Anisotropic spin system, quantized Dirac monopole and the Berry phase

Abstract

The Berry phase, acquired by an anisotropic spin system with spinJ≥1 when it is adiabatically rotated in a closed circuit, is considered to be associated with a non-quantized Dirac monopole, and has a geometrical as well as a topological component owing to the Dirac string. Here, it is argued that the Berry phase of a spin state withJ≥1 can be associated with a quantized Dirac monopole when the corresponding spin is taken to be an entangled state of a composite system of 1/2 spins. Evidently, this avoids the contribution of the Dirac string, and the Berry phase is purely geometrical in nature.