Direct evidence of bulk dynamic vortex matter reordering in the order-disorder transitional region

PASQUINI, GABRIELA; MARZIALI BERMUDEZ, MARIANO; BEKERIS, VICTORIA; ESKILDSEN, MORTEN RING

El Escorial

Workshop; 15th International Workshop on Vortex Matter in Superconductors; 2015

The order-disorder transition (ODT) in the vortex matter of type II superconductors and the associated history effects have been subject of study and controversies for many years. In materials with very low pinning, as clean NbSe2, most of the vortex phase diagram is well described by a dislocation free Bragg Glass (BG) phase that, with increasing field and/or temperature undergoes an ODT to a strongly pinned disordered phase. The fingerprint of the transition is the sudden rise of the effective pinning known as Peak Effect (PE). While a field-cooled (FC) vortex lattice (VL) remains trapped below the PE in a disordered metastable configuration, it can reach the stable ordered BG with the help of high dc currents or large shaking magnetic fields. In this framework, the existence of an in-between transitional region has been reported[1,2], where effective pinning can be partially decreased or even increased by applying dc currents or ac fields, yielding highly reproducible intermediate final responses independent of any previous history. In our earlier work[1,3] we have proposed that these ?in-between" responses are originated from bulk VL configurations with intermediate degree of disorder, accessible from stationary dynamic states. In this work[4] we present results obtained in a recent experiment, combining small angle neutron scattering (SANS), with in-situ linear ac susceptibility measurements in a clean NbSe2 single crystal. Our findings show a clear connection between the linear ac response, related to the effective pinning, and the bulk spatial correlation of the VL. Configurations with intermediate degree of disorder were obtained by shaking the VL in the proposed transitional region, and remained traceable down to low temperatures. Our findings support the existence of the transitional region where, for the first time, a dynamic reordering into robust bulk configurations with intermediate degree of disorder was directly evidenced.