Charge dynamics in underdoped Nd2-xCexCuO4: 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 Nd2-xCexCuO4 (0less than or equal toxless than or equal to0.15). In the optical spectra of underdoped crystals (x<0.15), a notable pseudogap is observed at low temperatures. A Drude-like response evolves concomitantly with pseudogap formation. Both the magnitude (Delta(PG)) and onset temperature (T-*) of the pseudogap decrease with electron doping, while holding the relation that Delta(PG)approximate to10k(B)T(*). The Delta(PG) is comparable to the magnitude of the pseudogap at around (pi/2,pi/2) in the photoemission spectra reported by Armitage [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 1/tau(omega) of the x=0.10-0.15 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 T-* 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 (pi,0), 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 B-1g Raman spectra below T-*. The pseudogap phenomenon in the electron-doped cuprate has been argued comparatively with that of the hole-doped cuprate.