BIAS-DEPENDENT STM IMAGES OF CHARGE-DENSITY WAVES ON TAS2

The atomic and electronic structures of the 4Hb polytype (alternating layers of T and H types) of TaS2 have been studied with a scanning tun neling microscope, operating in air at room temperature. On areas with presumably the T-type layer being the surface, a commensurate root 13 x root 13 charge-density wave (CDW) superlattice is observed, indepen dent of sample bias. On tile presumably H-type surface, however, image s are bias dependent. Increase of the positive sample bias from about 50 to 170 mV results in a continuous change of images from a dominant atomic pattern with a very weak superimposed CDW modulation to a fully developed commensurate root 13 x root 13 cow superlattice. With a neg ative bias (down to -300 mV), however, there is hardly any indication of a CDW modulation. The bias-dependent behavior can be explained by t he energy dependence of the probability of electron tunneling from the tip through top (H type) to the second (T type) layer.