Quantum phase crossover and chaos in a generalized Jahn–Teller lattice model

Abstract

The generalized multispin Jahn–Teller model on a finite lattice or formally equivalent Dicke model extended to two long-wavelength coherent bosons of different frequencies is shown to exhibit a crossover between the polaron-modified ‘quasi-normal’ and the squeezed ‘radiation’ domains. We investigate the effects of two kinds of interfering fluctuations on the phase crossover and on statistical characteristics of boson complex spectra. (i) Fluctuations in the electron subsystem—finite-size quantum fluctuations—are responsible for the dephasing of the coherence in the radiation domain and for the moderate occupation of the excited states in the normal domain. In the quasiclassical limit, radiation phase implies the existence of a coherent acoustic super-radiant phase. (ii) Level-spacing fluctuations in the excited boson level subsystem with strong level repulsions. Related probability distributions are shown to be non-universally spread between the limiting universal Wigner–Dyson and Poisson distributions. We proved that the difference in boson frequencies is responsible for reaching the most stochastic limit of the Wigner–Dyson distribution. Instanton lattice as a sequence of tunneling events in the most chaotic radiation domain exhibits maximal number of level-avoidings (repulsions). The non-universality of the distributions is caused by boson correlations which compete with the level repulsions.