In-plane conductivity of a layered large-bipolaron liquid
  Distinctive normal-state properties of cuprate superconductors follow from their charge carriers forming a large-bipolaron liquid. The very weak scattering of the slow-moving heavy-massed excitations of the liquid by acoustic phonons yields a scattering rate that is less than the Debye frequency. The moderate liquid mobility, greater than 1 cm2/V-sec at 300 K, results as the weak scattering of the liquid compensates for its large mass. In resolution of a long-standing dilemma, the dc resistivity resulting from scattering by acoustic phonons remains nearly proportional to temperature to well below the Debye temperature. Above the Debye frequency the frequency-dependent conductivity is dominated by excitation and photo-ionization of the self-trapped electronic carriers of the large-bipolarons. Below the Debye frequency the frequency-dependent conductivity is dominated by the Drude-like collective motion of the large-bipolaron liquid. The gap between these two domains sharpens with decreasing temperature as phonon scattering of the liquid diminishes. The high-frequency electronic excitations survive in the superconducting state. 
David Emin
cond-mat.supr-con