Four-fold structure of vortex core states in Bi2Sr2CaCu2O8 (Bi2212)
  We present a detailed study of vortex core spectroscopy in slightly overdoped Bi2Sr2CaCu2O8 using a low temperature scanning tunneling microscope. Inside the vortex core we observe a four-fold symmetric modulation of the local density of states with an energy-independent period of (4.3\pm 0.3)a0. Furthermore we demonstrate that this square modulation is related to the vortex core states which are located at ~6 meV. Since the core-state energy is proportional to the superconducting gap magnitude, our results strongly suggest the existence of a direct relation between the superconducting state and the local electronic modulations in the vortex core. 
Giorgio Levy (1), Martin Kugler (1), Alfred A. Manuel (1), Oystein   Fischer (1) and Ming Li (2) ((1) University of Geneve, ...