Comparative study of superconducting and normal-state anisotropy in Fe1+yTe0.6Se0.4 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 (ρab) and along the c axis (ρc). To precisely measure ρab and ρc 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 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 mc*/mab* 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 τab/τc∼ 7.8. The τab/τc is found to be independent of the excess Fe.

Source-reported materials — not catalogue approval

Similar papers