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Standard concepts of nuclear physics explaining the systematics of ground state spins in nuclei by the presence of specific coherent terms in the nucleon-nucleon interaction were put in doubt by the observation that these systematics can be reproduced with high probability by randomly chosen rotationally invariant interactions. We review the recent development in this area, along with new original results of the authors. The self-organizing role of geometry in a finite mesoscopic system explains the main observed features in terms of the created mean field and correlations that are considered in analogy to the random phase approximation.

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In a frequency range where a microwave resonator simulates a chaotic quantum billiard, we have measured moduli and phases of reflection and transmission amplitudes in the regimes of both isolated and of weakly overlapping resonances and for resonators with and without time-reversal invariance. Statistical measures for S -matrix fluctuations were determined from the data and compared with extant and/or newly derived theoretical results obtained from the random-matrix approach to quantum chaotic scattering. The latter contained a small number of fit parameters. The large data sets taken made it possible to test the theoretical expressions with unprecedented accuracy. The theory is confirmed by both a goodness-of-fit-test and the agreement of predicted values for those statistical measures that were not used for the fits, with the data.

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