arXiv subjects

Mai Suan Li

Publications and source records attributed to Mai Suan Li.

At least 37 records · Page 2Linked to original sources

arXiv2003

Paramagnetic Meissner effect and related dynamical phenomena

The present review is given on the paramagnetic Meissner effect observed in conventional and ceramic high-Tc superconductors. We discuss two mechanisms leading to this phenomenon: the d-wave and the flux compensation. It is shown that the chiral glass phase may occur in granular superconductors possessing d-wave pairing symmetry. Dynamical phenomena such as AC susceptibility, compensation effect, anomalous microwave absorption, aging effect, AC resistivity and enhancement of critical current due to the external electric field are considered.

cond-mat.supr-con↗

arXiv2003

Flux-lattice melting in two-dimensional disordered superconductors

The flux line lattice melting transition in two-dimensional pure and disordered superconductors is studied by a Monte Carlo simulation using the lowest Landau level approximation and quasi-periodic boundary condition on a plane. The position of the melting line was determined from the diffraction pattern of the superconducting order parameter. In the clean case we confirmed the results from earlier studies which show the existence of a quasi-long range ordered vortex lattice at low temperatures. Adding frozen disorder to the system the melting transition line is shifted to slightly lower fields. The correlations of the order parameter for translational long range order of the vortex positions seem to decay slightly faster than a power law (in agreement with the theory of Carpentier and Le Doussal) although a simple power law decay cannot be excluded. The corresponding positional glass correlation function decays as a power law establishing the existence of a quasi-long range ordered positional glass formed by the vortices. The correlation function characterizing a phase coherent vortex glass decays however exponentially ruling out the possible existence of a phase coherent vortex glass phase.

cond-mat.supr-con↗

arXiv2002

Dependence of folding rates on protein length

Using three-dimensional Go lattice models with side chains for proteins, we investigate the dependence of folding times on protein length. In agreement with previous theoretical predictions, we find that the folding time grows as a power law with the chain length N with exponent $λ\approx 3.6$ for the Go model, in which all native interactions (i.e., between all side chains and backbone atoms) are uniform. If the interactions between side chains are given by pairwise statistical potentials, which introduce heterogeneity in the contact energies, then the power law fits yield large $λ$ values that typically signifies a crossover to an underlying activated process. Accordingly, the dependence of folding time is best described by the stretched exponential \exp(\sqrt{N}). The study also shows that the incorporation of side chains considerably slows down folding by introducing energetic and topological frustration.

cond-mat.soft↗

arXiv2002

Folding in lattice models with side chains

The folding kinetics of three-dimensional lattice Go models with side chains is studied using two different Monte Carlo move sets. A flexible move set based on single, double and triple backbone moves is found to be far superior compared to the standard Monte Carlo dynamics. It was found that the folding time grows as a power law with the chain length and the corresponding exponent $λ\approx 3.6$ for Go models. The study shows that the incorporation of side chains dramatically slows down folding rates.

cond-mat.soft↗

arXiv2002

Nonlinear AC resistivity in s-wave and d-wave disordered granular superconductors

We model s-wave and d-wave disordered granular superconductors with a three-dimensional lattice of randomly distributed Josephson junctions with finite self-inductance. The nonlinear ac resistivity of these systems was calculated using Langevin dynamical equations. The current amplitude dependence of the nonlinear resistivity at the peak position is found to be a power law characterized by exponent $α$. The later is not universal but depends on the self-inductance and current regimes. In the weak current regime $α$ is independent of the self-inductance and equal to 0.5 or both of s- and d-wave materials. In the strong current regime this exponent depends on the screening. We find $α\approx 1$ for some interval of inductance which agrees with the experimental finding for d-wave ceramic superconductors.

cond-mat.supr-con↗

arXiv2001

Heat capacity of mesoscopically disordered superconductors:implications to MgB$_{2}$

Electronic heat capacity was calculated for a mesoscopically disordered s-wave superconductor treated as a spatial ensemble of domains with a continuously varying superconducting properties. The domains are assumed to have sizes larger than the coherence length. Each domain is characterized by a certain critical temperature. The averaging over the superconducting gap distribution leads to quadratic temperature dependence of the specific heat at low temperatures, whereas its anomaly at the critical temperature is substantially smeared. The results explain well the heat capacity data for MgB$_{2}$, where a multiple-gap structure is observed.

cond-mat.supr-con↗

arXiv2001

Collective Dynamics of One-Dimensional Charge Density Waves

The effect of disorder on the static and dynamic behaviour of one-dimensional charge density waves at low temperatures is studied by analytical and numerical approaches. In the low temperature region the spatial behaviour of the phase-phase correlation function is dominated by disorder but the roughness exponent remains the same as in the pure case. Contrary to high dimensional systems the dependence of the creep velocity on the electric field is described by an analytic function.

cond-mat.stat-mech↗

arXiv2001

Microwave absorption in s- and d-wave disordered superconductors

We model s- and d-wave ceramic superconductors with a three-dimensional lattice of randomly distributed Josephson junctions with finite self-inductance. The field and temperature dependences of the microwave absoption are obtained by solving the corresponding Langevin dynamical equations. We find that at magnetic field H=0 the microwave absoption of the s-wave samples, when plotted against the field, has a minimum at any temperature. In the case of d-wave superconductors one has a peak at H=0 in the temperature region where the paramagnetic Meissner effect is observable. These results agree with experiments. The dependence of the microwave absorption on the screening strength was found to be nontrivial due to the crossover from the weak to the strong screening regime.

