High magnetic field studies of the vortex lattice structure in YBa2 Cu3 O7

We report on small-angle neutron scattering (SANS) measurements of the vortex lattice (VL) in twin-free YBa2Cu3O7, extending the previously investigated maximum magnetic field of <11 T up to 16.7 T with the field applied parallel to the c axis. This is a microscopic study of vortex matter in this region of the superconducting phase. We find that the high magnetic field VL displays a rhombic structure, with a field-dependent coordination that passes through a square configuration with no lock-in to a field-independent structure. The VL pinning reduces with increasing temperature but is seen to affect the VL correlation length even above the irreversibility temperature of the lattice structure. At high field and temperature we observe a VL melting transition, which appears to be first order. The vortex liquid phase above the transition does not give a detectable SANS signal. © 2014 American Physical Society.