In this model of orbital order in paramagnetic LaMnO3, the investigations of coupling between spin, orbital, and lattice degrees of freedom have been started by performing model average spin state calculations, which allow one to treat an orbital order in a paramagnetic state.

Arthur Freeman, Northwestern University

Research Objectives
Our research focuses on the structural, electronic, and magnetic properties of metal/ceramic interfaces. We investigate the total energies, band structures, and Fermi surfaces of double-layered colossal magnetoresistance (CMR) oxides with different doping (doping level and type of dopant), with different Mn-O octahedral distortions corresponding to temperature and doping variation, and with different spin configurations (ferromagnetic [FM] and antiferromagnetic [AFM]).

Computational Approach
The first-principles calculations within the local density approximation are performed by solving the Kohn-Sham equations self-consistently in a variational approach using the highly precise full-potential linearized augmented plane wave (FLAPW) method, the full-potential linear muffin-tin orbital (FLMTO) method, and the tight-binding linear muffin tin orbital method within the atomic sphere approximation (LMTO-ASA), and with the local spin density approximation with on-site Coulomb interaction taken into account (LSDA+U).

Accomplishments
Our major accomplishment with the Cray T3E and the IBM SP was the full and optimized parallelization of the all-electron FLAPW method, done together with Andrew Canning at NERSC. To our knowledge, this is the first all-electron electronic structure method that has been successfully parallelized.

We have investigated the influence of the Coulomb interaction parameter on both the electronic structure and the magnetic ordering of LaSr₂Mn₂O₇. The main factors governing band formation in this manganite are (1) the exchange splitting with almost unoccupied minority spin Mn-3d states, (2) the ligand field splitting of the t_2g and e_g states, and (3) further splitting of the e_g states with increase of U, which we consider an essential factor in the layered manganites. We have examined FM and A-type AFM ground states of this compound by comparing their total energy differences for a number of U values (from 0 to 10 eV) and found that the experimentally observed magnetic ordering is reproduced within LSDA+U only for U > 7 eV — in contrast with the 3D manganites (LaMnO₃), where LSDA gives the true magnetic ground state.

Our first-principles FLMTO-GGA electronic structure calculations of the new medium-T_C superconductor (MTSC) MgB₂ and related diborides indicate that superconductivity in these compounds is related to the existence of p_x,y-band holes at the G point. Based on these calculations, we explained the absence of medium-T_C superconductivity for BeB₂, AlB₂, ScB₂, and YB₂. The simulations of a number of MgB₂-based ternary systems using a supercell approach demonstrates that the electron doping of MgB₂ and the creation of isoelectronic defects in the boron sublattice are not favorable for superconductivity, and that a possible way of searching for similar MTSC should be via hole doping of MgB₂ or CaB₂ or via creating layered superstructures of the MgB₂/CaB₂ type.

Significance
There is a great demand for simple but realistic models describing essential electronic interactions in CMR materials. A detailed investigation of electronic structures is therefore important for a better understanding of the physics of CMR materials and for potential device applications. The investigations on properties and fundamental electronic structure characteristics of the new superconductor MgB₂ and related binary and ternary compounds will provide a theoretical basis for experimental efforts of solid-state chemists and physicists in searching for new superconductors with unusual and valuable properties.

Publications
J. E. Medvedeva, V. I. Anisimov, M. A. Korotin, O. N. Mryasov, and A. J. Freeman, "Coulomb correlations and magnetic ordering in double-layered manganites: LaSr₂Mn₂O₇," J. Magnetism and Magnetic Mater. (in press).

N. I. Medvedeva, A. L. Ivanovskii, J. E. Medvedeva, and A. J. Freeman, "Electronic structure of superconducting MgB₂ and related binary and ternary borides," Phys. Rev. B 64, 020502(R) (2001).

I. G. Kim, J. I. Lee, B. I. Min, and A. J. Freeman, "Surface electronic structures of superconducting thin film MgB₂(0001)," Phys. Rev. B 64, 020508(R) (2001).