钌氧化合物中奇异物理性质的理论研究

The layered perovskites (Ca1-xSrx)n+1RunO3n+1 have attracted considerable interest in condensed matter physics over the last decades. These compounds exhibit a wide variety of interesting physics phenomena, such as superconductivity, colossal magnetoresistance, and Mott metal-insulator transition. It has been found that the density functional theory (DFT) failed to explain these phenomena. The failure of DFT is mainly due to its disregard for electron correlation. To overcome the shortcomings of DFT, this research will apply DFT+U+SO method to the present system to explain the novel phenomena. The research will focus on Ca2RuO4 and Ca3Ru2O7. There are several phase transitions in Ca2RuO4, and its ferromagnetic phase has an unusual small magnetic moment. These properties of Ca2RuO4 are related to the interplay or competition of multiple interactions, showing very rich physics. Ca3Ru2O7 has a unique bulk spin valve effect. For this reason, this compound is believed to have a good opportunity to be a new functional material. The aim of this research is to explain the novel phenomena in (Ca1-xSrx)n+1RunO3n+1, clarify the mechanism, and promote the related applying research.

钌氧化合物（Ca1-xSrx）n+1RunO3n+1是近年来受到广泛关注的一个体系。这一系列化合物展现出了许多奇异的物理现象，包括超导电性、超巨磁矩、莫特金属-绝缘相变等等。研究发现密度泛函理论（DFT）在解释这些问题时遇到了很大困难，其主要原因是DFT忽视了体系中的电子关联。为了克服DFT方法的困难，本项目计划采用DFT+U+SO方法对这一体系中的奇异物理现象进行理论研究，研究对象以Ca2RuO4和Ca3Ru2O7为主。Ca2RuO4中存在着多种物理相变，同时其铁磁相的磁矩异乎寻常的小，这些现象涉及到多种相互作用在同一体系中的竞争与耦合，具有丰富的物理内涵。Ca3Ru2O7则有独特的体材料自旋阀效应，被认为是极具应用潜力的新型功能材料。此项研究的目的是解释这些物理现象，揭示其中的物理机制，并在此基础上推动应用研究的进步。

采用第一性原理计算结合紧束缚模型的方案研究了Ca2RuO4、Ca2RhO4、Sr2IrO4、LaFeAsO等系统中的金属绝缘相变及磁矩异常等问题。在Ca2RuO4中，我们证明其铁磁相的磁矩异常减小来自于自旋轨道耦合与磁交换劈裂的竞争，由此提出了一个紧束缚模型，可以统一的解释Ca2RuO4的反铁磁绝缘相、顺磁绝缘相、铁磁金属相。在Ca2RhO4中，我们发现自旋轨道耦合可以诱导一个反铁磁金属绝缘相变，但是由于自旋轨道耦合与磁交换劈裂的竞争，其绝缘相不是通常认为的J=1/2态，。在Sr2IrO4，我们提出了一个紧束缚模型，证明其金属绝缘相变兼具Mott与Slater两种特性。在LaFeAsO，我们的计算表明电子关联可以增强反铁磁近邻的轨道杂化，这一图像可以解释铁基材料中的磁矩远小于能带理论的预言，进一步可以解释铁基超导中存在的负压力-Tc系数。