59 research outputs found
Thermally fluctuating superconductors in two dimensions
We describe the different regimes of finite temperature dynamics in the
vicinity of a zero temperature superconductor to insulator quantum phase
transition in two dimensions. New results are obtained for a low temperature
phase-only hydrodynamics, and for the intermediate temperature quantum-critical
region. In the latter case, we obtain a universal relationship between the
frequency-dependence of the conductivity and the value of the d.c. resistance.Comment: Presentation completely revised; 4 pages, 2 figure
The Superconductor-Insulator Transition in a Tunable Dissipative Environment
We study the influence of a tunable dissipative environment on the dynamics
of Josephson junction arrays near the superconductor-insulator transition. The
experimental realization of the environment is a two dimensional electron gas
coupled capacitively to the array. This setup allows for the well-controlled
tuning of the dissipation by changing the resistance of the two dimensional
electron gas. The capacitive coupling cuts off the dissipation at low
frequencies. We determine the phase diagram and calculate the temperature and
dissipation dependence of the array conductivity. We find good agreement with
recent experimental results.Comment: 4 pages, 4 .eps figures, revte
Interaction-induced Bose Metal in 2D
We show here that the regularization of the conductivity resulting from the
bosonic interactions on the `insulating' (quantum disordered) side of an
insulator-superconductor transition in 2D gives rise to a metal with a finite
conductivity, , as temperature tends to zero. The Bose
metal is stable to weak disorder and hence represents a concrete example of an
interaction-induced metallic phase. Phenomenological inclusion of dissipation
reinstates the anticipated insulating behaviour in the quantum-disordered
regime. Hence, we conclude that the traditionally-studied
insulator-superconductor transition, which is driven solely by quantum
fluctuations, corresponds to a superconductor-metal transition. The possible
relationship to experiments on superconducting thin films in which a
low-temperature metallic phase has been observed is discussed.Comment: A figure has been added and the physics has been clarified. To appear
in PR
On the Coexistence of Diagonal and off-Diagonal Long-Range Order, a Monte Carlo Study
The zero temperature properties of interacting 2 dimensional lattice bosons
are investigated. We present Monte Carlo data for soft-core bosons that
demonstrate the existence of a phase in which crystalline long-range order and
off-diagonal long-range order (superfluidity) coexist. We comment on the
difference between hard and soft-core bosons and compare our data to mean-field
results that predict a larger coexistence region. Furthermore, we determine the
critical exponents for the various phase transitions.Comment: 7 pages and 8 figures appended in postscript, KA-TFP-93-0
Molecular Hydrogen Emission from Protoplanetary Disks II. Effects of X-ray Irradiation and Dust Evolution
Detailed models for the density and temperature profiles of gas and dust in
protoplanetary disks are constructed by taking into account X-ray and
ultraviolet (UV) irradiation from a central T Tauri star, as well as dust size
growth and settling toward the disk midplane. The spatial and size
distributions of dust grains in the disks are numerically computed by solving
the coagulation equation for settling dust particles. The level populations and
line emission of molecular hydrogen are calculated using the derived physical
structure of the disks. X-ray irradiation is the dominant heating source of the
gas in the inner disk region and in the surface layer, while the far UV heating
dominates otherwise. If the central star has strong X-ray and weak UV
radiation, the H2 level populations are controlled by X-ray pumping, and the
X-ray induced transition lines could be observable. If the UV irradiation is
strong, the level populations are controlled by thermal collisions or UV
pumping, depending on the properties of the dust grains in the disks. As the
dust particles evolve in the disks, the gas temperature at the disk surface
drops because the grain photoelectric heating becomes less efficient, while the
UV radiation fields become stronger due to the decrease of grain opacity. This
makes the H2 level populations change from local thermodynamic equilibrium
(LTE) to non-LTE distributions, which results in changes to the line ratios of
H2 emission. Our results suggest that dust evolution in protoplanetary disks
could be observable through the H2 line ratios. The emission lines are strong
from disks irradiated by strong UV and X-rays and possessing small dust grains;
such disks will be good targets in which to observe H2 emission.Comment: 33 pages, accepted for publication in the Astrophysical Journa
Fluctuation Conductivity in Insulator-Superconductor Transitions with Dissipation
We analyze here the fluctuation conductivity in the vicinity of the critical
point in a 2D Josephson junction array shunted by an Ohmic resistor.We find
that at the Gaussian level, the conductivity acquires a logarithmic dependence
on when the dissipation is sufficiently small. In the renormalized
classical regime, this logarithmic dependence gives rise to a leveling-off of
the resistivity at low to intermediate temperatures when fluctuations are
included. We show, however, that this trend does not persist to T=0 at which
point the resistivity vanishes. The possible relationship of the leveling of
the resistivity to the low temperature transport in granlar superconductors is
discussed.Comment: 4 page
Flux-noise spectra around the Kosterlitz-Thouless transition for two-dimensional superconductors
The flux-noise spectra around the Kosterlitz-Thouless transition are obtained
from simulations of the two-dimensional resistively shunted junction model. In
particular the dependence on the distance between the pick-up coil and the
sample is investigated. The typical experimental situation corresponds to the
large- limit and a simple relation valid in this limit between the complex
impedance and the noise spectra is clarified. Features, which distinguish
between the large- and small- limit, are identified and the possibility of
observing these features in experiments is discussed.Comment: 12 pages including 8 figures, submitted to Phys. Rev.
Giant Shapiro steps for two-dimensional Josephson-junction arrays with time-dependent Ginzburg-Landau dynamics
Two-dimensional Josephson junction arrays at zero temperature are
investigated numerically within the resistively shunted junction (RSJ) model
and the time-dependent Ginzburg-Landau (TDGL) model with global conservation of
current implemented through the fluctuating twist boundary condition (FTBC).
Fractional giant Shapiro steps are found for {\em both} the RSJ and TDGL cases.
This implies that the local current conservation, on which the RSJ model is
based, can be relaxed to the TDGL dynamics with only global current
conservation, without changing the sequence of Shapiro steps. However, when the
maximum widths of the steps are compared for the two models some qualitative
differences are found at higher frequencies. The critical current is also
calculated and comparisons with earlier results are made. It is found that the
FTBC is a more adequate boundary condition than the conventional uniform
current injection method because it minimizes the influence of the boundary.Comment: 6 pages including 4 figures in two columns, final versio
Vortex Fluctuations in High-Tc Films: Flux Noise Spectrum and Complex Impedance
The flux noise spectrum and complex impedance for a 500 {\AA} thick YBCO film
are measured and compared with predictions for two dimensional vortex
fluctuations. It is verified that the complex impedance and the flux noise
spectra are proportional to each other, that the logarithm of the flux noise
spectra for different temperatures has a common tangent with slope , and that the amplitude of the noise decreases as , where is
the height above the film at which the magnetic flux is measured. A crossover
from normal to anomalous vortex diffusion is indicated by the measurements and
is discussed in terms of a two-dimensional decoupling.Comment: 5 pages including 4 figures in two columns, to appear in Phys. Rev.
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