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 YNi2B2C 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 (100) (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 (110) and the peak of N-s(E, r) splits into four peaks toward (110). 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.