Deviations from the extended London model at high magnetic fields in YBa$_2$Cu$_3$O$_7$

TL;DR

This study investigates the vortex lattice in YBa2Cu3O7 at high magnetic fields up to 25.9 T, revealing deviations from the extended London model and suggesting the influence of Pauli paramagnetism at high fields.

Contribution

The paper provides high-field neutron scattering data showing the vortex lattice behavior in YBa2Cu3O7, highlighting deviations from existing models at fields above 12 T.

Findings

Vortex lattice structure evolves with magnetic field and temperature.

Form factor remains nearly constant above 12 T, contrary to expectations.

Possible role of Pauli paramagnetic moments at high fields.

Abstract

We report on the evolution with magnetic field and temperature of the vortex lattice (VL) in fully-oxygenated YBa2Cu3O7 as studied by time-of-flight small angle neutron scattering. Using the HFM/EXED beamline, we have obtained data up to 25.9 T - much higher than that available previously. Our VL structure results indicate the progressive suppression by field of the superconductivity along the crystallographic b (CuO chain) direction. The intensity of the diffracted signal reveals the spatial variation of magnetization caused by the VL (the "form factor"). Instead of a rapid fall-off with field, as seen in superconductors with smaller upper critical fields, we find that the form factor is almost constant with field above about 12 T. We speculate that this is due to Pauli paramagnetic moments, which increase at high fields due to alignment of the spins of quasiparticles in the vortex cores.