We report on small-angle neutron scattering studies of the intrinsic vortex lattice (VL) structure in detwinned YBa2Cu3O7 at 2 K, and in fields up to 10.8 T. Because of the suppressed pinning to twin-domain boundaries, a new distorted hexagonal VL structure phase is stabilized at intermediate fields. It is separated from a low-field hexagonal phase of different orientation and distortion by a first-order transition at 2.0(2) T that is probably driven by Fermi surface effects. We argue that another first-order transition at 6.7(2) T, into a rhombic structure with a distortion of opposite sign, marks a crossover from a regime where Fermi surface anisotropy is dominant, to one where the VL structure and distortion is controlled by the order-parameter anisotropy.
@article{arxiv.0810.1947,
title = {Fermi surface and order parameter driven vortex lattice structure transitions in twin-free YBa2Cu3O7},
author = {J. S. White and V. Hinkov and R. W. Heslop and R. J. Lycett and E. M. Forgan and C. Bowell and S. Straessle and A. B. Abrahamsen and M. Laver and C. D. Dewhurst and J. Kohlbrecher and J. L. Gavilano and J. Mesot and B. Keimer and A. Erb},
journal= {arXiv preprint arXiv:0810.1947},
year = {2009}
}