4,933 research outputs found
Domain wall description of superconductivity
In the present work we shall address the issue of electrical conductivity in
superconductors in the perspective of superconducting domain wall solutions in
the realm of field theory. We take our set up made out of a dynamical complex
scalar field coupled to gauge field to be responsible for superconductivity and
an extra scalar real field that plays the role of superconducting domain walls.
The temperature of the system is interpreted through the fact that the soliton
following accelerating orbits is a Rindler observer experiencing a thermal
bath.Comment: 9 pages, 5 figures, Latex. Version to appear in PL
Spatial variations of the fine-structure constant in symmetron models
We investigate the variation of the fine-structure constant, {\alpha}, in
symmetron models using N-body simulations in which the full spatial
distribution of {\alpha} at different redshifts has been calculated. In
particular, we obtain simulated sky maps for this variation, and determine its
power spectrum. We find that in high-density regions of space (such as deep
inside dark matter halos) the value of {\alpha} approaches the value measured
on Earth. In the low-density outskirts of halos the scalar field value can
approach the symmetry breaking value and leads to significantly different
values of {\alpha}. If the scalar-photon coupling strength {\beta}{\gamma} is
of order unity we find that the variation of {\alpha} inside dark matter halos
can be of the same magnitude as the recent claims by Webb et al. of a dipole
variation. Importantly, our results also show that with low-redshift symmetry
breaking these models exhibit some dependence of {\alpha} on lookback time (as
opposed to a pure spatial dipole) which could in principle be detected by
sufficiently accurate spectroscopic measurements, such as those of ALMA and the
ELT-HIRES.Comment: 11 pages, 9 figure
Diamagnetic response of cylindrical normal metal - superconductor proximity structures with low concentration of scattering centers
We have investigated the diamagnetic response of composite NS proximity
wires, consisting of a clean silver or copper coating, in good electrical
contact to a superconducting niobium or tantalum core. The samples show strong
induced diamagnetism in the normal layer, resulting in a nearly complete
Meissner screening at low temperatures. The temperature dependence of the
linear diamagnetic susceptibility data is successfully described by the
quasiclassical Eilenberger theory including elastic scattering characterised by
a mean free path l. Using the mean free path as the only fit parameter we found
values of l in the range 0.1-1 of the normal metal layer thickness d_N, which
are in rough agreement with the ones obtained from residual resistivity
measurements. The fits are satisfactory over the whole temperature range
between 5 mK and 7 K for values of d_N varying between 1.6 my m and 30 my m.
Although a finite mean free path is necessary to correctly describe the
temperature dependence of the linear response diamagnetic susceptibility, the
measured breakdown fields in the nonlinear regime follow the temperature and
thickness dependence given by the clean limit theory. However, there is a
discrepancy in the absolute values. We argue that in order to reach
quantitative agreement one needs to take into account the mean free path from
the fits of the linear response. [PACS numbers: 74.50.+r, 74.80.-g]Comment: 10 pages, 9 figure
Vortex avalanches in the non-centrosymmetric superconductor Li2Pt3B
We investigated the vortex dynamics in the non-centrosymmetric superconductor
Li_2Pt_3B in the temperature range 0.1 K - 2.8 K. Two different logarithmic
creep regimes in the decay of the remanent magnetization from the Bean critical
state have been observed. In the first regime, the creep rate is
extraordinarily small, indicating the existence of a new, very effective
pinning mechanism. At a certain time a vortex avalanche occurs that increases
the logarithmic creep rate by a factor of about 5 to 10 depending on the
temperature. This may indicate that certain barriers against flux motion are
present and they can be opened under increased pressure exerted by the
vortices. A possible mechanism based on the barrier effect of twin boundaries
is briefly discussed
On the Stability of Fundamental Couplings in the Galaxy
Astrophysical tests of the stability of Nature's fundamental couplings are a
key probe of the standard paradigms in fundamental physics and cosmology. In
this report we discuss updated constraints on the stability of the
fine-structure constant and the proton-to-electron mass ratio
within the Galaxy. We revisit and improve upon the analysis by
Truppe {\it et al.} by allowing for the possibility of simultaneous variations
of both couplings and also by combining them with the recent measurements by
Levshakov {\it et al.} By considering representative unification scenarios we
find no evidence for variations of at the 0.4 ppm level, and of
at the 0.6 ppm level; if one uses the Levshakov bound on as a prior,
the bound is improved to 0.1 ppm. We also highlight how these
measurements can constrain (and discriminate among) several fundamental physics
paradigms.Comment: 7 pages, 1 figur
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