arXiv subjects

Enzo Granato

Publications and source records attributed to Enzo Granato.

At least 19 recordsLinked to original sources

arXiv2021

Global exploration of phase behavior in frustrated Ising models using unsupervised learning techniques

We apply a set of machine-learning (ML) techniques for the global exploration of the phase diagrams of two frustrated 2D Ising models with competing interactions. Based on raw Monte Carlo spin configurations generated for random system parameters, we apply principal-component analysis (PCA) and auto-encoders to achieve dimensionality reduction, followed by clustering using the DBSCAN method and a support-vector machine classifier to construct the transition lines between the distinct phases in both models. The results are in very good agreement with available exact solutions, with the auto-encoders leading to quantitatively superior estimates, even for a data set containing only 1400 spin configurations. In addition, the results suggest the existence of a relationship between the structure of the optimized auto-encoder latent space and physical characteristics of both systems. This indicates that the employed approach can be useful in perceiving fundamental properties of physical systems in situations where \emph{a priori} theoretical insight is unavailable.

cond-mat.stat-mech↗

arXiv2020

Disorder-induced superconductor to insulator transition and finite phase stiffness in two-dimensional phase-glass models

We study numerically the superconductor to insulator transition in two-dimensional phase-glass (or chiral-glass) models with varying degree of disorder. These models describe the effects of gauge disorder in superconductors due to random negative Josephson-junction couplings, or $π$ junctions. Two different models are considered, with binary and Gaussian distribution of quenched disorder, having nonzero mean. Monte Carlo simulations in the path-integral representation are used to determine the phase diagram and critical exponents. In addition to the usual superconducting and insulating phases, a chiral-glass phase occurs for sufficiently large disorder, with random local circulating currents of different chiralities. A transition from superconductor to insulator can take place via the intermediate chiral-glass phase. We find, however, that the chiral-glass state has a finite phase stiffness, being still a superconductor, instead of the Bose metal, which has been suggested by mean-field theory.

cond-mat.supr-con↗

arXiv2019

Critical exponents and fine-grid vortex model of the dynamic vortex Mott transition in superconducting arrays

We study the dynamic vortex Mott transition in two-dimensional superconducting arrays in a magnetic field with $f$ flux quantum per plaquette. The transition is induced by external driving current and thermal fluctuations near rational vortex densities set by the value of $f$, and has been observed experimentally from the scaling behavior of the differential resistivity. Recently, numerical simulations of interacting vortex models have demonstrated this behavior only near fractional $f$. A fine-grid vortex model is introduced, which allows to consider both the cases of fractional and integer $f$. The critical behavior is determined from a scaling analysis of the current-voltage relation and voltage correlations near the transition, and by Monte Carlo simulations. The critical exponents for the transition near $f=1/2$ are consistent with the experimental observations and previous numerical results from a standard vortex model. The same scaling behavior is obtained for $f=1$, in agreement with experiments. However, the estimated correlation-length exponent indicates that even at integer $f$, the critical behavior is not of mean-field type.

cond-mat.supr-con↗

arXiv2018

Critical behavior of dynamic vortex Mott transition in superconducting arrays at fractional vortex densities

We study the differential resistivity transition of two-dimensional superconducting arrays induced by an external driving current, in the presence of thermal fluctuations and a magnetic field corresponding to $f$ flux quantum per plaquette. Recent experiments have identified this transition as a dynamic vortex Mott insulator transition at vortex densities near rational values of $f$. The critical behavior is determined from a scaling analysis of the current-voltage relation near the transition, obtained by Monte Carlo simulations of a Josephson-junction array model in the vortex representation. For a square-lattice array, the critical exponents obtained near $f=1/2$ are consistent with the experimental observations. The same scaling behavior is observed near $f=1/3$. For a honeycomb array, although similar results are obtained for $f=1/3$, the transition is absent for $f=1/2$, consistent with an incommensurate vortex phase.

cond-mat.supr-con↗

arXiv2017

Random gauge models of the superconductor-insulator transition in two-dimensional disordered superconductors

We study numerically the superconductor-insulator transition in two-dimensional inhomogeneous superconductors with gauge disorder, described by four different quantum rotor models: a gauge glass, a flux glass, a binary phase glass and a Gaussian phase glass. The first two models, describe the combined effect of geometrical disorder in the array of local superconducting islands and a uniform external magnetic field while the last two describe the effects of random negative Josephson-junction couplings or $π$ junctions. Monte Carlo simulations in the path-integral representation of the models are used to determine the critical exponents and the universal conductivity at the quantum phase transition. The gauge and flux glass models display the same critical behavior, within the estimated numerical uncertainties. Similar agreement is found for the binary and Gaussian phase-glass models. Despite the different symmetries and disorder correlations, we find that the universal conductivity of these models is approximately the same. In particular, the ratio of this value to that of the pure model agrees with recent experiments on nanohole thin film superconductors in a magnetic field, in the large disorder limit.

cond-mat.supr-con↗

arXiv2016

Magnetic flux disorder and superconductor-insulator transition in nanohole thin films

