Anomalous microwave conductivity due to collective transport in the pseudogap state of cuprate superconductors

and underdoped YBa2Cu3O6.5 is found to be anomalous in that Rs(T.Tc)fiXs(T.Tc) in the pseudogap state. This implies plasmonlike response and negative permittivities «8(v),0 at microwave frequencies indicating non-Fermi-liquid transport in the ab plane. The anomalous microwave response is shown to arise from a collective mode characterized by a plasma frequency v pCM;0.1 eV and extremely low damping G CM ;10 25 ‐10 24 eV, distinctly different from those observed at optical frequencies.