Speaker: Janez Bonca, University of Ljubljana

Time: 10:00-11:00am

Location: BRIM 311

Abstract: We use density matrix renormalization group (DMRG) and variational exact diagonalization (VED) to calculate the single-electron removal spectral weight for the Hubbard-Holstein model at low electron densities. Tuning the strength of the electron-phonon coupling and of the Hubbard repulsion allows us to contrast the results for a liquid of polarons versus a liquid of bipolarons. The former shows spectral weight up to the Fermi energy, as expected for a (uncorrelated) metal. The latter has a gap in its spectral weight, set by the bipolaron binding energy, although this is also a (strongly correlated) metal. This difference suggests that angle-resolved photoemission spectroscopy could be used to identify liquids of pre-formed pairs. Furthermore, we show that the liquid of bipolarons is well approximated by an ensemble of bosons that are hard-core in momentum space, filling the states inside the Fermi sea but otherwise non-interacting [1]. We have also generalized the calculation to allow for a finite optical phonon dispersion. The spectral function is significantly influenced by the dispersion, particularly in systems with strong electron-phonon coupling. The sign of the curvature of the phonon band plays a crucial role in the distribution of spectral weight [2].

References

• [1] K. Kovaˇc, A. Nocera, A. Damascelli, J. Bonˇca, and M. Berciu, to appear in PRL, arXiv:2409.14490

• [2] K. Kovaˇc and J. Bonˇca, arXiv:2410.11537