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We present an ab initioanalysis for the ground-state properties of a correlated organic compound $\kappa$-(BEDT-TTF)2Cu(NCS)2. First, we derive an effective two-dimensional low-energy model from first principles, having short-ranged transfers and short-ranged Coulomb and exchange interactions. Then, we perform many-variable variational Monte Carlo calculations for this model and draw a ground-state phase diagram as functions of scaling parameters for the onsite and off-site interactions. The phase diagram consists of three phases; a paramagnetic metallic phase, an antiferromagnetic (Mott) insulating phase, and a charge-ordered phase. In the phase diagram, the parameters for the real compound are close to the first-order Mott transition, being consistent with experiments. We show that the off-site Coulomb and exchange interactions affect the phase boundary; (i) they appreciably stabilize the metallic state against the Mott insulating phase and (ii) enhance charge fluctuations in a wide parameter region in the metallic phase. We observe arc-like structure in Fermi surface around the region where the charge fluctuations are enhanced. Possible relevance of the charge fluctuations to the experimentally observed dielectric anomaly in the $\kappa$-BEDT-TTF family compounds is also pointed out.

We study how Co- and Ni-substitution affect the electronic structure of the iron-based superconductor, LaFeAsO. We perform ab initiosupercell calculations and unfold the first Brillouin zone (BZ) to calculate the spectral function in the BZ for the normal cell. The charge density distribution in real space shows that doped extra electrons are trapped around Co (Ni) atom. This seems to suggest that carriers are not doped by Co(Ni)-substitution. However, the present momentum-space analysis indicates that the Fermi-surface volume indeed expands by substitutions, which can be well described by the rigid-band shift approximation. By taking into account this effective doping, we discuss whether the sign-reversing $s$-wave ($s_{\pm}$-wave) scenario is compatible with experiments.

Effective low-energy Hamiltonians for several different families of iron-based superconductors are compared after deriving them from the downfolding scheme based on first-principles calculations. Systematic dependences of the derived model parameters on the families are elucidated, many of which are understood from the systematic variation of the covalency between Fe-3dand pnictogen-/chalcogen-porbitals. First, LaFePO, LaFeAsO (1111), BaFe2As2(122), LiFeAs (111), FeSe, and FeTe (11) have overall similar band structures near the Fermi level, where the total widths of 10-fold Fe-3dbands are mostly around 4.5 eV. However, the derived effective models of the 10-fold Fe-3dbands ($d$ model) for FeSe and FeTe have substantially larger effective onsite Coulomb interactions $U\sim 4.2$ and 3.4 eV, respectively, after the screening by electrons on other bands and after averaging over orbitals, as compared to ${\sim}2.5$ eV for LaFeAsO. The difference is similar in the effective models containing porbitals of As, Se or Te ($d p$ or $d p p$ model), where $U$ ranges from ${\sim}4$ eV for the 1111 family to ${\sim}7$ eV for the 11 family. The exchange interaction $J$ has a similar tendency. The family dependence of models indicates a wide variation ranging from weak correlation regime (LaFePO) to substantially strong correlation regime (FeSe). The origin of the larger effective interaction in the 11 family is ascribed to smaller spread of the Wannier orbitals generating larger bare interaction, and to fewer screening channels by the other bands. This variation is primarily derived from the distance $h$ between the pnictogen/chalcogen position and the Fe layer: The longer $h$ for the 11 family generates more ionic character of the bonding between iron and anion atoms, while the shorter $h$ for the 1111 family leads to more covalent-bonding character, the larger spread of the Wannier orbitals, and more efficient screening by the anion porbitals. The screened interaction of the $d$ model is strongly orbital dependent, which is also understood from the Wannier spread. The $d p$ and $d p p$ models show much weaker orbital dependence. The larger $h$ also explains why the 10-fold 3dbands for the 11 family are more entangled with the smearing of the “pseudogap” structure above the Fermi level seen in the 1111 family. While the family-dependent semimetallic splitting of the bands primarily consists of $d_{yz}/d_{zx}$ and $d_{x^{2}-y^{2}}$ orbitals, the size of the pseudogap structure is controlled by the hybridization between these orbitals and $d_{xy}/d_{3z^{2}-r^{2}}$: A large hybridization in the 1111 family generates a large “band-insulating”-like pseudogap (hybridization gap), whereas a large $h$ in the 11 family weakens them, resulting in a “half-filled” like bands of orbitals. This may enhance strong correlation effects in analogy with Mott physics and causes the orbital selective crossover in the three orbitals. On the other hand, the geometrical frustration $t'/t$, inferred from the ratio of the next-nearest transfer $t'$ to the nearest one $t$ of the $d$ model is relatively larger for the 1111 family than the 11 one. The models comprehensively derived here may serve as a firm starting basis of understanding both common and diverse properties of the iron-based superconductors including magnetism and superconductivity.

We derive an effective low-energy Hamiltonian for potassium loaded zeolite A, a unique ferromagnet from non-magnetic elements. We perform ab initiodensity functional calculations and construct maximally localized Wannier functions for low-energy states made from potassium $s$ electrons. The resulting Wannier orbitals, spreading widely in the alminosilicate cage, are found to be the superatomic$s$ and $p$ orbitals in the confining potential formed by the host cage. We then make a tight-binding model for these superatomic orbitals and introduce interaction parameters such as the Hubbard $U$. After mean-field calculations for the effective model, we find that ab initiospin density functional results are well reproduced by choosing appropriate sets of the interaction parameters. The interaction parameters turn out to be as large as the band width, ${\sim}0.5$ eV, indicating the importance of electron correlation, and that the present system is an interesting analog of correlated multi-orbital transition metal oxides.

Atomic and electronic structures of semiconducting liquid and amorphous selenium are studied with a chain model including an a nisotropic interaction between the Se atoms due to the p-lone pair on each atom.

By using a variational Monte Carlo method, we examine an effective low-energy model for LaFeAsO derived from an ab initiodownfolding scheme. We show that quantum and many-body fluctuations near the antiferromagnetic (AF) quantum critical point largely reduce the antiferromagnetic ordered moment. Our derived model not only quantitatively reproduces the small ordered moment in LaFeAsO, but also accounts for the diversity from LaFePO, BaFe2As2to FeTe. Electron correlation is found to determine the observed material dependence. We also find that LaFeAsO is subject to large orbital fluctuations, sandwiched by the AF Mott insulator and weakly correlated metals. The orbital fluctuations and Dirac-cone dispersion hold keys for the diverse magnetic properties.

An ab initiodownfolding method is formulated to construct low-dimensional models for correlated electrons. In addition to band downfolding by a constrained random phase approximation formulated for 3D models, screening away from the target layer (chain) is further involved. Eliminating the off-target degrees of freedom, namely, dimensional downfolding, yields ab initiolow-dimensional models. The method is applied to derive a 2D model for the layered superconductor LaFeAsO, where interlayer screening crucially makes the effective interaction short-ranged and reduces the onsite Coulomb interactions by 10--20% compared with the 3D model for the five iron-3dorbitals.