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Electronic Structure Calculations of Strongly Correlated Molecular Solids Using the Dynamical Mean Field Theory

Electronic Structure Calculations of Strongly Correlated Molecular Solids Using the Dynamical Mean Field Theory
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We have studied electronic structures and magnetic properties of strongly correlated molecular solids using the first principles electronic structure theories, such as the density functional theory (DFT), the DFT+U, and the DFT+dynamical mean-field theory (DFT+DMFT). We focus on two categories of strongly correlated molecular solids, hydrocarbon superconductors and O(2p) magnetic oxides. For hydrocarbon superconductors, we have investigated the electronic structures and magnetic properties of K3picene having Tc=18 K. It has been shown that active bands for the superconductivity near the Fermi level have the multi-orbital character with the hybridization of lowest unoccupied molecular orbital (LUMO) and LUMO+1 of the picene molecule. We have shown that K3picene is located in the vicinity of Mott transition from the estimation of the ratio of the Coulomb interation U and the bandwidth W. We suggest that K3picene is a strongly correlated correlated electron system with the multi-orbital character and the three dimensional electronic structure. We have extended our study for K3picene to correlated electronic structures and the phase diagram of overall electron-doped hydrocarbon solids, based on the DFT+DMFT method. We have shown that the ground state of hydrocarbon-based superconductors such as electron-doped picene and coronene is a multi-band Fermi liquid, while that of non-superconducting electron-doped pentacene is a single-band Fermi liquid near the metal-insulator transition. The size of the molecular orbital energy splitting between LUMO and LUMO+1 plays a key role in producing the superconductivity of electron-doped hydrocarbon solids. The multi-band nature of hydrocarbon solids is essential for the superconductivity by inducing metallic states even in the presence of the strong electronic correlation and enhancing the density of states at the Fermi level. For O(2p) magnetic oxides, we have investigated the electronic structures and magnetic properties of a spin-orbital-lattice coupled system, KO2, using the DFT+U. KO2 exhibits the concomitant emergence of antiferromagnetism (AFM) and structural transition (rotation of molecular axes), which originates from open-shell 2p electrons of O2 molecules. We have shown that the insulating state of the high symmetry (high temperature) phase of KO2 arises from the combined effect of spin-orbit coupling (SO) and the strong Coulomb correlation of O(2p) electrons. In contrast, for the low symmetry phase (low temperature) of KO2 with tilted O2 molecular axes, the band gap and the ferro-orbital (FO) ordering are driven by the combined effects of the crystal-field (CF) and the strong Coulomb correlation. We have verified that the emergence of the O(2p) FO ordering is essential to achive the observed AFM structure for KO2. We have also investigated electronic structures and magnetic properties of O2MF6 (M=Sb, Pt), which are composed of two building blocks of strongly correlated electrons: O2 dioxygenyls and MF6 octahedra, by employing the DFT+U. For O2SbF6, as a reference system of O2PtF6, we have shown that the Coulomb correlation of O(2p) electrons drives the Mott insulating state, similar to KO2. For O2PtF6, we have demonstrated that the Mott insulating state is induced by the combined effects of the Coulomb correlation of O(2p) and Pt(5d) electrons and the SO interaction of Pt(5d) states. The role of the SO interaction in forming the Mott insulating state of O2PtF6 is similar to the case of Sr2IrO4 that is a prototype of a SO induced Mott system with Jeff = 1/2. Next, we have extended our studies for KO2 to the nite temperature electronic structure using the DFT+DMFT. We have shown that KO2 exhibits the orbital fluctuation feature at high temperature due to the degenerate pi_g orbital. Upon cooling, the orbital fluctuation is suppressed by the Jahn-Teller type CF that becomes stronger with the lowering of structural symmetry, and then the FO ordering emerges at low temperature. We have shown that the FO ordering feature distinguishes KO2 from RbO2 and CsO2 in that the latter two seem to have antiferro-orbital orderings at low temperature, indicating that the underlying physics are different between them. We propose that the suppression of the orbital fluctuation in KO2 can be observed by thermal conductivity measurement.
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