Correlated Nanostructures

Most experiments examining the electronic properties of nanostructured materials have focused on simple metals and semiconductors. We are very interested in applying nanoscale transport techniques to strongly correlated materials, systems in which the simple single-electron approach of conventional band theory fails. Such nanostructure-based experiments, while challenging, can (1) apply large electric fields without the need for large voltages, discriminating between different physical processes; (2) probe inhomogeneous systems on a scale smaller than their inhomogeneity; and (3) enable sensitive studies of noise and contact effects not readily performed in macroscopic structures. We published a review article about this exciting topic here, as well as an overview of Rice University research on strongly correlated materials.

One example of a strongly correlated material is VO2. This material has a high temperture metallic state with a rutile (tetragonal) crystal structure, and a low temperature insulating state with a monoclinic crystal structure, separated by a first-order "metal-insulator" phase transition at around 65 \(^{o}\)C. This transition is very dramatic, with a 10000\(\times\) change in the electronic conductivity. For decades the roles of electron-electron interactions (particularly from the half-filled d band of the V ions) and the electron-phonon interactions (as seen in the structural change at the transition) have been debated. Current understanding is that both strong e-e and e-ph interactions are important.

Working with single-crystal nanobeams of VO2, we have found that this material can be reversibly doped by intercalation of atomic hydrogen. (It turns out that it has been known for over forty years that atomic hydrogen is readily taken up by the structurally related semiconductor, TiO2.) This doping process suppresses the insulating state (!), both by changing the effective V d occupancy and by expanding the lattice slightly. One of our ongoing efforts is the detailed understanding of the resulting low temperature conducting state. We now have a greater understanding of the resulting structures and the nature of the hydrogen diffusion process.

Current investigations are focused on "bad" or "strange" metals where the conventional quasiparticle picture of low energy electronic excitations may fail. We are interested in examining charge transport on mesoscopic scales. If the long-lived, low energy excitations in these systems do not look like conventional electrons, how should one think about them and their quantum corrections to classical conduction? Similarly, what information can shot noise reveal about the nature of charge transport in such systems - are these weird materials just masking essentially ordinary Fermi liquids, or is the situation more complex? We are looking at magnetotransport at small scales in correlated oxides (see here and here for recent results), and using tunneling shot noise spectroscopy to assess charge carriers in non-Fermi liquids.

This work is supported by the Material Sciences and Engineering Division of the Department of Energy's Office of Basic Energy Sciences.