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

Abstract

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 ${\gamma }_H$ 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 ${\gamma }_{\rho }$ was obtained by measuring the resistivity with current flowing in the $ab$ plane (${\rho }_{ab}$) and along the $c$ axis (${\rho }_c$). To precisely measure ${\rho }_{ab}$ and ${\rho }_c$ 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 ${\gamma }_H$ decreases from a value dependent on the amount of excess Fe at $T_{\mathrm{c}}$ to a common value $\sim$ 1 at 2 K. The different ${\gamma }_H$ at $T_{\mathrm{c}}$ ($\sim$1.5 for $y$ = 0, and 2.5 for $y$ = 0.14) suggests that the anisotropy of effective mass $m_c^{\ast }/m_{ab}^{\ast }$ increases from $\sim$ 2.25 ($y$ = 0) to 6.25 ($y$ = 0.14) with the excess Fe. The almost isotropic ${\gamma }_H$ at low temperatures is due to the strong spin paramagnetic effect at $H\parallel ab$. By contrast, the ${\gamma }_{\rho }$ shows a much larger value of $\sim$ 17 ($y$ = 0) to $\sim$ 50 ($y$ = 0.14) at the temperature just above $T_{\mathrm{c}}$. Combined the results of ${\gamma }_H$ and ${\gamma }_{\rho }$ 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 ${\tau }_{ab}$/${\tau }_c$ $\sim$ 7.8. The ${\tau }_{ab}$/${\tau }_c$ is found to be independent of the excess Fe.

to be published in Phy. Rev. B

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