【作者】 张勇；

【导师】 赵勇；

【作者基本信息】 西南交通大学 ， 材料学， 2009， 博士

【摘要】 自1986年发现了第一种高温超导材料——镧钡铜氧化物,铜基超导材料成为全世界物理学家的研究热点。然而直至今日,对于铜基超导材料的高温超导机制,物理学界仍未形成一致看法,这也使得高温超导成为当今凝聚态物理学中最大的谜团之一,因此很多科学家都希望在铜基超导材料以外再找到新的高温超导材料,从而能够使高温超导机制更加明朗。虽然目前发现的非铜氧化合物超导体T_c远低于铜氧化合物,但是对该体系的研究有助于理解高温超导机制,为探究高温超导机制提供一个更清晰的体系,找寻其他支持超导电性的结构规律,为最终揭开超导电性的深层机制提供了新的条件和机遇。因此,充分了解某些典型非铜氧化合物超导体的结构规律,将大大拓宽该类材料的探索和研究范围,这对于发现新的超导体具有重要意义。本论文主要分三部分,第一部分为BaBi_1-xPb_xO₃的掺杂行为对超导性能及电子结构的影响;第二部分为新型铁基超导材料的超导性能及电子结构研究;第三部分为NaCoO_x的掺杂行为对电磁性质的影响。主要内容安排如下:第一章概述了超导体的基本物理性质;然后介绍了铜氧化物的结构特征、超导相图及超导机制;最后是本论文的选题意义和论文主要研究内容。第二章对非铜基氧化物超导体进行了综述,介绍了高温超导体BaBiO₃体系的结构特征、元素替代效应和制备方法;介绍了层状钴类氧化物超导体的晶体结构、电子结构、超导相图和磁性质;重点介绍了新型铁基超导材料的研究现状、结构特征、元素替代效应和超导机制。第三章采用X光电子能谱（XPS）研究了BaPb_0.75-xHg_xBi_0.25O₃（BPHBO）系列超导体的电子结构,发现Pb4f和O1s芯能级随Hg掺入向低能级方向移动,同时Bi4f芯能级随Hg掺入向高能级方向移动,结果及分析显示Hg掺杂引起的电子结构系统改变最终影响系统的超导电性。在低掺杂量范围（x<0.2）超导电性被抑制是由于载流子浓度减小所致,在掺杂范围0.3<x≤0.4超导电性的恢复是由于新的金属-绝缘体转变使系统从绝缘体转变为载流子类型以空穴为主的金属态。第四章采用电阻测量和XPS方法对新型铁基超导材料SmFeAsO_1-XF_X的超导性能和电子结构进行了研究,结果表明随F掺杂量增大,系统从金属性自旋密度波状态转变为超导态,同时价带谱位于费米能级下0.2eV处的小峰逐渐消失并在费米能级附近形成一个较宽平台。从F掺杂导致的O1s和Sm 3d芯能级谱峰位移值推导出费米能级位移速度为dE_F/dx≌0.03 eV/at%F原子。结果显示自旋密度波状态来源于低自旋态的Fe3d电子,随F掺杂量增大,自旋密度波状态逐渐被抑制并且能级展宽。第五章通过磁性测量和XPS研究考察了Ir掺杂对SmFeAsO超导电性及能带结构的影响,实验结果表明Ir掺杂在Fe位,直接向Fe-As导电层注入电子,与氟掺杂相比较,显示其对超导电性的影响更为直接,少量掺杂即对超导性能产生强烈影响,故产生超导电性可调掺杂范围很小。揭示了SmFeAsO的磁有序与Fe3d电子的低自旋态相关联,且掺Ir抑制其低自旋态。当Ir掺入系统时,Fe3d电子的低自旋态被抑制,形成的超导样品SmFe_1-xIr_xAsO中Fe3d电子从低自旋态转变为高自旋态。所有结果显示,该系统存在电子的强关联效应。第六章对铜基及非铜基氧化物超导材料的电子结构进行比较。第七章主要研究NaCoO_x的磁性能随掺杂元素种类的变化关系。采用快速加热法成功制备出多晶Na_0.7Co_1-xAl_xO₂,名义掺杂量最高达到0.3。在5-300K测量不同掺杂量多晶样品的直流磁化率,比较掺杂量对直流磁化率的影响关系。发现低掺杂量样品总体为顺磁态,高掺杂量样品出现冻结温度约为13K的自旋玻璃态转变。第八章对全文的工作进行了总结。更多还原

