Translational–rotational interaction in the dynamics and thermodynamics of a 2D atomic crystal with a molecular impurity

Abstract

The interaction between the rotational degrees of freedom of a diatomic molecular impurity and the phonon excitations of a two-dimensional atomic matrix commensurate with a substrate is investigated theoretically. It is shown, that the translational-rotational interaction changes the form of the rotational kinetic energy operator as compared to the corresponding expression for a free rotator, and also renormalizes the parameters of the crystal field without change in its initial form. The contribution of the impurity rotational degrees of freedom to the low-temperature heat capacity for a dilute solution of diatomic molecules in an atomic two-dimensional matrix is calculated. The possibility of experimental observation of the effects obtained is discussed.

Low-energy excitations in helium-like dimer within an exact diagonalization approach

“…The exchange (of about 12 K) on the interatomic bond depends on the interatomic distance, so that there is a coupling between spin and phonon degrees of freedom which leads to renormalization of phonon spectrum in condensed phase as compared to the corresponding spinless medium. This effect can be interpreted as a direct analog of translation-rotation interaction in molecular cryocrystals [38][39][40][41].…”

Microwave spectroscopy of superfluid He II

“…(3) that spin subsystem plays the role of an external force for phonon excitations, and when approaching some 'resonant' wavelength the phonon dispersion curve will be 'subsided' due to loading from spin degrees of freedom (roton minimum of dispersion curve). Mechanically, it is rather evident effect which is just direct analog of translationrotation interaction in molecular cryocrystals [18][19][20] and could be a justification of the phonon-'roton' coupling in He II (phonons are translation excitations in the medium, but spin degrees of freedom have the well-known transformation properties of angular momentum). In this connection could be clarified the physical sense of term 'roton', introduced by I.E.…”