High-field superconducting phase diagram of carbon-substituted MgB2 single crystals

A comprehensive study of high-field superconducting phase diagram of carbon-substituted MgB2 single crystals Mg(B1−xCx)2 in a wide range of carbon content (x = 0-7.5%) is presented. Highly sensitive techniques of magnetic torque and rf penetration depth are employed to investigate both thermodynamic and pinning properties in strong magnetic fields of up to 40 T. The results indicate remarkably large enhancement of the irreversibility field Hirr, reaching 33 T at 3-5% carbon substitution, although superconducting transition temperature Tc monotonously decreases with carbon content. The peak effect corresponding to the vortex order-disorder transition is also modified substantially, and the transition field shifts to the lower field region in a systematic way. These changes upon carbon substitution are understood mainly in terms of an impurity effect.