Pii: S0022-3697(98)00160-7

The ab-plane reflectance of a Nd1.85Ce0.15CuO4 single crystal (Tc 1⁄4 23 K) has been measured from < 30 to 9500 cm above and below Tc, and the optical properties calculated from a Kramers–Kronig analysis. A rich phonon spectrum is observed with some new spectral features appearing below < 100 K. The normal-state optical conductivity may be described by a Drude-like component and an overdamped mid infrared component. Below Tc the plasma frequency of the condensate is qpS < 10 000 cm. The London penetration depth is determined to be lab 1⁄4 1600 6 100 Å, which places it well off the Uemura line. Estimates of the electron– phonon coupling from normal-state transport measurements yield l tr , 0.5. 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 to that in other hole-doped superconducting cuprates. q 1998 Elsevier Science Ltd. All rights reserved In most high-temperature (high-Tc) superconducting cuprates, such as YBa2Cu3O71d and La21xSrxCuO4, the charge carriers are doped holes. However, in Nd21xCexCuO4, where superconductivity is induced by substituting Nd with Ce, the CuO2 planes are believed to be doped with electrons [1, 2] as well as holes [3]. The Nd21xCe xCuO4 system has attracted a great deal of interest because of its possible conventional BCS s-wave pairing in the superconducting state, as opposed to the unconventional d-wave behavior proposed for the holedoped cuprates [4, 5]. The microwave surface impedance measurements on both thin films and single crystals have shown evidence for a conventional BCS s-wave behavior with a gap of 2D < 4kBTc [6–8]. Tunneling measurements have also shown a resemblance to conventional superconductors [9]. However, the magnetic field dependence of the specific heat anomaly [10, 11] and thin film transmission [12] of Nd21xCe xCuO4 both show a non-BCS-like behavior. Whether or not this system can be considered as a conventional BCS-type superconductor is an important question, given the strong evidence that the other holedoped cuprates are not. The optical properties of materials in the normal and superconducting states can provide a wealth of information about the electrodynamics of the system, and can yield important information about the nature of the superconductivity. Large, single crystals of Nd1.85Ce0.15CuO4 were grown from a CuO-based flux using a directional solidification technique [13]. During the course of the growth, the flux was allowed to flow out of the crucible at the end of the growth process, so that free-standing crystals are left in the bottom of the crucible that do not need to be mechanically separated from the flux. To induce superconductivity, the crystals were annealed in an inert gas atmosphere. Both resistivity and magnetization showed a sharp superconducting transition at 23 K, with a width of