DYNAMICAL SCREENING IN THE SCANNING TUNNELING MICROSCOPE AND METAL-INSULATOR-METAL JUNCTIONS

We investigate electron tunneling in a system consisting of two curved metal surfaces separated by insulator or vacuum. In particular, we ca lculate the modifications of the tunneling barrier due to dynamical sc reening, i.e., interaction with charge fluctuations: We apply our gene ral results to the planar metal-insulator-metal (MIM) junction, and to the scanning tunneling microscope (STM), describing the tip and the s ample surface in STM by two rotational hyperboloids. We analyze the in fluence of the shape, dielectric properties, and work functions of bot h metals on the tunneling characteristics in the MIM and STM systems. For metals with different plasma frequencies, charge-fluctuation modes are effectively decoupled, and the electron interaction with these mo des is significantly different than in the case of like metals, causin g asymmetry in the barrier and also in the tunneling currents and cond uctivities. We also show that, for geometrical reasons, the tunneling barrier in the STM is lowered near the tip apex, which leads to focusi ng of the tunneling current and increased lateral resolution of STM.