Yoshihide Yoshimoto scite author profile

We derive effective Hubbard-type Hamiltonians of -(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 ¼ Cu(NCS) 2 and 0.055 eV for a Mott insulator X ¼ 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. Organic conductors with BEDT-TTF molecules [where BEDT-TTF is bis(ethylenedithio)-tetrathiafulvalene, abbreviated as ET hereafter], (ET) 2 X with a number of choices of anions X, offer a variety of prototypical behaviors of strongly correlated electron systems with two-dimensional (2D) anisotropies.1) Examples range from correlated metals with superconductivity at low temperatures to Mott insulators either with a quantum spin liquid or with antiferromagnetic, charge-density or spin-Peierls orders. Intriguing Mott transitions are also found. They are all in front of recent active research for unconventional quantum phases and quantum critical phenomena in nature, while their essences of physics are still under strong debates.In particular, an unconventional nonmagnetic Mott-insulating phase is found near the Mott transition in the -type structure of ET molecules, X ¼ Cu 2 (CN) 3 referred to as -CN, where no magnetic order is identified down to the temperature T ¼ 0:03 K, four orders of magnitude lower than the antiferromagnetic spin-exchange interaction J $ 250 K.2) The emergence of the quantum spin liquid near the Mott transition has been predicted in earlier numerical studies, [3][4][5] while the full understanding of the spin liquid needs more thorough studies. It is also crucially important to elucidate the real relevance of the theoretical findings to the real -ET compounds. Most of numerical 6) and theoretical 7) studies have also been performed for a simplified single-band 2D Hubbard model based on an empirical estimate of parameters combined with extended Hückel calculations. 8,9) A more realistic description of -ET compounds is certainly needed beyond the empirical model.Another fundamental finding achieved in this series of compound is the unconventional Mott transition found for X ¼ Cu[N(CN) 2 ]Cl under pressure.10) The novel universality class of the Mott transition is in good agreement with the marginal quantum criticality at the meeting point of the symmetry breaking and topological change. [11][12][13][14] Because of its significance to the basic understanding on the physics of quantum criticality, the relevance of theoretical concept to the experimental observation needs to be further examined on the realistic and first-principles grounds. Furthermore, an unconventional superco...

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Formation of a two-dimensional single-component correlated electron system and band engineering in the nickelate superconductor NdNiO2

Nomura

et al. 2019

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Motivated by the recent experimental discovery of superconductivity in the infinite-layer nickelate Nd0.8Sr0.2NiO2 [Li et al., Nature 572, 624 (2019)], we study how the correlated Ni 3d x 2 −y 2 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 3d x 2 −y 2 , Nd 5d 3z 2 −r 2 , 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 3d x 2 −y 2 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 3d x 2 −y 2 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 3d x 2 −y 2 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. arXiv:1909.03942v2 [cond-mat.supr-con]

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Ab initiotwo-dimensional multiband low-energy models of EtMe3Sb[Pd(dmit)2]2and
Nakamura
¹
,
Yoshimoto
²
,
Imada
³

2012
Phys. Rev. B

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We present ab initio two-dimensional extended Hubbard-type multiband models for EtMe3Sb[Pd(dmit)2]2 and κ-(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 κ-BEDT-TTF salt, the HOMO-antibonding/bonding states are considered. Accordingly, a threeband model for the Pd(dmit)2 salt and a two-band model for the κ-(BEDT-TTF) salt are derived. We derive single band models for the HOMO-antibonding state for both of the compounds as well. The HOMO antibonding band of the Pd(dmit)2 salt has a triangular structure of the transfers with a one-dimensional anisotropy in contrast to the nearly equilateral triangular structure predicted in the extended Hückel results. The ratio of the larger interchain transfer t b to the intrachain transfer ta is around t b /ta ∼ 0.82. Our calculated screened onsite interaction U and the largest offsite interaction V are ∼0.7 eV and ∼0.23 eV, respectively, for EtMe3Sb[Pd(dmit)2]2 and ∼0.8 eV and ∼0.2 eV for κ-(BEDT-TTF)2Cu(NCS)2. These values are large enough compared to transfers t as ∼55 meV for the Pd(dmit)2 salt and ∼65 meV for the κ-BEDT-TTF one, and the resulting large correlation strength (U −V )/t ∼ 10 indicates that the present compounds are classified as the strongly correlated electron systems. In addition, the validity whether the present multiband model can be reduced to the single-band model for the HOMO-antibonding state, widely accepted in the literature, is discussed. For this purpose, we estimated the order of vertex corrections ignored in the cRPA downfolding to the single band model, which is given by W ′ /D, where W ′ is a fullscreened-interaction matrix element between the HOMO-antibonding and other bands away from the fermi level (namely HOMO-bonding or LUMO-bonding/antibonding bands), whereas D is the energy distance between the fermi level and the bands away from the fermi level. In the present materials, W ′ /D estimated as 0.3-0.5 signals a substantial correction and thus the exchange process between the low-energy HOMO-antibonding and other bands away from the fermi level may play a key role to the low-energy ground state. This supports that the minimal models to describe the low-energy phenomena of the organic compounds are the multiband models and may not be reduced to the single-band model.

