Volume 88, Number 5, December 2009
Article Number 56005
Number of page(s) 6
Section Condensed Matter: Structural, Mechanical and Thermal Properties
Published online 16 December 2009
EPL, 88 (2009) 56005
DOI: 10.1209/0295-5075/88/56005

Dynamics of one-dimensional and two-dimensional helium adsorbed on carbon nanotubes

S. O. Diallo1, 2, B. Fåk3, M. A. Adams4, O. E. Vilches5, M. R. Johnson6, H. Schober6 and H. R. Glyde2

1   Ames Laboratory, Iowa State University - Ames, IA 50011, USA
2   Department of Physics and Astronomy, University of Delaware - Newark, DE 19716-2570, USA
3   Commissariat à l'Energie Atomique, INAC, SPSMS - 38054 Grenoble Cedex 9, France, EU
4   ISIS Facility, Rutherford Appleton Laboratory - Didcot, OX11 0QX, UK, EU
5   Department of Physics, University of Washington - Seattle, WA 98195-1560, USA
6   Institut Laue-Langevin - BP 156, 38042 Grenoble, France, EU

received 14 September 2009; accepted in final form 16 November 2009; published December 2009
published online 16 December 2009

We present inelastic neutron scattering measurements of the dynamics of helium adsorbed on carbon nanotube bundles. The goal is to determine the vibrational properties of the 1D and 2D quantum solids that are stabilized on the nanotube bundle surfaces at different fillings of 4He. The mean square vibrational amplitude in the 1D solid is large with a Lindemann ratio of $\gamma _{1D}=(\langle u^{2}\rangle )^{1/2}/a_{1}=0.25$ comparable to bulk solid 4He. The $\gamma _{2D}$ is significantly smaller. The frequency density of states of the 2D solid, $g(\omega)$, has a gap at $\omega \simeq $ 0.75 meV consistent with a commensurate lattice. The 1D solid has no gap or a gap smeared by disorder. The 1D and 2D $g(\omega)$ are well described by dispersion curves having no gap and a gap, respectively, with some vibration along additional dimensions indicated.

61.05.F- - Neutron diffraction and scattering.
67.80.B- - Solid 4He. - Structure, lattice dynamics and sound.

© EPLA 2009