Imaging of a vortex lattice transition in YNi2B2C by scanning tunneling spectroscopy

The vortex lattices in YNi2B2C under the magnetic fields H up to 3 T applie d along both the a and the c axes have been studied by scanning tunneling s pectroscopy at 4.2 K- The vortex lattice transition has been found to occur in different manners for H parallel to a and H parallel to c; in H paralle l to a a slightly distorted hexagonal vortex lattice has been found to tran sform to a nearly square one above 1.0 T with increasing H, while in H para llel to c the transition occurs at a much lower field around 0.1 T The unco nventional steep increase of the quasiparticle density of states outside th e vortex core has also been found well below H-c2.