Two patterns of PT-symmetry breakdown in a non-numerical six-state simulation

Abstract: Three-parametric family of non-Hermitian but ${\cal PT}-$symmetric six-by-six matrix Hamiltonians $H^{(6)}(x,y,z)$ is considered. The ${\cal PT}-$symmetry remains spontaneously unbroken (i.e., the spectrum of the bound-state energies remains real so that the unitary-evolution stability of the quantum system in question is shown guaranteed) in a non-empty domain ${\cal D}^{(physical)}$ of parameters $x,y,z$. The construction of the exceptional-point (EP) boundary $\partial{\cal D}^{(physical)}$ of the physical domain is preformed using an innovative non-numerical implicit-function-construction strategy. The topology of the resulting EP boundary of the spontaneous ${\cal PT}-$symmetry breakdown (i.e., of the physical "horizon of stability") is shown similar to its much more elementary $N=4$ predecessor. Again, it is shown to consist of two components, viz., of the region of the quantum phase transitions of the first kind (during which at least some of the energies become complex) and of the quantum phase transitions of the second kind (during which some of the level pairs only cross but remain real).