Fig. 5: Exact diagonalization.

Low-energy spectra of Eq. (3) for different χ. System size is N = 10. Red and orange straight lines are the reference and ground-state energies, correspondingly at χ = 0. They are repeated on all panels as a guide for eye. When χ = 0, energies are straight lines with slopes (11). For larger χ, energies are curved down, due to the level repulsion. Simultaneously the gaps open up.