First observation of the fourfold-symmetric and quantum regime vortex core in YNi2B2C by scanning tunneling microscopy and spectroscopy

The local quasi-particle density of states, N s ( E , r ), in the isolated vortex of YNi 2 B 2 C has been measured at 0.46 K by low-temperature scanning tunneling microscopy and spectroscopy. The vortex core is found to be fourfold-symmetric star-shaped in real space: Near E = 0 meV, N s ( E , r ) extends toward (the a -axis) and has a peak at the center of the vortex core. With increasing energy, N s ( E , r ) comes to extend toward and the peak of N s ( E , r ) splits into four peaks toward . N s ( E , r ) in the vortex core exhibits electron–hole asymmetry with the intensity, indicating the first observation of the vortex core in a quantum regime.