Optical properties of ${\mathbf{Nd}}_{1.85}$${\mathbf{Ce}}_{0.15}$${\mathbf{CuO}}_4$

The $\mathrm{ab}$-plane reflectance of a Nd${}_{1.85}$Ce${}_{0.15}$CuO${}_4$ single crystal ${(T}_c=23\mathrm{}$ K) has been measured from $\approx 35$ to 9500 cm${}^{-1}$ at temperatures above and below $T_c,$ and the optical properties calculated from a Kramers-Kronig analysis. A rich phonon spectrum is observed, and there are a number of $c$-axis infrared and Raman modes that are observed at low temperature which are believed to be activated by disorder; several of these modes show evidence for electron-phonon coupling. The normal-state optical conductivity may be described by a Drude-like component and an overdamped midinfrared component. The Drude-like component narrows rapidly; at 30 K the mean free path is estimated to be $\approx 750$ Å. Below $T_c$ the Drude carriers collapse into the condensate; the plasma frequency of the $\delta$ function is determined to be ${\omega }_{\mathrm{pS}}\approx 10000$ cm${}^{-1}$. The small coherence length $({\xi }_0\approx 70$ Å) places this material into the clean limit $({\xi }_0/l\ll 1)$. The London penetration depth is determined to be ${\lambda }_{\mathrm{ab}}=1600\mathrm{}\pm 100$ Å, which places it well off the Uemura line. Estimates of the electron-phonon coupling from normal-state transport measurements of ${\lambda }_{\mathrm{tr}}<0.5\mathrm{}$, and the absence of Holstein sidebands below $T_c,$ indicate that the carriers that participate in superconductivity are weakly coupled to the phonons. The small values for the penetration depth and the electron-phonon coupling constant suggest that the superconductivity in this material is not due to the electron-phonon mechanism, and is different than in other hole-doped superconducting cuprates.