How T-invariance violation leads to an enhanced backscattering with increasing openness of a wave-chaotic system

We report on the experimental investigation of the dependence of the elastic enhancement, i.e., enhancement of scattering in backward direction over scattering in other directions of a wave-chaotic system with partially violated time-reversal (T ) invariance on its openness. The elastic enhancement factor is a characteristic of quantum chaotic scattering which is of particular importance in experiments, like compound-nuclear reactions, where only cross sections, i.e., the moduli of the associated scattering matrix elements are accessible. In the experiment a quantum billiard with the shape of a quarter bow-tie, which generates a chaotic dynamics, is emulated by a flat microwave cavity. Partial T-invariance violation of varying strength 0 < xi < 1 is induced by two magnetized ferrites. The openness is controlled by increasing the number M of open channels, 2 < M < 9, while keeping the internal absorption unchanged. We investigate the elastic enhancement as function of xi and find that for a fixed M it decreases with increasing time-reversal invariance violation, whereas it increases with increasing openness beyond a certain value of xi > 0.2. The latter result is surprising because it is opposite to that observed in systems with preserved T invariance (xi = 0). We come to the conclusion that the effect of T -invariance violation on the elastic enhancement then dominates over the openness, which is crucial for experiments which rely on enhanced backscattering, since, generally, a decrease of the openness is unfeasible. Motivated by these experimental results we, furthermore, performed theoretical investigations based on random matrix theory which confirm our findings.