Kazuma Nakamura

Comparison of Ab initio Low-Energy Models for LaFePO, LaFeAsO, BaFe2As2, LiFeAs, FeSe, and FeTe: Electron Correlation and Covalency

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-3 d and pnictogen-/chalcogen- p orbitals. First, LaFePO, LaFeAsO (1111), BaFe 2 As 2 (122), LiFeAs (111), FeSe, and FeTe (11) have overall similar band structures near the Fermi level, where the total widths of 10-fold Fe-3 d bands are mostly around 4.5 eV. However, the derived effective models of the 10-fold Fe-3 d bands ( d model) for FeSe and FeTe have substantially larger effective onsite Coulomb interactions U ∼4.2 and 3.4 eV, respectively, after the screening by electrons on other bands and after averaging over orbitals, as compared to ∼2.5 eV for LaFeAsO. The difference is similar in the effective models containing p orbitals of As, Se...

Ab initio Derivation of Low-Energy Model for Iron-Based Superconductors LaFeAsO and LaFePO

Effective Hamiltonians for LaFeAsO and LaFePO are derived from the downfolding scheme based on first-principles calculations and provide insights for newly discovered superconductors in the family of LnFeAsO 1- x F x , Ln = La, Ce, Pr, Nd, Sm, and Gd. Extended Hubbard Hamiltonians for five maximally localized Wannier orbitals per Fe are constructed dominantly from five-fold degenerate iron-3 d bands. They contain parameters for effective Coulomb and exchange interactions screened by the polarization of other electrons away from the Fermi level. The onsite Coulomb interaction estimated as 2.2–3.3 eV is compared with the transfer integrals between the nearest-neighbor Fe-3 d Wannier orbitals, 0.2–0.3 eV, indicating moderately strong electron correlation. The Hunds rule coupling is found to be 0.3–0.6 eV. The derived model offers a firm basis for further studies on physics of this family of materials. The effective models for As and P compounds turn out to have very similar screened interactions with slight...

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 Huckel results, especially much stronger onsite interaction U ∼0.8 eV ( U / t ∼12–15) than the Huckel 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.

Ab initio evidence for strong correlation associated with Mott proximity in iron-based superconductors.

We predict that iron-based superconductors discovered near d(6) configuration (5 Fe 3d orbitals filled by 6 electrons) is located on the foot of an unexpectedly large dome of correlated electron matter centered at the Mott insulator at d(5) (namely, half filling). This is based on the many-variable variational Monte Carlo results for ab initio low-energy models derived by the downfolding. The d(5) Mott proximity extends to subsequent emergence of incoherent metals, orbital differentiations due to the Mott physics, and Hunds rule coupling, followed by antiferromagnetic quantum criticality, in quantitative accordance with available experiments.

Comparative study of hybrid functionals applied to structural and electronic properties of semiconductors and insulators

We present a systematic study that clarifies validity and limitation of current hybrid functionals in density functional theory for structural and electronic properties of various semiconductors and insulators. The three hybrid functionals, PBE0 by Perdew, Ernzerhof, and Becke, HSE by Heyd, Sucseria, and Ernzerhof, and a long-range corrected (LC) functional, are implemented in a well-established plane-wave-basis-set scheme combined with norm-conserving pseudopotentials, thus enabling us to assess applicability of each functional on equal footing to the properties of the materials. The materials we have examined in this paper range from covalent to ionic materials as well as a rare-gas solid whose energy gaps determined by experiments are in the range of 0.6 eV - 14.2 eV: i.e., Ge, Si, BaTiO

First-principles calculation of transition-metal impurities in LaFeAsO

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Ab initioderivation of electronic low-energy models for C60and aromatic compounds

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First-principles calculation of effective onsite Coulomb interactions of 3 d transition metals: Constrained local density functional approach with maximally localized Wannier functions

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High-temperature superconductivity in layered nitridesβ-LixMNCl (M=Ti, Zr, Hf): Insights from density functional theory for superconductors

-GaN, diamond, MgO, NaCl, LiCl, Kr, and LiF. We find that the calculated bulk moduli by the hybrid functionals show better agreement with the experiments than the generalized gradient approximation (GGA) provides, whereas the calculated lattice constants by the hybrid functionals and GGA show comparable accuracy. The calculated energy band gaps and the valence-band widths for the ten prototype materials show substantial improvement using the hybrid functional compared with GGA. In particular, it is found that the band gaps of the ionic materials as well as the rare-gas solid are well reproduced by the LC-hybrid functional, whereas those of covalent materials are well described by the HSE functional. We also examine exchange effects due to short-range and long-range components of the Coulomb interaction and propose an optimum recipe to the short-range and long-range separation in treating the exchange energy.

Mott Transition and Phase Diagram of κ-(BEDT-TTF)2Cu(NCS)2 Studied by Two-Dimensional Model Derived from Ab initio Method

Department of Physics, Kyoto University, and JST TRIP, Sakyo-ku, Kyoto, 606-8502, Japan(Dated: October 5, 2010)We present a systematic ab initio study based on density-functional calculations to understandimpurity effects in iron-based superconductors. Effective tight-binding Hamiltonians for the d-bandsof LaFeAsO with various transition-metal impurities such as Mn, Co, Ni, Zn, and Ru are constructedusing maximally-localized Wannier orbitals. Local electronic structures around the impurity arequantitatively characterized by their onsite potential and transfer hoppings to neighboring sites.We found that the impurities are classified into three groups according to the derived parameters:For Mn, Co, and Ni, their impurity-3dlevels measured from the Fe-3dlevel are nearly 0.3 eV, −0.3eV, and −0.8 eV, respectively, while, for the Zn case, the dlevel is considerably deep as −8 eV. Forthe Ru case, although the onsite-level difference is much smaller as O(0.1) eV, the transfer integralsaround the impurity site are larger than those of the pure system by 20% ∼30%, due to the largespatial spread of the Ru-4dorbitals. We also show that, while excess carriers are tightly trappedaround the impurity site (due to the Friedel sum rule), there is a rigid shift of band structure nearthe Fermi level, which has the same effect as carrier doping.