Multi-gap superconductivity and Shubnikov-de Haas oscillations in single crystals of the layered boride OsB_2

Abstract

Single crystals of superconducting OsB_2 [T_c = 2.10(5) K] have been grown using a Cu-B eutectic flux. We confirm that OsB_2 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 OsB_2 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)/\ell = 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_BT_c = 1.9 and \Delta_2(T = 0)/k_BT_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 OsB_2. An anomalous increase in \Delta C at T_c of unknown origin is found in finite H; e.g., \Delta C/\gamma T_c \approx 2.5 for H \approx 25 Oe.