523 research outputs found
Anomalous Quantum Diffusion at the Superfluid-Insulator Transition
We consider the problem of the superconductor-insulator transition in the
presence of disorder, assuming that the fermionic degrees of freedom can be
ignored so that the problem reduces to one of Cooper pair localization. Weak
disorder drives the critical behavior away from the pure critical point,
initially towards a diffusive fixed point. We consider the effects of Coulomb
interactions and quantum interference at this diffusive fixed point. Coulomb
interactions enhance the conductivity, in contrast to the situation for
fermions, essentially because the exchange interaction is opposite in sign. The
interaction-driven enhancement of the conductivity is larger than the
weak-localization suppression, so the system scales to a perfect conductor.
Thus, it is a consistent possibility for the critical resistivity at the
superconductor-insulator transition to be zero, but this value is only
approached logarithmically. We determine the values of the critical exponents
and comment on possible implications for the interpretation of
experiments
Evolution of the Density of States Gap in a Disordered Superconductor
It has only recently been possible to study the superconducting state in the
attractive Hubbard Hamiltonian via a direct observation of the formation of a
gap in the density of states N(w). Here we determine the effect of random
chemical potentials on N(w) and show that at weak coupling, disorder closes the
gap concurrently with the destruction of superconductivity. At larger, but
still intermediate coupling, a pseudo-gap in N(w) remains even well beyond the
point at which off-diagonal long range order vanishes. This change in the
elementary excitations of the insulating phase corresponds to a crossover
between Fermi- and Bose-Insulators. These calculations represent the first
computation of the density of states in a finite dimensional disordered fermion
model via the Quantum Monte Carlo and maximum entropy methods.Comment: 4 pages, 4 figure
Thickness-Magnetic Field Phase Diagram at the Superconductor-Insulator Transition in 2D
The superconductor-insulator transition in ultrathin films of amorphous Bi
was tuned by changing the film thickness, with and without an applied magnetic
field. The first experimentally obtained phase diagram is mapped as a function
of thickness and magnetic field in the T=0 limit. A finite size scaling
analysis has been carried out to determine the critical exponent product vz,
which was found to be 1.2 for the zero field transition, and 1.4 for the finite
field transition. Both results are different from the exponents found for the
magnetic field tuned transition in the same system, 0.7.Comment: 4 pages, 4 figure
Evidence of Vortices on the Insulating Side of the Superconductor-Insulator Transition
The magnetoresistance of ultrathin insulating films of Bi has been studied
with magnetic fields applied parallel and perpendicular to the plane of the
sample. Deep in the strongly localized regime, the magnetoresistance is
negative and independent of field orientation. As film thicknesses increase,
the magnetoresistance becomes positive, and a difference between values
measured in perpendicular and parallel fields appears, which is a linear
function of the magnetic field and is positive. This is not consistent with the
quantum interference picture. We suggest that it is due to vortices present on
the insulating side of the superconductor-insulator transition.Comment: 4 pages, 3 figure
Dual theory of the superfluid-Bose glass transition in disordered Bose-Hubbard model in one and two dimensions
I study the zero temperature phase transition between superfluid and
insulating ground states of the Bose-Hubbard model in a random chemical
potential and at large integer average number of particles per site. Duality
transformation maps the pure Bose-Hubbard model onto the sine-Gordon theory in
one dimension (1D), and onto the three dimensional Higgs electrodynamics in two
dimensions (2D). In 1D the random chemical potential in dual theory couples to
the space derivative of the dual field, and appears as a random magnetic field
along the imaginary time direction in 2D. I show that the transition from the
superfluid state in both 1D and 2D is always controlled by the random critical
point. This arises due to a coupling constant in the dual theory with replicas
which becomes generated at large distances by the random chemical potential,
and represents a relevant perturbation at the pure superfluid-Mott insulator
fixed point. At large distances the dual theory in 1D becomes equivalent to the
Haldane's macroscopic representation of disordered quantum fluid, where the
generated term is identified with random backscattering. In 2D the generated
coupling corresponds to the random mass of the complex field which represents
vortex loops. I calculate the critical exponents at the superfluid-Bose glass
fixed point in 2D to be \nu=1.38 and z=1.93, and the universal conductivity at
the transition \sigma_c = 0.26 e_{*}^2 /h, using the one-loop field-theoretic
renormalization group in fixed dimension.Comment: 25 pages, 6 Postscript figures, LaTex, references updated, typos
corrected, final version to appear in Phys. Rev. B, June 1, 199
Critical behavior at superconductor-insulator phase transitions near one dimension
I argue that the system of interacting bosons at zero temperature and in
random external potential possesses a simple critical point which describes the
proliferation of disorder-induced topological defects in the superfluid ground
state, and which is located at weak disorder close to and above one dimension.
