Observation of a Triangular to Square Flux Lattice Phase Transition in YBCO

TL;DR

This study used small-angle neutron scattering to observe a phase transition in the flux lattice structure of YBCO, revealing a smooth change from triangular to square coordination at high magnetic fields, consistent with d-wave theories.

Contribution

It provides direct experimental evidence of a flux lattice phase transition in YBCO at high magnetic fields, confirming theoretical predictions about flux lattice symmetry.

Findings

Flux lattice changes from triangular to square near 11 T

Square flux lattice orientation matches d-wave theory predictions

Flux lattice transition occurs smoothly with increasing magnetic field

Abstract

We have used the technique of small-angle neutron scattering to observe magnetic flux lines directly in an YBCO single crystal at fields higher than previously reported. For field directions close to perpendicular to the CuO2 planes, we find that the flux lattice structure changes smoothly from a distorted triangular co-ordination to nearly perfectly square as the magnetic induction approaches 11 T. The orientation of the square flux lattice is as expected from recent d-wave theories, but is 45 deg from that recently observed in LSCO.