Laser-induced thermoelectric effects in an STM junction

Abstract

The spatial and temporal temperature distribution for a conical shape metal emitter under laser irradiance is obtained analytically using the Green function method. In this study the tip is modeled as an infinite cone of half angle θ 0 which can vary between 0 and 1 2 π . The full three-dimensional heat diffusion equation is solved simultaneously with the Fourier equation for the heat flux assuming no radiation losses. The general solution is obtained for an arbitrary temporal and spatial distribution of the irradiance. With uniform irradiance the temperature rise in the conical tip varies almost linearly with the laser intensity and heating time and depends strongly on cone angle and thermal properties. For a tungsten tip, the temperature increase is about 2–3 orders of magnitude above ambient for a typical laser intensity of about 1 MW/cm 2 in a time of a few hundred nanoseconds. To study Joule heating and the Thomson thermoelectric effect in the STM junction we first obtained the current distribution inside the emitter. Using this as a volume heat source, we used the Green function method to solve the heat conduction equation. The resulting temperature gradient was then used to obtain the thermoelectric potential. For a tungsten tip at ∼ 1000 K the thermoelectric effect generates a bias voltage on the order of 10 mV.