Vortex pinning in electron-doped cuprate superconductor La_2-xCe_xCuO_4-y

We have measured the vortex pinning strength in thin films of the electron-doped cuprate La2-xCexCuO4 for various doping x from x = 0.075 (T-c = 25 K) to x = 0.15 (T-c = 12.5 K). The optimal doping for this compound is x approximate to 0.11, at which T-c = 29 K. Films were prepared by a molecular-beam epitaxy method. A structural XRD investigation revealed no doping-induced lattice strain. We use a two-coil technique at frequencies from 10 to 100 kHz. The superfluid density n(s)(0) grows rapidly with x on the underdoped side and decreases slowly on the overdoped side. The Labusch parameter kappa, i.e. the spring constant for a pinned vortex, shows large variations as the doping changes. It increases with x rapidly to a maximum at slight underdoping and then it decreases on the overdoped side more rapidly than n(s)(0). Even at optimal doping kappa(0) is an order of magnitude lower than the highest values observed in YBCO.