Photocurrent effects in the scanning tunneling microscope

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Abstract
We present a theory for the currents induced in a scanning tunneling microscope by an incident electromagnetic wave. This is accomplished using a nonequilibrium Green-function formalism in a tight-binding representation, which allows us to obtain the nonlinear response of the system to any desired order in the applied field. In particular, an explicit expression for the direct component of the induced photocurrent is obtained. An approximate description of the electromagnetic field at the microscope interface and of the tip and sample surface densities of states is then used to analyze this expression numerically. We found that the direct photocurrent presents a maximum as a function of the tip-sample distance around 2 Å away from close contact. The possible use of this effect in order to calibrate the tip-sample distance is proposed.