Magnetic Quantum Rotors

A new quantum system, magnetic quantum rotors, is presented. A magnetic rotor—a rotor possessing nonzero spin—is exemplified by the oxygen molecule. Of the exchange, spin-orbital, and spin-figure interactions contributed by the spin degree of freedoms only the latter was taken into consideration in this study. Phase diagrams of magnetic quantum linear rotors in a crystal field are studied for a system where all angular momentum states are allowed (all-J system) and for one where angular momentum is restricted to have odd values (odd-J system). It is found that depending on the crystal field and the value of the spin-figure coupling constant the orientational phase transition line can display a set of critical points (one, two, three or none in the case of the all-J systems and one, two or no critical points in the case of the odd-J systems) which are responsible for the topology of the phase diagram.