Invited Paper TF-ThM1
Superconductivity in Very Clean and Doped MgB₂ Thin Films

Thursday, October 18, 2007, 8:00 am, Room 613/614

MgB₂, with a T_c of 40 K, is arguably the first superconductor to show clear multiband superconductivity and two energy gaps. It has the highest T_c among BCS superconductors, which makes it very attractive for many applications. The interplay between the interband and intraband scattering has resulted in many unusual properties. In this talk, I will review our results on MgB₂ thin films made by hybrid physical-chemical vapor deposition (HPCVD). Very clean epitaxial MgB₂ thin films on (0001) SiC and sapphire substrates have been obtained with a slight increase of T_c from the bulk value due to the epitaxial strain. From the normal state magnetoresistance measurement, we have shown a complex dependence on the orientation of the applied magnetic field, with a maximum magnetoresistance (MR~136%) observed at the field H parallel to c plane at low temperature. However, the angular dependence changes dramatically as the temperature increases, and at high temperatures, the magnetoresistance maximum changes to the H perpendicular to c direction. We attribute the results to the multiple bands with different Fermi surface topology and different electron-phonon coupling in MgB₂. By introducing carbon carrying gas during HPCVD deposition, C-doping in the films was achieved. The upper critical field was found to increase with C doping. The upper critical field can be increased to above 60 T in H parallel to ab direction and 40 T in H perpendicular to ab direction. C-doped polycrystalline films coated on SiC filbers also show H_c2 above 55 T, which make it possible as a coated conductor for high field applications.

¹ Work supported by NSF and conducted in collaboration with B. T. Liu, Y. F. Hu, J. Chen, V. Ferrando, P. Orgiani, Q. R. Feng, H. Gao, L. Shan, H. H. Wen, A. Gurevich, D.C. Larbalestier, J. B. Betts, C. H. Mielke.