Microwave conductivity of patterned YBa/sub 2/Cu/sub 3/O/sub 7/ thin films

We have used coplanar resonators to provide a sensitive measure of the ab-plane microwave complex conductivity /spl sigma/=/spl sigma//sub 1/-i/spl sigma//sub 2/ at 8 and 16 GHz of high quality, patterned thin films of YBa/sub 2/Cu/sub 3/O/sub 7-/spl sigma// on MgO substrates. No BCS coherence effects were observed in /spl sigma/1(T) below T/sub c/. Instead, we observe a broad peak centred between 40 and 50 K, which was present in all of the samples. The T dependence of the magnetic penetration depth /spl lambda/(T)=(/spl omega//spl mu//sub 0//spl sigma//sub 2/(T))/sup -1/2/ also differed from that expected for a BCS superconductor in the London limit. This difference is most noticeable at low T, where instead of the usual exponential T dependence, we observe a term in T/sup 2/ for every sample. We discuss briefly the effects of sample oxygenation on these results.>