Peak effect and square-to-rhombic vortex lattice transition in La2-xSrxCuO4

A theory of structural phase transition of the vortex lattice in tetragonal superconductors is constructed based on the self consistent harmonic approximation for lattice anharmonicities, within the fourfold-symmetric generalization of the London model. Thermal fluctuations on the mesoscopic scale are strong enough to affect the location of the square to rhomb transition line in the T-H plane. We find that the slope of the transition line is generally negative: thermal fluctuations favor the more symmetric square lattice. The calculated transition line is concave, and fits the experimental line deduced in LaSCO crystals from the onset of the second magnetization peak. Near the transition line the "squash" modulus Csq=2(C11+C12)-C66 is softened leading to enhancement of the critical current, born out in the experiment as a second magnetization peak.