发展实空间格林函数方法对铁基超导中电子向列相的研究

Iron-based superconductor is a new type of high temperature superconductor which has been widely studied in recent years. It’s a typical strongly correlated multi-orbital system which has multiple energy bands across the Fermi surface. Many experiments show that the electronic states of iron-based superconductors have a remarkable C2 symmetry feature in the plane of Fe-As/Se. Its origin is usually attributed to orbital fluctuations or spin scenarios, but it is still a challenge to distinguish the primary driving force. As the nematic instability is a characteristic feature of the normal state from which the superconductivity emerges, the study for nematic phase is essential to understand the pairing mechanism, structure transition and magnetic physics. For the need of theoretical research, we have developed an effective method based on Green’s function and high-order decoupling scheme, which can treat the real space fluctuation induced by disorder and the multi-orbital correlation on the same footing. We applied this method to a three-orbital Hubbard model with random distributed dopants, iron vacancies and multi-orbital electron correlation to find their effects on the nematic phase, which is helpful for us to reveal the physical origin of nematic phase and its relationship with superconducting pairing.

铁基超导是近年来被广泛研究的一种新型的高温超导材料，其具有多条穿过费米面的能带，是一种典型的多轨道强关联材料。许多实验结果揭示了铁基超导体中电子态性质有着显著的铁砷/铁硒面内二重对称特征，来源于旋转对称破缺的电子向列相。其起源通常归因于轨道涨落或自旋涨落，但尚不清楚何种为主要的驱动机制。对于超导相产生的母态——电子向列相的研究，将有助于我们理解铁基超导材料的超导配对机制，同时，对其结构相变机理、磁性物理等方面的研究也至关重要。为了理论研究的需要，我们发展了一套基于格林函数和高阶截断近似的有效方法，既能准确地描述无序导致的实空间涨落和局域化特征，又能够合理地处理多轨道关联效应。我们将其应用于一个三轨道铁基超导Hubbard模型，对无序掺杂、铁空位等引入的点缺陷以及多轨道电子关联对铁基超导中电子向列相的影响，进行一次系统的研究和梳理，力图揭示电子向列相的物理起源及其与超导配对的关系。

铁基超导中各向异性的电导及其起源机制，目前仍然是一个有争议的问题。该项目基于一个三轨道铁基超导Hubbard模型，对无序掺杂和多轨道电子关联对体系各向异性电导的影响进行了细致的研究，取得了一定的成果。我们首先发展了一套超越平均场近似的数值方法，在Hubbard模型中可以重现费米能级处的近藤峰，这是一般格林函数截断算法所不具备的。其次，我们利用这套方法，对铁基超导材料中各向异性的电子向列相的形成机制进行了探索。我们发现调节洪德耦合的强度会改变体系中电子态的各向异性，在J/U=0.12时，体系的各向异性电导达到峰值。此外，杂质原子的引入会在多轨道体系中引入外源性的局域电子向列相，并且与洪德耦合调节的内源性向列相形成竞争。这些结果对于我们理解在不同铁基超导体中电导的各向异性及其在掺杂条件下的矛盾表现提供了一个合理的解释。