Date of Award

Fall 2012

Degree Type

Thesis

Degree Name

Master of Science (MSc)

Department

Physics and Astronomy

Supervisor

Takashi Imai

Language

English

Abstract

In this thesis we present a 77Se NMR study of the iron selenide based high-temperature superconductor KxFe2−ySe2 (Tc = 33 K). We observe NMR lineshapes as narrow as ∼ 4.5 kHz with an applied field along the crystal c-axis, and find no evidence for the co-existence of magnetic order with superconductivity. With an applied field along the ab plane, however, the lineshape splits into two peaks of equal intensities at all temperatures, suggesting that the tetragonal fourfold symmetry of the average structure may be locally lowered by vacancy superstructure. Knight shift data indicate that spin susceptibility decreases progressively with temperature, similar to other iron arsenide high-Tc systems. In the nuclear spin-lattice relaxation rate 1/T1 we observe no Hebel-Slichter coherence peak, nor any enhancement in low frequency antiferromagnetic spin fluctuations in 1/T1T. We also report on the effects of sulphur (S) substitution on the selenium sites in this system by conducting 77Se NMR measurements on KxFe2−ySe2−zSz (z = 0.8, 1.6). We find that both spin susceptibility and low frequency spin fluctuations are suppressed with increasing S content along with Tc.

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