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Transcriptional activation of p62/A170/ZIP during the formation of the aggregates: possible mechanisms and the role in Lewy body formation in Parkinson's disease

et al. 2004

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First-principles calculation of effective onsite Coulomb interactions of3dtransition metals: Constrained local density functional approach with maximally localized Wannier functions

et al. 2006

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We present a new ab initio method for calculating effective onsite Coulomb interactions of itinerant and strongly correlated electron systems. The method is based on constrained local density functional theory formulated in terms of maximally localized Wannier functions. This scheme can be implemented with any basis, and thus allows us to perform the constrained calculation with plane-wave-based electronic-structure codes. We apply the developed method to the evaluation of the onsite interaction of 3d transition-metal series. The results are discussed using a heuristic formula for screened Coulomb interactions.Properties of itinerant and strongly correlated electron systems are widely discussed with phenomenological Hamiltonians such as Anderson-impurity [1] and Hubbard models [2]. These models embody essential aspects of correlated electrons, and can be solved numerically or sometimes analytically for special cases. However, parameters in the models are often determined empirically so that the employed model should reproduce experimental results of interest.Recently, there has been growing interest in constructing model Hamiltonians from first principles [3]. The principal motivation for such a study is a so-called long-standing "beyond LDA" problem. One of recent major approaches toward correlated electrons from first principles is combining densityfunctional theory within local density approximation (LDA) with dynamical mean-field theory (DMFT) [4]. The main idea of this method is to map a first-principles Hamiltonian onto a lattice-fermion model to which the DMFT method is applicable, so developments of tractable but rational mapping techniques are highly needed.An important notice in constructing ab initio model Hamiltonians lies in an evaluation of an onsite interaction parameter known as the Hubbard U . Transfer parameters can be easily obtained by representing a one-body ab initio Hamiltonian with a spatially localized (or atomic) orbital, while the U parameter evaluated with a one-center Coulomb integral of the atomic orbital gives a rather large value relative to that deduced from experiments [5,6]. This is because a screening effect of surrounding valence electrons is completely neglected in the evaluation of the value. A representative treatment for calculating U including the screening effect is a constrained approach [7,8]. It gives an optimally screened U , because, in that calculation, the U value is obtained from a response of the system to a change in a local charge density, thus incorporating the valence-electron-screening effect statically into the U calculation.An important point in performing the constrained calculation is a choice of basis functions to define the charge density of localized electrons. The use of an atom-centered localized basis set like a linear muffin-tin orbital (LMTO) [9] facilitates this definition, so constrained calculations to date have been performed with LMTO-based electronic-structure codes. The method has been widely applied to investigations of effective Hubbar...

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Data fitting with a spline using a real-coded genetic algorithm

Yoshimoto

Harada

Yoshimoto

2003

Computer-Aided Design

147

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Hypertrophy of IMC of carotid artery in Parkinson's disease is associated with l-DOPA, homocysteine, and MTHFR genotype

Nakaso

Yasui

Kowa

et al. 2003

Journal of the Neurological Sciences

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Reversible local-modification of surface structure on clean Ge(001) by scanning tunneling microscopy below 80 K

et al. 2004

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