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이관우,Lee, K.W. 한국초전도학회 2010 Progress in superconductivity Vol.12 No.1
In the superconducting $Sr_2VO_3FeAs$, containing bimetallic layers, with maximum $T_c{\approx}\;46\;K$ correlation effects on V ions have been investigated using LDA+U method. Within the local density approximation (LDA) this system has the one-third filled $t_{2g}$ manifold of V, decomposed into $d_{xy}$ of bandwidth W=2 eV and nearly degenerate $d_{zx}d_{yz}$ of W=1 eV. Consideration of correlation effects leads to a metal-insulator transition on V ions $t^{2\uparrow}_{2g}\;{\rightarrow}\;d^{1\uparrow}_{xz}\;d^{1\uparrow}_{yz}$ at the critical on-site Coulomb repulsion $U_c$= 3.5 eV. At U=4 eV, the electronic structure, in which V ions are insulating, leads to several van Hove singularities near $E_F$ and similar Fermiology with other pnictides. Applying U to V ions results in increasing Fe moment as well as V moment, indicating somewhat hybridization between Fe and V ions even though this system is strongly 2-dimesional. Our results show possible importance of correlation effects on this system.
이관우,Lee, K.W. 한국초전도학회 2009 Progress in superconductivity Vol.10 No.2
NaFeAs recently observed superconductivity with the maximum $T_c{\approx}25$ K is investigated using first principles approach. We will address briefly the electronic structure and contrast other superconducting pnictides. This system shows strong two-dimensionality and reduction of flatness in the Fermi surfaces undermines tendencies of magnetic or charge instabilities. As observed in other superconducting pnictides, $Q_M=(\pi,\pi,0)$ antiferromagnetic ordering, which has not been observed clearly yet in this compound, is energetically favored. However, contrast to other superconducting pnictides, the density of states in this ordering shows considerable electron-hole asymmetry, implying efficiency of hole-doping than electron-doping to enhance $T_c$.