366 research outputs found
Local contribution of a quantum condensate to the vacuum energy density
We evaluate the local contribution g_[mu nu]L of coherent matter with
lagrangian density L to the vacuum energy density. Focusing on the case of
superconductors obeying the Ginzburg-Landau equation, we express the
relativistic invariant density L in terms of low-energy quantities containing
the pairs density. We discuss under which physical conditions the sign of the
local contribution of the collective wave function to the vacuum energy density
is positive or negative. Effects of this kind can play an important role in
bringing about local changes in the amplitude of gravitational vacuum
fluctuations - a phenomenon reminiscent of the Casimir effect in QED.Comment: LaTeX, 8 pages. Final journal versio
On Electric Fields in Low Temperature Superconductors
The manifestly Lorentz covariant Landau-Ginzburg equations coupled to
Maxwell's equations are considered as a possible framework for the effective
description of the interactions between low temperature superconductors and
magnetic as well as electric fields. A specific experimental set-up, involving
a nanoscopic superconductor and only static applied fields whose geometry is
crucial however, is described, which should allow to confirm or invalidate the
covariant model through the determination of the temperature dependency of the
critical magnetic-electric field phase diagram and the identification of some
distinctive features it should display.Comment: 14 pages (Latex) + 2 postscript figure
Tunneling conductance of graphene ferromagnet-insulator-superconductor junctions
We study the transport properties of a graphene ferromagnet-insulator
superconductor (FIS) junction within the Blonder-Tinkham-Klapwijk formalism by
solving spin-polarized Dirac-Bogoliubov-de-Gennes equation. We find that the
retro and specular Andreev reflections in the graphene FIS junction are
drastically modified in the presence of exchange interaction and that the
spin-polarization () of tunneling current can be tuned from the positive
to negative value by bias voltage (). In the thin-barrier limit, the
conductance of a graphene FIS junction oscillates as a function of barrier
strength . Both the amplitude and phase of the conductance oscillation
varies with the exchange energy . For (Fermi energy), the
amplitude of oscillation decreases with . For ,
the amplitude of oscillation increases with , where
( is the applied electrostatic potential on
the superconducting segment of the junction). For , the
amplitude of oscillation decreases with again. Interestingly, a
universal phase difference of in exists between the
curves for and . Finally, we find that the transitions
between retro and specular Andreev reflections occur at and
, and hence the singular behavior of the conductance near
these bias voltages results from the difference in transport properties between
specular and retro Andreev reflections.Comment: Accepted for publication in Physical Review
Bernoulli potential in type-I and weak type-II superconductors: III. Electrostatic potential above the vortex lattice
The electrostatic potential above the Abrikosov vortex lattice, discussed
earlier by Blatter {\em et al.} {[}PRL {\bf 77}, 566 (1996){]}, is evaluated
within the Ginzburg-Landau theory. Unlike previous studies we include the
surface dipole. Close to the critical temperature, the surface dipole reduces
the electrostatic potential to values below a sensitivity of recent sensors. At
low temperatures the surface dipole is less effective and the electrostatic
potential remains observable as predicted earlier.Comment: 8 pages 5 figure
A 43-GHz Survey in the ELAIS N2 Area
We describe a survey in the ELAIS N2 region with the VLA at 43.4 GHz, carried
out with 1627 independent snapshot observations in D-configuration and covering
about 0.5 square degrees. One certain source is detected, a
previously-catalogued flat-spectrum QSO at z=2.2. A few (<5) other sources may
be present at about the 3sigma level, as determined from positions of
source-like deflections coinciding with blue stellar objects, or with sources
from lower-frequency surveys. Independently we show how all the source-like
detections identified in the data can be used with a maximum-likelihood
technique to constrain the 43-GHz source counts at a level of ~7 mJy. Previous
estimates of the counts at 43 GHz, based on lower-frequency counts and spectral
measurements, are consistent with these constraints, although the present
results are suggestive of somewhat higher surface densities at the 7 mJy level.
They do not provide direct evidence of intrusion of a previously unknown source
population, although the several candidate sources need examination before such
a population can be ruled out.Comment: 13 pages, 11 figures, 1 table; accepted for publication in Mon. Not
R. Astr. So
A Note on Einstein Sasaki Metrics in D \ge 7
In this paper, we obtain new non-singular Einstein-Sasaki spaces in
dimensions D\ge 7. The local construction involves taking a circle bundle over
a (D-1)-dimensional Einstein-Kahler metric that is itself constructed as a
complex line bundle over a product of Einstein-Kahler spaces. In general the
resulting Einstein-Sasaki spaces are singular, but if parameters in the local
solutions satisfy appropriate rationality conditions, the metrics extend
smoothly onto complete and non-singular compact manifolds.Comment: Latex, 13 page
Electrical transport measurements in the superconducting state of Bi2212 and Tl2201
Precise measurements of the in-plane microwave surface impedance of
high-quality single crystals of Bi2212 and Tl2201 are used to probe the
relaxation time of nodal quasiparticles in the d-wave superconducting state
through a two-fluid analysis of the microwave conductivity. While this analysis
requires us to posit a form for the frequency-dependent quasiparticle
conductivity, we clearly demonstrate that the extraction of the relaxation rate
is quite insensitive to the assumed shape of the quasiparticle spectrum. The
robustness of the analysis is rooted in the oscillator-strength sum rule and
the fact that we simultaneously measure the real and imaginary parts of the
conductivity. In both Bi2212 and Tl2201 we infer a linear temperature
dependence of the transport relaxation rate 1/tau and a small but finite
zero-temperature intercept. The linear temperature dependence of 1/tau is in
accord with expectations for weak elastic scattering in an unconventional
superconductor with line nodes and a small residual density of states. The same
analysis reveals an onset of inelastic scattering at higher temperatures
similar to that seen in the YBCO superconductors. Finally we extrapolate the
two-fluid model over a range of frequencies up to five times the measurement
frequency, where the extrapolation predicts behaviour that is qualitatively
similar to terahertz conductivity data on Bi2212 thin films. While relaxation
rates in Bi2212 and Tl2201 are substantially higher than in YBCO there are
qualitative similarities between all three materials, and the differences can
likely be attributed to varying levels of static disorder. We therefore
conclude that a universal picture of quasiparticle scattering in the cuprates
is emerging.Comment: 10 pages, 9 figure
Thermo-magnetic hysteretic properties resembling superconductivity in the normal state of La1.85Sr0.15CuO4
We have performed detailed magnetic and thermal hysteresis experiments in the
normal-state magnetization of La1.85Sr0.15CuO4 single crystal. Using a
combination of in-field and in-zero-magnetic-field measurements at different
stages of thermal history of the sample, we identified subtle effects
associated with the presence of magnetic signatures which resemble those below
the superconducting transition temperature (Tc=36 K) but survive up to 250 K.Comment: One file including text and figure
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