【Abstract】 Since the discovery of the La-Ba-Cu-0 high-T_c superconductor in 1986, extensive research activities have been devoted to this system due to the high T_c. However, the underlying physical mechanism for the high-T_c superconductivity （SC） of copper oxides is still under debate. It has been one of the most challenging problems in condensed material physics to clarify the mechanism, and many scientists hope to find new high-T_c superconductor other than the copper oxides to explain the high-T_c superconductivity. Although the T_c of the copper-free oxide superconductors is much lower than that of copper oxides, they provides a more clear system to uncover the mechanism of the high-T_c superconductivity and to explore other structural characteristics supporting the superconductivity. Therefore, it is very important to fully understand the structural characteristics of the copper-free oxide superconductors, which will broaden the research scope of superconductive materials.This dissertation mainly focuses on three parts. The first part is to investigate the effects of the doping behaviors of BaBi_1-xPb_xO₃ and Ba_1-xK_xBiO₃ superconductors on the electronic structures and superconductivity. The second part is to examine the effects of doping behaviors of NaCoO_x on the electro-magnetic properties. The last parts is to discuss the superconductivity and the electronic structures of the novel iron pnictides superconductor. The main contents are presented as follows:In chapter 1, a brief introduction was made to the fundamental properties of superconductors such as the superconductivity, crystal structure, mechanism models, significance of the current research and main contents of this dissertation.In chapter 2, the copper-free high-T_c superconductors were summarized. The structural characteristics, element substitute effect and preparation methods of the BaBiO₃ superconductors were introduced. The crystal structures, electronic structures, superconductivity phase maps and magnetism of the cobalt oxide superconductor system were briefly reviewed. Emphases were placed to introduce the research status, structural characteristics, element substitute effect, and mechanism of the novel iron based superconductive materials.In chapter 3, the electronic structures of BaPb_0.75-xHg_xBi_0.25O₃ （BPHBO） system were studied with x-ray photoelectron spectroscopy （XPS）. It was indicated that the core levels of Pb4f and O1s states shifted towards lower region with the mercury doping, and that of Bi4f state shifted towards higher region. The results revealed that the mercury doping induced systematic changes of the electronic structures, consequently affecting the superconductivity. The suppression of superconductivity at low doping levels （x<0.2） was attributed to the suppressed charge-disproportionate state, whereas the recovery in the doping level of 0.3<x≤0.4 was due to a new M-I transition, which converted the system from an insulating state to metallic one with the dominant carriers of charge-being holes.In chapter 4, the electronic structures of Fe-based superconductor SmFeAsO_1-xF_x were examined with the resistivity measurement and XPS. With the increase in the fluorine doing levels, the system changed from a metallic spin-density-wave state to a superconductive state. The small peak centered at 0.2 eV below the Fermi level in the valence band gradually disappeared and a broad plateau near the Fermi level formed. The shift of the Fermi level with a rate of dE_F/dx≌0.03 eV/at% after fluorine doping was obtained from the shift of the binding energy of O1s and Sm 3d core-levels. The results revealed that the spin-density-wave state originated from the low-spin state of the Fe3d electrons, which was gradually suppressed with the band width expansion after fluorine doping.In chapter 5, the electronic structures of Fe-based superconductor SmFe_1-xIr_xAsO were studied with the magnetism measurement and XPS. It indicated that element Ir was doped on the Fe site, which directly introduced electrons to the conductive Fe-As layer. Therefore, compare to F doping, Ir doping inicated more significant effects on the superconductivity. The magnetism order of SmFeAsO was closely related with the Fe3d low spin state, which was suppressed by Ir doping through the suppressing magnetism of the 3d itinerant electrons as mentioned previously. In superconductivity samples the 3d itinerant electrons were mainly in the high spin state. All the results suggested the strong correlation occurred between the electrons in this system.In chapter 6, the electronic structures of copper oxide superconductor were compared with those of copper-free oxide superconductor.In chapter 7, single phase polycrystalline samples Na_0.7Co_1-xAl_xO₂ （x = 0, 0.05, 0.10, 0.15, 0.20, 0.25, and 0.30） were prepared by solid state reaction. The magnetic properties were characterized by dc and ac magnetic susceptibility measurements from 5 to 300 K. Samples with lower doping amounts （x = 0, 0.05, and 0.10） showed paramagnetic behaviors, but those with higher doping amounts （x = 0.20, 0.25, and 0.30） showed spin-glass behaviors with a freezing temperature （T_f） of about 13 K.In chapter 8, the whole dissertation was sum-up.更多还原