arXiv subjects

Enzo Granato

Publications and source records attributed to Enzo Granato.

32 records · Page 2Linked to original sources

arXiv2004

Nonlinear sliding friction of adsorbed overlayers on disordered substrates

We study the response of an adsorbed monolayer on a disordered substrate under a driving force using Brownian molecular-dynamics simulation. We find that the sharp longitudinal and transverse depinning transitions with hysteresis still persist in the presence of weak disorder. However, the transitions are smeared out in the strong disorder limit. The theoretical results here provide a natural explanation for the recent data for the depinning transition of Kr films on gold substrate.

cond-mat.mtrl-sci↗

arXiv2003

Resistive transition in $π$-junction superconductors

The resistivity behavior of inhomogeneous superconductors with random $π$ junctions, as in high-$T_c$ materials with d-wave symmetry, is studied by numerical simulation of a three-dimensional XY spin glass model. Above a concentration threshold of antiferromagnetic couplings, a resistive transition is found in the chiral-glass phase at finite temperatures and the critical exponents are determined from dynamic scaling analysis. The power-law exponent for the nonlinear contribution found in recent resistivity measurements is determined by the dynamic critical exponent of this transition.

cond-mat.supr-con↗

arXiv2003

Two dynamic exponents in the resistive transition of fully frustrated Josephson-junction arrays

We study the resistive transition in Josephson-junction arrays at $f=1/2$ flux quantum per plaquette by dynamical simulations of the resistively-shunted-junction model. The current-voltage scaling and critical dynamics of the phases are found to be well described by the same critical temperature and static exponents as for the chiral (vortex-lattice) transition. Although this behavior is consistent with a single transition scenario, where phase and chiral variables order simultaneously, two different dynamic exponents result for phase coherence and chiral order.

cond-mat.supr-con↗

arXiv2000

Phase-coherence transition in granular superconductors with $π$ junctions

We study the three-dimensional XY-spin glass as a model for the resistive behavior of granular superconductors containing a random distribution of $π$ junctions, as in high-$T_c$ superconducting materials with d-wave symmetry. The $π$ junctions leads to quenched in circulating currents (chiralities) and to a chiral-glass state at low temperatures, even in the absence of an external magnetic field. Dynamical simulations in the phase representation are used to determine the nonlinear current-voltage characteristics as a function of temperature. Based on dynamic scaling analysis, we find a phase-coherence transition at finite temperature below which the linear resistivity should vanish and determine the corresponding critical exponents. The results suggest that the phase and chiralities may order simultaneously for decreasing temperatures into a superconducting chiral-glass state.

cond-mat.supr-con↗

arXiv2000

Phase-coherence threshold and vortex-glass state in diluted Josephson-junction arrays in a magnetic field

We study numerically the interplay of phase coherence and vortex-glass state in two-dimensional Josephson-junction arrays with average rational values of flux quantum per plaquette $f$ and random dilution of junctions. For $f=1/2$, we find evidence of a phase coherence threshold value $x_s$, below the percolation concentration of diluted junctions $x_p$, where the superconducting transition vanishes. For $x_s < x < x_p$ the array behaves as a zero-temperature vortex glass with nonzero linear resistance at finite temperatures. The zero-temperature critical currents are insensitive to variations in $f$ in the vortex glass region while they are strongly $f$ dependent in the phase coherent region.

cond-mat.supr-con↗

arXiv2000

Transverse thermal depinning and nonlinear sliding friction of an adsorbed monolayer

We study the response of an adsorbed monolayer under a driving force as a model of sliding friction phenomena between two crystalline surfaces with a boundary lubrication layer. Using Langevin-dynamics simulation, we determine the nonlinear response in the direction transverse to a high symmetry direction along which the layer is already sliding. We find that below a finite transition temperature, there exist a critical depinning force and hysteresis effects in the transverse response in the dynamical state when the adlayer is sliding smoothly along the longitudinal direction.

cond-mat.stat-mech↗

arXiv1999

Nonlinear response scaling of the two-dimensional XY spin glass in the Coulomb-gas representation

Vortex critical dynamics of the two dimensional XY spin glass is studied by Monte Carlo methods in the Coulomb-gas representation. A scaling analysis of the nonlinear response is used to calculate the correlation length exponent $ν$ of the zero-temperature glass transition. The estimate, $ν=1.3(2)$, is in agreement witth a recent estimate in the phase representation using the same analysis and indicates that the relevant length scale for vortex motion is set by the spin-glass correlation length and that spin and chiralities may order with different correlation length exponents.

cond-mat.stat-mech↗

arXiv1998

Dynamical transitions and sliding friction in the two-dimensional Frenkel-Kontorova model

The nonlinear response of an adsorbed layer on a periodic substrate to an external force is studied via a two dimensional uniaxial Frenkel-Kontorova model. The nonequlibrium properties of the model are simulated by Brownian molecular dynamics. Dynamical phase transitions between pinned solid, sliding commensurate and incommensurate solids and hysteresis effects are found that are qualitatively similar to the results for a Lennard-Jones model, thus demonstrating the universal nature of these features.

