Charge dynamics in underdoped ${\mathrm{Nd}}_{2-x}{\mathrm{Ce}}_x{\mathrm{CuO}}_4:$  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 ${\mathrm{Nd}}_{2-x}{\mathrm{Ce}}_x{\mathrm{CuO}}_4$ $(0\mathrm{}<~x<~0.15).$ In the optical spectra of underdoped crystals $\mathrm{}(x<\mathrm{}0.15\mathrm{}),$ a notable pseudogap is observed at low temperatures. A Drude-like response evolves concomitantly with pseudogap formation. Both the magnitude $\left({\Delta }_{\mathrm{PG}}\right)$ and onset temperature ${(T}^{*})$ of the pseudogap decrease with electron doping, while holding the relation that ${\Delta }_{\mathrm{PG}}\approx {10k}_{\mathrm{B}}{T}^{*}.$ The ${\Delta }_{\mathrm{PG}}$ is comparable to the magnitude of the pseudogap at around $(\pi /2,\pi /2)$ 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 $1/\tau \left(\omega \right)$ of the $x=0.10\mathrm{}–0.15$ 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 $B_{1\mathrm{g}}$ Raman spectra below $T^{*}.$ The pseudogap phenomenon in the electron-doped cuprate has been argued comparatively with that of the hole-doped cuprate.