On the dynamical invariants and the geometric phases for a general spin system in a changing magnetic field

We consider a class of general spin Hamiltonians of the form H-S(t) = H-0(t ) + H ' (t), where H-0(t) and H ' (t) describe the dipole interaction of th e spins with an arbitrary time-dependent magnetic field and the internal in teraction of the spins, respectively. We show that if H ' (t) is rotational ly invariant, then H-S(t) admits the same dynamical invariant as H-0(t). A direct application of this observation is a straightforward rederivation of the results of Yan et al. (Phys. Lett. A 251 (1999) 289, 259 (1999) 207) o n the Heisenberg spin system in a changing magnetic field. (C) 2001 Elsevie r Science B.V. All rights reserved.