新型巡游电子系统中的关联效应和量子相变

The interplay between electron correlations and quantum fluctuations, which may result in various competing or co-existing quantum states, has been an important theme in modern condensed matter physics. A new challenge to this theme comes from the recently discovered iron-based superconductors, where several new phenomena, including the co-existence of superconductivity and magnetic order, the crossover from antiferromagnetism or superconductivity to heavy fermions, as well as various new properties associated with the Fe-vacancy ordering, has been emerged in addition to the unconventional superconductivity. ..In this Project, we will systematically investigate the roles the electron correlations play on the aforementioned novel phenemena in two prototypes of multi-orbital magnetic metals as exemplified by the rare earth iron pnictides and the Fe-vacancy ordered iron chalcogenides. For this purpose, a combinational approach to the microscopic many-body models describing the interacting electrons will be adopted, mainly based on the mean field and effective quantum field theories, assisted by the efficient numerical renormalization group, the first principle simulations and some other numerical methods. The main topics will be focused on but not limited to : (1) the interplay of the 3d-electrons and 4f-electrons in the rare earth iron pnictides; (2) the interplay among the 3d-electron Coulomb interaction, the Hund's coupling, the spin-orbit coupling in the vacancy ordered iron chalcogenides. In these tetrahedral-bonded multi-orbital materials the strengths of certain correlations are expected to be tunable under physical or chemical pressures. We will then explore new and rich physics driven by these tunable correlation effects such as Fermi surface transformations, crossover from superconductivity to heavy fermions, metal-insulator transitions, proximity of superconductivity to magnetic orders, as well as various quantum phase transitions. ..While in the course of our investigations we can hopefully explain interesting low temperature transport and magnetic properties and predict several new and novel quantum phenomena in these magnetic metallic materials, the insights gained from this Project should be very helpful for us to understand the underlying physics driven by various correlations and quantum fluctuations in a wider class of correlated electron systems.

电子关联与量子涨落及由此所致量子态之竞争、共存是凝聚态物理的重要课题。铁基超导化合物作为新型多轨道磁性金属体系，不仅可呈超导电性，还呈超导与磁性共存、超导向重费米子过渡、空位有序等独特而重要的新现象，从而产生诸多新问题与新挑战。本项目拟以稀土铁砷材料和含铁空位铁硒材料为例，从相互作用多电子模型出发，发展运用平均场和有效场论等分析方法，结合数值重整化群、第一性原理计算等数值方法，系统性地研究这类新型巡游电子系统两个方面的电子关联效应。主要内容包括：（1）由3d电子和4f电子的相互作用导致的超导或反铁磁坏金属到重费米子的过渡及伴随的费米面拓扑结构变化、量子相变；（2）由空位有序和相互作用联合驱动的金属绝缘体相变、轨道有序、及超导与磁性的共存或近邻效应等。这些研究旨在认识复杂磁性金属材料中各种关联作用随掺杂压力等参数调控的规律以及它们对超导电性、低温运输与磁性的影响，解释或预言新的物理现象。

本项目针对凝聚态电子系统中的新奇量态和丰富的低温物理性质，研究这些新奇量子态背后的微观机理，认识由电子关联和量子涨落导致的量子态之间的竞争和共存现象。.. 本项目具体研究对象以铁基超导化合物为主，同时也关注其它新型过渡金属化合物，特别是具有类似铁砷结构的镍基、铬基多轨道金属体系，以探讨与电子关联效应和量子磁性相关的共性问题。这些新型多轨道磁性金属体系通常不仅呈超导电性，还呈现超导与磁性共存、超导向重费米子过渡、量子相变、空位有序等独特而重要的新现象。我们尤其注重和相关实验的结合，并直接参与一些实验组的合作，达到用理论推动实验研究、或直接解释实验现象的目的。.. 本项目的主要研究成果有：.（1）结合平均场理论和数值对角方法确定了122-铁砷超导材料的锌掺杂、关联效应和无序对超导转变温度的影响，成功解释相关锌掺杂实验现象；.（2）用Monte Carlo和短时动力学数值模拟方法研究了推广J1-J2模型中的块状反铁磁结构相变现象，成功解释铁空位超导材料块状反铁磁性的起源和相变临界温度；.（3）与实验结合研究了一类具有铁砷材料1111-结构的稀土镍基材料的输运性质，研究了材料的电子结构并发现这一材料出现局域量子临界性的理论和实验证据；.（4）用泛函重整化群方法研究了多轨道5d系统如掺杂铱氧化合物的超导电性，发现这类材料在电子和空穴掺杂下超导配对对称性的不同特征；.（5）建立并研究了基于5d-4f体系中自旋轨道耦合和近藤耦合竞争的微观模型，由此提出拓扑绝缘体到近藤绝缘体的量子相变理论和拓扑近藤绝缘体的新机理；.（6）针对新型准一维钾铬砷超导材料建立微观原子轨道模型并推导出该模型的分子轨道能带以及由相互作用导致的Luttinger液体理论，解释了相关实验现象；.（7）结合实验和第一性原理方法系统研究了一类新型拓扑半金属材料XPn2的电子结构。.. 本项目成员在本项目经费的资助下共发表SCI 论文约20篇，其中1篇 Nature Materials, 2篇 Physical Review Letters, 11篇 Physical Review B，这些论文均标注本项目经费资助。