Multi-band superconductivity in LaFeAsO_{0.9}F_{0.1} single crystals probed by high-field vortex torque magnetometry

Abstract

To probe manifestations of multiband superconductivity in oxypnictides, we measured the angular dependence of the magnetic torque $\ddot{I}„(\hat{I}¸)$ 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 ${\hat{I}}^3=(m_c/m_{ab}{)}^{1/2}$ from $\ddot{I}„(\hat{I}¸)$. We show that ${\hat{I}}^3$ 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 $\hat{I}{»}_{ab}$ and ${\hat{I}}^3$ suggest the onset of suppression of a superconducing gap at $H\approx H_{c2}/3$.

7 pages, 7 figures