Collective rotational tunneling of methyl groups and quantum solitons in 4methylpyridine: Neutron scattering studies of single crystals
Abstract
The structure of the 4methylpyridine crystal has been determined at 10 and 260 K with the singlecrystal neutrondiffraction technique. The spacegroup symmetries are I4_{1}/a and I4_{1}/amd, respectively. In both cases, there are eight molecular entities in the unit cell. The rotational axes of the methyl groups are aligned along the c axis. The shortest intermolecular distances occur between facetoface methyl groups. The next shortest distances correspond to infinite chains of rotors parallel to the orthogonal axes a and b. The angular probability densities of the methyl groups are clear evidence of orientational disorder. The incoherent scattering function S(Q_{a},Q_{b},ω) has been measured with the inelastic neutron scattering technique at 1.7 K. The energytransfer ranges, ħω=±(500±60)μeV, encompass transitions due to rotational tunneling. The anisotropy in Q is distinctive of dynamics in one dimension. These are represented with the quantum sineGordon equation that is an approximation to the Hamiltonian for an infinite chain of coupled rotors. Spots of intensity observed for neutronenergy loss at Q_{a} or Q_{b}≈1.55 Å^{1} are distinctive of stationary states for breathers. Weaker peaks at Q_{a} or Q_{b}≈1.0 Å^{1} reveal collective tunneling. This is a thermally activated process arising from rather heavy pseudoparticles composed of large numbers (from 22 to 25) of kinks or antikinks. Quantization of the kinetic momentum arises from the chain discreteness and from conservation of the angular momentum. Traveling states are stationary when the kinetic energy is within the tunneling energy band. In the ground state, the chain dynamics are represented with a single collective angular coordinate. The incoherent scattering function for neutronenergygain reveals bound excited states with lifetimes of several days.
 Publication:

Physical Review B
 Pub Date:
 December 2003
 DOI:
 10.1103/PhysRevB.68.224301
 Bibcode:
 2003PhRvB..68v4301F
 Keywords:

 63.20.Dj;
 25.40.Dn;
 33.20.Tp;
 Phonon states and bands normal modes and phonon dispersion;
 Elastic neutron scattering;
 Vibrational analysis