Multiband superconductivity in LaFeAsO_0.9F_0.1 single crystals probed by high-field vortex torque magnetometry

To probe manifestations of multiband superconductivity in oxypnictides, we measured the angular dependence of the magnetic torque τ(θ) in the mixed state of LaO_0.9F_0.1FeAs single crystals as a function of temperature T and magnetic fields H up to 18 T. The paramagnetic contribution of the Fe ions is properly treated in order to extract the effective mass anisotropy parameter γ=(m_c/m_ab)1 ^/2 from τ(θ). We show that γ depends strongly on both T and H, reaching a maximum value of ~10 followed by a decrease toward values close to 1 as T is lowered. The observed field dependencies of the London penetration depth λ_ab and γ suggest the onset of suppression of a superconducing gap at H_c2/3.