Vortex pinning in electron‐doped cuprate superconductor La 2– x Ce x CuO 4

We have measured the vortex pinning strength in thin films of the electron‐doped cuprate La 2– x Ce x CuO 4 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 ≈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 κ , 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 κ (0) is an order of magnitude lower than the highest values observed in YBCO.