NMR characterization of sulphur substitution effects in the KxFe2−ySe2

Abstract

We present a 77 Se NMR study of the effect of S substitution in the high Tc superconductor, KxFe2−ySe2−zSz, in a temperature range up to 250 K. We examine two S concentrations, with z = 0.8 (Tc ∼ 26 K) and z = 1.6 (non-superconducting). The samples containing sulphur exhibit broader NMR lineshapes than the KxFe2Se2 sample due to local disorder in the Se environment. Our Knight shift 77 K data indicate that in all samples uniform spin susceptibility decreases with temperature, and that the magnitude of the Knight shift itself decreases with increased S concentration. In addition, S substitution progressively suppresses low frequency spin fluctuations. None of the samples exhibit an enhancement of low frequency antiferromagnetic spin fluctuations (AFSF) near Tc in 1/T1T, as seen in FeSe.

Evolution of the Pauli spin-paramagnetic effect on the upper critical fields of single-crystallineKxFe2−ySe2−zSz

“…33,36 The decrease in α is in agreement with suppression of spin susceptibility and spin excitations in K x Fe 2−y Se 1−z S z . 37 In addi-tion, sulfur substitution might also change the electronic structure leading to reduced multiband effects. Further theoretical work is necessary to clarify this.…”

Spin fluctuations in iron pnictides and chalcogenides: From antiferromagnetism to superconductivity

“…Only Ni, Co and Cr atoms could be homogenously doped into the superconducting stripes of the phase-separated K x Fe 1− y Se 2 system, leading to a rapid suppression of T c by just a few percent of the dopant atoms [259]. A much more gradual and accurate control of T c could be achieved by replacing Se with S [284,[290][291][292][293] or Te [294][295][296], resulting in phase diagrams like the one presented in Fig. 6, continuously connecting the superconducting K x Fe 2− y Se 2 with the semiconducting end member compound, K x Fe 2− y S 2 .…”