Infrared Properties of Electron Doped Cuprates: Tracking Normal State   Gaps and Quantum Critical Behavior in Pr(2-x)Ce(x)CuO(4)
  We report the temperature dependence of the infrared-visible conductivity of Pr(2-x)Ce(x)CuO(4) thin films. When varying the doping from a non-superconducting film (x = 0.11) to a superconducting overdoped film (x = 0.17), we observe, up to optimal doping (x = 0.15), a partial gap opening. A model combining a spin density wave gap and a frequency and temperature dependent self energy reproduces our data reasonably well. The magnitude of this gap extrapolates to zero for x ~ 0.17 indicating the coexistence of magnetism and superconductivity in this material and the existence of a quantum critical point at this Ce concentration. 
A. Zimmers, J.M. Tomczak, R.P.S.M. Lobo, N. Bontemps, C.P. Hill, M.C.   Barr, Y. Dagan, R.L. Greene, A.J. Millis, C.C. Homes