arXiv subjects

M. R. Kolahchi

Publications and source records attributed to M. R. Kolahchi.

9 recordsLinked to original sources

arXiv2026

Superconducting Diode Effect in Josephson $φ_0$ Junction

We investigate the superconducting diode effect in a Josephson junction with a ferromagnetic weak link in the presence of Rashba spin-orbit coupling. While the standard $φ_0$ junction model is valid only for weak exchange fields $h \lesssim T_c$, realistic ferromagnetic barriers typically operate in the regime $h \gg T_c$, where the conventional Ginzburg-Landau gradient expansion becomes insufficient and requires the inclusion of higher order gradient terms to ensure stability of the free energy. By extending the free energy with such higher order term, we demonstrate that the linear gradient coupling alone leads to finite momentum Cooper pairing while leaving the critical current symmetry unchanged. The interplay between this linear term and the higher order gradient one leads to the Josephson diode effect.

cond-mat.supr-con↗

arXiv2021

Anomalous Gilbert Damping and Duffing Features of the SFS {\boldmath $φ_0$} Josephson Junction

We demonstrate unusual features of phase dynamics, IV-characteristics and magnetization dynamics of the $φ_0$ Josephson junction at small values of spin-orbit interaction, ratio of Josephson to magnetic energy and Gilbert damping. In particular, an anomalous shift of the ferromagnetic resonance frequency with an increase of Gilbert damping is found. The ferromagnetic resonance curves show the Duffing oscillator behaviour, reflecting the nonlinear nature of Landau-Lifshitz-Gilbert (LLG) equation. Based on the numerical analysis of each term in LLG equation we obtained an approximated equation demonstrated both damping effect and Duffing oscillator features. The resulting Duffing equation incorporates the Gilbert damping in a special way across the dissipative term and the restoring force. A resonance method for the determination of spin-orbit interaction in noncentrosymmetric materials which play the role of barrier in $φ_0$ junctions is proposed.

cond-mat.supr-con↗

arXiv2014

Structured Chaos in a Devil's Staircase of the Josephson Junction

The phase dynamics of Josephson junctions under external electromagnetic radiation is studied through numerical simulations. Current-voltage characteristics, Lyapunov exponents and Poincare sections are analyzed in detail. It is found that the subharmonic Shapiro steps at certain parameters are separated by structured chaotic windows. By performing a linear regression on the linear part of the data, a fractal dimension of D = 0.868 is obtained, with an uncertainty of +-0.012. The chaotic regions exhibit scaling similarity and it is shown that the devil's staircase of the system can form a backbone that unifies and explains the highly correlated and structured chaotic behavior. These features suggest a system possessing multiple complete devil's staircases. The onset of chaos for subharmonic steps occurs through the Feigenbaum period doubling scenario. Universality in the sequence of periodic windows is also demonstrated. Finally the influence of the radiation and Josephson junction parameters on the structured chaos is investigated and it is concluded that the structured chaos is a stable formation over a wide range of parameter values.

cond-mat.supr-con↗

arXiv2013

Devil's Staircases and Continued Fractions in the Josephson Junctions

The detailed numerical simulations of the IV-characteristics of Josephson junction under external electromagnetic radiation show devil's staircases within different bias current intervals. We have found that the observed steps form very precisely continued fractions. Increasing of the amplitude of radiation shifts the devil's staircases to higher Shapiro steps. The algorithm of appearing and detection of the subharmonics with increasing radiation amplitude is proposed. We demonstrate that subharmonic steps registered in the famous experiments by A. H. Dayem and J. J. Wiegand [Phys. Rev 155, 419 (1967)] and J. Clarke [Phys. Rev. B 4, 2963 (1971)] also form continued fractions.

cond-mat.supr-con↗

arXiv2012

Manifestation of Resonance-Related Chaos in Coupled Josephson Junctions

Chaotic features of systems of coupled Josephson junctions are studied. Manifestation of chaos in the temporal dependence of the electric charge, related to a parametric resonance, is demonstrated through the calculation of the maximal Lyapunov exponent, phase-charge and charge-charge Lissajous diagrams and correlation functions. The number of junctions in the stack strongly influences the fine structure in the current voltage characteristics and a strong proximity effect results from the nonperiodic boundary conditions. The observed resonance-related chaos exhibits intermittency over a range of conditions and parameters. General features of the system are analyzed by means of a linearized equation and the criteria for a breakpoint region with no chaos are obtained. Such criteria could clarify recent experimental observations of variations in the power output from intrinsic Josephson junctions in high temperature superconductors.

cond-mat.supr-con↗

arXiv2011

Character of ground state of an aperiodic frustrated Josephson junction array

We study the energy spectrum for an aperiodic Josephson junction ladder, as a function of frustration. Frustration is brought about by application of a transverse magnetic field, and aperiodicity is imposed by the arrangement of plaquettes with two incommensurate areas. We study the effect of the incommensurate plaquette areas in conjunction with that of the aperiodicity. The structure of the energy spectrum at deep minima is shown to be described by a model that treats the plaquettes independently. The energy spectrum is a quasiperiodic function of frustration; short range correlations in the arrangement of plaquettes have a small effect on the energy power spectrum.

cond-mat.supr-con↗

arXiv2009

Effect of coupling between junctions on superconducting current and charge correlations in intrinsic Josephson junctions

Charge formations on superconducting layers and creation of the longitudinal plasma wave in the stack of intrinsic Josephson junctions change crucially the superconducting current through the stack. Investigation of the correlations of superconducting currents in neighboring Josephson junctions and the charge correlations in neighboring superconducting layers allows us to predict the additional features in the current--voltage characteristics. The charge autocorrelation functions clearly demonstrate the difference between harmonic and chaotic behavior in the breakpoint region. Use of the correlation functions gives us a powerful method for the analysis of the current--voltage characteristics of coupled Josephson junctions.

cond-mat.supr-con↗

arXiv2007

Study of sediment transport in the saltation regime

We present a study of sediment transport in the creeping and saltation regime. In our model, a bed of particles is simulated with the conventional event-driven method. The particles are considered as hard disks in a 2d domain, with periodic boundary conditions in horizontal direction. The flow of the fluid over this bed of particles is modeled by imposing a force on each particle that depends on the velocity of the fluid and its height above the bed. We considered two velocity profiles for the fluid, parabolic and logarithmic. The first one models laminar flow and the second corresponds to turbulent flow. For each case we investigated the behavior of the saturated flux. We found that for the logarithmic profile, the saturated flux shows a quadratic increase with the strength of the flow, and for parabolic profile, a cubic increase. The velocity distribution functions are used to interpret the results.

cond-mat.other↗

arXiv2007

Rod-like Polyelectrolyte Brushes with Mono- and Multivalent Counterions

A model of rod-like polyelectrolyte brushes in the presence of monovalent and multivalent counterions but with no added-salt is studied using Monte Carlo simulation. The average height of the brush, the histogram of rod conformations, and the counterion density profile are obtained for different values of the grafting density of the charge-neutral wall. For a domain of grafting densities, the brush height is found to be relatively insensitive to the density due to a competition between counterion condensation and inter-rod repulsion. In this regime, multivalent counterions collapse the brush in the form of linked clusters. Nematic order emerges at high grafting densities, resulting is an abrupt increase of the brush height.

cond-mat.soft↗