Superconductivity in boron-doped diamond, synthesized at high pressure

Diamonds, synthesized at high pressures and high temperatures in the presence of boron, are superconductors below T-c = 2-6 K. Synthesis in the systems boron-graphite and B4C-graphite at P = 9 GPa and T = 2500-2800 K results in formation of polycrystalline carbonado-like diamonds. Synthesis by decomposition of orthocarborane or naphthalene (with addition of boron) gives small single crystals and intergrowth plates. Dense superconducting bodies can be prepared by compacting these single crystal particles at P = 8 GPa and T = 1800 K. The dependence of the characteristic Raman mode frequency of diamond on unit cell volume points to softening of the optical phonon upon boron doping. These observations, together with tunneling measurements on pressure-synthesized samples, are consistent with a BCS-like phonon-assisted mechanism for superconductivity in diamond.