microfilm:

A model for quantized gravity coupled to matter in the form of a single scalar field is investigated in four dimensions. For the metric degrees of freedom we employ Regge’s simplicial discretization, with the scalar field defined at the vertices of the four-simplices. We examine how the continuous phase transition found earlier, separating the smooth from the rough phase of quantized gravity, is influenced by the presence of scalar matter. A determination of the critical exponents seems to indicate that the effects of matter are rather small, unless the number of scalar flavors is large. Close to the critical point where the average curvature approaches zero, the coupling of matter to gravity is found to be weak. The nature of the phase diagram and the values for the critical exponents suggest that gravitational interactions increase with distance. 1.

The lattice formulation of quantum gravity provides a natural framework in which nonperturbative properties of the ground state can be studied in detail. In this paper we investigate how the lattice results relate to the continuum semiclassical expansion about smooth manifolds. As an example we give an explicit form for the lattice ground-state wave functional for semiclassical geometries. We then do a detailed comparison between the more recent predictions from the lattice regularized theory and results obtained in the continuum for the nontrivial ultraviolet fixed point of quantum gravity found using weak field and nonperturbative methods. In particular we focus on the derivative of the beta function at the fixed point and the related universal critical exponent for gravitation. Based on recently available lattice and continuum results we assess the evidence for the presence of a massless spin-two particle in the continuum limit of the strongly coupled lattice theory. Finally we compare the lattice prediction for the vacuum-polarization induced weak scale dependence of the gravitational coupling with recent calculations in the continuum, finding similar effects.

The dual approach to the Ginzburg-Landau theory of a Bardeen-Cooper-Schrieffer superconductor is reviewed. The dual theory describes a grand canonical ensemble of fluctuating closed magnetic vortices, of arbitrary length and shape, which interact with a massive vector field representing the local magnetic induction. When the critical temperature is approached from below, the magnetic vortices proliferate. This is signaled by the disorder field, which describes the loop gas, developing a non-zero expectation value in the normal conducting phase. It thereby breaks a global U(1) symmetry. The ensuing Goldstone field is the magnetic scalar potential. The superconducting-tonormal phase transition is studied by applying renormalization group theory to the dual formulation. In the regime of a second-order transition, the critical exponents are given by those of a superfluid with a reversed temperature axis. I.

We investigate, by experiments and numerical simulations, thermal activation processes of Josephson tunnel junctions in the presence of microwave radiation. When the applied signal resonates with the Josephson plasma frequency oscillations, the switching current may become multi-valued in a temperature range far exceeding the classical to quantum crossover temperature. Plots of the switching currents traced as a function of the applied signal frequency show very good agreement with the functional forms expected from Josephson plasma frequency dependencies on the bias current. Throughout, numerical simulations of the corresponding thermally driven classical Josephson junction model show very good agreement with the experimental data. 1 The Josephson tunnel junction is an intriguing solid state physics system due to the macroscopic quantum nature of the variables describing the governing equations [1]; irradiation of junctions with microwave (ac) radiation has produced a number of significant nonlinear phenomena such as chaos and phase-locking observed both in experiments and theoretical

Abstract. We present evidence for a close analogy between the nonlinear behavior of a pulsed microwave-driven Josephson junction at low temperature and the experimentally observed behavior of Josephson systems operated below the quantum transition temperature under similar conditions. We specifically address observations of Ramsey-type fringe oscillations, which can be understood in classical nonlinear dynamics as results of slow transient oscillations in a pulsed microwave environment. Simulations are conducted to mimic experimental measurements by recording the statistics of microwave-induced escape events from the anharmonic potential well of a zero-voltage state. Observations consistent with experimentally found Ramsey-type oscillations are found in the classical model. 1.

Conservation laws preserving algorithms for spin dynamics simulations

Abstract. We present evidence for a close analogy between the nonlinear behavior of a pulsed microwave-driven Josephson junction at low temperature and the experimentally observed behavior of Josephson systems operated below the quantum transition temperature under similar conditions. We specifically address observations of Ramsey-type fringe oscillations, which can be understood in classical nonlinear dynamics as results of slow transient oscillations in a pulsed microwave environment. Simulations are conducted to mimic experimental measurements by recording the statistics of microwave-induced escape events from the anharmonic potential well of a zero-voltage state. Observations consistent with experimentally found Ramsey-type oscillations are found in the classical model. 1.

We study the flux noise SΦ(ω) and finite frequency conductivity σ1(ω) in two dimensional unfrustrated Josephson junction arrays (JJA’s), by numerically solving the equations of the coupled overdamped resistively-shuntedjunction model with Langevin noise. We find that SΦ(ω) ∝ ω−3/2 at high frequencies ω and flattens at low ω, indicative of vortex diffusion, while σ1 ∝ ω−2 at sufficiently high ω. Both quantities show clear evidence of critical slowing down and possibly scaling behavior near the Kosterlitz-Thouless-Berezinskii (KTB) transition. The critical slowing down of SΦ, but not its frequency dependence, is in agreement with recent experiments on Josephson junction arrays. PACS numbers: 74.50.+r, 74.40.+k, 64.60.Fr 1 Josephson junction arrays (JJA’s) and thin-film superconductors are excellent model systems for studying vortex dynamics. At zero magnetic field, such systems are believed to undergo a Kosterlitz-Thouless-Berezinskii 1,2 (KTB) transition at a temperature TKTB. At temperatures below TKTB the vortices and anti-vortices are bound into pairs, whereas

Abstract. We investigate the results of recently published experiments on the quantum behavior of Josephson circuits in terms of the classical modelling based on the resistively and capacitivelyshunted (RCSJ) junction model. Our analysis shows evidence for a close analogy between the nonlinear behavior of a pulsed microwave-driven Josephson junction at low temperature and low dissipation and the experimental observations reported for the Josephson circuits. Specifically, we demonstrate that Rabi-oscillations, Ramsey-fringes, and spin-echo observations are not phenomena with a unique quantum interpretation. In fact, they are natural consequences of transients to phase-locking in classical nonlinear dynamics and can be observed in a purely classical model of a Josephson junction when the experimental recipe for the application of microwaves is followed and the experimental detection scheme followed. We therefore conclude that classical nonlinear dynamics can contribute to the understanding of relevant experimental observations of Josephson response to various microwave perturbations at very low temperature and low dissipation. 1.