Myung Joon Han

Electron-Hole Symmetry and Magnetic Coupling in Antiferromagnetic LaFeAsO

When either electron or hole doped at concentrations x approximately 0.1, the LaFeAsO family displays remarkably high temperature superconductivity with Tc up to 55 K. In the most energetically stable Q-->M=(pi,pi,0) antiferromagnetic (AFM) phase comprised of tetragonal-symmetry breaking alternating chains of aligned spins, there is a deep pseudogap in the Fe 3d states centered at the Fermi energy arising from light carriers (m* approximately 0.25-0.33), and very strong magnetophonon coupling is uncovered. Doping (of either sign) beyond x approximately 0.08 results in heavy carriers per Fe (by roughly an order of magnitude) with a large Fermi surface. Calculated Fe-Fe transverse exchange couplings Jij(R) reveal that exchange coupling is strongly dependent on both the AFM symmetry and on the Fe-As distance.

Doping driven (pi, 0) nesting and magnetic properties of Fe_{1+x}Te superconductors.

To understand newly discovered superconductivity in Fe-based systems, we investigate the electronic structure and magnetic properties of Fe_{1+x}Te using first-principles density functional calculations. While the undoped FeTe has the same Fermi surface nested at (pi,pi) as in Fe arsenides, doping by approximately 0.5 electrons reveals a novel square-type Fermi surface showing a strong (pi,0) nesting and leading to a different magnetic structure. Our result strongly supports the same mechanism of superconductivity in chalcogenides as in the arsenides, reconciling theory with existing experiments. The calculated magnetic interactions are found to be critically dependent on doping and notably different from the arsenides.

Anisotropy, itineracy, and magnetic frustration in high-Tc iron pnictides.

Using first-principles density functional theory calculations combined with insight from a tight-binding representation, dynamical mean field theory, and linear response theory, we have extensively investigated the electronic structures and magnetic interactions of nine ferropnictides representing three different structural classes. The calculated magnetic interactions are found to be short range, and the nearest (J_{1a}) and next-nearest (J2) exchange constants follow the universal trend of J_{1a}/2J_{2} approximately 1, despite their itinerant origin and extreme sensitivity to the z position of As. These results bear on the discussion of itineracy versus magnetic frustration as the key factor in stabilizing the superconducting ground state. The calculated spin-wave dispersions show strong magnetic anisotropy in the Fe plane, in contrast with cuprates.

Dynamical Mean-Field Theory of Nickelate Superlattices

Dynamical mean-field methods are used to calculate the phase diagram, many-body density of states, relative orbital occupancy, and Fermi-surface shape for a realistic model of LaNiO(3)-based superlattices. The model is derived from density-functional band calculations and includes oxygen orbitals. The combination of the on-site Hunds interaction and charge transfer between the transition metal and the oxygen orbitals is found to reduce the orbital polarization far below the levels predicted either by band-structure calculations or by many-body analyses of Hubbard-type models which do not explicitly include the oxygen orbitals. The findings indicate that heterostructuring is unlikely to produce one band-model physics and demonstrate the fundamental inadequacy of modeling the physics of late transition-metal oxides with Hubbard-like models.

Electronic structure and magnetic properties of small manganese oxide clusters

To investigate the electronic structure and magnetic properties of manganese oxide clusters, we carried out first-principles electronic structure calculations for small MnO clusters. Among various structural and magnetic configurations of the clusters, the bulklike [111]-antiferromagnetic ordering is found to be favored energetically, while the surface atoms of the clusters exhibit interesting electronic and magnetic characteristics which are different from their bulk ones. The distinct features of the surface atoms are mainly attributed to the reduction of Mn coordination numbers and the bond-length contractions in the clusters, which may serve as a key factor for the understanding of physical and chemical properties of magnetic oxide nanoparticles.

Chemical control of orbital polarization in artificially structured transition-metal oxides: La2NiXO6 (X=B, Al, Ga, In) from first principles

The application of modern layer-by-layer growth techniques to transition-metal oxide materials raises the possibility of creating new classes of materials with rationally designed correlated electron properties. An important step toward this goal is the demonstration that electronic structure can be controlled by atomic composition. In compounds with partially occupied transition-metal d shells, one important aspect of the electronic structure is the relative occupancy of different d orbitals. Previous work has established that strain and quantum confinement can be used to influence orbital occupancy. In this paper we demonstrate a different modality for orbital control in transition-metal oxide heterostructures, using density-functional band calculations supplemented by a tight-binding analysis to show that the choice of nontransition-metal counterion X in transition-metal oxide heterostructures composed of alternating LaNiO3 and LaXO3 units strongly affects orbital occupancy, changing the magnitude and in some cases the sign of the orbital polarization.

