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arXiv:quant-ph/0010087 · Journal of Physics A Mathematical and General

Complex Calogero model with real energies

Miloslav Znojil, Miloš Tater

Published Feb 19, 2001• 82 citations
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Abstract

Recently, the symmetry of many single-particle non-Hermitian Hamiltonians has been conjectured to be sufficient to keep their spectrum real. We demonstrate this and how the similar concept of a `weakened Hermiticity' can be extended to some exactly solvable two- and three-particle models.
DOI
10.1088/0305-4470/34/8/321

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The recently proposed complexification of Hamiltonians which keeps the spectra real (and is usually called PT symmetry) is re-interpreted here as a certain natural linear-algebraic alternative to Hermiticity. The juxtaposition is mediated by the Feshbachian projection on a model space which reduces the difference just to a sign of a propagator in the effective Hamiltonian. The problem of norms and orthogonality is clarified and a few perturbation aspects are mentioned. A remarkable simplicity of the secular polynomial is conjectured. The mechanism of the PT symmetry breaking is clarified via a separable approximation scheme.
Although parity-time (PT)-symmetric Hamiltonians are often associated with real energy spectra, PT symmetry is neither a sufficient nor a necessary condition for a real spectrum. More generally, real spectra are associated with the broader class of pseudo-Hermitian Hamiltonians, of which PT-symmetric Hamiltonians constitute a simple subclass. Here, we investigate the nonequilibrium dynamics of the imbalanced-pairing Kitaev model, a prototypical pseudo-Hermitian system, under a linearly time-dependent chemical potential. The dynamics are analyzed within the biorthogonal framework using the concept of dynamical quantum phase transitions (DQPTs). We show that, under a linear ramp protocol, DQPTs occur only when the post-ramp Hamiltonian possesses a real energy spectrum. For positive values of the non-Hermiticity parameter (γ>0γ>0γ>0), where the energy spectrum remains entirely real, a ramp crossing a single quantum critical point gives rise to a single family of critical times, analogous to the Hermitian case. Furthermore, for ramps crossing two critical or exceptional points, the critical sweep velocity above which DQPTs disappear decreases as the non-Hermiticity parameter is reduced and vanishes in the staggered-pairing limit, γ=−1γ=-1γ=−1.
The coupling of non-Hermitian -symmetric Hamiltonians to standard Hermitian Hamiltonians, each of which individually has a real energy spectrum, is explored by means of a number of soluble models. It is found that in all cases the energy remains real for small values of the coupling constant, but becomes complex if the coupling becomes stronger than some critical value. For a quadratic non-Hermitian -symmetric Hamiltonian coupled to an arbitrary real Hermitian -symmetric Hamiltonian, the reality of the ground-state energy for small enough coupling constant is established up to second order in perturbation theory.