Superconducting and normal-state properties of the layered boride OsB₂

OsB₂ crystallizes in an orthorhombic structure (Pmmn) which contains alternate boron and osmium layers stacked along the c axis. The boron layers consist of puckered hexagons as opposed to the flat graphite-like boron layers in MgB₂. OsB₂ is reported to become superconducting below 2.1 K. We report results of the dynamic and static magnetic susceptibilities, electrical resistivity, Hall effect, heat capacity, and penetration depth measurements on arc-melted polycrystalline samples of OsB₂ to characterize its superconducting and normal-state properties. These measurements confirmed that OsB₂ becomes a bulk superconductor below T_c=2.1 K. Our results indicate that OsB₂ is a moderate-coupling type-II superconductor with an electron-phonon coupling constant λ_ep≈0.4–0.5, a small Ginzburg-Landau parameter κ∼1–2, and an upper critical magnetic field H_c2(0.5 K)∼420 Oe for an unannealed sample and H_c2(1 K)∼330 Oe for an annealed sample. The temperature dependence of the superfluid density n_s(T) for the unannealed sample is consistent with an s-wave superconductor with a slightly enhanced zero temperature gap Δ(0)=1.9k_BT_c and a zero temperature London penetration depth λ(0)=0.38(2) μm. The n_s(T) data for the annealed sample show deviations from the predictions of the single-band s-wave BCS model. The magnetic, transport, and thermal properties in the normal state of isostructural and isoelectronic RuB₂, which is reported to become superconducting below 1.6 K, are also reported.