We study the superconductor-insulator transition in nanohole ultrathin films in a transverse magnetic field by numerical simulation of a Josephson-junction array model. Geometrical disorder due to the random location of nanoholes in the film corresponds to random flux in the array model. Monte Carlo simulation in the path-integral representation is used to determine the critical behavior and the universal resistivity at the transition as a function of disorder and average number of flux quanta per cell, $f_o$. The resistivity increases with disorder for noninteger $ f_o$ while it decreases for integer $ f_o$, and reaches a common constant value in a vortex-glass regime above a critical value of the flux disorder $D_f^c$. The estimate of $D_f^c$ and the resistivity increase for noninteger $ f_o$ are consistent with recent experiments on ultrathin superconducting films with positional disordered nanoholes.

cond-mat.supr-con↗

arXiv2016

Superconductor-insulator transition of Josephson-junction arrays on a honeycomb lattice in a magnetic field

We study the superconductor to insulator transition at zero temperature in a Josephson-junction array model on a honeycomb lattice with $f$ flux quantum per plaquette. The path integral representation of the model corresponds to a (2+1)-dimensional classical model, which is used to investigate the critical behavior by extensive Monte Carlo simulations on large system sizes. In contrast to the model on a square lattice, the transition is found to be first order for $f=1/3$ and continuous for $f=1/2$ but in a different universality class. The correlation-length critical exponent is estimated from finite-size scaling of vortex correlations. The estimated universal conductivity at the transition is approximately four times its value for $f=0$. The results are compared with experimental observations on ultrathin superconducting films with a triangular lattice of nanoholes in a transverse magnetic field.

cond-mat.supr-con↗

arXiv2016

Minimum energy path for the nucleation of misfit dislocations in Ge/Si(001) heteroepitaxy

A possible mechanism for the formation of a 90° misfit dislocation at the Ge/Si(001) interface through homogeneous nucleation is identified from atomic scale calculations where a minimum energy path connecting the coherent epitaxial state and a final state with a 90° misfit dislocation is found using the nudged elastic band method. The initial path is generated using a repulsive bias activation procedure in a model system including 75000 atoms. The energy along the path exhibits two maxima in the energy. The first maximum occurs as a 60° dislocation nucleates. The intermediate minimum corresponds to an extended 60° dislocation. The subsequent energy maximum occurs as a second 60° dislocation nucleates in a complementary, mirror glide plane, simultaneously starting from the surface and from the first 60° dislocation. The activation energy of the nucleation of the second dislocation is 30% lower than that of the first one showing that the formation of the second 60° dislocation is aided by the presence of the first one. The simulations represent a step towards unraveling the formation mechanism of 90° dislocations, an important issue in the design of growth procedures for strain released Ge overlayers on Si(100) surfaces, and more generally illustrate an approach that can be used to gain insight into the mechanism of complex nucleation paths of extended defects in solids.

cond-mat.mtrl-sci↗

arXiv2013

Resistive transition in frustrated Josephson-junction arrays on a honeycomb lattice

We use driven Monte Carlo dynamics to study the resistive behavior of superconducting Josephson junction arrays on a honeycomb lattice in a magnetic field corresponding to $f$ flux quantum per plaquette. While for $f=1/3$ the onset of zero resistance is found at nonzero temperature, for $f=1/2$ the results are consistent with a transition scenario where the critical temperature vanishes and the linear resistivity shows thermally activated behavior. We determine the thermal critical exponent of the zero-temperature transition for $f=1/2$, from a dynamic scaling analysis of the nonlinear resistivity. The resistive behavior agrees with recent results obtained for the phase-coherence transition from correlation length calculations and with experimental observations on ultra-thin superconducting films with a triangular pattern of nanoholes.

cond-mat.supr-con↗

arXiv2012

Zero-temperature transition and correlation-length exponent of the frustrated XY model on a honeycomb lattice

Phase coherence and vortex order in the fully frustrated XY model on a two-dimensional honeycomb lattice are studied by extensive Monte Carlo simulations using the parallel tempering method and finite-size scaling. No evidence is found for an equilibrium order-disorder or a spin/vortex-glass transition, suggested in previous simulation works. Instead, the scaling analysis of correlations of phase and vortex variables in the full equilibrated system is consistent with a phase transition where the critical temperature vanishes and the correlation lengths diverge as a power-law with decreasing temperatures and corresponding critical exponents $ν_{ph}$ and $ν_{v}$. This behavior and the near agreement of the critical exponents suggest a zero-temperature transition scenario where phase and vortex variables remain coupled on large length scales.

cond-mat.stat-mech↗

arXiv2010

Non-monotonic velocity dependence of atomic friction

We study the velocity dependence of the frictional force of the tip of an atomic force microscope as it is dragged across a surface, taking into account memory effects and thermal fluctuations. Memory effects are described by a coupling of the tip to low frequency excitation modes of the surface in addition to the coupling to the periodic corrugation potential. We find that when the excitation mode frequency is comparable to the characteristic frequency corresponding to the motion of the tip across the surface, the velocity dependence of the frictional force is non monotonic, displaying a velocity range where the frictional force can decrease with increasing velocity. These results provide theoretical support for the interpretation of recent experiments which find a frictional force that decreases with velocity on surfaces covered with a monolayer.

