Ab initio two-dimensional multiband low-energy models of EtMe_3Sb[Pd(dmit)_2]_2 and \kappa-(BEDT-TTF)_2Cu(NCS)_2 with comparisons to single-band models

We present ab initio two-dimensional extended Hubbard-type multiband models for EtMe_3Sb[Pd(dmit)_2]_2 and \kappa-(BEDT-TTF)_2Cu(NCS)_2, after a downfolding scheme based on the constrained random phase approximation (cRPA) and maximally-localized Wannier orbitals, together with the dimensional downfolding. In the Pd(dmit)_2 salt, the antibonding state of the highest occupied molecular orbital (HOMO) and the bonding/antibonding states of the lowest unoccupied molecular orbital (LUMO) are considered as the orbital degrees of freedom, while, in the \kappa-BEDT-TTF salt, the HOMO-antibonding/bonding states are considered. Accordingly, a three-band model for the Pd(dmit)_2 salt and a two-band model for the \kappa-(BEDT-TTF) salt are derived. We derive single band models for the HOMO-antibonding state for both of the compounds as well.Comment: 10 pages, 9 figures, 3 tables; submitted to Physical Review

Ab initio GW plus cumulant calculation for isolated band systems: Application to organic conductor (TMTSF)2PF6 and transition-metal oxide SrVO3

We present ab initio GW plus cumulant-expansion calculations for an organic compound (TMTSF)2PF6 and a transition-metal oxide SrVO3. These materials exhibit characteristic low-energy band structures around the Fermi level, which bring about interesting low-energy properties; the low-energy bands near the Fermi level are isolated from the other bands, and, in the isolated bands, unusually low-energy plasmon excitations occur. To study the effect of this low-energy-plasmon fluctuation on the electronic structure, we calculate spectral functions and photoemission spectra using the ab initio cumulant expansion of the Green’s function based on the GW self-energy. We found that the low-energy plasmon fluctuation leads to an appreciable renormalization of the low-energy bands and a transfer of the spectral weight into the incoherent part, thus resulting in an agreement with experimental photoemission data

Optical Absorption Study by Ab initio Downfolding Approach: Application to GaAs

We examine whether essence and quantitative aspects of electronic excitation spectra are correctly captured by an effective low-energy model constructed from an {\em ab initio} downfolding scheme. A global electronic structure is first calculated by {\em ab initio} density-functional calculations with the generalized gradient approximation. With the help of constrained density functional theory, the low-energy effective Hamiltonian for bands near the Fermi level is constructed by the downfolding procedure in the basis of maximally localized Wannier functions. The excited states of this low-energy effective Hamiltonian ascribed to an extended Hubbard model are calculated by using a low-energy solver. As the solver, we employ the Hartree-Fock approximation supplemented by the single-excitation configuration-interaction method considering electron-hole interactions. The present three-stage method is applied to GaAs, where eight bands are retained in the effective model after the downfolding. The resulting spectra well reproduce the experimental results, indicating that our downfolding scheme offers a satisfactory framework of the electronic structure calculation, particularly for the excitations and dynamics as well as for the ground state.Comment: 14 pages, 6 figures, and 1 tabl

Ab initio Derivation of Low-Energy Model for κ-ET Type Organic Conductors

We derive effective Hubbard-type Hamiltonians of κ-(BEDT-TTF) 2 X, using an ab initio downfolding technique, for the first time for organic conductors. They contain dispersions of the highest occupied Wannier-type molecular orbitals with the nearest neighbor transfer t ∼0.067 eV for a metal X =Cu (NCS) 2 and 0.055 eV for a Mott insulator X =Cu 2 (CN) 3, as well as screened Coulomb interactions. It shows unexpected differences from the conventional extended Hückel results, especially much stronger onsite interaction U ∼0.8 eV (U / t ∼12–15) than the Hückel estimates (U /t ∼7–8) as well as an appreciable longer-ranged interaction. Reexamination on physics of this family of materials is required from this realistic basis

Formation of 2D single-component correlated electron system and band engineering in the nickelate superconductor NdNiO2

Motivated by the recent experimental discovery of superconductivity in the infinite-layer nickelate Nd0.8Sr0.2NiO2 [Li et al., Nature (London) 572, 624 (2019)], we study how the correlated Ni 3dx2−y2 electrons in the NiO2 layer interact with the electrons in the Nd layer. We show that three orbitals are necessary to represent the electronic structure around the Fermi level: Ni 3dx2−y2, Nd 5d3z2−r2, and a bonding orbital made from an interstitial s orbital in the Nd layer and the Nd 5dxy orbital. By constructing a three-orbital model for these states, we find that the hybridization between the Ni 3dx2−y2 state and the states in the Nd layer is tiny. We also find that the metallic screening by the Nd layer is not so effective in that it reduces the Hubbard U between the Ni 3dx2−y2 electrons just by 10%–20%. On the other hand, the electron-phonon coupling is not strong enough to mediate superconductivity of Tc∼10 K. These results indicate that NdNiO2 hosts an almost isolated correlated 3dx2−y2 orbital system. We further study the possibility of realizing a more ideal single-orbital system in the Mott-Hubbard regime. We find that the Fermi pockets formed by the Nd-layer states dramatically shrink when the hybridization between the interstitial s state and Nd 5dxy state becomes small. By an extensive materials search, we find that the Fermi pockets almost disappear in NaNd2NiO4 and NaCa2NiO3