Comparative study of superconducting and normal-state anisotropy in Fe$_{1+y}$Te$_{0.6}$Se$_{0.4}$ superconductors with controlled amounts of interstitial excess Fe
We report a systematic study of the superconducting (SC) and normal-state anisotropy of Fe$_{1+y}$Te$_{0.6}$Se$_{0.4}$ single crystals with controlled amounts of excess Fe ($y$ = 0, 0.07, and 0.14). The SC state anisotropy $γ_{H}$ was obtained by measuring the upper critical fields under high magnetic fields over 50 T for both $H\parallel ab$ and $H\parallel c$. On the other hand, the normal state anisotropy $γ_ρ$ was obtained by measuring the resistivity with current flowing in the $ab$ plane ($ρ_{ab}$) and along the $c$ axis ($ρ_c$). To precisely measure $ρ_{ab}$ and $ρ_c$ in the same part of a specimen avoiding the variation dependent on pieces or parts, we adopt a new method using a micro-fabricated bridge with an additional neck part along $c$ axis. The $γ_{H}$ decreases from a value dependent on the amount of excess Fe at $T_{\rm{c}}$ to a common value $\sim$ 1 at 2 K. The different $γ_{H}$ at $T_{\rm{c}}$ ($\sim$1.5 for $y$ = 0, and 2.5 for $y$ = 0.14) suggests that the anisotropy of effective mass $m_c^*/m_{ab}^*$ increases from $\sim$ 2.25 ($y$ = 0) to 6.25 ($y$ = 0.14) with the excess Fe. The almost isotropic $γ_{H}$ at low temperatures is due to the strong spin paramagnetic effect at $H\parallel ab$. By contrast, the $γ_ρ$ shows a much larger value of $\sim$ 17 ($y$ = 0) to $\sim$ 50 ($y$ = 0.14) at the temperature just above $T_{\rm{c}}$. Combined the results of $γ_{H}$ and $γ_ρ$ near $T_{\rm{c}}$, we found out that the discrepant anisotropies between the SC and normal states originates from a large anisotropy of scattering time $τ_{ab}$/$τ_c$ $\sim$ 7.8. The $τ_{ab}$/$τ_c$ is found to be independent of the excess Fe.