Charge dynamics in underdoped Nd_2−xCe_xCuO₄: Pseudogap and related phenomena

We have investigated the temperature and doping variations of optical and transport properties in the electron-doped high-T_c cuprate crystals Nd_2−xCuO₄(0 < ∼ 0.15). In the optical spectra of underdoped crystals (0 <.15), a notable pseudogap is observed at low temperatures. A Drude-like response evolves concomitantly with pseudogap formation. Both the magnitude (γPG) and onset temperature T*, in the photoemission spectra reported by Armitage et al. [Phys. Rev. Lett. 88, 257001 (2002)], which indicates that the pseudogap appearing in the optical spectra is identical to that discerned by the photoemission spectroscopy. The scattering rate spectra of crystals show a kink structure at around 0.07 eV, which can be ascribed not to the pseudogap but to the electron-phonon coupling. In accordance with the evolution of the Drude response, the in-plane resistivity begins to decrease rapidly at around in the underdoped region. The out-of-plane resistivity shows an even more distinct decrease below T_*. This is because the interplane charge transport is governed by electronic states at around where the quasiparticle spectral weight is accumulated in the case of the electron-doped system. This is contrary to the hole-doped case with the pseudogap around this point. The origin of the pseudogap has been ascribed to the antiferromagnetic spin correlation, which is consistent with the evolution of a two-magnon band in the Raman spectra below T_*The pseudogap phenomenon in the electron-doped cuprate has been argued comparatively with that of the hole-doped cuprate.