Dependence of the Superconducting Transition Temperature on the Doping Level in Single-Crystalline Diamond Films

Homoepitaxial diamond layers doped with boron in the 10²⁰--10²¹ cm^-3 range are shown to be type II superconductors with sharp transitions (~0.2 K) at temperatures increasing from 0 to 2.1 K with boron contents. The critical concentration for the onset of superconductivity in those 001-oriented single-crystalline films is about 5--7 10²⁰ cm^-3. The H-T phase diagram has been obtained from transport and ac-susceptibility measurements down to 300 mK.