发展实空间格林函数理论研究铜替位铁基超导  

Real-space Green's function method for Fe-based superconductors with Cu substitution

作　　者：刘洋[1] 宋筠[1] 

机构地区：[1]北京师范大学物理学系,北京100875

出　　处：《中国科学：物理学、力学、天文学》2016年第8期60-68,共9页Scientia Sinica Physica,Mechanica & Astronomica

基　　金：国家自然科学基金资助项目(编号:11174036;11474023)

摘　　要：基于格林函数运动方程,发展了一套新的实空间多轨道格林函数方法,能够在实空间内对所有格点和轨道的格林函数进行同步求解,因此适用于研究因掺杂和空位等引入了无序效应的多轨道强关联体系.在文章中,讨论了如何合理地对多轨道体系的高阶格林函数进行截断近似,并详细介绍了在实空间开展矩阵化自洽求解时所需引入的数值计算技术方法.利用这一方法研究了用于描述铜替位掺杂的铁基超导的非均匀三轨道Hubbard模型,并重点探讨铜杂质对轨道选择莫特相变的影响.在轨道选择莫特相变中,只有d_(xy)轨道会发生莫特金属-绝缘体相变,而简并的d_(xz/yz)轨道仍然处于金属相.我们的计算结果显示,掺杂可以在d_(xy)轨道的莫特能隙中引入局域的隙间态,从而会显著提高发生轨道选择相变的洪德耦合临界值J_c.We develop a novel real-space Green＇s function method for the study of the Cu-substituted Fe-based superconductors based on the Green＇s function equation of motion approach. As an extension of the Hubbard-I approximation for the one-band Hubbard model, a series of decoupling scheme are introduced for the multi-orbital Hubbard model. Since the real-space fluctuation induced by the disorder effect can be treated exactly, our method is very suitable for the study of the strongly correlated multi-orbital systems with the presence of impurities. An inhomogeneous three-orbital Hubbard model is employed to involve the electrons in the three dxy, dxz, and dxz orbitals of the iron and cupper atoms in Cu-substituted Fe-based superconductors. When the orbital-selective Mott transition happens in the iron-based superconductors, a Mott gap can be found in the dxy orbital, while the degenerated dxzlyz orbitals remain metallic. With the increase of the Cu substitution, the dopants will induce in-gap states near the Fermi level within the Mott gap of the dxy orbital, and the Mott gap can be fulfilled by the in-gap states at higher doping cases. In addition, we find that the in-gap states only appear in a few discrete lattice points, where the occupancy of dxy orbital is away from half filling. It is also found that all these electronic states are Anderson localized. Meanwhile, the orbital polarization of the whole system increases with the enhancement of the real space fluctuation of the orbital occupancy induced by the Cu substitution. We also find that the Hund＇s rule coupling has strong effect in suppressing the real space fluctuation, which causes the in-gap states to vanish. As a result, the dxy orbital will restore to Mott-Hubbard insulator, suggesting that the critical value Jc of Hund＇ s rule coupling for the Mort-Hubbard transition will be pushed up significantly.

关 键 词：三轨道Hubbard模型 轨道选择莫特相变 铁基超导 铜替位掺杂 

分 类 号：O511.3[一般工业技术—材料科学与工程]