Lator, Elijah; (2019) Superconductivity In Layered Transition Metal (Di)chalcogenides: Iron Selenide And Niobium Diselenide. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

This thesis presents a study on a series of layered transition metal-based superconductors, namely, niobium diselenide NbSe2 and iron selenide FeSe, and their intercalation compounds. Both NbSe2 and FeSe have received extensive scientific attention in recent years due to the coexistence of charge density waves, superconductivity and exotic pairing mechanisms at relatively high superconducting transition temperatures Tc. The intercalation of layered materials is known to tune their superconducting properties but an in depth understanding of superconductivity in these compounds remains lacking, partly because of a lack of high quality pure and single crystal samples on which to measure. In this work, a novel synthesis route is shown to create new single crystal forms of layered materials in order to achieve new high-quality superconductors. In the first part, NbSe2 is successfully intercalated with lithium to form LiNbSe2, a new superconductor with a Tc of 9.0 K, compared to 7.2 K for unintercalated NbSe2, confirmed by magnetometry and resistivity measurements. LiNbSe2 properties and structure are investigated with X-ray diffraction, Raman spectroscopy, magnetometry and resistivity, and DFT calculations. Upon intercalation phonons are found to have reduced energy due to an increase in the in-plane lattice parameters and shift of XRD 2θ values show the unit cell c-axis increases from 1.255(1) nm to 1.352(1) nm. The second part of the thesis investigates the superconductivity in FeSe and the intercalation compounds MXFe2Se2, where M is lithium, potassium or barium, and X is ammonia or methylamine. Metal intercalation is known to increase Tc from 8 K to 45 K. However, the mechanism is unknown not least because no one has successful intercalated single crystals required for many measurements. FeSe single crystals were synthesised using KCl/AlCl3 flux method. Single crystals were then successfully intercalated with metal via low temperature liquid ammonia and liquid methylamine method. The fabrication of FeSe and the intercalation using both condensed methods allow to discover new superconductors Li(NH3)Fe2Se2 (Tc = 30.5 K), K(NH3)Fe2Se2 (Tc = 30 K), K(CH3NH2)Fe2Se2 (Tc = 11.45 K), including record superconductor in these compounds of Tc at 47 K for Li(CH3NH2)Fe2Se2. These findings open up many new opportunities to realize FeSe-based pure and single phase superconductors with high Tc and to investigate the superconducting tuning in layered transition metals via dimensionality and doping, using scientific tools such as ARPES, STM and neutron scattering.

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