Quantum limiting behaviors of a vortex core in an anisotropic gap   superconductor
  Quantized bound states at a vortex core are discretized in YNi$_2$B$_2$C. By using scanning tunneling spectroscopy with an unprecedented 0.1 nm spatial resolution, we find and identify the localized spectral structure, where in addition to the first main peak with a positive low energy, a second subpeak coming from the fourfold symmetric gap structure is seen inside the energy gap. Those spectral features are understood by solving the Bogoliubov-de Gennes equation for a fully three-dimensional gap structure. A particle-hole asymmetric spectrum at the core site and quantum oscillation in the spectra are clearly observed. 
S. Kaneko, K. Matsuba, M. Hafiz, K. Yamasaki, E. Kakizaki, N. Nishida,   H. Takeya, K. Hirata, T. Kawakami, T. Mizushima, K....
cond-mat.supr-con