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 $\gamma =(m_c\mathrm{/}{m}_{ab}{)}^{1\mathrm{/}2}$ 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 $H\approx H_{c2}\mathrm{/}3$.