title: Hexagonal and square flux line lattices in CeCoIn5.
abstract: Using small-angle neutron scattering, we have imaged the magnetic flux line lattice (FLL) in the d-wave heavy-fermion superconductor CeCoIn5. 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 d-wave order parameter symmetry.
authors.affiliation: DPMC, University of Geneva, 24 Quai E.-Ansermet, CH-1211 Genève 4, Switzerland. morten.eskildsen@physics.unige.ch
authors.forename: Morten Ring
authors.initials: MR
authors.lastname: Eskildsen
authors.forename: Charles D
authors.initials: CD
authors.lastname: Dewhurst
authors.forename: Bart W
authors.initials: BW
authors.lastname: Hoogenboom
authors.forename: Cedomir
authors.initials: C
authors.lastname: Petrovic
authors.forename: Paul C
authors.initials: PC
authors.lastname: Canfield