Nonlocal Coulomb interaction and spin-freezing crossover as a route to valence-skipping charge order

Ryee, Siheon; Sémon, P.; Han, Myung Joon; Choi, Sangkook

doi:10.1038/s41535-020-0221-9

Cited by 16 publications

(9 citation statements)

References 61 publications

(95 reference statements)

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“…For example, in the multi-orbital Fe-based superconductors [8,9], the on-site Hunds coupling (J) promotes bad metallic behavior in their normal phase [10][11][12][13]. This gives rise to the new concept of the Hund's metal [14,15], which plays the role of a reliable reference system for Fe-based superconducting materials [12,[14][15][16][17][18][19][20] and ruthenates [12,[21][22][23][24].…”

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confidence: 99%

Infinite-layer nickelates as Ni-eg Hund's metals

Kang¹,

Melnick²,

Sémon³

et al. 2020

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The recent and exciting discovery of superconductivity in the hole-doped infinite-layer nickelate Nd 1−δ Sr δ NiO2 draws strong attention to correlated quantum materials. From a theoretical view point, this new class of unconvential superconducting materials provide an opportunity to unveil new physics in correlated quantum materials. Here we study the temperature and doping dependence of the local spectrum of the charge, spin and orbital susceptibilities from first principles. By using ab initio LQSGW+DMFT methodology, we show that onsite Hund's coupling in Ni-d orbitals gives rise to multiple signatures of Hund's metallic phase in Ni-eg orbitals. The proposed picture of the nickelates as an eg (two orbit) Hund's metal differs from the picture of the Fe-based superconductors as a five orbital Hund's metal as well as the picture of the cuprates as doped charge transfer insulators. Our finding unveils a new class of the Hunds metals and has potential implications for the broad range of correlated two orbital systems away from half-filling.

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confidence: 99%

Infinite-layer nickelates as Ni-eg Hund's metals

Kang¹,

Melnick²,

Sémon³

et al. 2020

Preprint

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“…Furthermore, a strong Hund's coupling can even lead to the formation of an inhomogeneous, charge-ordered or charge-disproportionated, insulating phase, distinct from the usual (homogeneous) Mott-insulator. The existence of such a charge disproportionated insulator (CDI) has been demonstrated both for a two-orbital and a threeorbital Hubbard model [6][7][8], which are applicable to materials with partially filled e g and t 2g sub-shells. Similar physics have also been discussed in the context of valenceskipping metals and insulators resulting from a negative effective Coulomb interaction [9].…”

Section: Introductionmentioning

confidence: 99%

“…The formation of a charge-disproportionated insulating phase has been observed, e.g., in the series of rare earth nickelates, RNiO 3 [6,[10][11][12][13][14][15]. Here, the Ni cations are in a formal d 7 electron configuration, which then disproportionates according to 2d 7 → d 6 + d 8 (or, equivalently, 2d 8 L → d 8 L 2 + d 8 , where L denotes a ligand hole in the surrounding oxygen network [16,17]). Since in this case the t 2g subshell is always completely filled, with on average one electron per site in the e g states, the underlying physics can be well described within a minimal two-orbital Hubbard model.…”

Section: Introductionmentioning

confidence: 99%

“…Since in this case the t 2g subshell is always completely filled, with on average one electron per site in the e g states, the underlying physics can be well described within a minimal two-orbital Hubbard model. Furthermore, the charge disproportionation lowers the symmetry of the crystal structure and couples strongly with a lattice distortion corresponding to a breathing mode of the octahedral network with R + 1 symmetry, resulting in a three-dimensional checkerboard-like pattern of alternating small and large oxygen octahedra surrounding the nominal d 6 and d 8 Ni sites, respectively. Similar behavior has also been observed for the alkaline earth ferrate CaFeO 3 [18][19][20][21][22] and in PbCrO 3 [23,24].…”

