过渡金属化合物中新型非常规超导体的探索

发布时间：2018-04-24 23:19

本文选题：非常规超导体 + 电子关联系统　；参考：《南京大学》2016年博士论文

【摘要】：非常规超导体是不能用以声子作为配对媒的传统BCS理论理解的超导体,主要有铜氧化物超导体、铁基超导体、重费米子超导体和有机超导体等。经过近半个世纪的研究,物理学家们虽然认为非常规超导的诱发与磁性涨落和量子临界现象有关,但至今仍无法对其找到完备而统一的描述方式。因此,发现新型非常规超导体系以提供新的研究平台,并对其进行物性研究得到更多非常规超导机理的线索,就成了非常重要的课题。本篇论文基于寻找非常规超导材料的基本原则性思路,对探索基于过渡金属化合物的新型超导体做了一些有益尝试。本篇论文的第一部分首先概括了常规超导现象及物理图像,并简单介绍了铜氧化物超导体、铁基超导体、重费米子超导体和有机超导体四类非常规超导体,然后对历史上发现非常规超导的各种尝试做了回顾和梳理,并尝试提出了探索非常规超导材料的基本原则性思路,最后介绍了设计钴、镍基高温超导的一个实例。论文的第二部分介绍了我们所使用的晶体生长、表征方法,和电输运测量、热输运测量、磁性测量、高压测量的实验及分析方法。论文第三部分我们以LaRu2P2材料为平台对Ru基122系统中可能存在的电子关联效应进行研究。我们在LaRu2P2单晶中观测到了超导转变温度Tc随压力P的增强,这不能由麦克米兰理论完全理解。关于正常态电阻以及上临界场的自洽分析暗示某些额外关联性质的电子-玻色子相互作用可能在压力下被诱发,成为超导性质增强的来源。基于电子及声子结构的第一性原理计算,我们分析了关联增强可能的原因,并对Tc-p中相图的dome状结构做出了解释。我们的实验表明LaRu2P2与自旋涨落为配对胶的铁基超导有一些共通之处,这或许会在将来激发Ru基系统中关联效应诱发新型非常规超导的研究。论文第四部分是为发现新超导做出的一些尝试。我们首先研究了EuPdAs多晶样品,通过电阻和磁化数据我们可以看到样品常压下在180 K存在的相变,当我们对样品施加静水压,仅仅0.48 GPa就将180 K相变完全压制,并且电阻在压力下单调降低,然而加压直至1.90 GPa都并未看到超导在低温被诱发。这说明180 K相变可能仅仅是一个价态转换相变,而与长程磁序无关。为了实现一些奇异电子性质,我们认为使用电子掺杂调节Pd原子的4d轨道占据应当是必要的途径。接着我们看到了MnP多晶在高温区的铁磁相变与低温区的螺旋磁性相变,磁化上螺旋磁性态出现了回滞,而高温区的铁磁态并未建立回滞。我们发现在高场下螺旋磁性相变会被压制,使系统在低温区为铁磁态,而静水压下300K左右的铁磁相变将缓慢往低温移动。我们还讨论了绝缘态RuP向金属态CrP的系列掺杂实验,并在Cr60%处看到了可能对形成超导态有利的疑似量子临界现象。论文第五部分首先介绍了一些量子自旋液体态(QSL)的理论背景,之后对Kagome格子中的非常规超导材料LaRu3Si2进行了化学掺杂研究。我们对La位和Ru位进行了不同稀土与过渡金属的掺杂,得到了从LaRu3Si2至CeRu3S i2勺全相图,并发现稀土与碱土金属的掺杂对超导只有较微弱的压制作用。特别是在Ru位用Cr和Ni磁性元素进行的掺杂,对超导的压制作用远远比不上同为磁性原子的Fe元素。通过居里外斯定律拟合分析,我们得到杂质元素Cr和Ni局域磁矩非常小,显示它们的确未像Fe元素那样建立较强的局域磁性散射中心,而仅仅扮演了类似巡游磁矩的角色。论文第六部分是全文的总结。
[Abstract]:Unconventional superconductors are superconductors that can not be understood by traditional BCS theory with phonons as pairing medium. They are mainly copper oxide superconductors, iron based superconductors, heavy fermion superconductors and organic superconductors. After nearly half a century of research, physicists believe that the induced and magnetic fluctuations and quantum critical phenomena of unconventional superconductors are considered. So far, it is not possible to find a complete and unified description of it. Therefore, it is very important to find a new unconventional superconductor system to provide a new research platform and to study its physical properties to get more unconventional superconducting mechanisms. This article is based on the basic principles of finding unconventional superconducting materials. The first part of this paper first summarizes the conventional superconductivity and physical images, and briefly introduces the four kinds of unconventional superconductors, such as copper oxide superconductors, Tie Jichao conductors, heavy fermion superconductors and organic superconductors. All attempts to find unconventional superconductivity have been reviewed and combed in history, and the basic principles for exploring unconventional superconducting materials are put forward. Finally, an example of the design of cobalt and nickel based high temperature superconductors is introduced. The second part of the thesis introduces the crystal growth, characterization, electrical transport measurement and thermal transport measurements we have used. In the third part of the paper, we have studied the possible electronic correlation effect in the Ru based 122 system with the LaRu2P2 material as a platform. We observed that the superconducting transition temperature Tc is enhanced with the pressure P in the LaRu2P2 