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Physical Review C

Bifurcation sets of the self-consistent flow in generalized SU(2) models

C. E. Vignolo, Dora M. Jezek, E. Susana Hernández

Published Jul 1, 1988• 4 citations
Publisher

Abstract

We analyze the possible bifurcations of the stationary points of the self-consistent flow on the Bloch sphere. We propose a generalized SU(2) model Hamiltonian for which, by means of a geometrically simple classification, we find the topologically invariant regions of the mean field flow in the space of interaction parameters. The borders of these regions are the bifurcation sets corresponding to general nonthermodynamic phase transitions. When these separatrices are crossed, the mean field flow undergoes a qualitative change. We also examine the consequences of the catastrophical configurations on the exact dynamics in quasispin space.

DOI
10.1103/physrevc.38.506

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We analyze the possible bifurcations of the stationary points of the self-consistent flow on the Bloch sphere. We propose a generalized SU(2) model Hamiltonian for which, by means of a geometrically simple classification, we find the topologically invariant regions of the mean field flow in the space of interaction parameters. The borders of these regions are the bifurcation sets corresponding to general nonthermodynamic phase transitions. When these separatrices are crossed, the mean field flow undergoes a qualitative change. We also examine the consequences of the catastrophical configurations on the exact dynamics in quasispin space.
The structural instabilities or general nonthermodynamic phase transitions of the time-dependent Hartree-Fock flow on the Bloch sphere are investigated for different SU(2) model Hamiltonians by means of a simple geometrical construction. In particular, the generalized Lipkin-Meshkov-Glick model is found to exhibit a variety of instabilities, opposite to the standard Lipkin-Meshkov-Glick model which possesses only one ground-state (thermodynamic) phase transition. The relationship between the fixed points of the time-dependent Hartree-Fock flow and the tangency points between energy surfaces and the Bloch sphere is established. It is found that different Hamiltonians of the class under study give rise to flow patterns whose invariant sets may contain either rotations or librations, which in turn may be degenerate as well as nondegenerate, in contrast to previously investigated models.
We describe the motion of an SU(1,1) pseudospin vector in the frame of the mean-field approximation induced by the variational principle on linear-plus-quadratic Hamiltonians. The dynamics of the SU(1,1) states of the Perelomov type obeys a nonlinear Bloch or torquelike equation, and each orbit can be interpreted as the intersection of two quadrics, one representing the energy surface and the other the group manifold, both in the space of the averaged algebra generators or semiclassical pseudospin. The fixed points of the flow can be also determined by resorting to strictly geometric considerations. The evolution of the phase diagram in parameter space is investigated as well for selected Hamiltonians. The bifurcation sets are constructed and the nonthermodynamic phase transitions can be clearly identified for the systems under consideration.
We investigate the time-dependent Hartree-Fock flow pattern of a two-level many fermion system interacting via a two-body interaction which does not preserve the parity symmetry of standard SU(2) models. The geometrical features of the time-dependent Hartree-Fock energy surface are analyzed and a phase instability is clearly recognized. The time evolution of one-body observables along self-consistent and exact trajectories are examined together with the overlaps between both orbits. Typical time scales for the determinantal motion can be set and the validity of the time-dependent Hartree-Fock approach in the various regions of quasispin phase space is discussed.
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.
To find out how quantum bifurcation phenomena manifest themselves in the space of states, various periodic trajectories accompanied with several sets of bifurcated trajectories are studied for a simple quantum model Hamiltonian under a time-dependent mean-field approximation. Making use of the quantum-classical correspondence, a method of classifying the eigenstates and of visualizing the global structures of the whole space of eigenstates is proposed. By means of the profile functions of the eigenstates, appropriate sets of eigenstates representing the quantum correspondents of the bifurcations are found. The idea of quantum bifurcations is supported by similar behavior of the energy vs period time ( E-T ) plots of the periodic trajectories and the eigenstates.
The thermal dynamics of the two-dimensional Ising model and quantum dynamics of the one-dimensional transverse-field Ising model (TFIM) are mapped to one another through the transfer-matrix formalism. We show that the fermionised TFIM undergoes a Fermi-surface topology-changing Lifshitz transition at its critical point. We identify the degree of freedom which tracks the Lifshitz transition via changes in topological quantum numbers (e.g., Chern number, Berry phase etc.). An emergent SU(2)SU(2)SU(2) symmetry at criticality is observed to lead to a topological quantum number different from that which characterises the ordered phase. The topological transition is also understood via a spectral flow thought-experiment in a Thouless charge pump, revealing the bulk-boundary correspondence across the transition. The duality property of the phases and their entanglement content are studied, revealing a holographic relation with the entanglement at criticality. The effects of a non-zero longitudinal field and interactions that scatter across the singular Fermi surface are treated within the renormalisation group (RG) formalism. The analysis reveals that the critical point of the 1D TFIM and the 1D spin-1/2 Heisenberg chain are connected via a line of SU(2)SU(2)SU(2)-symmetric theories. We extend our analysis to show that the classical to quantum correspondence links the critical theories of Ising models in various dimensions holographically through the universal effective Hamiltonian that describes the Lifshitz transition of the 1D TFIM. We obtain in this way a unified perspective of transitions in Ising models that lie beyond the traditional Ginzburg-Landau-Wilson paradigm. We discuss the consequences of our results for similar topological transitions observed in classical spin models, topological insulators, superconductors and lattice gauge-field theories which are related to the Ising universality class.