cond-mat.mes-hall↗

arXiv2009

Spin-size disorder model for granular superconductors with charging effects

A quantum pseudo-spin model with random spin sizes is introduced to study the effects of charging-energy disorder on the superconducting transition in granular superconducting materials. Charging-energy effects result from the small electrical capacitance of the grains when the Coulomb charging energy is comparable to the Josephson coupling energy. In the pseudo-spin model, randomness in the spin size is argued to arise from the inhomogeneous grain-size distribution. For a particular bimodal spin-size distribution, the model describes percolating granular superconductors. A mean-field theory is developed to obtain the phase diagram as a function of temperature, average charging energy and disorder.

cond-mat.supr-con↗

arXiv2008

Phase and vortex correlations in Josephson-junction arrays at irrational frustration

Phase coherence and vortex order in a Josephson-junction array at irrational frustration are studied by extensive Monte Carlo simulations using the parallel tempering method. A scaling analysis of the correlation length of phase variables in the full equilibrated system shows that the critical temperature vanishes with a power-law divergent correlation length and critical exponent $ν_{ph}$, in agreement with recent results from resistivity scaling analysis. A similar scaling analysis for vortex variables reveals a different critical exponent $ν_{v}$, suggesting that there are two distinct correlation lengths associated with a decoupled zero-temperature phase transition.

cond-mat.supr-con↗

arXiv2007

Zero-temperature resistive transition in Josephson-junction arrays at irrational frustration

We use a driven Monte Carlo dynamics in the phase representation to determine the linear resistivity and current-voltage scaling of a two-dimensional Josephson-junction array at an irrational flux quantum per plaquette. The results are consistent with a phase-coherence transition scenario where the critical temperature vanishes. The linear resistivity is nonzero at any finite temperatures but nonlinear behavior sets in at a temperature-dependent crossover current determined by the thermal critical exponent. From a dynamic scaling analysis we determine this critical exponent and the thermally activated behavior of the linear resistivity. The results are in agreement with earlier calculations using the resistively shunted-junction model for the dynamics of the array. The linear resistivity behavior is consistent with some experimental results on arrays of superconducting grains but not on wire networks, which we argue have been obtained in a current regime above the crossover current.

cond-mat.supr-con↗

arXiv2005

Field-induced superconductor to insulator transition in Josephson-junction ladders

The superconductor to insulator transition is studied in a self-charging model for a ladder of Josephson-junctions in presence of an external magnetic field. Path integral Monte Carlo simulations of the equivalent (1+1)-dimensional classical model are used to study the phase diagram and critical behavior. In addition to a superconducting (vortex-free) phase, a vortex phase can also occur for increasing magnetic field and small charging energy. It is found that an intervening insulating phase separates the superconducting from the vortex phases. Surprisingly, a finite-size scaling analysis shows that the field-induced superconducting to insulator transition is in the KT universality class even tough the external field breaks time-reversal symmetry.

cond-mat.supr-con↗

arXiv2005

Pattern Selection in a Phase Field Model for Directional Solidification

A symmetric phase field model is used to study wavelength selection in two dimensions. We study the problem in a finite system using a two-pronged approach. First we construct an action and, minimizing this, we obtain the most probable configuration of the system, which we identify with the selected stationary state. The minimization is constrained by the stationary solutions of stochastic evolution equations and is done numerically. Secondly, additional support for this selected state is obtained from straightforward simulations of the dynamics from a variety of initial states.

cond-mat.stat-mech↗

arXiv2005

Resistivity scaling and critical dynamics of fully frustrated Josephson-junction arrays with on-site dissipation

We study the scaling behavior and critical dynamics of the resistive transition in Josephson-junction arrays, at f=1/2 flux quantum per plaquette, by numerical simulation of an on-site dissipation model for the dynamics. The results are compared with recent simulations using the resistively-shunted-junction model. For both models, we find that the resistivity scaling and critical dynamics of the phases are well described by the same critical temperature as for the chiral (vortex-lattice) transition, with a power-law divergent correlation length. The behavior is consistent with the single transition scenario, where phase and chiral variables order at the same temperature, but with different dynamic exponents z for phase coherence and chiral order.

cond-mat.supr-con↗

arXiv2004

Critical behavior and driven Monte Carlo dynamics of the XY spin glass in the phase representation

A driven Monte Carlo dynamics is introduced to study resistivity scaling in XY-type models in the phase representation. The method is used to study the phase transition of the three-dimensional XY spin glass with a Gaussian coupling distribution. We find a phase-coherence transition at finite temperature in good agreement with recent equilibrium Monte Carlo simulations which shows a single (spin and chiral) glass transition. Estimates of the static and dynamic critical exponents indicate that the critical behavior is in the same universality class as the the model with a bimodal coupling distribution. Relevance of these results for $π$-junction superconductors is also discussed.

cond-mat.dis-nn↗