Covalency, double-counting, and the metal-insulator phase diagram in transition metal oxides

Dynamical mean field theory calculations are used to show that for late transition metal oxides a critical variable for the Mott/charge-transfer transition is the number of d electrons, which is determined by charge transfer from oxygen ions. Insulating behavior is found only for a narrow range of d occupancy, irrespective of the size of the intra-d Coulomb repulsion. The result is useful in interpreting “density functional + U ” and “density functional plus dynamical mean field” methods in which additional correlations are applied to a specific set of orbitals and an important role is played by the “double counting correction” which dictates the occupancy of these correlated orbitals. General considerations are presented and are illustrated by calculations for two representative transition metal oxide systems: layered perovskite Cu-based high-Tc materials, an orbitally nondegenerate electronically quasi-two-dimensional system, and pseudocubic rare earch nickelates, an orbitally degenerate electronically three-dimensional system. Density functional calculations yield d occupancies very far from the Mott metal-insulator phase boundary in the nickelate materials, but closer to it in the cuprates, indicating the sensitivity of theoretical models of the cuprates to the choice of double counting correction, and corroborating the critical role of lattice distortions in attaining the experimentally observed insulating phase in the nickelates.

Direct theoretical evidence for weaker correlations in electron-doped and Hg-based hole-doped cuprates.

Many important questions for high-Tc cuprates are closely related to the insulating nature of parent compounds. While there has been intensive discussion on this issue, all arguments rely strongly on, or are closely related to, the correlation strength of the materials. Clear understanding has been seriously hampered by the absence of a direct measure of this interaction, traditionally denoted by U. Here, we report a first-principles estimation of U for several different types of cuprates. The U values clearly increase as a function of the inverse bond distance between apical oxygen and copper. Our results show that the electron-doped cuprates are less correlated than their hole-doped counterparts, which supports the Slater picture rather than the Mott picture. Further, the U values significantly vary even among the hole-doped families. The correlation strengths of the Hg-cuprates are noticeably weaker than that of La2CuO4. Our results suggest that the strong correlation enough to induce Mott gap may not be a prerequisite for the high-Tc superconductivity.

Competition between structural distortion and magnetic moment formation in fullerene C20

We investigate the effect of on-site Coulomb interactions on the structural and magnetic ground state of C(20) fullerene based on density-functional-theory calculations within the local density approximation (LDA) plus on-site Coulomb corrections (LDA+U). The total energies of the high symmetry (I(h)) and the distorted (D(3d)) structures of C(20) are calculated for different spin configurations. The ground state configurations are found to depend on the forms of exchange-correlation potentials and the on-site Coulomb interaction parameter U, reflecting the subtle nature of the competition between Jahn-Teller distortion and magnetic instability in the C(20) fullerene. While the nonmagnetic state of the distorted D(3d) structure is robust for small U, a magnetic ground state of the undistorted I(h) structure emerges for U larger than 4 eV when the LDA exchange-correlation potential is employed.

Spin-orbital entangled molecular j eff states in lacunar spinel compounds

The entanglement of the spin and orbital degrees of freedom through the spin-orbit coupling has been actively studied in condensed matter physics. In several iridium oxide systems, the spin-orbital entangled state, identified by the effective angular momentum jeff, can host novel quantum phases. Here we show that a series of lacunar spinel compounds, GaM4X8 (M=Nb, Mo, Ta and W and X=S, Se and Te), gives rise to a molecular jeff state as a new spin-orbital composite on which the low-energy effective Hamiltonian is based. A wide range of electron correlations is accessible by tuning the bandwidth under external and/or chemical pressure, enabling us to investigate the cooperation between spin-orbit coupling and electron correlations. As illustrative examples, a two-dimensional topological insulating phase and an anisotropic spin Hamiltonian are investigated in the weak and strong coupling regimes, respectively. Our finding can provide an ideal platform for exploring jeff physics and the resulting emergent phenomena.