Using small-angle neutron scattering, we have imaged the magnetic flux line lattice (FLL) in the -wave heavy-fermion superconductor . At low fields we find a hexagonal FLL. Around 0.6 T this undergoes what is most likely a first-order transition to square symmetry, with the nearest neighbors oriented along the gap node directions. This orientation of the square FLL is consistent with theoretical predictions based on the -wave order parameter symmetry.
- Received 25 November 2002
- Published 7 May 2003
© 2003 The American Physical Society