Special Colloquium, March 4, 2004

Studies of molecular motion using the scanning tunneling microscope

Jay A. Gupta

IBM Almaden Research Center,
San Jose, California

The scanning tunneling microscope (STM) can be used to study the motion of atoms or molecules in controlled environments that can be engineered on the atomic scale. As an example, the quantum tunneling of single carbon monoxide molecules can be harnessed for the transmission of one bit across a surface. The mechanism for this information transmission is based on metastable arrangements of molecules called chevrons. In analogy to a row of toppling dominoes, these chevrons can be linked in series to transport information and perform computation on the nanometer scale.

Due to the light mass and correspondingly large zero point motion, atomic and molecular hydrogen are often delocalized to some degree on metal surfaces at low temperature. In atomic hydrogen, this delocalization is apparent as hopping between lattice sites via quantum tunneling. Both single H atoms and collections of atoms in ‘troughs’ on (110) surfaces exhibit this kind of motion. Delocalization of adsorbed molecular hydrogen is sufficiently pronounced that individual molecules are not resolved in STM images. Within a range of surface coverage, tunneling spectra reveal prominent nonlinearities that deceptively resemble the quasiparticle excitation spectrum of a Tc>200K BCS superconductor. An additional negative differential resistance feature appears as the STM tip is moved closer to the surface. These nonlinearities are attributed to states of adsorbed H2 that are coupled by discrete excitations which do not readily correspond to any known rotational, vibrational or electronic transition of H2. A model for understanding these nonlinearities is developed by extending the framework for inelastic tunneling spectroscopy to include saturation effects. This model can give information on excited state lifetimes and explains a characteristic lineshape. These studies were motivated by the possibility of achieving novel phases of hydrogen when the strength of intermolecular interactions is mediated by surface electronic structure.

10:30 a.m., Smith Laboratory, Room 1094

Reception in Smith 1094 at 10:00 a.m.