Quantum dynamics of molecules in 4He nano-droplets: Microscopic superfluidity

Abstract

High resolution spectroscopy of doped molecules in He nano-droplets and clusters gives a signature of superfluidity in microscopic system, termed as microscopic superfluidity. Ro-vibrational spectrum of HeN-M clusters is studied with the help of some important observations, revealed from experiments (viz., localised and orderly arrangement of He atoms, although, being free to move in the order of their locations; individual He atoms can not be tagged as normal/ superfluid, etc.) and other factors (e.g., consideration that the 4 4 4 4 4 He atoms which happen to fall in the plane of rotation of a molecule, render a equipotential ring and thus, do not take part in rotation; etc.) which effect the rotational and vibrational spectrum of the system. This helps us in successfully explaining the experimental findings which state that the rotational spectrum of clusters have sharp peaks (indicating that the molecule rotates like a free rotor) and moment of inertia and vibrational frequency shift have a non-trivial dependence on N.

On the free rotation of a molecule embedded in helium-4 clusters

“…This undoubtedly proves the absence of p = 0 condensate in these systems and we use these aspects of Jain's NCMT [23] to frame a model which provides a better account for the typical nature of non-trivial dependence of B on N revealed from experimental observations on selected M:He N clusters. In this context it may be mentioned that our intial efforts [26] tried to use some simple thoughts to explain the effect by presuming that: (i) each added atom which takes the cluster from M:He N to M:He N +1 can significantly change the positions of other 4 He atoms from the axis of rotation, and (ii) with N increasing beyond its certain value (depending on several physico-chemical aspects of M), 4 He atoms start occupying the second position from M (e.g. in M(zero)-4 He(first)-4 He(second)) and these atoms interact so weekly with the rotor-part of cluster (M and few 4 He atoms, -at first position, which interact directly with M) that they do not follow the rotation of the rotor.…”