single crystal. This can not be fully understood by Macmillan theory. The self consistent analysis of the normal resistance and the upper critical field suggests that the electron boson interaction of some additional associated properties may be induced under pressure and becomes the source of the enhancement of superconducting properties. Based on the first principle calculation of the electron and phonon structures, we analyzed the reasons for the correlation enhancement and the dome structure of the phase diagram in Tc-p We have made an explanation. Our experiments show that LaRu2P2 has some similarities with the iron base superconductor of the spin fluctuation, which may stimulate the study of the new unconventional superconductivity induced by the association effect in the Ru based system in the future. The fourth part is the attempt to find new superconductivity. We first studied the EuPdAs polycrystal sample, Through the resistance and magnetization data we can see the phase transition at 180 K under the normal pressure. When we apply the static pressure to the sample, only 0.48 GPa will completely suppress the 180 K phase transition, and the resistance is monotonically reduced under pressure. However, the pressure until 1.90 GPa does not see the superconductor being induced at low temperature. This indicates that the 180 K phase transition may be only only possible. In order to realize some singular electron properties, we think that the use of electronic doping to regulate the 4D orbit of Pd atoms should be a necessary way to realize some singular electron properties. Then we see the ferromagnetic phase transition of MnP polycrystals in the high temperature region and the spiral magnetic phase transition in the low temperature region, and the appearance of the spiral magnetic state on the magnetization. We found that the ferromagnetic state of the high temperature region did not establish hysteresis. We found that the magnetic phase transition in the high field will be suppressed and the system is ferromagnetic in the low temperature region, and the ferromagnetic phase transition of about 300K under static water pressure will move slowly to the low temperature. We also discuss the series doping experiments of the insulating state RuP to the metal state CrP, and see it at Cr60%. In the fifth part of this paper, the theoretical background of some quantum spin fluid states (QSL) is introduced, and then the chemical doping of the unconventional superconducting material LaRu3Si2 in the Kagome lattice is studied. The doping of different rare earth and transition metals in the La and Ru sites is obtained. The full phase diagram from LaRu3Si2 to CeRu3S I2 spoon is found, and the doping of rare earth and alkaline earth metal has only a weaker suppression effect on superconductivity. Especially in the Ru bit doping with Cr and Ni magnetic elements, the suppression of superconducting is far less than the Fe element of the magnetic atom. The local magnetic moments of elements Cr and Ni are very small, showing that they do not establish strong local magnetic scattering centers like Fe elements, but only play a role similar to the parade magnetic moment. The sixth part of the paper is the summary of the full text.

【学位授予单位】：南京大学
【学位级别】：博士
【学位授予年份】：2016
【分类号】：O469

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