Condensed-Matter Physics & Materials Science Seminar

Vortex phases in type-II superconductors are manifestations of a complex interplay among various underlying interactions; intervortex repulsions tend to drive the vortices into forming an ideal crystalline lattice with long-range order (LRO), while disorder-induced pinning deforms the Abrikosov vortices. The competition between such interactions in a vortex system creates phases with very interesting behaviors, which have been the subject of intense study. The well-known “peak” effect has been shown to be a first order transition from a quasi-ordered Bragg glass phase into a disordered phase in weakly-disordered systems. The purpose of our work has been to explore vortex structures in strongly-disordered type-II superconductor V-21at.%Ti which exhibits a peak effect in its phase diagram using ac magnetic susceptibility and small-angle neutron scattering (SANS). In this system, the peak effect appears only at fields higher than 3.4 T. The sample is characterized by strong atomic disorder. Vortex states with field-cooled thermal histories show that both deep in the mixed state, as well as close to the peak effect, there exist no long-range orientationally ordered vortex lattices. The SANS scattering radial widths reveal vortex states ordered in the sub-μm scale. We conjecture that
the peak effect in this system is a remnant of the Bragg glass disordering transition, but occurs on submicron length scales due to the presence of strong atomic disorder on larger length scales.