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

Yue Sun; Yongqiang Pan; Nan Zhou; Xiangzhuo Xing; Zhixiang Shi; Jinhua Wang; Zengwei Zhu; Akira Sugimoto; Toshikazu Ekino; Tsuyoshi Tamegai; Haruhisa Kitano

doi:10.1103/PhysRevB.103.224506

paper

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

arXiv:2105.10093 · doi:10.1103/PhysRevB.103.224506

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 $γ_{H}$ was obtained by measuring the upper critical fields under high magnetic fields over 50 T for both $H ∥ ab$ and $H ∥ c$ . 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 $T_{c}$ to a common value $\sim$ 1 at 2 K. The different $γ_{H}$ at $T_{c}$ ( $\sim$ 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 $H ∥ ab$ . 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 $T_{c}$ . Combined the results of $γ_{H}$ and $γ_{ρ}$ near $T_{c}$ , 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.

arXiv commentto be published in Phy. Rev. B

Comparative study of superconducting and normal-state anisotropy in Fe1+y​Te0.6​Se0.4​ superconductors with controlled amounts of interstitial excess Fe

Abstract

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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