arXiv subjects

Mohammad Kohandel

Publications and source records attributed to Mohammad Kohandel.

23 records · Page 2Linked to original sources

arXiv2003

Disorder-Driven Magnetic Field Dependence of the Internal Field Distribution in the Bragg Glass Phase of Type-II Superconductors

We use the replica variational method to study the effects of weak point disorder on the variance of the internal field distribution measured in NMR and muon-spin rotation experiments in type-II superconductors. We show that for a simple model there is significant magnetic field dependence which is extrinsic and disorder-driven, and does not have a microscopic (non $s-$wave pairing) origin. Results are presented where we examine the dependence of the magnetic field variance upon the strength of the applied external field.

cond-mat.dis-nn↗

arXiv2002

Hexatic-Herringbone Coupling at the Hexatic Transition in Smectic Liquid Crystals: 4-$ε$ Renormalization Group Calculations Revisited

Simple symmetry considerations would suggest that the transition from the smectic-A phase to the long-range bond orientationally ordered hexatic smectic-B phase should belong to the XY universality class. However, a number of experimental studies have constantly reported over the past twenty years "novel" critical behavior with non-XY critical exponents for this transition. Bruinsma and Aeppli argued in Physical Review Letters {\bf 48}, 1625 (1982), using a $4-ε$ renormalization-group calculation, that short-range molecular herringbone correlations coupled to the hexatic ordering drive this transition first order via thermal fluctuations, and that the critical behavior observed in real systems is controlled by a `nearby' tricritical point. We have revisited the model of Bruinsma and Aeppli and present here the results of our study. We have found two nontrivial strongly-coupled herringbone-hexatic fixed points apparently missed by those authors. Yet, those two new nontrivial fixed-points are unstable, and we obtain the same final conclusion as the one reached by Bruinsma and Aeppli, namely that of a fluctuation-driven first order transition. We also discuss the effect of local two-fold distortion of the bond order as a possible missing order parameter in the Hamiltonian.

cond-mat.dis-nn↗

arXiv1999

Instability and Fluctuations of Flux Lines with Point Impurities in a Parallel Current

A parallel current can destabilize a single flux line (FL), or an array of FLs. We consider the effects of pinning by point impurities on this instability. The presence of impurities destroys the long-range order of a flux lattice, leading to the so called Bragg glass (BrG) phase. We first show that the long-range topological order of the BrG is also destroyed by a parallel current. Nonetheless, some degree of short-range order should remain, whose destruction by thermal and impurity fluctuations, as well as the current, is studied here. To this end, we employ a cage model for a single FL in the presence of impurities and current, and study it analytically (by replica variational methods), and numerically (using a transfer matrix technique). The results are in good agreement, and in conjunction with a Lindemann criterion, provide the boundary in the magnetic field--temperature plane for destruction of short-range order. In all cases, we find that the addition of impurities or current (singly or in combination) leads to further increase in equilibrium FL fluctuations. Thus pinning to point impurities does not stabilize FLs in a parallel current $j_z$, although the onset of this instability is much delayed due to large potential barriers that diverge as $j_z^{-μ}$.

cond-mat.stat-mech↗

arXiv1999

Numerical Study of the Vortex Phase Diagram Using the Bose Model in STLS approximation

We study the phase diagram of the flux lines using the mapping to 2D bosons in the self-consistent-field approximation of Singwi, Tosi, Land, and Sjolander (STLS). The pair correlation function, static structure factor, interaction energy, and spectrum of the excited energies are calculated over a wide range of the parameters in this approximation. These quantities are used for studying the melting transition from the Abrikosov lattice into the entangled vortex liquid. The resulting $B-T$ phase diagram is in good agreement with the known estimates for the vortex lattice melting and the Monte Carlo simulations. We also discuss the effect of van der Waals interaction, induced by thermal fluctuations, together with the repulsion potential on the phase diagram.

cond-mat.supr-con↗

arXiv1998

Melting of Flux Lines in an Alternating Parallel Current

We use a Langevin equation to examine the dynamics and fluctuations of a flux line (FL) in the presence of an {\it alternating longitudinal current} $J_{\parallel}(ω)$. The magnus and dissipative forces are equated to those resulting from line tension, confinement in a harmonic cage by neighboring FLs, parallel current, and noise. The resulting mean-square FL fluctuations are calculated {\it exactly}, and a Lindemann criterion is then used to obtain a nonequilibrium `phase diagram' as a function of the magnitude and frequency of $J_{\parallel}(ω)$. For zero frequency, the melting temperature of the mixed phase (a lattice, or the putative "Bose" or "Bragg Glass") vanishes at a limiting current. However, for any finite frequency, there is a non-zero melting temperature.

cond-mat.stat-mech↗