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 Fe1+yTe0.6Se0.4 single crystals with controlled amounts of excess Fe (y = 0, 0.07, and 0.14). The SC state anisotropy γH 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 γρ was obtained by measuring the resistivity with current flowing in the ab plane (ρ ab ) and along the c axis (ρc). To precisely measure ρ ab and ρc 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 γH decreases from a value dependent on the amount of excess Fe at Tc to a common value ∼ 1 at 2 K. The different γH at Tc (∼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 ∼ 2.25 (y = 0) to 6.25 (y = 0.14) with the excess Fe. The almost isotropic γH at low temperatures is due to the strong spin paramagnetic effect at H ab. By contrast, the γρ shows a much larger value of ∼ 17 (y = 0) to ∼ 50 (y = 0.14) at the temperature just above Tc. Combined the results of γH and γρ near Tc, we found out that the discrepant anisotropies between the SC and normal states originates from a large anisotropy of scattering time τ ab /τc ∼ 7.8. The τ ab /τc is found to be independent of the excess Fe.