糖心TV

Recent angle-resolved photoemission spectroscopy (ARPES) has identified that a finite-range Fröhlich electron-phonon interaction (EPI) with c-axis polarized optical phonons is important in cuprate superconductors. Thus, it is interesting to determine what sorts of phonon-mediated unconventional pairing are possible. To this end, schemes that can cope beyond the limited range of applicability for BCS theory need to be developed. In this talk, I describe results from two major extensions: A high-order weak-coupling expansion (extended Eliashberg theory) and an advanced continuous-time quantum Monte-Carlo algorithm (CTQMC). I first demonstrate how expanding the electron self-energy to second order in the binding energy leads to the conclusion that d-wave superconductivity can be mediated by phonons, with Coulomb repulsion stabilising d- over s- wave pairing [1]. Eliashberg theories may be used for moderate electron-phonon couplings, however, to determine exact results at intermediate to strong couplings we use a CTQMC algorithm that we recently developed for studying bipolarons (two electrons bound by phonons). The CTQMC algorithm is excellent for investigating unconventional pairing possibilities. We have investigated the effective mass and binding energies of singlet and triplet real-space bipolarons on a number of lattice types [2,3]. We demonstrate that bipolarons can be simultaneously small and light, provided that suitable conditions on the electron-phonon and electron-electron interaction and lattice type are satisfied. Such light small bipolarons are a necessary precursor to high-temperature Bose-Einstein Condensation (BEC) in solids. The light bipolaron mass is shown to be universal in systems made of triangular plaquettes, due to a novel crab-like motion. We discuss the conditions under which such particles may form a BEC with an exceptionally high transition temperature [2].