Section: Introductionmentioning

confidence: 99%

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Charge disproportionation and Hund's insulating behavior in a five-orbital Hubbard model applicable to $d^4$ perovskites

Merkel,

Ederer

2021

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We explore the transition to a charge-disproportionated insulating phase in a five-orbital cubic tight-binding model applicable to transition-metal perovskites with a formal d 4 occupation of the transition-metal cation, such as ferrates or manganites. We use dynamical mean-field theory to obtain the phase diagram as a function of the average local Coulomb repulsion U and the Hund's coupling J. The main structure of the phase diagram follows from the zero band-width (atomic) limit and represents the competition between high-spin and low-spin homogeneous and an inhomogeneous charge-disproportionated state. This results in two distinct insulating phases: the standard homogeneous Mott insulator and the inhomogeneous charge-disproportionated insulator. We characterize the unconventional nature of this Hund's insulating state. Our results are consistent with previous studies of two-and three-orbital models applicable to isolated t2g and eg subshells, respectively, with the added complexity of the low-spin/high-spin transition. We also test the applicability of an effective two-orbital (eg-only) model with disordered S = 3/2 t2g core spins. Our results show that the overall features of the phase diagram in the high-spin region are well described by this simplified two-orbital model but also that the spectral features exhibit pronounced differences compared to the full five-orbital description.

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“…Many attempts to account for spatial fluctuations diagrammatically beyond DMFT were made [2], such as the GW+DMFT approach [3][4][5][6][7][8][9], the dynamical vertex approximation (DΓA) [10,11], the dual fermion (DF) [12][13][14][15] and dual boson (DB) [16][17][18][19][20] theories including their diagrammatic Monte Carlo realizations [21][22][23], as well as the recently introduced triply irreducible local expansion (TRILEX) [24][25][26], and the dual TRILEX (D-TRILEX) [27] method. However, only few of these approaches, namely the GW+DMFT [4, [28][29][30][31][32][33], the quasi-particle self-consistent GW (QSGW) merged with DMFT [34][35][36][37], and the ab initio DΓA [38][39][40], are currently available in a multi-orbital realization. Among them, only the ab initio DΓA allows to treat different collective electronic fluctuations on equal footing.…”

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confidence: 99%

Orbital Isotropy of Magnetic Fluctuations in Correlated Electron Materials Induced by Hund’s Exchange Coupling

et al. 2021

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Characterizing non-local magnetic fluctuations in materials with strong electronic Coulomb interactions remains one of the major outstanding challenges of modern condensed matter theory. Here, we address the spatial symmetry and orbital structure of magnetic fluctuations in perovskite materials, within an anisotropic threeorbital model of cubic t 2g symmetry. To this effect, we develop a consistent multi-orbital diagrammatic extension of dynamical mean field theory, which allows for a proper description of many-body effects. We find that the form of spatial spin fluctuations is governed by the local Hund's coupling. For small values of the coupling, magnetic fluctuations are anisotropic in orbital space, which reflects the symmetry of the considered t 2g model. Large Hund's coupling enhances collective spin excitations, which mixes orbital and spatial degrees of freedom, and magnetic fluctuations become orbitally isotropic. Remarkably, this effect can be seen only in two-particle quantities; single-particle observables remain anisotropic for any value of the Hund's coupling. Importantly, we find that the orbital isotropy can be induced in both half-filled and doped cases, where the magnetic instability is associated with different, antiferromagnetic and ferromagnetic modes, respectively.

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Nonlocal Coulomb interaction and spin-freezing crossover as a route to valence-skipping charge order

Cited by 16 publications

References 61 publications

Infinite-layer nickelates as Ni-eg Hund's metals

Infinite-layer nickelates as Ni-eg Hund's metals

Charge disproportionation and Hund's insulating behavior in a five-orbital Hubbard model applicable to $d^4$ perovskites

Orbital Isotropy of Magnetic Fluctuations in Correlated Electron Materials Induced by Hund’s Exchange Coupling

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