Mott insulator phases and first-order melting in BSCCO crystals with periodic surface holes

TL;DR

This study investigates how periodic surface holes in BSCCO crystals influence vortex matter, revealing Mott insulator phases and a unique first-order melting transition within the vortex lattice.

Contribution

It demonstrates the creation of Mott insulator phases and identifies a novel first-order melting transition occurring beneath surface holes in high-T_c superconductors.

Findings

Irreversibility line shifts to higher fields and temperatures

Reduced vortex compressibility at integer matching fields

First-order melting occurs within Mott insulator regions

Abstract

We measured the effects of periodic surface holes, created using a focused ion beam, on the phase diagram of the vortex matter in high-T_c Bi_2Sr_2CaCu_2O_{8+\delta} crystals. Differential magneto-optical measurements show that the irreversibility line is shifted to higher fields and temperatures, with respect to the pristine melting line. The irreversibility line displays weak field dependence between integer matching fields indicating multiple-flux-quanta pinning at holes. We find reduced equilibrium compressibility of the vortex matter at integer matching fields, which is strong evidence for the existence of thermodynamic Mott insulator phases. Shaking with a transverse ac field surprisingly reveals first-order melting that is not shifted with respect to the pristine melting line and that seems to occur within the Mott insulator regions. This melting is understood to be the first-order transition in the bulk of the crystal beneath the surface holes. The transition is visible at the surface, despite the reduced vortex compressibility in the top layer.