Superconducting properties of disordered lanthanum strontium copper oxide epitaxial thin films켜쌓기 성장 시킨 무질서한 란타늄 스트론튬 구리산화물 박막의 초전도 특성 연구

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High-$T_{c}$ cuprate superconductors, unlike conventional superconductors, have a significantly high critical temperature $T_{c}$. Since they are discovered, many researchers have studied for about 30 years. The main interest has been focused on improving the $T_{C}$ for applications. At the same time, researches are also conducted to understand the mechanism of superconductivity. High-$T_{c}$ superconductors cannot be completely explained by the well-known Bardeen-Cooper-Schrieffer (BCS) theory and have a d-wave gap structure that is different from the conventional s-wave gap. In the $\textbf{k}$ reciprocal space, it is observed that the gap is dependent on the $\textbf{k}$ with gap closing at the nodes leading to the pseudo-gap structure. Phase diagram of the high-$T_{c}$ cuprate superconductors appear through hole-doping or by controlling the amount of oxygen contents. In the case of La$_{2-x}$Sr$_{x}$CuO$_{4}$, substitution of Sr for La element provides a hole in a CuO$_2$ conduction plane. In the bulk single crystal, it is known that superconductivity appears in the range of $0.05\leq x \leq0.3$. It is also considered to be a quasi-two dimensional system because of the layered perovskite structure. Superconductor-to-insulator transitions (SITs) reveal the transition model and conduction mechanism result in the observed phase change, and thereby play an important role in understanding of the superconductivity in each system. Especially, the SITs in thin films can be induced by applying magnetic field, carrier density control by using gate voltage, and disorder effects caused by controlling film thickness. Among various parameters, the disorder effect which inevitably arises in thin films has a huge influence on the superconductivity of thin films. In this dissertation, we investigate the SIT and describe anomalous electrical transport phenomena in the La$_{2-x}$Sr$_{x}$Cu$_{4}$ as changing Sr doping ratio and film thickness.The optimal-doped La$_{1.85}$ Sr$_{0.15}$CuO$_{4}$ thin films are deposited with changing film thickness to investigate the disorder effect. The $R_{s}-T$ curves show that the SIT occurs with the film thickness control, that is not induced from the crystal structural or stoichiometric variations as characterized by X-ray diffraction and transmission electron microscopy (TEM). Also, a highly disordered thin film shows a clear field-driven SIT with a significantly low critical magnetic field. Being consistent with the previous theoretical work by M. P. A. Fisher, the observed transport property is attributed to a bosonic transition rather than a fermionic one. Finite-size scaling indicates the classical percolation model plays a crucial role in the quantum phase transition. For a better understanding of the disorder effect, we systematically measure $R_{s}-T$ curves for various Sr doping ratios and film thicknesses. As a result, the disorder effect does not only suppress the superconductivity but also shifts the superconducting dome to the over-doped regime in the phase diagram. Additionally, this shift is supported by an opposite tendency of the enhancement of $T_{c}$ with the decrease of film thickness in the highly over-doped La$_{1.7}$Sr$_{0.3}$CuO$_{4}$ thin films. In 5-nm-thick La$_{1.7}$Sr$_{0.3}$CuO$_{4}$ thin films, two-step phase transitions are observed by tuning the magnetic field, suggesting the existence of two conduction channels. Lastly, anomalous electronic transport behavior is observed in the La$_{2-x}$Sr$_{x}$CuO$_{4}$ thin films depending on AC frequency. The enormous phase angle rotation is observed and the magnitude even reaches 180$^{\circ}$ corresponding to the `negative resistance' regime. The huge phase angle rotation exhibits a strong correlation with $T_{c}$ and carrier density controlled by Sr doping.
Advisors
Yang, Chan Horesearcher양찬호researcher
Description
한국과학기술원 :물리학과,
Publisher
한국과학기술원
Issue Date
2019
Identifier
325007
Language
eng
Description

학위논문(박사) - 한국과학기술원 : 물리학과, 2019.8,[vi, 98 p. :]

Keywords

High-T_{c} Superconductor; oxide thin film; phase diagram; superconductor-to-insulator transition; electronic transport; 고온초전도; 산화물 박막; 상도표; 초전도-절연체 상변화; 전기적 수송현상

URI
http://hdl.handle.net/10203/284521
Link
http://library.kaist.ac.kr/search/detail/view.do?bibCtrlNo=927176&flag=dissertation
Appears in Collection
PH-Theses_Ph.D.(박사논문)
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