Interplay of iron and rare-earth magnetic order in rare-earth iron pnictide superconductors under magnetic field

The magnetic properties of iron pnictide superconductors with magnetic rare-earth ions under strong magnetic field are investigated based on the cluster self-consistent field method. Starting from an effective Heisenberg model, we present the evolution of magnetic structures on magnetic field in RFeAsO(R = Ce, Pr, Nd, Sm, Gd, and Tb) and RFe_2As_2(R =Eu) compounds. It is found that spin-flop transition occurs in both rare-earth and iron layers under magnetic field, in good agreement with the experimental results. The interplay between rare-earth and iron spins plays a key role in the magneticfield-driven magnetic phase transition, which suggests that the rare-earth layers can modulate the magnetic behaviors of iron layers. In addition, the factors that affect the critical magnetic field for spin-flop transition are also discussed.     

Ferromagnetism in Layered Metallic Fei/4TaS2 in the Presence of Conventional and Dirac Carriers

We present the microscopic origin of the ferromagnetism of Feo.25TaS2 and its finite-temperature magnetic properties.The band structures of Feo.25TaS2 are first obtained by the first-principles calculations and it is found that both conventional and Dirac carriers coexist in metallic Feo.25TaS2-Accordingly,considering the spin-orbit coupling of Fe 3d ion,we derive an effective Ruderman-Kittle-Kasuya-Yosida-type Hamiltonian between Fe spins in the presence of both the conventional parabolic-dispersion and the Dirac linear-dispersion carriers,which contains a Heisenberg-like,an Ising-like and an XY-like term.In addition,we obtain the ferromagnetic Curie temperature Tc by using the cluster self-consistent Held method.Our results could address not only the high ferromagnetic Curie temperature but also the large magnetic anisotropy in Fex TaS2.     

多轨道Hubbard模型的隶玻色子数值算法研究

通过综合模式搜索法、广义Lagrange乘子法、以及转轴法等多种数值方法, 建立了一套针对多轨道Hubbard模型隶玻色子解法的数值优化方法. 该数值方法能够在考虑晶场劈裂、轨道间跳跃以及真实能带结构基础上, 利用隶玻色子方法计算实际关联电子材料的性质. 首先利用该方法计算了两轨道体系的Mott金属-绝缘体转变性质, 得到了与目前已有工作一致的结果; 然后利用该方法讨论了Coulomb关联对三轨道体系Na_xCoO₂的影响. 结果表明: 在中间关联情况下由e_g^'轨道形成的六个小Fermi面消失, 原因是由于电子关联导致该轨道上的空穴数随U减少. 这些结果也证实了算法的正确性和有效性. 关键词： 多轨道Hubbard模型 隶玻色子 Mott转变 xCoO₂')" href="#">Na_xCoO₂     

关联电子体系中的轨道物理

轨道自由度为凝聚态关联电子材料带来丰富多彩的新量子相的同时也导致了复杂性,对理解强关联电子体系的本质带来了挑战。文章系统地介绍了多轨道关联电子体系的各种物理性质。首先简要地介绍了轨道自由度在过渡金属氧化物和稀土化合物中的重要作用和轨道序的物理图象;其次论述了轨道极化、轨道序及其与晶格畸变的联系;然后讨论了轨道序的理论研究及其在实验上的可能探测;接着介绍了多轨道体系中的金属—绝缘体相变和多轨道超导电性的新特征;最后简短讨论了目前轨道物理研究中面临的问题和挑战。     

Effects of anharmonic lattice distortion on orbital and magnetic orderings in KCuF3

Lattice, magnetic and orbital structures in KCuF3 are self-consistently determined by our cluster self-consistent field approach based on a spin-orbital-lattice Hamiltonian. Two stable structures are obtained and found to be degenerate, which confirms the presence of the coexistent phases observed experimentally. We clearly show that due to the inherent frustration, the ground state of the system only with the superexchange interaction is degenerate; while the Jahn-Teller distortion, especially the anharmonic effect, stabilizes the orbital ordered phase at about 23% in the x2-y2 orbit and at 77% in the 3z2-r2 orbit. Meanwhile the magnetic moment of Cu is considerably reduced to 0.56μB, and magnetic coupling strengths are highly anisotropic, Jz/Jxy ≈ 18. These results are in good agreement with the experiments, implying that the anharmonic Jahn-Teller effect plays an essential role in stabilising the orbital ordered ground state of KCuF3.     

Orbital Order and Orbital Excitations in Degenerate Itinerant Electron Systems

We present a theory of orbital ordering in orbital-degenerate itinerant electron systems. The orbital instability in a two-orbital degenerate Hubbard model is investigated in the random phase approximation （RPA）. After demonstrating the criteria for the formation of orbital ordering or the orbital density wave ordering, we find that the orbital and the spin-orbital collective excitation spectra in the ferro-orbital ordered phase exhibit finite gaps. The possible application of the present theory in orbital-ordered 4d compounds is also discussed.     

Variational Monte Carlo study of the nematic state in iron-pnictide superconductors with a five-orbital model

We perform a variational Monte Carlo study of the nematic state in iron-pnictide superconductors within a realistic five-orbital model.Our numerical results show that the nematic state,formed by introducing an anisotropic hopping order into the projected wave function,is not stable unless the off-site Coulomb interaction V exceeds a critical value.This demonstrates that V plays a key role in forming the nematic state in iron-pnictide superconductors.In the nematic state,the orbital order and the anisotropic spin correlations are consistent with the experimental observations.We argue that the experimentally observed anisotropic magnetic couplings and structural transition are associated with the nematic state and can be understood in a unified framework.     

Electrical Control of Edge Magnetism in Two-Dimensional Buckled Honeycomb Lattice

We theoretically study indirect spin coupling strength between two magnetic impurities located on honeycomb Kane-Mele zigzag ribbon （KMZR） with periodic and open boundary （PB and OB）. We show that spin interaction J in PB ribbons displays an AFM-FM oscillating behavior with increasing the staggered potential and electron density, and approaches to maximum at the edges. While the spin coupling in OB KMZR shows a trivial smooth AFM coupling with varying staggered potential. Such a novel J（ A） behavior is the combining effect of finite size, topological edge states and inversion symmetry breaking induced by the staggered potential. We propose that one could control the edge magnetism electrically in two-dimensional buckled honeycomb materials, e.g., silicence, germanene and stanene.     

Non-Adiabatic Effects of Superconductor Silane under High Pressure

We present the investigations of non-adiabatic effects by including vertex corrections in the standard Eliashberg theory and show that high phonon frequency is unfavorable to superconductivity in the regime of strong vertex correction. This means that it is hard to find high-transition-temperature superconductors in the compounds with light elements if the non-adiabatic effects are strong. The interplay interaction between non-adiabatic effect and Coulomb interaction makes the transition temperature of silane superconductor not so high as predicted by the standard Eliashberg theory.