arXiv subjects

Gergely T. Zimanyi

Publications and source records attributed to Gergely T. Zimanyi.

25 records · Page 2Linked to original sources

arXiv1997

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.

cond-mat.supr-con↗

arXiv1997

Revisiting the Theory of Finite Size Scaling in Disordered Systems: νCan Be Less Than 2/d

For phase transitions in disordered systems, an exact theorem provides a bound on the finite size correlation length exponent: ν_{FS}<= 2/d. It is believed that the true critical exponent νof a disorder induced phase transition satisfies the same bound. We argue that in disordered systems the standard averaging introduces a noise, and a corresponding new diverging length scale, characterized by ν_{FS}=2/d. This length scale, however, is independent of the system's own correlation length ξ. Therefore νcan be less than 2/d. We illustrate these ideas on two exact examples, with ν< 2/d. We propose a new method of disorder averaging, which achieves a remarkable noise reduction, and thus is able to capture the true exponents.

cond-mat.dis-nn↗

arXiv1997

Direct Mott Insulator-to-Superfluid Transition in the Presence of Disorder

We introduce a new renormalization group theory to examine the quantum phase transitions upon exiting the insulating phase of a disordered, strongly interacting boson system. For weak disorder we find a direct transition from this Mott insulator to the Superfluid phase. In d > 4 a finite region around the particle-hole symmetric point supports this direct transition, whereas for 2=< d <4 perturbative arguments suggest that the direct transition survives only precisely at commensurate filling. For strong disorder the renormalization trajectories pass next to two fixed points, describing a pair of distinct transitions; first from the Mott insulator to the Bose glass, and then from the Bose glass to the Superfluid. The latter fixed point possesses statistical particle-hole symmetry and a dynamical exponent z, equal to the dimension d.

cond-mat↗

arXiv1996

Quantum Fluctuations and Resistivity of Thin Superconducting Wires

We present a microscopic study of the quantum fluctuations of the superconducting order parameter in thin homogeneous superconducting wires at all temperatures below $T_C$. The rate of quantum phase slip processes determines the resistance $R(T)$ of the wire, which is observable in very thin wires, even at low temperature. Furthermore, we predict a new low temperature metallic phase below a critical wire-thickness in the 10nm range, in which quantum phase slips proliferate.

cond-mat↗

arXiv1996

The Moving Glass Phase of Dirty Type II Superconductors

We study numerically the motion of vortices in dirty type II superconductors. In two dimensions at strong driving currents, vortices form highly correlated ``static channels''. The static structure factor exhibits convincing scaling behaviour, demonstrating quasi long range translational order in the transverse direction. However order in the longitudinal direction is only short range. We clearly establish the existence of a finite transverse critical current, suggesting strong barriers against small transverse driving forces. These results are most consistently interpreted in terms of the recently proposed moving glass picture, modified to account for the strong anisotropy.

cond-mat↗

arXiv1996

Theory for Quantum Spin and Vortex Glasses

We develop a theory for the quantum vortex glass, with both the coupling strengths and the site energies disordered. This model is closely related to XY spin glasses and bosons in random media. For properly chosen distributions of the site disorder the phase diagram consists of a superfluid phase, and Weak and Strong Glass regions, dominated by long range and local fluctuations, respectively. The most direct experimental manifestation of the Strong Glass transition is the lack of divergence of the corresponding non-linear susceptibility.

cond-mat↗

arXiv1994

Tunneling in the Electron Box in the Nonperturbative Regime

We study charging effects and tunneling in the single electron box. Tunneling mixes different charge states and in the nonperturbative regime the charge in the island may be strongly screened. When charge states are nearly degenerate the screening of the charge is strong even in the weak tunneling regime. Virtual tunneling processes reduce both the level splitting and the tunneling strength. The charge on the island and the decay rates are calculated. In the strong tunneling regime also nondegenerate states are affected by tunneling. Strong-coupling scaling renormalizes the effective capacitance, a result which we confirm by Monte Carlo simulations. The tunneling strength scales to smaller values into the regime where the weak-coupling scaling applies. We propose a two stage scaling procedure providing the unified picture for the problem. The scaling analysis is also extended to superconducting tunnel junctions with finite subgap conductance. (9 figures upon request)

cond-mat↗