at a particular point on the surface were averaged over 100 measurements in the same voltage range of | V | ≤ 4 mV with 512 steps each in this voltage span. Measurements (1) and (3) were performed away from a vortex and show the superconductive density of states. The spectrum at the vortex (2) clearly shows the enhancement at zero bias of the conductance due to the Andreev bound states. The observation of this state indicates the stable and reproducible positioning of the tip at the center of the vortex. We thank R. Hoffmann, B. Pilawa, C. Debuschewitz and W. Wulfhekel for helpful dis- cussions. Many thanks to C. P´erez Le´ on for the help with the assembly of the electronics. We thank D. Rosenmann at the Argonne Nation Laboratory, Condensed Matter Division, for providing us the NbSe 2 sample. VI. ACKNOWLEDGMENTS [1] G. Binnig et al. , Appl. Phys. Lett. 40 , 178 (1982). [2] E. W. Hudson et al. , Nature 411 , 920 (2001). [3] T. Hanaguri et al. , Nature 430 , 1001 (2004). [4] G. R. Stewart et al. , Phys. Rev. Lett. 52 , 679 (1984). [5] Ø. Fischer et al. , Rev. Mod. Phys. 79 , 353 (2007). [6] F. Mugele et al. , Rev. Sci. Inst. 69 , 1765 (1998). [7] C. Debuschewitz et al. , J. Low Temp. Phys. 147 , 525 (2007). [8] Macor is a registered trademark of Corning, Inc., One Riverfront Plaza, Corning, NY 14831, USA. [9] D. W. Pohl, Rev. Sci. Instrum. 58 , 54 (1987). [10] H. -J. G¨ untherodt, R. Wiesendanger, Scanning Tunneling Microscopy I, Springer Verlag (1994). [11] H. Bando et al. , Jap. J. Appl. Phys. 26 , L41 (1987). [12] R. V. Coleman et al. , J. Vac. Sci. Technol. A 6 , 338 (1987). [13] H. F. Hess et al. , Phys. Rev. Lett. 69 , 2138 (1992). [14] I. Guillamon et al. , Phys. Rev. B 77 , 134505 (2008). 16