This makes it possible to address the critical behavior at the superfluid-Bose
glass transition in dirty boson systems by expanding around the lower critical
dimension d=1. Within the formulated renormalization procedure near d=1 the
dynamical critical exponent is obtained exactly and the correlation length
exponent is calculated as a Laurent series in the parameter \sqrt{\epsilon},
with \epsilon=d-1: z=d, \nu=1/\sqrt{3\epsilon} for the short range, and z=1,
\nu=\sqrt{2/3\epsilon}, for the long-range Coulomb interaction between bosons.
The identified critical point should be stable against the residual
perturbations in the effective action for the superfluid, at least in
dimensions 1\leq d \leq 2, for both short-range and Coulomb interactions. For
the superfluid-Mott insulator transition in the system in a periodic potential
and at a commensurate density of bosons I find \nu=(1/2\sqrt{\epsilon})+
1/4+O(\sqrt{\epsilon}), which yields a result reasonably close to the known XY
critical exponent in d=2+1. The critical behavior of the superfluid density,
phonon velocity and the compressibility in the system with the short-range
interactions is discussed.Comment: 23 pages, 1 Postscript figure, LaTe
Detection of adeno-associated virus type 2 genome in cervical carcinoma
Adeno-associated virus (AAV) can impair the replication of other viruses. Adeno-associated virus seroprevalences have been reported to be lower among women with cervical cancer. In-vitro, AAV can interfere with the production of human papillomavirus virions. Adeno-associated virus-2 DNA has also been detected in cervical cancer tissue, although not consistently. To evaluate the role of AAV infection in relation to invasive cervical cancer, we performed a nested case–control study within a retrospectively followed population-based cohort. A total of 104 women who developed invasive cervical cancer on average 5.6 years of follow-up (range: 0.5 months–26.2 years) and 104 matched control-women who did not develop cervical cancer during the same follow-up time were tested for AAV and human papillomavirus by polymerase chain reaction. At baseline, two (2%) case-women and three (3%) control-women were positive for AAV-2 DNA. At the time of cancer diagnosis, 12 (12%) case-women and 3 (3%) matched control-women were positive for AAV-2 DNA. Persisting AAV infection was not evident. In conclusion, AAV-2 DNA was present in a low proportion of cervical cancers and we found no evidence that the presence of AAV in cervical smears of healthy women would be associated with reduced risk of cervical cancer
Cognitive Changes and Quality of Life in Neurocysticercosis: A Longitudinal Study
Neurocysticercosis (NCC) is one of the most common parasitic infections of the central nervous system. Cognitive changes have been frequently reported with this disease but have not been well studied. Our study team recruited a group of new onset NCC cases and a matched set of healthy neighborhood controls and new onset epilepsy controls in Lima, Peru for this study. A neuropsychological battery was administered at baseline and at 6 months to all groups. Brain MRI studies were also obtained on NCC cases at baseline and at 6 months. Newly diagnosed patients with NCC had mild cognitive deficits and more marked decreases in quality of life at baseline compared with controls. Improvements were found in both cognitive status and quality of life in patients with NCC after treatment. This study is the first to assess cognitive status and quality of life longitudinally in patients with NCC and provides new data on an important clinical morbidity outcome
The Pioneer Anomaly
Radio-metric Doppler tracking data received from the Pioneer 10 and 11
spacecraft from heliocentric distances of 20-70 AU has consistently indicated
the presence of a small, anomalous, blue-shifted frequency drift uniformly
changing with a rate of ~6 x 10^{-9} Hz/s. Ultimately, the drift was
interpreted as a constant sunward deceleration of each particular spacecraft at
the level of a_P = (8.74 +/- 1.33) x 10^{-10} m/s^2. This apparent violation of
the Newton's gravitational inverse-square law has become known as the Pioneer
anomaly; the nature of this anomaly remains unexplained. In this review, we
summarize the current knowledge of the physical properties of the anomaly and
the conditions that led to its detection and characterization. We review
various mechanisms proposed to explain the anomaly and discuss the current
state of efforts to determine its nature. A comprehensive new investigation of
the anomalous behavior of the two Pioneers has begun recently. The new efforts
rely on the much-extended set of radio-metric Doppler data for both spacecraft
in conjunction with the newly available complete record of their telemetry
files and a large archive of original project documentation. As the new study
is yet to report its findings, this review provides the necessary background
for the new results to appear in the near future. In particular, we provide a
significant amount of information on the design, operations and behavior of the
two Pioneers during their entire missions, including descriptions of various
data formats and techniques used for their navigation and radio-science data
analysis. As most of this information was recovered relatively recently, it was
not used in the previous studies of the Pioneer anomaly, but it is critical for
the new investigation.Comment: 165 pages, 40 figures, 16 tables; accepted for publication in Living
Reviews in Relativit
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