One-dimensional Peierls phase separation in the dilute carrier density limit

We use Density Matrix Renormalization Group to study a one-dimensional chain with Peierls electron-phonon coupling describing the modulation of the electron hopping due to lattice distortion. We demonstrate the appearance of an exotic phase-separated state, which we call Peierls phase separation, in the limit of very dilute electron densities, for sufficiently large couplings and small phonon frequencies. This is unexpected, given that Peierls coupling mediates effective pair-hopping interactions that disfavor phase clustering. The Peierls phase separation consists of a homogenous, dimerized, electron-rich region surrounded by electron-poor regions, which we show to be energetically more favorable than a dilute liquid of bipolarons. This mechanism qualitatively differs from that of typical phase separation in conventional electron-phonon models that describe the modulation of the electrons potential energy due to lattice distortions. Surprisingly, the electron-rich region always stabilizes a dimerized pattern at fractional densities, hinting at a non-perturbative correlation-driven mechanism behind phase separation.