Multi-band superconductivity in LaFeAsO_(0.9)F_(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 $\tau(\theta)$ in the mixed state of LaO$_{0.9}$F$_{0.1}$FeAs 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 $\gamma=(m_c/m_{ab})^{1/2}$ from $\tau(\theta)$. We show that $\gamma$ depends strongly on both $T$ and $H$, reaching a maximum value of $\sim$ 10 followed by a decrease towards values close to 1 as $T$ is lowered. The observed field dependencies of the London penetration depth $\lambda_{ab}$ and $\gamma$ suggest the onset of suppression of a superconducing gap at $H \approx H_{c2}/3$.