Multigap superconductivity and Shubnikov-de Haas oscillations in single crystals of the layered boride OsB2

Single crystals of superconducting OsB2 [T-c=2.10(5) K] have been grown using a Cu-B eutectic flux. We confirm that OsB2 crystallizes in the reported orthorhombic structure (space group Pmmn) at room temperature. Both the normal and superconducting state properties of the crystals are studied using various techniques. Heat capacity versus temperature C(T) measurements yield the normal state electronic specific heat coefficient gamma=1.95(1) mJ/mol K-2 and the Debye temperature Theta(D)=539(2) K. The measured frequencies of Shubnikov-de Haas oscillations are in good agreement with those predicted by band structure calculations. Magnetic susceptibility chi(T, H), electrical resistivity rho(T), and C(T, H) measurements (H is the magnetic field) demonstrate that OsB2 is a bulk low-kappa [kappa(T-c)=2(1)] type-II superconductor that is intermediate between the clean and dirty limits [xi(T=0)/l=0.97)] with a small upper critical magnetic field H-c2(T=0) = 186(4) Oe. The penetration depth is lambda(T=0)=0.300 mu m. An anomalous (not single-gap BCS) T dependence of lambda was fitted by a two-gap model with Delta(1)(T=0)/k(B)T(c)=1.9 and Delta(2)(T=0)/k(B)T(c)=1.25, respectively. The discontinuity in the heat capacity at T-c, Delta C/gamma T-c=1.32, is smaller than the weak-coupling BCS value of 1.43, consistent with the two-gap nature of the superconductivity in OsB2. An anomalous increase in Delta C at T-c of unknown origin is found in finite H; e.g., Delta C/gamma T-c approximate to 2.5 for H approximate to 25 Oe.