Polarons
Recent studies have shed some more light on the nature of excitations that make up the broad, gapped spectral feature through detailed studies of Ca2-xNaxCuO2Cl2 as a function of doping. Quasiparticles are coherent electron states that form in metals, while “polarons” are the exact opposite—incoherent excitations of electrons strongly coupled to a bath of lattice vibrations and commonly found in insulators. This work suggests that the spectral intensity near the Fermi level in the metallic copper-oxides evolves from a polaron-like distribution of intensity in the pseudo-gaped insulators. Further work shows that these polaronic excitations may be related to a charge order in the pseudogap region of the phase diagram, seen also by Scanning Tunneling Microscopy.
