Charge dynamics in underdoped Nd 2 − x Ce x CuO 4 : Pseudogap and related phenomena

We have investigated the temperature and doping variations of optical and transport properties in the electron-doped high-${T}_{\mathrm{c}}$ cuprate crystals ${\mathrm{Nd}}_{2\ensuremath{-}x}{\mathrm{Ce}}_{x}{\mathrm{CuO}}_{4}$ $(0<~x<~0.15).$ 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 $({\ensuremath{\Delta}}_{\mathrm{PG}})$ and onset temperature ${(T}^{*})$ of the pseudogap decrease with electron doping, while holding the relation that ${\ensuremath{\Delta}}_{\mathrm{PG}}\ensuremath{\approx}{10k}_{\mathrm{B}}{T}^{*}.$ The ${\ensuremath{\Delta}}_{\mathrm{PG}}$ is comparable to the magnitude of the pseudogap at around $(\ensuremath{\pi}/2,\ensuremath{\pi}/2)$ 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 $1/\ensuremath{\tau}(\ensuremath{\omega})$ 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 $(\ensuremath{\pi},0),$ 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 ${B}_{1\mathrm{g}}$ Raman spectra below ${T}^{*}.$ The pseudogap phenomenon in the electron-doped cuprate has been argued comparatively with that of the hole-doped cuprate.