Optical properties of (Pr,Ce)2CuO4
  We studied the optical conductivity of electron doped Pr{1-x)Ce(x)CuO(4) from the underdoped to the overdoped regime. The observation of low to high frequency spectral weight transfer reveals the presence of a gap, except in the overdoped regime. A Drude peak at all temperatures shows the partial nature of this gap. The close proximity of the doping at which the gap vanishes to the antiferromagnetic phase boundary leads us to assign this partial gap to a spin density wave. 
A. Zimmers, N. Bontemps, R.P.S.M. Lobo, C.P. Hill, M.C. Barr, R.L.   Greene, C.C. Homes, and A.J. Millis