Anisotropic peak effect due to structural phase transition in the vortex lattice

Abstract

It is shown that the recently observed new peak effect in YBCO could be explained by the softening of the vortex lattice due to a structural phase transition in the vortex lattice. At this transition square lattice transforms into a distorted hexagonal one. While conventional peak effect is associated with the softening of shear modes (elastic modulus c(66) vanishes) at melting, in this case the relevant mode is "squash" mode(c(11) + c(22) - 2c(12) vanishes).