Magnetic-field tuned anisotropy in superconducting RbxFe2−ySe2

Abstract

The anisotropic superconducting properties of a Rb${}_{x}$Fe${}_{2\ensuremath{-}y}$Se${}_{2}$ single crystal with ${T}_{\mathrm{c}}\ensuremath{\simeq}32$ K were investigated by means of superconducting quantum interference device (SQUID) and torque magnetometry, probing the effective magnetic penetration depth ${\ensuremath{\lambda}}_{\mathrm{eff}}$ and the magnetic penetration depth anisotropy ${\ensuremath{\gamma}}_{\ensuremath{\lambda}}$. Interestingly, ${\ensuremath{\gamma}}_{\ensuremath{\lambda}}$ is found to be temperature independent in the superconducting state but strongly field dependent: ${\ensuremath{\gamma}}_{\ensuremath{\lambda}}(0.2\phantom{\rule{0.28em}{0ex}}\mathrm{T})<4$ and ${\ensuremath{\gamma}}_{\ensuremath{\lambda}}(1.4\phantom{\rule{0.28em}{0ex}}\mathrm{T})>8$. This unusual anisotropic behavior, together with a large zero-temperature ${\ensuremath{\lambda}}_{\mathrm{eff}}(0)\ensuremath{\simeq}1.8$ $\ensuremath{\mu}$m, is possibly related to a superconducting state heavily biased by the coexisting antiferromagnetic phase.