  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.
