DISTANCE-DEPENDENT STM-STUDY OF THE W(110) C-R(15X3) SURFACE/

The W(110)/C-R(15 x 3) reconstructed surface has been studied by STM a t variable tip-surface separation controlled by the tunneling gap resi stance. A pronounced dependence of the STM image contrast as a functio n of tip height has been observed which is explained by the suppressio n of higher Fourier components, i.e. small wavelength features, with i ncreasing tip height and an additional spatial dependence of the decay length of the surface wave-function. As an important implication of o ur study we have found that STM images of non-trivial surface structur es can depend critically on the tunneling gap resistance.