Statistical fluctuations of matrix elements in regular and chaotic systems

A combination of semiclassical arguments and random-matrix theory is used to analyze transition strengths in quantum systems whose associated classical systems are chaotic. The mean behavior is found semiclassically while the local fluctuations are characterized by a Porter-Thomas distribution. The methods are tested numerically for a system with two degrees of freedom, the coupled-rotators model. The deviations of the strength distribution from a Porter-Thomas one when the system is nonchaotic are also investigated. It is found that the distribution gets gradually wider as the classical system becomes more regular.