Comparative study of superconducting and normal-state anisotropy in Fe1+yTe0.6Se0.4 superconductors with controlled amounts of interstitial excess Fe

We report a systematic study of the superconducting (SC) and normal-state anisotropy of ${\mathrm{Fe}}_1+y{\mathrm{Te}}_0.6{\mathrm{Se}}_0.4$ single crystals with controlled amounts of excess Fe ($y$ = 0, 0.07, and 0.14). The SC state anisotropy ${\ensuremath{\gamma}}_H$ was obtained by measuring the upper critical fields under high magnetic fields over 50 T for both $H\ensuremath{\parallel}ab$ and $H\ensuremath{\parallel}c$. On the other hand, the normal state anisotropy ${\ensuremath{\gamma}}_{\ensuremath{\rho}}$ was obtained by measuring the resistivity with current flowing in the $ab$ plane (${\ensuremath{\rho}}_ab$) and along the $c$ axis (${\ensuremath{\rho}}_c$). To precisely measure ${\ensuremath{\rho}}_ab$ and ${\ensuremath{\rho}}_c$ in the same part of a specimen avoiding the variation dependent on pieces or parts, we adopt a new method using a microfabricated bridge with an additional neck part along the $c$ axis. The ${\ensuremath{\gamma}}_H$ decreases from a value dependent on the amount of excess Fe at ${T}_{\mathrm{c}}$ to a common value $\ensuremath{\sim}1$ at 2 K. The different ${\ensuremath{\gamma}}_H$ at ${T}_{\mathrm{c}}$ ($\ensuremath{\sim}1.5$ for $y$ = 0, and 2.5 for $y$ = 0.14) suggests that the anisotropy of effective mass ${m}_c^*/{m}_ab^*$ increases from $\ensuremath{\sim}2.25$ ($y$ = 0) to 6.25 ($y$ = 0.14) with the excess Fe. The almost isotropic ${\ensuremath{\gamma}}_H$ at low temperatures is due to the strong spin paramagnetic effect at $H\ensuremath{\parallel}ab$. By contrast, the ${\ensuremath{\gamma}}_{\ensuremath{\rho}}$ shows a much larger value of $\ensuremath{\sim}17$ ($y$ = 0) to $\ensuremath{\sim}50$ ($y$ = 0.14) at the temperature just above ${T}_{\mathrm{c}}$. Combined the results of ${\ensuremath{\gamma}}_H$ and ${\ensuremath{\gamma}}_{\ensuremath{\rho}}$ near ${T}_{\mathrm{c}}$, we found out that the discrepant anisotropies between the SC and normal states originates from a large anisotropy of scattering time ${\ensuremath{\tau}}_ab/{\ensuremath{\tau}}_c\phantom{\rule{4pt}{0ex}}\ensuremath{\sim}$ 7.8. The ${\ensuremath{\tau}}_ab/{\ensuremath{\tau}}_c$ is found to be independent of the excess Fe.