon
prior results, including the contributions of vertex corrections and Fermi
liquid corrections in our calculations of universal limit electrical, thermal,
and spin conductivity. We find that while vertex corrections modify electrical
conductivity and Fermi liquid corrections renormalize both electrical and spin
conductivity, only thermal conductivity maintains its universal value,
independent of impurity scattering or Fermi liquid interactions. Hence, low
temperature thermal conductivity measurements provide the most direct means of
obtaining the velocity anisotropy for high T_c cuprate superconductors.Comment: 22 pages, 6 figures; revised version to be published in Phys Rev
Anisotropic Optical Conductivity of Nd2-xCexCuO4 Thin Films
Opticcal conductivity spectra of Nd2-xCexCuO4 thin films,
measured by the reflectance-transmittance method (R-T method) which has been
proposed to investigate far-infrared spectroscopy, are investigated based on
the anisotropic pairing model. Precise measurements of the frequency-dependent
conductivity enable us to examine quantitatively the nature of the
superconducting gap through infrared properties in the electron-doped high-Tc
superconductors. We show that the behavior of optical conductivity
is consistent with the anisotropic superconducting gap and is well explained by
the formula for d-wave pairing in the low-energy regime of the far-infrared
region. Our results suggest that the electron-doped cuprate superconductors
Nd2-xCexCuO4 have nodes in the superconducting gap.Comment: 4 pages, 3 figure
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