QUANTUM SIGNATURES OF CHAOS IN INTEGRABLE SYSTEMS

Two fundamental quantum signatures of classically chaotic behaviour, l arge second derivatives and the logarithmic time barrier separating cl assical from quantum dynamics, have been observed near the separatrix of an integrable spin system. A systematic study in the neighbourhood of the separatrix reveals that the error in semiclassical eigenvalues (measured in units of the quantum level spacing) approaches a finite v alue at the classical limit. The generic factor which limits the valid ity of the correspondence principle is the presence of classical insta bilities; chaotic behaviour is one such case; the separatrix of an int egrable system is another.