Superconductivity in Bulk, Hole-Doped Diamond*

Diamonds, synthesized at high pressure (9 GPa) and high temperature (2500-2800 K) in the systems boron carbide-graphite and boron-graphite, are heavily hole-doped by incorporation of boron into the diamond lattice. These diamonds were characterized by: X-ray diffraction, Raman scattering, NMR, SQUID magnetometry, calorimetry, Hall effect, resistivity and magnetic susceptibility measurements. They show an expanded ($\sim $1 {\%} in volume) lattice with a softened zone-centre optical phonon mode and reduced Debye temperature, and exhibit bulk superconductivity below Tc $\sim $4 K. Upper critical field, specific heat and resistivity measurements provide a consistent set of materials parameters that favor a conventional, weak coupling electron-phonon interpretation of the superconducting mechanism at high hole doping. Preliminary measurements of conductance spectra, obtained with contacts fabricated at the surface of these hole-doped diamonds, indicate the appearance of superconducting gap below Tc.