1. Theoretical Solid State Physics Marvin L. Cohen and Steven G. Louie, University of California at Berkeley, DMR-0087088 Carbon nanotubes possess novel properties that are of fundamental scientific interest and significant technological potential. For example, their unique optical properties make them promising candidates as nanoscale opto- electronic devices or photo-markers in various applications. However, quantum many-electron effects often dramatically modify the properties of reduced dimensional systems. We have performed first-principles calculations to investigate electron-hole interaction (excitonic) effects on the optical spectra of single-walled carbon nanotubes. We discovered that excitonic effects are orders of magnitude larger in the nanotubes than bulk carbon materials and they qualitatively alter the optical response of both semiconducting and metallic tubes. These large many-electron effects elucidate recent experimental findings and explain the discrepancies between previous theories and experiments. Optical absorption spectrum (top panel) and spatial correlation of the photo-excited electron- hole pair (exciton) for state A 1 ’ (bottom panel) of a (8,0) carbon nanotube. Spataru, Ismail-Beigi, Benedict & Louie, Phys. Rev. Lett. (2004).

2. Theoretical Solid State Physics Marvin L. Cohen and Steven G. Louie, University of California at Berkeley, DMR-0087088 Structurally, boron nitride (BN) nanotubes are identical to carbon nanotubes except that the carbon atoms are alternatively replaced by boron and nitrogen atoms. This class of nanotubes, whose existence was first predicted by theory, can now be synthesized in abundance. Unlike carbon nanotubes, the BN nanotubes however are large gap semiconductors. Our ab initio calculations show that the band gap of BN nanotubes can be significantly reduced by a transverse electric field. This gap reduction arises from a giant Stark effect. The gap modulation increases with tube diameter and is nearly independent of chirality. This effect provides a possible way to tune the band gap of BN nanotubes for various applications. Tuning the band gap of BN nanotube with transverse electric field. Khoo, Mazzoni, and Louie, Phys. Rev. B (2004).

3. Theoretical Solid State Physics Marvin L. Cohen and Steven G. Louie, University of California at Berkeley, DMR-0087088 The energetics of face-centered-cubic Cs 3 C 60, which is a material of great interest as a possible high transition- temperature superconductor, is studied. At the optimized lattice constant the volume per C 60 is found to be smaller than the the most-stable hexagon- coordination A15 phase, while the total energy of the fcc phase is higher than the A15 phase. These results indicate that a low-temperature and high-pressure synthesis method might be a possible way to produce the fcc Cs 3 C 60 phase. In addition, it is also found that the A15 Cs 3 C 60 should show a phase transformation from a hexagon- coordination phase to a pentagon- coordination phase under hydrostatic pressure. Total energies of the fcc phase (solid line), two bco phases (+ and x) and two A15 phases (dashed lines) as a function of the volume per C60. Saito, Umemoto, Louie & Cohen, Solid Stat. Commun. (2003).

4. Theoretical Solid State Physics Marvin L. Cohen and Steven G. Louie, University of California at Berkeley, DMR-0087088 How mechanical energy dissipates (or friction) at the nanoscale remains an open issue which is central to the understanding and efficient operation of nanomachines. We have carried out molecular dynamics simulations to study dissipation in carbon nanotube oscillators of lengths of tens of nanometers. The principal source of friction is found to be the ends of the tubes and hence dynamical friction is virtually independent of the overlap area between tubes. As a result of this, tube commensuration does not lead to significantly increased frictional forces. The friction force is found to depend strongly and nonlinearly on the relative velocity of the tubes. It is suggested that a strong velocity dependence and strong contributions from surface edges may be quite general features of friction at the nanoscale. Molecular dynamics simulation of energy dissipation in double-walled carbon nanotube oscillators. Tangney, Louie and Cohen, Phys. Rev. Lett. (2004).