Mahesh Chandran

Domain regime in two-dimensional disordered vortex matter

A detailed numerical study of the real-space configuration of vortices in disordered superconductors using two-dimensional London-Langevin model is presented. The magnetic field B is varied between 0 and B c 2 for variouspinning strengths A. For weak pinning, an inhomogeneous disordered vortex matter is observed, in which the topologically ordered vortex lattice survives in large domains. The majority of the dislocations in this state are confined to the grain boundaries/domain walls. Such quasiordered configurations are observed in the intermediate fields, and we refer it as the domain regime (DR). The DR is distinct from the low-field and the high-fields amorphous regimes which are characterized by a homogeneous distribution of defects over the entire system. Analysis of the real-space configuration suggests domain-wall roughening as a possible mechanism for the crossover from the DR to the high-field amorphous regime. The DR also shows a sharp crossover to the high-temperature vortex liquid phase. The domain size distribution and the roughness exponent of the lattice in the DR are also calculated. The results are compared with some of the recent Bitter decoration experiments.

Field distribution in thin superconductors: effect of sample shape

Abstract Flux distribution in a finite-sized thin superconductor in transverse magnetic field is strongly influenced by the sample shape. This is investigated by direct numerical simulation of the field distribution in a two-dimensional Josephson junction array (JJA). The effect of non-local interaction of fields and currents in thin continuum superconductor is considered by including the mutual inductance between all pairs of plaquettes in the array. In small applied fields, for a rectangular array, the flux penetration occurs predominantly through the middle of the edges. The line originating from the corners (discontinuity lines) at which the flux distribution shows sharp ridges, form strong barriers to the flux-motion, gives rise to independent domains. In large external field (field greater than that requred for full-penetration of the array), the low-field distribution crosses over to a new kind in which vortices accumulate close to the discontinuity lines. This field distribution closely resembles the ‘mountain-pass’ type, observed by Brawner et al. [Nature 358, (1992) 567]. A qualitative explanation to this is presented by considering the influence of the discontinuity lines on the penetrating vortices. Effect of inner boundaries on the flux distribution in a multiply-connected geometry is studied by simulating the square array with a central hole in transverse magnetic field. The field-distribution is influenced by the interaction of the discontinuity lines originating from the external and internal boundary which depends upon the dimension and orientation of the central hole region.

Critical state in a Josephson junction array: effect of transport currents

We present a detailed simulation of a Josephson junction array in the presence of a varying external field and a transport current. We show that the field profiles in the array are similar to the profiles generated by the critical state model for an infinite slab of hard type-II superconductor. A discrete equation similar to Beans model is introduced to describe the field profiles in the array.

Metastability and uniqueness of vortex states at depinning.

We present results from numerical simulations of the transport of vortices in the zero-field-cooled (ZFC) and the field-cooled (FC) state of a type-II superconductor. In the absence of an applied current I, we find that the FC state has a lower defect density than the ZFC state, and is stable against thermal cycling. On the other hand, by cycling I, surprisingly, we find that the ZFC state is the stable state. The FC state is metastable as manifested by increasing I to the depinning current I(c), in which case the FC state evolves into the ZFC state. We also find that all configurations acquire a unique defect density at the depinning transition independent of the history of the initial states.

Critical state in thin superconductors: a Josephson junction array analogy

Abstract The Josephson junction array is numerically simulated to study the vortex dynamics in a thin superconductor in a transverse magnetic field. The non-local interaction between currents and fields is incorporated in the dynamics by considering the full inductance matrix of the array. The field and the current distribution in the array are obtained for various applied fields which shows striking similarity with that obtained in the critical state model for a superconductor in the 2D limit (thin film geometry). The cross-over from a non-linear distribution of the field for the 2D limit (demagnetisation factor N → 1) to a linear distribution for the 3D case (N = 0) is studied phenomenologically by increasing the thickness of the superconducting islands forming the array. Increasing the thickness of the array effectively reduces the strength of the non-local interaction between currents and fields. The M-H curve and the field distribution within the array are calculated for field independent and field dependent critical currents of the Josephson junction. We also probe in detail the evolution of the field and the current distribution towards their equilibrium values after an external field is applied. The results are discussed and compared with analytical results of the critical state model for a thin superconducting strip in a transverse magnetic field.

Modulation in the non-uniformity of the vortex state across the peak effect: evidence via the magnetic quadrupole moment in a very clean crystal of 2H-NbSe2 for H∥c

Abstract The non-uniform magnetic character of the mixed (vortex) state is known to result in a magnetic quadrupolar response of a type-II superconductor. The nucleation of stronger pinned regions coexisting with weaker pinned regions across the peak effect (PE) in a weakly pinned superconductor has been demonstrated using ac scanning Hall bar microscopy by Marchevsky et al. [Nature 409 (2001) 591]. Coexistence of this nature could produce an imprint in the measured quadrupolar response of such samples. We present successful detection of such an imprint across the PE region of a very clean crystal of 2H–NbSe 2 for H ∥ c via a well-established procedure of preferentially recording a signal arising from the non-uniformity in the magnetization of the sample in a vibrating sample magnetometer.