Flux-flow resistivity and penetration depth measurements of BaFe$_2$(As,P)$_2$: semi-quantitative estimates of gap anisotropy

Flux-flow measured by using a microwave technique is unique method to investigate quasiparticles in the vortex core. By measuring the magnetic-field dependence of the flux-flow resistivity, $\rho_f$, of several Fe-based SCs, we found that $\rho_f(H)$ is expressed as $\rho_f/\rho_n=\alpha H/H_{\rm c2}$ with $\alpha$ strongly depends on materials, suggesting that $\rho_f(H)$ of Fe-based SCs is dominated by the gap structure. By comparing these with the penetration depth data, we also found that $\alpha$ becomes larger when the gap function is more anisotropic [1,2]. To make this gap-anisotropy scenario more convincing, we focused on BaFe$_2$(As,P)$_2$ (P=30, 45\%), and found that the penetration depth increased in proportion to $T^{1.5-1.7}$. The fractional exponent can be understood by assuming that these materials have gaps with lines of nodes and deeply-warped nodeless gaps. As for flux-flow, $\rho_f(H)$ showed a large gradient of $\alpha>2.5$, similar to that of SrFe$_2$(As,P)$_2$ [2], pointing to highly anisotropic gaps in a consistent manner. These results support the gap-anisotropy scenario. [1]T. Okada $et\ al.$, PRB {\bf 86}, 064516 (2012); Physica C {\bf 484}, 27 (2013); $ibid$ {\bf 494}, 109 (2013) [2]H. Takahashi $et\ al.$, PRB {\bf 86}, 144525 (2012).