Quantum Coherence in Nanostructures

Electrons are quantum mechanical objects, yet in daily experience one does not worry about quantum interference effects when, for example, turning on the lights. The reason for this is decoherence. As electrons interact inelastically with their environment (including other electrons), their quantum mechanical phase relationships become scrambled on a time scale called the coherence time (and a corresponding distance scale, the coherence length). At room temperature, this length scale is ~ 1 nm. At low temperatures, however, it can reach easily accessible scales and produce significant quantum corrections to the electrical properties of conducting systems.

Information about decoherence processes must be inferred from measurements of those corrections. An important outstanding question is, Do different quantum corrections probe the same decoherence physics? In the past we have concentrated on measuring universal conductance fluctuations and weak localization.

We have used these measurements to probe the nature of quantum-enhanced noise in normal metal nanowires, including the surprising effects of surface chemistry on the source and distribution of that noise. These measurements also revealed interesting magnetic properties of partially oxidized titanium.

We have also performed similar measurements in ferromagnetic metal nanowires as well as InMnAs dilute magnetic semiconductor nanowires. Quantum coherence effects in ferromagnetic systems remain relatively unexplored experimentally, and ferromagnetism leads to new collective modes (e.g., spin waves) that may profoundly affect electronic coherence.

This work had been supported by the DOE and the Packard Foundation.

While we are not currently pursuing this project actively, the general topic of coherence remains of much interest to us. It is interesting to ask, what coherence effects are seen and how may they be understood in materials where the conventional quasiparticle picture of electronic excitations is inadequate? This is one aspect of our work on correlated nanostructures.