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The superconducting properties research of iron based-122 by transport and scanning micro-squid measurements

Abstract : More than twenty years after the discovery of high temperature superconductors, the underlying physical mechanism is still not well understood. In 2008, the discovery of a new family of high temperature superconductors, the iron-based superconductors, provided us a new chance to understand the high temperature superconductivity. Synthesizing high quality sample, detecting the basic superconducting properties, the gap structure and order parameter symmetry are essential steps in revealing the mechanism and application of new superconductors. This dissertation describes the growth of Ba₁₋ₓKₓFe₂As₂ single crystals and the study of superconducting properties, gap structure and order parameter on Ba-122 iron-based superconductors with resistivity, Hall probe, point contact Andreev reflection spectroscopy and scanning nano-squid microscopy measurements. Some historical events concerning superconductivity are recalled, and some key properties and theories of superconductivity are presented in Chapter 1. Then we will briefly introduce the discovery and current research situation of the iron-based superconductors. In Chapter 2, the growth procedure of Ba₁₋ₓKₓFe₂As₂ single crystals with self-flux method, and the characterization of the crystals with diffraction and energy dispersive analysis of x-ray, AC susceptibility and resistivity measurements are described. Then we report some results from temperature dependent resistivity measurements on Ba₁₋ₓKxFe₂As₂ (0.23 ≤ x ≤ 0.4) single crystals in magnetic fields up to 9 T and angle dependent resistivity measurements on Ba₀⋅₆K₀⋅₄Fe₂As₂ single crystals. In Chapter 3, we introduce some details about a Hall probe measurement system we built. Then we present local and global magnetization measurements on high pressure SmFeAsO₀⋅₉F₀⋅₁ polycrystals and Ba₀⋅₆K₀⋅₄Fe₂As₂ single crystals with Hall probe and VSM. In Chapter 4, we give a brief introduction about point contact Andreev reflection spectroscopy, then we report the measurements on Ba₀⋅₆K₀⋅₄Fe₂As₂ single crystal and a series of electron-doped BaFe₂₋ₓNiₓAs₂ single crystals over a wide doping range. In Chapter 5, the development of a scanning nano-SQUID force microscope and measurements performed on a 80 nm Rhenium film are presented. The microscope can take topographic and magnetic images simultaneously. The maximal scanning range is 70µm x 85µm and the magnetic resolution is about 1,5 x 10⁻⁴Φ₀/√Hz. In Chapter 6, we present some results from lower critical field, tunnel diode oscillator, heat capacity and scanning nano-squid microscopy measurements on systematic doped Ba(Fe₁₋ₓNiₓ)₂As₂ single crystals..Finally, in Chapter 7, a detailed summary is presented.
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Zhao-Sheng Wang. The superconducting properties research of iron based-122 by transport and scanning micro-squid measurements. Other [cond-mat.other]. Université de Grenoble; University of Chinese academy of sciences, 2012. English. ⟨NNT : 2012GRENY029⟩. ⟨tel-01128251⟩

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