Suppression of geometrical barrier in $Bi_2Sr_2CaCu_2O_{8+\delta}$ crystals by Josephson vortex stacks

TL;DR

This study demonstrates how Josephson vortex stacks can suppress geometrical barriers in Bi-2212 crystals, facilitating vortex entry and reducing hysteresis through flux channeling, supported by experimental imaging and modeling.

Contribution

It introduces a model explaining vortex chain protrusion caused by Josephson vortices and confirms the effect with experimental data and comparative measurements.

Findings

Josephson vortex stacks protrude into vortex-free regions

Vortex chains act as channels for vortex entry and exit

Model accounts for demagnetization effects and flux focusing

Abstract

Differential magneto-optics are used to study the effect of dc in-plane magnetic field on hysteretic behavior due to geometrical barriers in $Bi_2Sr_2CaCu_2O_{8+\delta}$ crystals. In absence of in-plane field a vortex dome is visualized in the sample center surrounded by barrier-dominated flux-free regions. With in-plane field, stacks of Josephson vortices form vortex chains which are surprisingly found to protrude out of the dome into the vortex-free regions. The chains are imaged to extend up to the sample edges, thus providing easy channels for vortex entry and for drain of the dome through geometrical barrier, suppressing the magnetic hysteresis. Reduction of the vortex energy due to crossing with Josephson vortices is evaluated to be about two orders of magnitude too small to account for the formation of the protruding chains. We present a model and numerical calculations that qualitatively describe the observed phenomena by taking into account the demagnetization effects in which flux expulsion from the pristine regions results in vortex focusing and in the chain protrusion. Comparative measurements on a sample with narrow etched grooves provide further support to the proposed model.