Ya-Min Quan

Band filling and correlation controlling electronic properties and magnetism in KxFe2−ySe2: a slave boson study

We investigate the electronic and magnetic properties of K(x)Fe(2-y)Se2 materials at different band fillings utilizing the multi-orbital Kotliar-Ruckenstein slave boson mean-field approach. We find that the ground state of KFe2Se2 is a paramagnetic (PM) bad metal with intermediate correlation, in contrast with the previous antiferromagnetic (AFM) results obtained by the local density approximation. Our PM metallic ground state suggests that KFe2Se2 is the parent phase of superconducting K(x)Fe(2-y)Se2, supporting a recent scanning tunneling spectroscopy experiment. For pure Fe2+-based systems, the ground state is a striped AFM (SAFM) metal with a spin density wave gap partially opened near the Fermi level. In comparison, for Fe3+-based compounds, besides SAFM, a Néel AFM metal without orbital ordering is observed, and an orbital selective Mott phase (OSMP) accompanied by an intermediate-spin to high-spin transition is also found, giving a possible scenario of an OSMP in K(x)Fe(2-y)Se2. These results demonstrate that the band filling and correlation control the Fermi surface topology, electronic state and magnetism in K(x)Fe(2-y)Se2.

Competition between crystal field splitting and Hund’s rule coupling in two-orbital magnetic metal-insulator transitions

Abstract Competition between crystal field splitting and Hund’s rule coupling in magnetic metal-insulator transitions of half-filled two-orbital Hubbard model is investigated by multi-orbital slave-boson mean field theory. We show that with the increase of Coulomb interaction, the system firstly transits from a paramagnetic (PM) metal to a Néel antiferromagnetic (AFM) Mott insulator, or to a nonmagnetic orbital insulator, depending on the competition of crystal field splitting and the Hund’s rule coupling. The AFM Mott insulating, PM metallic and orbital insulating phases are not, partially and fully orbital polarized, respectively. For a small JH and a finite crystal field, the orbital insulator is robust. These results demonstrate that large crystal field splitting favors the formation of the orbital insulating phase, while large Hund’s rule coupling tends to destroy it, driving the low-spin to high-spin transition.

Influence of electronic correlations on orbital polarizations in the parent and doped iron pnictides

Orbital polarization and electronic correlation are two essential aspects in understanding the normal-state and superconducting properties of multi-orbital FeAs-based superconductors. In this paper, we present a systematic study on the orbital polarization of iron pnictides from weak to strong Coulomb correlations within the Kotliar-Ruckenstein slave boson approach. The magnetic phase diagram of the two-orbital model for LaFeAsO clearly shows that a striped antiferromagnetic metallic phase with orbital polarization exists over a wide doping range, in addition to the Slater-type insulator, Mott insulator and paramagnetic phases. A reversal of the orbital polarization occurs in the intermediate correlation regime in the absence of the crystal field splitting; however, a small crystal field splitting considerably enhances the orbital polarization, and stabilizes the xz-type orbital order. We argue that the ferro-orbital polarization is characteristic of a density wave, and leads to a pseudogap-like behavior in the density of states.

Numerical optimization algorithm for rotationally invariant multi-orbital slave-boson method

Abstract We develop a generalized numerical optimization algorithm for the rotationally invariant multi-orbital slave boson approach, which is applicable for arbitrary boundary constraints of high-dimensional objective function by combining several classical optimization techniques. After constructing the calculation architecture of rotationally invariant multi-orbital slave boson model, we apply this optimization algorithm to find the stable ground state and magnetic configuration of two-orbital Hubbard models. The numerical results are consistent with available solutions, confirming the correctness and accuracy of our present algorithm. Furthermore, we utilize it to explore the effects of the transverse Hund’s coupling terms on metal–insulator transition, orbital selective Mott phase and magnetism. These results show the quick convergency and robust stable character of our algorithm in searching the optimized solution of strongly correlated electron systems.

Ferro-orbital order induced by electron-lattice coupling in orthorhombic iron pnictides

In this paper we explore the magnetic and orbital properties in iron pnictides based on the two-orbital as well as the five-orbital Hubbard models. These properties are closely related to a tetragonal-orthorhombic structural phase transition. The electron-lattice coupling, interplaying with electron-electron interaction, is self-consistently treated. Our results reveal that the orbital polarization favors the spin-density wave (SDW) in the orthorhombic phase. The ferro-orbital (FO) order only occurs in the orthorhombic phase rather than in the tetragonal one. For the five-orbital model, magnetic moments of Fe are small in the intermediate Coulomb interaction region in the striped antiferromagnetic phase. We also calculate the Fermi surface, which is anisotropic in the SDW/FO orthorhombic phase and agrees well with the recent angle-resolved photoemission spectroscopy experiments. These results suggest that the magnetic phase transition is driven by the FO order arising from the electron-lattice coupling.

A site-selective antiferromagnetic ground state in layered pnictide-oxide BaTi2As2O

The electronic and magnetic properties of BaTi2As2O have been investigated using both the first-principles and analytical methods. The full-potential linearized augmented plane-wave calculations show that the most stable state is a site-selective antiferromagnetic (AFM) metal with a 2×1×1 magnetic unit cell containing two nonmagnetic Ti atoms and two other Ti atoms with antiparallel moments. Further analysis to Fermi surface and spin susceptibility shows that the site-selective AFM ground state is driven by the Fermi surface nesting and the Coulomb correlation. Meanwhile, the charge density distribution remains uniform, suggesting that the phase transition at 200 K in experiment is a spin-density-wave transition.

Stability of Ferromagnetism and Ferromagnetic orbital selective Mott phase in three-orbital Hubbard model

In this paper we investigate the nature of ferromagnetic (FM) phase in three-orbital Hubbard model by using Kotliar-Ruckenstein slave-boson approach. We find that the FM metallic phase widely exists and stabilizes in magnetic phase diagram for large Hund’s rule coupling, and FM insulator is not seen. An FM orbital-selective Mott phase (OSMP), mainly due to large crystal field splitting and strong electronic correlation, is observed and identified as double-exchange FM. An antiferromagnetic (AFM) OSMP is also found. We conclude that large Coulomb correlation and Hund’s rule coupling is crucial for stable FM phase, and further crystal field splitting also enhance the stability of AFM and FM OSMP.

A three-dimensional tight-binding model and magnetic instability of iron selenide KFe2Se2

Abstract For a newly discovered iron-based high T c superconducting parent material KFe 2 Se 2 , we present an effective three-dimensional five-orbital tight-binding model by fitting the band structures. The three t 2 g -symmetry orbitals of the five Fe 3d orbitals mainly contribute to the electron-like Fermi surface, which has a three-dimensional character, in agreement with recent angle-resolved photoemission spectroscopy experiments. To understand the groundstate magnetic properties, the two- and three-dimensional spin and charge susceptibilities within the random phase approximation are investigated. It obviously shows a sharp peak at wave vector q ∼ ( π , π ) , indicating the magnetic instability of Neel-type antiferromagnetic rather than ( π / 2 , π / 2 )-type or stripe ( π , 0)-type antiferromagnetic ordering, while it exhibits a ferromagnetic instability between the nearest neighboring FeSe layers along c axis. The three-dimensional spin fluctuation may play an important role in pairing symmetry in superconducting materials A y Fe 2 − x Se 2 ( A = Rb , Cs , K , etc.).