Twist as a probe for phase structure

The use of twist is advocated as an efficient tool, in computer simulated lattice gauge theories, for a quick scan of phase diagrams and for determining phase structure. Physically, a twist is an excitation induced by a specific dislocation ('twisted boundary condition') in the system. It is shown that its effect can efficiently be measured and used to determine the thermodynamic phase of the system. An intriguing phenomenon occurs in case of nonabelian gauge theories: e.g. for SU(N), introducing a 'simple' twist leaves the minimum action unchanged, (but changes the dimension of the zero action manifold), a double twist (two simple ones perpendicular to one another) (when N=2 or 3) always raises it. Also reported are results obtained by applying this method to two-dimensional spin-spin systems, with the nonabelian symmetry groups O(3) and O(4). The results for O(4) are consistent with a disordered phase at all temperatures; those for O(3) are more difficult to interpret. (Auth.)