Spectra and magnetic properties of large spins in external fields

Spectra and magnetic properties of large spins J (e.g., spins possessed by ions or molecules), placed into a crystal electric field (CEF) of an arbitr ary symmetry point group, are shown to change drastically when J changes by 1/2 or 1. At a fixed field symmetry and configuration of its N extrema sit uated at the p-fold symmetry axis, physical characteristics of the spin dep end periodically on J with the period equal to p. The problem of the spectr um and eigenstates of the large spin J is equivalent to an analogous proble m for a scalar charged particle confined to a sphere S-2 and placed into th e magnetic field of the monopole with the charge J. This analogy, as well a s the strong difference between close values of J, stems from the Berry pha se occurring in the problem. For energies close to the extrema of the.CEF, the problem can be formulated as Harper's equation on the sphere. The (2J 1)-dimensional space of states is split into smaller multiplets of classic ally degenerated states. These multiplets in turn are split into submultipl ets of states transforming according to specific irreducible representation s of the symmetry group determined by J and p. We classify possible configu rations and corresponding spectra. Experimental realizations of large spins in a symmetric environment are proposed and physical effects observable in these systems are analyzed. [S1050-2947(99)00709-X].