Peak effect and the rhomb to square structural transition in the vortex lattice

Abstract

The theory of structural transformation of the vortex lattice in a fourfold symmetric type II superconductor in the presence of both thermal fluctuations and quenched disorder is constructed. We show that in the absence of pinning, the slope of the square to rhomb transition in the T-H plane is generally negative: thermal fluctuations favor a more symmetric square lattice. Disorder\'s influence on the slope is just the opposite- it favors a less symmetric rhombic phase in which the fourfold symmetry is spontaneously broken. The second magnetization peak line in LaSCO in a wide range of doping is well described as a result of the transition.