Dirac particles in twisted tubes

We consider the dynamics of a relativistic Dirac particle constrained to mo ve in the interior of a twisted tube by confining boundary conditions, in t he approximation that the curvature of the tube is small and slowly varying . In contrast with the nonrelativistic theory, which predicts that a partic le's spin does not change as the particle propagates along the tube, we fin d that the angular momentum eigenstates of a relativistic spin-1/2 particle may behave nontrivially. For example, a particle with its angular momentum initially polarized in the direction of propagation may acquire a nonzero component of angular momentum in the opposite direction on turning through 2 pi radians. Also, the usual nonrelativistic effective potential acquires an additional factor in the relativistic theory. (C) 1999 Academic Press.