Deviations from the extended London model at high magnetic fields in YBa2Cu3 O7

We report on the evolution with the magnetic field and the temperature of the vortex lattice (VL) in fully oxygenated YBa2Cu3O7 as studied by time-of-flight small-angle neutron scattering. Using the High Field Magnet/Extreme Environment Diractometer beamline at Helmholtz-Zentrum Berlin, we have obtained data up to 25.9 T - much higher than data available previously. Our VL structure results are consistent with the progressive suppression by the field of the superconductivity along the crystallographic b (CuO chain) direction and an accompanying shift of the nodal directions as the field is increased. The intensity of the diffracted signal reveals the spatial variation of magnetization caused by the VL (the "form factor"). Instead of a rapid falloff with the field, as seen in superconductors with smaller upper critical fields, we find that the form factor is almost constant with the field above ∼12 T. We speculate that this is due to Pauli paramagnetic effects.