Probing multi-band superconductivity and magnetism in SmFeAsO${}_{0.8}$F${}_{0.2}$ single crystals by high-field vortex torque magnetometry

Abstract

To probe manifestations of multiband superconductivity in oxypnictides, we measured the angular dependence of magnetic torque $\ddot{I}„(\hat{I}¸)$ in the mixed state of SmO${}_{0.8}$F${}_{0.2}$FeAs single crystals as functions of temperature $T$ and high magnetic field $H$ up to 30 T. We show that the effective mass anisotropy parameter ${\hat{I}}^3$ extracted from $\ddot{I}„(\hat{I}¸)$, can be greatly overestimated if the strong paramagnetism of Sm or Fe ions is not properly taken into account. The correctly extracted ${\hat{I}}^3$ depends on both $T$ and $H$, saturating at ${\hat{I}}^3\simeq 9$ at lower temperatures. Neither the London penetration depth nor the superfluid density is affected by high fields fields up to the upper critical field. Our results indicate two strongly-coupled superconducting gaps of nearly equal magnitudes.

5 pages, 5 figures