STM Junction Research Articles

Self-consistent theory for the DC Josephson effect in a superconducting STM junction

We present a microscopic theory for the DC Josephson tunnelling currents between a superconducting STM tip and the surface of a superconductor. Within this theory, the spatial variation of the order parameter in the presence of a stationary current is calculated in a self-consistent way at the interface region. The Hamiltonian modelling this system is written in a tight-binding representation and the superconducting state is treated in a mean-field (BCS-like) approximation. We use a non-equilibrium Green functions technique for the calculation of the phase-parameter profile and currents. The results are analyzed as a function of the tip-sample distance, tunnelling current intensity, total phase gradient through the interface and coherence length of the superconductors. The effects of self-consistency are found to be crucial in the close-contact regime when the tip-sample junction has a bottle-neck geometry.

Theory of light rectification in scanning tunneling microscopy in the presence of an adsorbed molecule

AbstractWe consider the problem of the rectified current induced by laser radiation in the STM junction when the tip is placed above a small molecule like CO or NO. This is calculated assuming a simple tight‐binding model for the tunneling junction including the adsorbate and using nonequilibrium Green's functions techniques. The coupling between tunneling electrons and the molecule vibrational modes is taken into account by a local electron‐phonon interaction term. In a second step we estimate the excitation rate of the molecule vibrations for a given laser power. This value is then used to obtain the relative change in the rectified current when the laser is in resonance with a molecule vibration. For a moderate laser power of 2 kW/cm2 a relative change of 1 to 3% is predicted.

Laser-induced thermoelectric effects in an STM junction

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.

Proposed use of a scanning-tunneling-microscope tunnel junction for the measurement of a tunneling time.

Recent measurements of dc current-voltage characteristics of scanning-tunneling-microscope (STN) junctions have confirmed their expected high rectification property. We propose to exploit this property (i) to study rectification at infrared and optical frequencies and (ii) to arrive at a procedural definition of an electron tunneling time. We envisage focusing a laser beam of linearly polarized light onto a STM junction and detecting the dc bias induced across the junction by the alternating, asymmetrical tunnel current. The asymmetry may be of geometrical, material, and/or thermal origin. The dc rectified bias should vanish at sufficiently high laser frequency for fixed tip-to-surface distance or when withdrawing the tip away from the surface for fixed laser frequency. The average electron tunneling time is then one half of the inverse cutoff frequency observed in such a laser-rectification experiment. Similar experiments have already been conducted successfully with metal-whisker diodes and have resulted in the recent redefinition of the meter. The new opportunity offered by STM is the controllability of the junction and its microscopic size, entailing high responsivity. Additional possible laser-STM studies are proposed.