Large microwave inductance of granular boron-doped diamond superconducting films

Boron-doped diamond granular thin films are known to exhibit superconductivity with an optimal critical temperature of Tc 1⁄4 7:2K. Here, we report the measured in-plane complex surface impedance of boron-doped diamond films in the microwave frequency range using a resonant technique. Experimentally measured inductance values are in good agreement with estimates obtained from the normal state sheet resistance of the material. The magnetic penetration depth temperature dependence is consistent with that of a fully gapped s-wave superconductor. Boron-doped diamond films should find application where high kinetic inductance is needed, such as microwave kinetic inductance detectors and quantum impedance devices. Published under an exclusive license by AIP Publishing. https://doi.org/10.1063/5.0051227 Diamond is the hardest known natural material and an excellent thermal conductor despite also being electrically insulating with a large bandgap. Doping diamond can create either semiconducting or metallic states. It was discovered from high pressure and high temperature (HPHT) synthesis techniques that bulk single-crystal diamond can be doped with boron (hole doping) to the point that superconductivity is observed with Tc ffi 2:3K. The first thin films of boron-doped diamond (BDD) were single-crystal like and had a transition temperature that increased with boron doping, but only slightly exceeded that obtained in bulk. Later it was discovered that higher B concentrations could be obtained by preparing the films with energetic and nonequilibrium methods, including microwave plasma-assisted chemical vapor deposition (MPCVD), which resulted in Tc ffi 4:2K. Upon further refinement, materials that have become known as nanocrystalline diamond (NCD) prepared by MPCVD showed even higher transition temperatures. A remarkable dome-shaped Tc vs doping curve has emerged from extensive studies of boron-doped granular diamond films, with a peak transition temperature of Tc ffi 7:2K. Meanwhile Moussa and Cohen have predicted that the transition temperature of B-doped diamond will grow to 55K with B concentration in the range of 20%–30%. There are also predictions of high transition temperature superconductivity in metastable crystals of boron–carbon compounds based on electronic and vibrational numerical calculations. Recently superconductivity with a zero-resistance Tc 1⁄4 24K has been discovered in 27 at. % B-doped rapidly quenched carbon. Meanwhile, BDD films have also found a broad set of uses in sensing, electroanalytics, catalysis, semiconductor devices, and as conducting contacts in various applications. The single-crystal-like B-doped diamond films show clear signs of BCS s-wave superconducting behavior in STM tunneling experiments, with no subgap states and a peak in the density of states consistent with Dð0Þ=kBTc 1⁄4 1:74. However, it was noted from scanning tunneling spectroscopy methods that boron-doped diamond NCD films displayed modulation of the superconducting order parameter between grains, strong superconducting fluctuations, and substantial tunneling between grains in the superconducting state, all consistent Appl. Phys. Lett. 118, 242601 (2021); doi: 10.1063/5.0051227 118, 242601-1 Published under an exclusive license by AIP Publishing Applied Physics Letters ARTICLE scitation.org/journal/apl