Research Objectives
To determine the structural, electronic, and magnetic properties of metal/ceramic interfaces (notably transition metals/BST interfaces); and to understand the effects of lattice distortions and magnetic canting in the colossal magneto-resistance (CMR) materials, using highly precise all-electron local-density total-energy methods.
Computional Approach
For several of our projects, we used the full-potential, linearized, augmented plane wave (FLAPW) method. The underlying concept is the partitioning of real space into spherical regions around atoms, and into interstitial and vacuum regions. This expands the charge, potential, and wave functions without any shape approximation, which makes it the most suitable and highly precise method for solids. One of the most important capabilities of FLAPW is the ability to calculate atomic forces and predict the total energy as a function of nuclear positions, which allows the determination of structural properties.
To further increase our capability of handling large complex systems to a new level and to exploit the computational power of massively parallel machines, we have parallelized our FLAPW code. The generalized eigenvalue equation has been solved by the newly developed parallelized QR (PQR) package, which is a portable eigen-solver written for distributed memory computers for solving the eigenvalues and vectors. We achieved good acceleration and reasonable scalability.
Accomplishments
We performed first-principles modeling of the leakage behavior of metal/BST (barium strontium titanate) thin film devices and found significant hybridization between metal d states and the O 2p-Ti 3d states. The Fermi levels of the metals lie in the gap of BST, and metal-induced gap states were observed. The issues of how to achieve low leakage current (through calculation of the Schottky barrier height), the bonding mechanism and structure of the interface have been addressed.
We determined from first-principles the magnetism in CMR materials and investigated the effects of lattice distortions on the competition between double exchange (DEX) and superexchange. We found that both global and internal lattice distortions dramatically influence the character of the exchange interactions. Distortions associated with typical Mn-O bond variations promote the DEX contribution; the bending of the Mn-O bonds in the (a-b) plane suppresses the DEX; and the tilting of the octahedra promotes non-Heisenberg contributions to the exchange interaction energy.
Significance
Our simulations on metal/BST systems provide information that helps direct the search and expedites the corresponding experimental work by steering the selection of materials for new metal electrode materials that lead to low leakage current for metal/BST/metal capacitors. The investigation on the CMR material helps us to understand its exchange coupling, which goes beyond the Heisenberg model.
Publications
Rao, F., M. Kim, A. J. Freeman, S. Tang, and M. Anthony. 1997. Structural and electronic properties of transition-metal/BaTiO3(001) interface, Phys. Rev. B 55:13953.
Mryasov, O. N., R. F. Sabiryanov, A. J. Freeman, S. S. Jaswal. 1997. Effect of lattice distortions on the competition between the double and superexchange mechanisms in LaMnO3, Phys. Rev. B 56:7255.
Charge density difference upon Pt adsorption on a slab of BaTiO3.