Microwave surface resistance of Hg-1212 thin films prepared through a Tl/Hg substitution process

Abstract HgBa 2 CaCu 2 O 6 +  δ thin films were prepared on lanthanum aluminate (LaAlO 3 ) as well as on CeO 2 buffered r -plane sapphire substrates through a conversion process using Tl 2 Ba 2 CaCu 2 O 8 +  δ films as well as thallium free amorphous Ba–Ca–Cu–O films as precursors. The c -axis oriented HgBa 2 CaCu 2 O 6 +  δ films show high critical current densities at 77 K: above 8 MA/cm 2 on LaAlO 3 and 6 MA/cm 2 on buffered sapphire. Using the dielectric resonator technique the microwave surface resistance R s was determined to be below 1 and 3.2 mΩ, at low fields and at 77 K and 17.8 GHz, for films on LaAlO 3 and buffered sapphire, respectively. However, several films show strong increases of R s with field amplitudes B HF  > 1 mT and even higher low field R s values of above 10 mΩ, which are remarkable higher than expected for high quality HgBa 2 CaCu 2 O 6 +  δ or Tl 2 Ba 2 CaCu 2 O 8 +  δ and YBa 2 Cu 3 O 7 −  δ thin films. We present R s measurements of HgBa 2 CaCu 2 O 6 +  δ films on both substrate materials and discuss possible defects and degradation mechanisms. The layer growth process, which includes melting phenomena instead of only cation-exchange of Tl with Hg, will be discussed. We consider the thallium to play a key role for the conversion process.