Reentrant Orientational Phase Transitions and Critical Points at Quantum Orientational Melting

Abstract

An effect is studied of the crystal field on a system of rotors under the conditions of quantum orientational melting. It is shown that even very small crystal fields can substantially change the behavior of the system: at a certain positive value of the crystal field, a critical point appears at the “classical” side of the phase separation line; another critical point appears at the “quantum” side with further increasing of the field; then these critical points degenerate into a multicritical point and then the phase separation line disappears. For negative crystal fields stable states of the system of rotors can exist with a negative order parameter which can be treated as the orientational analog of the easy-plane-type ordering in magnets.