Binary ruthenium pnictides, RuP and RuAs, with an orthorhombic MnP structure, were found to show a metal to a non-magnetic insulator transition at TMI = 270 K and 200 K, respectively. In the metallic region above TMI, a structural phase transition, a
ccompanied by a weak anomaly in the resistivity and the magnetic susceptibility, indicative of a pseudo-gap formation, was identified at Ts = 330 K and 280 K, respectively. These two transitions were suppressed by substituting Ru with Rh. We found superconductivity with a maximum Tc = 3.7 K and Tc =1.8 K in a narrow composition range around the critical point for the pseudo-gap phase, Rh content xc = 0.45 and xc = 0.25 for Ru1-xRhxP and Ru1-xRhxAs, respectively, which may provide us with a novel non-magnetic route to superconductivity at a quantum critical point.
