A Possible Ground State and Its Electronic Structure of a Mother Material (LaOFeAs) of New Superconductors

Abstract

The electronic and magnetic properties of the mother material LaOFeAs of new superconductors have been carefully studied using first-principles electronic structure calculations based on the generalized gradient approximation in the density functional theory. The present calculation predicts that the ground state of LaOFeAs is antiferromagnetic with a stripe type magnetic moment alignment leading to orthorhombic symmetry of the crystal. In this particular magnetic state, the density of states at the Fermi level is very small. On the other hand, LaOFeP has turned out to be paramagnetic and a good metal. Implications of the results regarding the experimental observations will also be presented. Discovery of a new superconductor always attracts strong attention of the scientific community particularly if the critical temperature T c is above 20 K. It is rather surprising that immediately after the recent news of superconductivity of LaO 1Àx F x FeAs 1) several papers, both theoretical and experimental, have been circulated in the community. There may be four main reasons why this discovery has produced such a strong impact. First, T c is relatively high 26 K. Second, it is a new finding that Fe, which is a typical magnetic element, seems to be participating to superconductivity. Third, the crystal takes again a layered structure and the doping region and the superconducting region are geometrically and electronically separated. Fourth, the material seems to have strong flexibility in the choice of constituent elements suggesting possibility of higher T c materials in the same category. 2)In order to go further in exploring possibility of higher T c materials, it is essential to understand the basic electronic properties of the mother material LaOFeAs and the role of partial replacement of O with F. In the analogy of high T c cuprates,3) important questions about LaOFeAs may be whether magnetism is involved or not and whether it is metallic or insulating. These questions have already been addressed by other works [4][5][6] in which standard DFT band calculations have been performed. At least within our knowledge, it was concluded that the system is metallic and nonmagnetic with possible strong antiferromagnetic (AFM) fluctuation. We have also performed similar calculations and have arrived at a conclusion that the ground state will be a particular AFM state. The density of states (DOS) at the Fermi level is very low suggesting that the system may be a bad metal.Briefly, our calculation is based on the PAW method 7) and the PBE version 8) of generalized gradient approximation in the density functional theory. Spin-orbit interaction was not taken into account. We used our in-house computer code named QMAS (Quantum MAterials Simulator) and have tested the reliability of the code in several ways. In relation to the present system, we have performed test calculations for LaOMnAs, LaONiP, and MnAs with NiAs structure. In all of these test calculations, we have confirmed consistency between our results and other existing o...

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