Research Expertise and Interest
physics, electronic properties of atomic-scale structures at surfaces, atomic-scale structures, morphology and dynamics of mesoscopic systems, atomic manipulation, visualizing low dimensional electronic behavior
Michael Crommie's main research interest is in exploring the local electronic properties of atomic-scale structures at surfaces. He is also interested in studying how local interactions between atomic-scale structures affect the morphology and dynamics of mesoscopic systems. His main experimental tool is the ultra-high vacuum cryogenic scanning tunnelling microscope, which can be used to both fabricate atomic-scale structures and probe them spectroscopically.
Atomic manipulation: The tip of a scanning tunneling microscope (STM) can be used to position individual atoms and molecules on a conducting surface, allowing the fabrication of precise atomic-scale structures. We have built a low temperature STM that is capable of performing atomic manipulation, and we are currently exploring the physical parameters necessary to controllably position atoms and molecules on different substrates. The ability to manipulate an atom depends on the relative strength of its coupling to the surface and to the STM tip. By characterizing these interactions we extend our ability to fabricate structures at the atomic-scale, and further our understanding of how adsorbates bind to surfaces.
Visualizing low dimensional electronic behavior: Electrons at some metal surfaces occupy a 2-dimensional surface state, known as a Shockley state. These electrons can be observed with an STM, allowing direct visualization of the quantum mechanical state density of 2-dimensional electronic wavefunctions. We are currently studying such systems in an effort to better understand how 2-d electrons interact with adsorbates, artificially fabricated atomic-scale structures, and surface reconstructions.
Magnetic nanostructures: One of the smallest magnetic structures imaginable is a single magnetic atom in a non-magnetic host. If the host is a metal, then spin-flip scattering between conduction electrons and the magnetic impurity leads to a many-body groundstate where an extended cloud of electrons screens the local impurity moment. This is known as the Kondo effect, and the influence of the "Kondo screening cloud" on macroscopic material properties has been well studied. We are currently using a low temperature STM to directly investigate the local electronic properties of individual Kondo impurities at a clean metal surface. We are using the atomic manipulation capabilities of our STM to probe magnetic interaction effects between individual Kondo impurities and also to study artificially fabricated clusters of magnetic atoms. These studies help us to understand how the properties of microscopic magnetic structures evolve with the overall size of a system.
Nanostructures on semiconductors: Microfabricated circuit elements on semiconducting substrates form the core of current high technology. We are using a low temperature STM to probe the limits of atomic manipulation on semiconducting surfaces. Single atom manipulation on semiconductors holds the possibility of providing us with the ability to fabricate 1-dimensional wires only a single atom wide. These structures are predicted to exhibit novel electronic behavior, such as the Luttinger liquid groundstate. Such studies bring us closer to the goal of exploring electrical circuitry at the ultimate level of miniaturization.
Carbon nanostructures: sp2 bonded carbon lattices have been shown to yield a fabulous diversity of molecular structures with striking electronic and structural properties. One exciting aspect of these "fullerines" is that their properties can be tuned through doping and structural modification, opening possibilities for new device applications. We are interested in characterizing the electronic properties of "bare" fullerine molecules, as well as atomically-fabricated and molecularly functionalized fullerine structures. Current projects involve C60 and carbon nanotubes deposited onto metal and semiconducting surfaces.