2,960 research outputs found
Broadband, unpolarized repumping and clearout light sources for Sr single-ion clocks
Future transportable optical clocks require compact and reliable light
sources. Here, broadband, unpolarized repumper and state clearout sources for
Sr+ single-ion optical clocks are reported. These turn-key devices require no
frequency stabilization nor external modulators. They are fiber based,
inexpensive, and compact. Key characteristics for clock operation are
presented, including optical spectra, induced light shifts and required
extinction ratios. Tests with an operating single-ion standard show a clearout
efficiency of 100%. Compared to a laser-based repumper, the achievable
fluorescence rates for ion detection are a few tens of per cent lower. The
resulting ion kinetic temperature is 1--1.5 mK, near the Doppler limit of the
ion system. Similar repumper light sources could be made for Ca+ (866 nm) and
Ba+ (650 nm) using semiconductor gain media.Comment: 4 pages, 6 figure
The Effect of Columnar Disorder on the Superconducting Transition of a Type-II Superconductor in Zero Applied Magnetic Field
We investigate the effect of random columnar disorder on the superconducting
phase transition of a type-II superconductor in zero applied magnetic field
using numerical simulations of three dimensional XY and vortex loop models. We
consider both an unscreened model, in which the bare magnetic penetration
length is approximated as infinite, and a strongly screened model, in which the
magnetic penetration length is of order the vortex core radius. We consider
both equilibrium and dynamic critical exponents. We show that, as in the
disorder free case, the equilibrium transitions of the unscreened and strongly
screened models lie in the same universality class, however scaling is now
anisotropic. We find for the correlation length exponent , and
for the anisotropy exponent . We find different dynamic
critical exponents for the unscreened and strongly screened models.Comment: 30 pages 12 ps figure
Theory of Magneto-resistance of Disordered Superconducting Films
Recent experimental studies of magneto-resistance in disordered
superconducting thin films reveal a huge peak (about 5 orders of magnitude).
While it may be expected that magnetic field destroys superconductivity,
leading to an enhanced resistance, attenuation of the resistance at higher
magnetic fields is surprising.
We propose a model which accounts for the experimental results in the entire
range of magnetic fields, based on the formation of superconducting islands due
to fluctuations in the superconducting order parameter amplitude. At strong
magnetic fields Coulomb blockade in these islands gives rise to negative
magneto-resistance. As the magnetic field is reduced the effect of Coulomb
blockade diminishes and eventually the magneto-resistance changes sign.
Numerical calculations show good qualitative agreement with experimental data.Comment: 4 pages, 4 figure
Localized Flux Lines and the Bose Glass
Columnar defects provide effective pinning centers for magnetic flux lines in
high-- superconductors. Utilizing a mapping of the statistical
mechanics of directed lines to the quantum mechanics of two--dimensional
bosons, one expects an entangled flux liquid phase at high temperatures,
separated by a second--order localization transition from a low--temperature
``Bose glass'' phase with infinite tilt modulus. Recent decoration experiments
have demonstrated that below the matching field the repulsive forces between
the vortices may be sufficiently large to produce strong spatial correlations
in the Bose glass. This is confirmed by numerical simulations, and a remarkably
wide soft ``Coulomb gap'' at the chemical potential is found in the
distribution of pinning energies. At low currents, the dominant transport
mechanism in the Bose glass phase proceeds via the formation of double kinks
between not necessarily adjacent columnar pins, similar to variable--range
hopping in disordered semiconductors. The strong correlation effects
originating in the long--range vortex interactions drastically reduce
variable--range hopping transport.Comment: 10 pages, latex ("lamuphys.sty" file included), 6 figures can be
obtained from the author ([email protected]); to appear in Proc. XIV
Sitges conference on "Complex Behaviour of Glassy Systems" (Springer--Verlag
Monte Carlo calculation of the current-voltage characteristics of a two dimensional lattice Coulomb gas
We have studied the nonlinear current-voltage characteristic of a two
dimensional lattice Coulomb gas by Monte Carlo simulation. We present three
different determinations of the power-law exponent of the nonlinear
current-voltage characteristic, . The determinations rely on
both equilibrium and non-equilibrium simulations. We find good agreement
between the different determinations, and our results also agree closely with
experimental results for Hg-Xe thin film superconductors and for certain single
crystal thin-film high temperature superconductors.Comment: late
Current--Voltage Characteristics of Two--Dimensional Vortex Glass Models
We have performed Monte Carlo simulations to determine current--voltage
characteristics of two different vortex glass models in two dimensions. The
results confirm the conclusions of earlier studies that there is a transition
at . In addition we find that, as , the linear resistance vanishes
exponentially, and the current scale, , where non-linearities appear in
the -- characteristics varies roughly as , quite different from the
predictions of conventional flux creep theory, . The results for
the two models agree quite well with each other, and also agree fairly well
with recent experiments on very thin films of YBCO.Comment: 18 pages with 10 figures available upon request from R. A. Hyman at
[email protected]. The only change in the new version is the
deletion of an unimportant comment.IUCM94-01
Vortex glass transition in a random pinning model
We study the vortex glass transition in disordered high temperature
superconductors using Monte Carlo simulations. We use a random pinning model
with strong point-correlated quenched disorder, a net applied magnetic field,
longrange vortex interactions, and periodic boundary conditions. From a finite
size scaling study of the helicity modulus, the RMS current, and the
resistivity, we obtain critical exponents at the phase transition. The new
exponents differ substantially from those of the gauge glass model, but are
consistent with those of the pure three-dimensional XY model.Comment: 7 pages RevTeX, 4 eps figure
Resistance scaling at the Kosterlitz-Thouless transition
We study the linear resistance at the Kosterlitz-Thouless transition by Monte
Carlo simulation of vortex dynamics. Finite size scaling analysis of our data
show excellent agreement with scaling properties of the Kosterlitz-Thouless
transition. We also compare our results for the linear resistance with
experiments. By adjusting the vortex chemical potential to an optimum value,
the resistance at temperatures above the transition temperature agrees well
with experiments over many decades.Comment: 7 pages, 4 postscript figures included, LATEX, KTH-CMT-94-00
Spin Bose Glass Phase in Bilayer Quantum Hall Systems at
We develop an effective spin theory to describe magnetic properties of the
Quantum Hall bilayer systems. In the absence of disorder this theory
gives quantitative agreement with the results of microscopic Hartree-Fock
calculations, and for finite disorder it predicts the existence of a novel spin
Bose glass phase. The Bose glass is characterized by the presence of domains of
canted antiferromagnetic phase with zero average antiferromagnetic order and
short range mean antiferromagnetic correlations. It has infinite
antiferromagnetic transverse susceptibility, finite longitudinal spin
susceptibility and specific heat linear in temperature. Transition from the
canted antiferromagnet phase to the spin Bose glass phase is characterized by a
universal value of the longitudinal spin conductance.Comment: 4 pages, 4 eps figure
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