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To explore the electronic structure of the first aromatic superconductor, potassium-doped solid picene which has been recently discovered by Mitsuhashi et al with the transition temperatures $T_c=7 - 20$ K, we have obtained a first-principles electronic structure of solid picene as a first step toward the elucidation of the mechanism of the superconductivity. The undoped crystal is found to have four conduction bands, which are characterized in terms of the maximally localized Wannier orbitals. We have revealed how the band structure reflects the stacked arrangement of molecular orbitals for both undoped and doped (K$_3$picene) cases, where the bands are not rigid. The Fermi surface for K$_3$picene is a curious composite of a warped two-dimensional surface and a three-dimensional one.
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We have obtained the first-principles electronic structure of solid coronene, which has been recently discovered to exhibit superconductivity with potassium doping. Since coronene, along with picene, the first aromatic superconductor, now provide a class of superconductors as solids of aromatic compounds, here we compare the two cases in examining the electronic structures. In the undoped coronene crystal, where the molecules are arranged in a herringbone structure with two molecules in a unit cell, the conduction band above an insulating gap is found to comprise four bands, which basically originate from the lowest two unoccupied molecular orbitals (doubly-degenerate, reflecting the high symmetry of the molecular shape) in an isolated molecule but the bands are entangled as in solid picene. The Fermi surface for a candidate of the structure of K$_x$coronene with $x=3$, for which superconductivity is found, comprises multiple sheets, as in doped picene but exhibiting a larger anisotropy with different topology.
Based on First-principles calculation, we have investigated electronic structure of a ZrCuSiAs structured superconductor LaNiPO. The density of states, band structures and Fermi surfaces have been given in detail. Our results indicate that the bonding of the La-O and Ni-P is strongly covalent whereas binding property between the LaO and NiP blocks is mostly ionic. Its also found that four bands are across the Fermi level and the corresponding Fermi surfaces all have a two-dimensional character. In addition, we also give the band decomposed charge density, which suggests that orbital components of Fermi surfaces are more complicated than cuprate superconductors.
Recent experiments showed the distinct observations on the transition metal ditelluride NiTe$_2$ under pressure: one reported a superconducting phase transition at 12 GPa, whereas another observed a sign reversal of Hall resistivity at 16 GPa without the appearance of superconductivity. To clarify the controversial experimental phenomena, we have carried out first-principles electronic structure calculations on the compressed NiTe$_2$ with structure searching and optimization. Our calculations show that the pressure can transform NiTe$_2$ from a layered P-3m1 phase to a cubic Pa-3 phase at $sim$10 GPa. Meanwhile, both the P-3m1 and Pa-3 phases possess nontrivial topological properties. The calculated superconducting $T_c$s for these two phases based on the electron-phonon coupling theory both approach 0 K. Further magnetic transport calculations reveal that the sign of Hall resistance for the Pa-3 phase is sensitive to the pressure and the charge doping, in contrast to the case of the P-3m1 phase. Our theoretical predictions on the compressed NiTe$_2$ wait for careful experimental examinations.
Recently, a new organic superconductor, K-intercalated Picene with high transition temperatures $T_c$ (up to 18,K) has been discovered. We have investigated the electronic properties of the undoped relative, solid picene, using a combination of experimental and theoretical methods. Our results provide detailed insight into the occuopied and unoccupied electronic states.
Based on the first-principles calculations, we have investigated the geometry, binding properties, density of states and band structures of the novel superconductor LaFe1-xCoxAsO and its parent compounds with the ZrCuSiAs structure. We demonstrate that La-O bond and TM-As (TM=Fe or Co) bond are both strongly covalent, while the LaO and TMAs layers have an almost ionic interaction through the Bader charge analysis. Partial substitution of iron with cobalt modify the Fermi level from a steep edge to a flat slope, which explains why in this system Co doping suppresses the spin density wave (SDW) transition.
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