Liang-Jian Zou

Composite polaron mechanism for colossal magnetoresistance in perovskite manganites

The mechanism of carriers accompanied by spin-wave cloud and Jahn–Teller phononic cloud, called composite polaron, is proposed to explain the unusual transport and thermodynamic properties in perovskite manganites over all the temperature and magnetic field range. It is shown that the energy bandwidth of composite polarons is strongly renormalized with the increases of temperature and magnetic field, the bandwidth narrows with elevated temperature, while it broadens with increasing magnetic field. The resistivity of composite polarons increases with elevated temperature strongly, reaches a maximum near the Curie temperature TC, and decreases with the increase of magnetic field. The specific heat of composite polarons also exhibits a maximum near TC. These results agree with experiments in a broad temperature and magnetic field range, suggesting that composite polarons contribute to the unusual properties in manganites.

Ground state of the double-exchange model

We invesitgate the electronic correlation effect on the ground-state properties of the double exchange model for manganites by using a semiclassical approach and the slave-boson technique. It is shown that magnetic feild has a similar effect on the canted angle between manganese spins as doping concentration does, and the canted angle exhibits weak dependence on the Coulomb interaction. We have shown that the phase separation may take place at low doping. In the slave-boson saddle-point approximation in the ferromagnetic metallic regime, the dependence of the magnetization and the Curie temperature on the doping concentration exhibits maxima near 1/3 doping. These results agree with experimental data and suggest that the electronic correlation plays an important role for understanding the ground-state properties of manganites.

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 2×1×1u2009magnetic 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 200u2009K in experiment is a spin-density-wave transition.

Spin diffusion dynamics in double exchange manganites

We present theoretical studies on the spin diffusion dynamics of the double exchange model including Jahn–Teller distortion for manganites. It is shown that due to the trapping of composite polarons in the magnetic transition regime, the spin diffusion dynamics becomes important. The spin diffusion coefficient obtained by our theory agrees well with experimental data.

Ground state properties of a high-spin Mn12O12 molecule in an organic compound

The electronic structure and the magnetism of a Mn12O12 molecule at ground state have been studied by density functional theory with a local spin density approximation. We have found that the magnetic moments of Mn ions in the tetrahedron and those of Mn ions in the crown of the Mn12O12 molecule align antiferromagnetically. The average moment per Mn ion is about 3.07μB in the tetrahedron and 4.07μB in the crown. The total spin amounts to 20.0μB which is in agreement with recent experimental results. The significant difference of magnetic moments between Mn ions at two sites is attributed to the different exchange splitting of 3d orbitals. However, the charge difference between the two kinds of Mn ions is as small as 0.22 electrons. The charge density and the spin density exhibit strong directional dependence, which indicates the strong anisotropy in this molecule.

Electric and geometric controlling of the magnetic coupling in Kane-Mele nanoribbons

In this paper, we show that indirect spin interaction J between two magnetic impurities located on honeycomb Kane-Mele zigzag/armchair ribbons (KMZR/KMAR) is easily controlled by staggered potential and geometry. We demonstrate that J in periodic-boundary KMZR reaches maximum at the edges, and oscillates between antiferromagnetic and ferromagnetic couplings when tuning the sublattice staggered potential Δ. The odd-even length effect of J in KMZR and the width dependence of J in KMAR are also presented. These results clearly demonstrate the unique role of topological edge states and finite-size effect in magnetic coupling of quantum spin Hall (QSH) ribbons, and the controllability of the edge magnetism, hence favoring the fabrication of the spintronic devices in two-dimensional buckled honeycomb materials, e.g., silicene and germanene.

Giant magnetoresistance induced by spin-correlation scattering in magnetic thin films and other compounds

We present the study of the giant magnetoresistance effect in ferromagnetically ordered thin film and bulk based on the Hund’s rule coupling between the mobile d electron and the core spin of Mn ions. It has been shown that the resistivity is proportional to the spin–spin correlation functions, a maximum resistivity appears near the critical point in absence of magnetic field and an applied field drives the resistivity peak to higher temperature and reduces the peak value, which is in agreement with the experiments. The giant magnetoresistance effect in thin film is attributed to the spin‐correlation‐dependent scattering and the low‐dimensional character.