The upper critical field, HC2, of Mg(B1-xCx)2 was measured in order to probe the maximum magnetic field range for superconductivity that could be attained by C doping. C-doped MgB2 filaments were prepared, and for C levels below 4% the transition temperatures were depressed by about 1K/%C and HC2(0) rose by about 5T/%C. This meant that 3.8%C substitution would depress TC from 39.2 to 36.2K and raise HC2(0) from 16.0 to 32.5T. These rises in HC2 were accompanied by a rise in resistivity at 40K from about 0.5 to about 10μcm.

Systematic Effects of Carbon Doping on the Superconducting Properties of Mg(B1-xCx)2. R.H.Wilke, S.L.Budko, P.C.Canfield, D.K.Finnemore, R.J.Suplinskas, S.T.Hannahs: Physical Review Letters, 2004, 92[21], 217003 (3pp)