Series expansions in closed and open quantum many-body systems with multiple quasiparticle types

The established approach of perturbative continuous unitary transformations (pCUTs) constructs effective quantum many-body Hamiltonians as perturbative series that conserve the number of one quasiparticle type. We extend the pCUT method to similarity transformations - dubbed ${\mathrm{p}\mathrm{c}\mathrm{s}\mathrm{t}}^{\mathtt{\text{++}}}$ - allowing for multiple quasiparticle types with complex-valued energies. This enlarges the field of application to closed and open quantum many-body systems with unperturbed operators corresponding to arbitrary superimposed ladder spectra. To this end, a generalized counting operator is combined with the quasiparticle generator for open quantum systems recently introduced by Schmiedinghoff and Uhrig (arXiv:2203.15532). The ${\mathrm{p}\mathrm{c}\mathrm{s}\mathrm{t}}^{\mathtt{\text{++}}}$ then yields model-independent block-diagonal effective Hamiltonians and Lindbladians allowing a linked-cluster expansion in the thermodynamic limit similar to the conventional pCUT method. We illustrate the application of the ${\mathrm{p}\mathrm{c}\mathrm{s}\mathrm{t}}^{\mathtt{\text{++}}}$ method by discussing representative closed, open, and non-Hermitian quantum systems.