Electron dynamics in Nd 1.85 Ce 0.15 CuO 4+δ : Evidence for the pseudogap state and unconventional c-axis response

Infrared reflectance measurements were made with light polarized along the a and c axes of both superconducting and antiferromagnetic phases of electron doped ${\mathrm{Nd}}_{1.85}{\mathrm{Ce}}_{0.15}{\mathrm{CuO}}_{4+\ensuremath{\delta}}.$ The results are compared to characteristic features of the electromagnetic response in hole doped cuprates. Within the ${\mathrm{CuO}}_{2}$ planes the frequency dependent scattering rate, 1/\ensuremath{\tau}(\ensuremath{\omega}), is depressed below $\ensuremath{\sim}650 {\mathrm{cm}}^{\ensuremath{-}1};$ this behavior is a hallmark of the pseudogap state. While in several hole doped compounds the energy scales associated with the pseudogap and superconducting states are quite close, we are able to show that in ${\mathrm{Nd}}_{1.85}{\mathrm{Ce}}_{0.15}{\mathrm{CuO}}_{4+\ensuremath{\delta}}$ the two scales differ by more than one order of magnitude. Another feature of the in-plane charge response is a peak in the real part of the conductivity, ${\ensuremath{\sigma}}_{1}(\ensuremath{\omega}),$ at $50\char21{}110 {\mathrm{cm}}^{\ensuremath{-}1}$ which is in sharp contrast with the Drude-like response where ${\ensuremath{\sigma}}_{1}(\ensuremath{\omega})$ is centered at \ensuremath{\omega}=0. This latter effect is similar to what is found in disordered hole doped cuprates and is discussed in the context of carrier localization. Examination of the c-axis conductivity gives evidence for an anomalously broad frequency range from which the interlayer superfluid is accumulated. Compelling evidence for the pseudogap state as well as other characteristics of the charge dynamics in ${\mathrm{Nd}}_{1.85}{\mathrm{Ce}}_{0.15}{\mathrm{CuO}}_{4+\ensuremath{\delta}}$ signal global similarities of the cuprate phase diagram with respect to electron and hole doping.