Phase fluctuations dominated superconductivity in a thick Granular Boron-Doped Diamond film

Overview & Synopsis

We investigate the superconducting properties of a 1.2-μm-thick granular boron-doped diamond film, where tightly packed grains form a disordered network of Josephson junctions. The film exhibits a sharp superconducting transition at a critical temperature, T_c ~ 3 K. Its transport properties are well described by the Ambegaokar-Halperin theory of phase fluctuations. Direct measurements of the penetration depth using a two-coil mutual inductance technique reveal classic phase-fluctuation-dominated behavior: below T_c, the superfluid density (n_s) varies linearly over a wide temperature range - a hallmark of thermal phase fluctuations - before crossing over to a T^2 dependence at ultra-low temperatures. Both the large low-temperature penetration depth, λ(0) ~ 8.8 μm, and the slope of the linear ns variation in the thermal regime reveal a remarkably low superfluid stiffness in the range of 0.5-0.8 K. These results establish granular boron-doped diamond films as an excellent model system for investigating the role of long-wavelength phase fluctuations in three-dimensional superconductors.