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|Title:||NMR STUDY OF THE POTASSIUM IRON SELENIDE HIGH-TEMPERATURE SUPERCONDUCTOR|
|Department:||Physics and Astronomy|
|Keywords:||Superconductvitiy;nuclear magnetic resonance;potassium iron selenide;high-temperature iron-based superconductor;Condensed Matter Physics;Condensed Matter Physics|
|Abstract:||<p>In this thesis we present a <sup>77</sup>Se NMR study of the iron selenide based high-temperature superconductor K<sub>x</sub>Fe<sub>2−y</sub>Se<sub>2</sub> (T<sub>c</sub> = 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-T<sub>c</sub> systems. In the nuclear spin-lattice relaxation rate 1/T<sub>1</sub> we observe no Hebel-Slichter coherence peak, nor any enhancement in low frequency antiferromagnetic spin fluctuations in 1/T<sub>1</sub>T. We also report on the effects of sulphur (S) substitution on the selenium sites in this system by conducting <sup>77</sup>Se NMR measurements on K<sub>x</sub>Fe<sub>2−y</sub>Se<sub>2−z</sub>S<sub>z</sub> (z = 0.8, 1.6). We find that both spin susceptibility and low frequency spin fluctuations are suppressed with increasing S content along with T<sub>c</sub>.</p>|
|Appears in Collections:||Open Access Dissertations and Theses|
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