Effect of orbital symmetry of the tip on Scanning Tunneling Spectra of   Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$
  We discuss how variations in the scanning tunneling microscope (STM) tip, whether unintentional or intentional, can lead to changes in topographic images and dI/dV spectra. We consider the possibility of utilizing functionalized tips in order to improve the sensitivity of STM experiments to local irregularities at the surface or hidden below the surface layers. The change in the tip symmetry can radically alter the contrast of the topographic image due to changes in tip-surface overlap. The dI/dV curves change their shape according to which sample bands the tip orbital tends to overlap. In addition, relative phases between competing tunneling channels can be inverted by changing the tip symmetry, which could help reveal the origin of a local irregularity in tunneling spectrum. 
Ilpo Suominen, Jouko Nieminen, R.S. Markiewicz and A. Bansil
cond-mat.supr-con