Frozen fluctuations of a tunnel barrier: role of rare events

Small spatial fluctuations of the barrier thickness tend to create a very anomalous statistical distribution of the tunnel current (see figure). The tunnel current exhibits very large fluctuations and tends to be dominated by the very few sites where the barrier is the thinnest. This is reminiscent of Lévy flights, statistical processes dominated by rare events. Lévy flights (invented by the mathematician Paul Lévy in the 30's) play an increasingly important role in statistical physics to describe anomalous diffusion processes.

These broad distributions of tunnel currents are being studied experimentally at IPCMS by Victor da COSTA, Yves HENRY and Kamel OUNADJELA. With a modified atomic force microscope, they manage to image the tunnel current flowing through a thin oxide layer (the tunnel barrier) with a spatial resolution on the order of one nanometer. Their results confirm beautifully the theoretical expectations.

Time dependent fluctuations: quantum evaporation

Tunnelling might also be influenced by time-dependent fluctuations. Indeed, an electron going through a potential barrier is never isolated, but rather coupled to other electrons, phonons... These couplings induce dissipation whose effects are well understood in mesoscopic systems. Together with Dominique BOOSE (Laboratoire de Physique Théorique, Strasbourg), we have found that they can also induce a dramatic enhancement of the barrier transmission called 'quantum evaporation'.

Scanning tunnelling microscopy (STM) experiments

I am developing an STM experiment dedicated to the study of the time-dependent and statistical properties of tunnelling. Current focus include noise studies: the STM currents exhibit large low frequency fluctuations that significantly degrade the image quality.