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, $\sigma =(2/\pi) 4 e^2/h$, 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

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 $d$ between the pick-up coil and the sample is investigated. The typical experimental situation corresponds to the large-$d$ 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-$d$ limit, are identified and the possibility of observing these features in experiments is discussed.Comment: 12 pages including 8 figures, submitted to Phys. Rev.

Can Short-Range Interactions Mediate a Bose Metal Phase in 2D?

We show here based on a 1-loop scaling analysis that short-range interactions are strongly irrelevant perturbations near the insulator-superconductor (IST) quantum critical point. The lack of any proof that short-range interactions mediate physics which is present only in strong coupling leads us to conclude that short-range interactions are strictly irrelevant near the IST quantum critical point. Hence, we argue that no new physics, such as the formation of a uniform Bose metal phase can arise from an interplay between on-site and nearest-neighbour interactions.Comment: 3 pages, 1 .eps file. SUbmitted to Phys. Rev.

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 $T/(T-T_c)$ 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

Transport Properties near the z=2 Insulator-Superconductor Transition

We consider here the fluctuation conductivity near the point of the insulator-superconductor transition in a system of regular Josephson junction arrays in the presence of particle-hole asymmetry or equivalently homogeneous charge frustration. The transition is characterised by the dynamic critical exponent $z=2$, opening the possibility of the perturbative renormalization-group (RG) treatment. The quartic interaction in the Ginzburg-Landau action and the coupling to the Ohmic heat bath, giving the finite quasiparticle life-time, lead to the non-monotonic behavior of the dc conductivity as a function of temperature in the leading logarithmic approximation.Comment: Revised version for publication. To appear in PR