Superconducting properties of boron-doped diamond

The question of the nature of superconductivity in boron-doped diamond (synthesized at high pressures and high temperatures) is still open. Here we present consistent measurements of resistivity and specific-heat on two samples containing pure 13C and 12C, revealing a 0.2 K shift of the superconducting transition temperature Tc. Hall-coefficient measurements confirm equal charge-carrier concentrations in both samples. The results are interpreted as a carbon-isotope effect more than two times larger than expected from the most simple BCS model for phonon-mediated superconductivity. Additional analyses of microstructure and exact boron content of the superconducting material show the presence of highly boron-enriched amorphous boundaries between the grains. For these investigations high-resolution transmission-electron microscopy and electron-energy-loss spectroscopy were used.