Synthesis, crystal structure and properties of the new superconductors TaRuB and NbOsB

Two new ternary compounds TaRuB and NbOsB were synthesized by arc-melting and annealing at 1500-1850 °C. They crystallize in orthorhombic primitive structures with space group Pbam. Magnetic susceptibility, electrical resistivity, and specific heat measurements reveal bulk superconductivity for metallic TaRuB with a T(c) ≈ 4 K. Electronic structure calculations by DFT methods show that 4d and 5d transition-metal states dominate the density of states (DOS) at the Fermi level E(F) with a pronounced quasi one-dimensional behaviour along the [0 0 1] direction. Comparison of the calculated DOS at E(F) with specific heat data reveals a moderate electron-phonon coupling. Possible small boron vacancies could significantly reduce the DOS at E(F), hence decrease T(c) for samples annealed at higher temperatures. For NbOsB, the DOS(E(F)) is strongly reduced due to an increase of covalent bonding interactions between Os and B. Accordingly, a lower T(c) ≈ 1 K is observed.