cond-mat.stat-mech↗

arXiv1998

Charge glass in two-dimensional arrays of capacitively coupled grains with random offset charges

We study the effect of random offset charges in the insulator to conductor transition in systems of capacitively coupled grains, as realized in two-dimensional arrays of ultrasmall Josephson junctions. In presence of disorder, the conductive transition and charge ordering at nonzero gate voltages are both destroyed for any degree of disorder $σ$ at finite temperatures $T$, in the thermodynamic limit, but crossover effects will dominate at length scales smaller than $ξ_{σ, T}=T^{-ν} f(σT^{-ν})$, where $ν$ is the thermal critical exponent of the zero-temperature charge glass transition. The conductance is linear and thermally activated but nonlinear behavior sets in at a crossover voltage which decreases as temperature decreases. For large disorder, the results are supported by Monte Carlo dynamics simulations of a Coulomb gas with offset charges and are consistent with the thermally activated behavior found in recent experiments.

cond-mat.dis-nn↗

arXiv1998

Current-voltage scaling of chiral and gauge-glass models of two-dimensional superconductors

The scaling behavior of the current-voltage characteristics of chiral and gauge glass models of disordered superconductors, are studied numerically, in two dimensions. For both models, the linear resistance is nonzero at finite temperatures and the scaling analysis of the nonlinear resistivity is consistent with a phase transition at T=0 temperature characterized by a diverging correlation length $ξ\propto T^{-ν_{T}}$ and thermal critical exponent $ν_{T}$. The values of $ν_{T}$, however, are found to be different for the chiral and gauge glass models, suggesting different universality classes, in contrast to the result obtained recently in three dimensions.

cond-mat.supr-con↗

arXiv1997

Current-voltage characteristics of diluted Josephson-junction arrays: scaling behavior at current and percolation threshold

Dynamical simulations and scaling arguments are used to study the current-voltage (IV) characteristics of a two-dimensional model of resistively shunted Josephson-junction arrays in presence of percolative disorder, at zero external field. Two different limits of the Josephson-coupling concentration $p$ are considered, where $p_c$ is the percolation threshold. For $p$ $>$ $p_c$ and zero temperature, the IV curves show power-law behavior above a disorder dependent critical current. The power-law behavior and critical exponents are consistent with a simple scaling analysis. At $p_c$ and finite temperature $T$, the results show the scaling behavior of a T=0 superconducting transition. The resistance is linear but vanishes for decreasing $T$ with an apparent exponential behavior. Crossover to non-linearity appears at currents proportional to $% T^{1+ν_T}$, with a thermal-correlation length exponent $ν_T$ consistent with the corresponding value for the diluted XY model at $p_c$.

cond-mat.supr-con↗

arXiv1996

Current-voltage scaling of a Josephson-junction array at irrational frustration

Numerical simulations of the current-voltage characteristics of an ordered two-dimensional Josephson junction array at an irrational flux quantum per plaquette are presented. The results are consistent with an scaling analysis which assumes a zero temperature vortex glass transition. The thermal correlation length exponent characterizing this transition is found to be significantly different from the corresponding value for vortex-glass models in disordered two-dimensional superconductors. This leads to a current scale where nonlinearities appear in the current-voltage characteristics decreasing with temperature $T$ roughly as $T^2$ in contrast with the $T^3$ behavior expected for disordered models.

cond-mat↗

arXiv1995

Critical behavior of Josephson-junction arrays at f=1/2

The critical behavior of frustrated Josephson-junction arrays at $f=1/2$ flux quantum per plaquette is considered. Results from Monte Carlo simulations and transfer matrix computations support the identification of the critical behavior of the square and triangular classical arrays and the one-dimensional quantum ladder with the universality class of the XY-Ising model. In the quantum ladder, the transition can happen either as a simultaneous ordering of the $Z_2$ and $U(1)$ order parameters or in two separate stages, depending on the ratio between interchain and intrachain Josephson couplings. For the classical arrays, weak random plaquette disorder acts like a random field and positional disorder as random bonds on the $Z_2$ variables. Increasing positional disorder decouples the $Z_2$ and $U(1)$ variables leading to the same critical behavior as for integer $f$.

cond-mat↗

arXiv1995

Quantum critical phenomena in ladders of Josephson junctions in a magnetic field

A model of a ladder of Josephson junctions in a magnetic field is considered. The topological features of the zero-temperature phase diagram, as a function of charging energy and small deviations from commensurability of the vortex lattice, are strongly dependent on the field. In addition to superconductor-insulator transitions, commensurate-incommensurate transitions and depinning by quantum fluctuations of the vortex-lattice are also possible. the critical behavior of the superconductor-insulator transition at f = 1/2 is found to depend on the ratio between interchain and intrachain Josephson couplings and is in the universality of the classical XY-Ising model.

cond-mat↗