Klemen Kovač, mag. fiz. and prof. dr. Janez Bonča from our Department, in collaboration with researchers at the University of British Columbia (Canada), have published a paper entitled Signature of Preformed Pairs in Angle-Resolved Photoemission Spectroscopy in Physical Review Letters.

Their work applies density matrix renormalization group (DMRG) and variational exact diagonalization (VED) methods to the Holstein-Hubbard model. Two key findings emerge:

1. Preformed-Pair Signature: A clear gap appears below the Fermi energy in the spectral function when carriers form bipolaronic “preformed pairs.” Notably, this gap closes once the quasiparticles become unbound (polarons). Importantly, the gap does not imply an insulating ground state; instead, the bipolaron liquid remains a strongly correlated metal.
2. Effective Hard-Core Boson Description: This incoherent bipolaron liquid is well approximated by a Bose sea of hard-core bosons in momentum space, with otherwise minimal effective interactions.

These results offer fresh insight into how strong electron-boson coupling can lead to metallic states with preformed pairs—an important step in understanding correlated systems.