Investigation of Crystal Growth and Characterization in Multiferroic LiCu2O2 and Fe-based Superconductor
Date Issued
2012
Date
2012
Author(s)
Chao, Wei-Hsiang
Abstract
LiCu2O2 is a typical quasi-one dimensional helimagnetic material which also is the first member of the cuprate family found to be mutiferroics. Due to its unusual magnetic order at low temperature and low dimensional quantum spin systems with geometric frustration. Therefore, it has recently attracted a great deal of attention. More experiments clearly demonstrated that this s=1/2 magnetic system possess an incommensurate helimagnetic ground state during the spin ordering temperature about 22-24 K. In first part of this thesis, a systematic investigation of LiδCu2O2 (LCO) and LiδTMxCu2-xO2 (LTCO, TM = Ti, Ni and Zn) (δ = 1 - 1.2 and x = 0 - 0.1) were undertaken to determine growth parameters and crystal properties. Typical LiCu2O2 crystals with ab-plane habit not only show a clean X-ray diffraction pattern without impurities and high crystalinity but also exhibit untwined structure. A typical size of large single crystals LiCu2O2 was 9×6 mm2. The orbital character of the valence states of Cu and TM ions on different sites were investigated by polarization-dependent soft X-ray absorption (XAS) spectroscopy measurement with in- and out-of-plane X-ray E vectors. These crystals exhibit varied physical properties owing to successful incorporation of TM dopants.
On the other hand, in recent years the iron-based superconductors are well received by everyone interested in the issues. With layer structure iron-based quaternary oxypnictides, LaO1-xFxFeAs superconductors were found with its superconductivity transition TC ~ 26 K which is a relatively high transition temperature. Since then, more iron-based superconductors have been discovered and the transition temperature TC up to 56 K. The FexSe arouse the majority of interest because of its simplest crystal structure and with superconducting transition around 8 K without any carrier doping. In the iron-chacogenides system, excluding arsenic but with iron-pnictides have many common physical characteristics which can provide important information to understand the superconducting mechanism of high-temperature superconductors. In the second part we reported the growth and characterization of FexSe single crystals. High-pressure cubic-anvil technique successfully obtained FexSe single crystals of a size up to 2 × 2 mm2 at an optimum growth pressure of 2.8 GPa. As-grown crystals showed a characteristic of (101) crystal orientation with good out-of-plane and in-plane crystallinity. The maximum TC = 9 K in Fe1.03Se crystal was reached after appropriate heat treatment. The crystals exhibit more slight magnetic background both in ZFC and FZ curves in the normal state corroborate by susceptibility measurement. Such crystals have significantly purer quality compared to the polycrystalline samples and are promising for future investigation of superconductivity.
Subjects
crystal growth
mutiferroics
floating-zone method
high pressure method
iron-based superconductor
Type
thesis
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