Inelastic-neutron-scattering study at low temperature of the quantum sine-Gordon breather in 4-methyl-pyridine with partially deuterated methyl groups

Multistability of synchronous regimes in rotator ensembles

We study collective dynamics in rotator ensembles and focus on the multistability of synchronous regimes in a chain of coupled rotators. We provide a detailed analysis of the number of coexisting regimes and estimate in particular, the synchronization boundary for different types of individual frequency distribution. The number of wave-based regimes coexisting for the same parameters and its dependence on the chain length are estimated. We give an analytical estimation for the synchronization frequency of the in-phase regime for a uniform individual frequency distribution.

Exceptional discretizations of the sine-Gordon equation

The method of one-dimensional maps was recently introduced as a means of generating exceptional discretizations of the phi(4) theory, i.e., discrete phi(4) models which support kinks centered at a continuous range of positions relative to the lattice. In this paper, we employ this method to obtain exceptional discretizations of the sine-Gordon equation (i.e., exceptional Frenkel-Kontorova chains). We also use one-dimensional maps to construct a discrete sine-Gordon equation supporting kinks which move with arbitrary velocities without emitting radiation.

Rotation–libration and rotor–rotor coupling in 4-methylpyridine

The low temperature rotational dynamics of methyl groups in 4-methylpyridine is analyzed in terms of a model potential including rotation-libration and rotor-rotor coupling. The parameters of the model potential are adjusted by comparison of calculated with published and newly recorded inelastic neutron scattering spectra. Initial evaluations of the potential parameters of the model are obtained from molecular mechanics calculations. Experimental spectra are calculated from these potentials by numerical solution of Schrödinger's equation for clusters of coupled rotors embedded in a bigger ensemble of rotors treated in the mean field approximation. Adjustment of the potential parameters leads to excellent agreement with the experimental spectra of protonated 4-methylpyridine, measured at well-defined spin temperatures. At higher levels of deuteration, agreement with experiment is qualitative, only. The observed deviations are attributed to the increasing frustration of the system of coupled methyl groups and mutual localization, effects leading to a phase transition around 5.5 K in isotopic mixtures, as shown in diffraction experiments.

Discrete quantum breathers: what do we know about them?

The knowledge about discrete quantum breathers, accumulated during the last two decades, is reviewed. "Prehistory" of the problem is described and some important properties differentiating localized and extended vibrational modes are outlined. The state of art of our understanding of the principal features of the quantum discrete breathers is presented.

Quantitative atom-atom potentials from rotational tunneling: their extraction and their use

Rotational tunneling of small molecular groups has been the subject of considerable theoretical and experimental activity for several decades. Much of this activity has been driven by the promise of exploiting the extreme sensitivity of quantum tunneling to interatomic potentials, but until recently, there was no straightforward means by which quantitative information about these potentials could be extracted. This review explains how a quantitative method, suitable for general application, was developed. It then goes on to show how this has been used to understand tunneling systems for which no previous satisfactory explanation had been found. The application of the methodology, and its results, to other disciplines is discussed.