cond-mat.supr-con↗

arXiv2001

Nature of the ordering of the three-dimensional XY spin glass

Spin and chirality orderings of a three-dimensional XY spin glass are studied by extensive Monte Carlo simulations. By calculating an appropriately defined spin-overlap distribution function, we show that the finite-temperature chiral-glass transition does not accompany the standard spin-glass order, giving support to the spin-chirality decoupling picture. Critical behavior of the chiral-glass transition turns out to be different from that of the Ising spin glass. The chiral-glass ordered state exhibits a one-step-like peculiar replica-symmetry breaking.

cond-mat.dis-nn↗

arXiv2000

Aging Effect in Ceramic Superconductors

A three-dimensional lattice of the Josephson junctions with a finite self-conductance is employed to model ceramic superconductors. Using Monte Carlo simulations it is shown that the aging disappears in the strong screening limit. In the weeak screening regime aging is present even at low temperatures. For intermediate values of the self-inductance aging occurs at intermediate temperatures interval but is suppressed entirely at high and low temperatures. Our results are in good agreement with experiments.

cond-mat.dis-nn↗

arXiv2000

Structural Susceptibilities in Toy Models of Proteins

New definitions of the structural susceptibilities based on the fluctuations of distances to the native state of toy protein models are proposed. The calculation of such susceptibilities does not require the basin of native state and the folding temperature can be defined from their peak if the native conformation is compact. The number of peaks in the derivatives of distances to the native state with respect to temperature, when plotted versus temperature, may serve as a criterion for foldability. The thermodynamics quantities are obtained by Monte Carlo and molecular dynamics simulations.

cond-mat.stat-mech↗

arXiv2000

AC resistivity of d-wave ceramic superconductors

We model d-wave ceramic superconductors with a three-dimensional lattice of randomly distributed $π$ Josephson junctions with finite self-inductance. The linear and nonlinear ac resistivity of the d-wave ceramic superconductors is obtained as function of temperature by solving the corresponding Langevin dynamical equations. We find that the linear ac resistivity remains finite at the temperature $T_p$ where the third harmonics of resistivity has a peak. The current amplitude dependence of the nonlinear resistivity at the peak position is found to be a power law. These results agree qualitatively with experiments. We also show that the peak of the nonlinear resistivity is related to the onset of the paramagnetic Meissner effect which occurs at the crossover temperature $T_p$, which is above the chiral glass transition temperature $T_{cg}$.

cond-mat.dis-nn↗

arXiv2000

Dynamical chaos and power spectra in toy models of heteropolymers and proteins

The dynamical chaos in Lennard-Jones toy models of heteropolymers is studied by molecular dynamics simulations. It is shown that two nearby trajectories quickly diverge from each other if the heteropolymer corresponds to a random sequence. For good folders, on the other hand, two nearby trajectories may initially move apart but eventually they come together. Thus good folders are intrinsically non-chaotic. A choice of a distance of the initial conformation from the native state affects the way in which a separation between the twin trajectories behaves in time. This observation allows one to determine the size of a folding funnel in good folders. We study the energy landscapes of the toy models by determining the power spectra and fractal characteristics of the dependence of the potential energy on time. For good folders, folding and unfolding trajectories have distinctly different correlated behaviors at low frequencies.

cond-mat.stat-mech↗

arXiv1999

Spin analogs of proteins: scaling of "folding" properties

Reaching a ground state of a spin system is analogous to a protein evolving into its native state. We study the ``folding'' times for various random Ising spin systems and determine characteristic temperatures that relate to the ``folding''. Under optimal kinetic conditions, the ``folding'' times scale with the system size as a power law with a non-universal exponent. This is similar to what happens in model proteins. On the other hand, the scaling behavior of the characteristic temperatures is different than in model proteins. Both in the spin systems and in proteins, the folding properties deteriorate with the system size.

cond-mat.stat-mech↗

arXiv1999

Delineation of the Native Basin in Continuum Models of Proteins

We propose two approaches for determining the native basins in off-lattice models of proteins. The first of them is based on exploring the saddle points on selected trajectories emerging from the native state. In the second approach, the basin size can be determined by monitoring random distortions in the shape of the protein around the native state. Both techniques yield the similar results. As a byproduct, a simple method to determine the folding temperature is obtained.

cond-mat.soft↗

arXiv1999

Folding in two-dimenensional off-lattice models of proteins

Model off-lattice sequences in two dimensions are constructed so that their native states are close to an on-lattice target. The Hamiltonian involves the Lennard-Jones and harmonic interactions. The native states of these sequences are determined with a high degree of certainty through Monte Carlo processes. The sequences are characterized thermodynamically and kinetically. It is shown that the rank-ordering-based scheme of the assignment of contact energies typically fails in off-lattice models even though it generates high stability of on-lattice sequences. Similar to the on-lattice case, Go-like modeling, in which the interaction potentials are restricted to the native contacts in a target shape, gives rise to good folding properties. Involving other contacts deteriorates these properties.

cond-mat.soft↗

arXiv1999

Scaling of folding properties in simple models of proteins

Scaling of folding properties of proteins is studied in a toy system -- the lattice Go model with various two- and three- dimensional geometries of the maximally compact native states. Characteristic folding times grow as power laws with the system size. The corresponding exponents are not universal. Scaling of the thermodynamic stability also indicates size-related deterioration of the folding properties.

cond-mat.stat-mech↗

arXiv1999

Chiral Glass Phase in Ceramic Superconductors

A three-dimensional lattice of the Josephson junctions with a finite self-conductance is employed to model the ceramic superconductors. By Monte Carlo simulations it is shown that the chiral glass phase is stable in three dimensions even under the influence of screening. The nonlinear ac susceptibility and the compensation effect are also studied. The compensation effect is shown to be due to the existence of the chiral glass phase. In agreement with experiments, this effect is demonstrated to be present in the ceramic superconductors which show the paramagnetic Meissner effect.

cond-mat.dis-nn↗