Random-phase approximation and broken symmetry

Universal relationship between a quantum phase transition and instability points of classical systems

The direct and universal relationship between the accumulation of exceptional points in the quantum spectrum at a phase transition and the singularity of the classical action at a homoclinic point of the separatrix is investigated. The particular common features are the analytic structure and, related to it, instability and high sensitivity leading generically to the onset of chaos in both cases under perturbation.

Decoherence and thermalization in a simple bosonic system

Properties of a parameter-dependent quantum system with the Hamiltonian H(lambda) randomized by fluctuations of the parameter lambda in a narrow range are investigated. The model employed (the interacting boson model-1) exhibits a crossover behavior at a critical parameter value. Due to the fluctuations, individual eigenstates /psi(alpha)(lambda)> of the Hamiltonian become statistical ensembles of states [density matrices rho(alpha)(lambda)], which allows us to study effects related to the decoherence and thermalization. In the decoherence part, we evaluate von Neumann and information entropies of the density matrices rho(alpha)(lambda) and the overlaps of the eigenstates of the density matrix with various physically relevant bases. An increased decoherence at the " phase transitional" point and an exceptional role of the dynamic-symmetry U(5) basis are discovered. In the part devoted to the thermalization, we develop a method of how a given density matrix rho(alpha)(lambda) can be represented by an equivalent canonical (thermal) ensemble. Thermodynamic consequences of the quantum "phase transition" (related, in particular, to the specific heat of the thermal equivalent) are discussed.

Quantum phase transitions studied within the interacting boson model

We study quasicritical phenomena in transitions between two "quantum phases" of a finite boson system, described by the interacting boson model 1 used in nuclear physics. The model is formulated in the algebraic framework and has a simple geometrical interpretation; the "phases" represented by dynamical symmetries U(5) and SU(3) correspond to spherical and deformed nuclear shapes. The quasicriticality of the U(5)-SU(3) transition is shown to be connected with the following phenomena simultaneously occurring in a narrow parameter region between the symmetries: (a) abrupt structural changes of eigenstates, (b) multiple avoided crossing of levels, (c) peaked density of exceptional points, (d) qualitative changes of the corresponding classical potential. We show that these spectroscopic features influence the dynamics of intersymmetry transitions in the model parameter space if the parameters themselves become dynamical variables.