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

TL;DR

This study experimentally and theoretically explores how partial violation of time-reversal invariance affects elastic enhancement and backscattering in wave-chaotic systems, revealing that T-invariance violation can increase backscattering with system openness.

Contribution

It provides the first experimental analysis of T-invariance violation effects on elastic enhancement in wave-chaotic systems, supported by random matrix theory.

Findings

Elastic enhancement decreases with T-invariance violation at fixed openness.

Elastic enhancement increases with openness beyond a certain T-invariance violation level.

Experimental results are confirmed by random matrix theory simulations.

Abstract

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.