Measurement of the magnetic penetration depth in p-doped superconducting diamond films

Boron-doped diamond becomes superconducting once a critical doping concentration of 4.5 x 10²⁰ cm^-3 is reached. Mutual inductance measurements with a two-coil setup have been performed to determine the magnetic penetration depth λ(T), which is a measure for the superfluid stiffnes θ ∝ 1/λ²(T). Two superconducting p-doped diamond films with thicknesses of 145 nm and 345 nm were investigated. At low temperatures these values agree reasonably with the values expected within BCS-theory using T_c, carrier density and mean free path determined from electric transport measurements. Magnetic penetration depths of 3.7 μm for the thinner and 2.6 μm for the thicker film have been found. λ decreases and accordingly θ increases with increasing film thickness. On the other hand, the superfluid stiffness drops by a factor of 2 or even more at T_c/2, i.e., much faster than expected from BCS-theory, but remains finite between T_c/2 < T < T_c. At present it is unclear, whether this behavior results from the proliferation of phase fluctuations already far below T_c or from a spatial inhomogeneity of the films.