Magnetic Field Effects in a Correlated Electron System with Spin-State Degree of Freedom — Implications for an Excitonic Insulator —

Tatsuno, Taro; Mizoguchi, Eriko; Nasu, Joji; Naka, Masaaki; Ishihara, Sumio

doi:10.7566/jpsj.85.083706

Cited by 42 publications

(45 citation statements)

References 38 publications

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“…In Ref. [15], the finite-temperature phase diagram was obtained by applying the mean-field approximation to the effective pseudo-spin model derived from the two-orbital Hubbard model. It was shown that the HL phase is realized down to T = 0 in contrast to Fig.…”

Section: A Finite T Phase Diagrammentioning

confidence: 99%

Optical response in the excitonic insulating state: Variational cluster approach

Otsuki

Naka

et al. 2020

Phys. Rev. B

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Optical responses in an excitonic insulating (EI) system with strong electron correlation are studied. We adopt the two-orbital Hubbard model with a finite energy difference between the two orbitals where the spin state degree of freedom exists. This model is analyzed by the variational cluster approach. In order to include the local electron correlation effect, the vertex correction is taken into account in the formulation of the optical conductivity spectra. We calculate a finite-temperature phase diagram, in which an EI phase appears between a low-spin band insulating state and a high-spin Mott insulating state. Characteristic components of the optical conductivity spectra consisting of a sharp peak and continuum appear in the EI phase. Integrated intensity almost follows the order parameter of the EI state, suggesting that this component is available to identify the EI phases and transitions.

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Section: A Finite T Phase Diagrammentioning

confidence: 99%

Optical response in the excitonic insulating state: Variational cluster approach

Otsuki

Naka

et al. 2020

Phys. Rev. B

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“…LaCoO 3 under a high magnetic field is another example of the possible realization of the EP, 21) which was substantiated by the theoretical calculations based on the two-orbital Hubbard and related models in two-dimension. 22,23) In these materials, cobalt ions are basically in the Co 3+ valence state with a 3d…”

Section: 20)mentioning

confidence: 99%

Low-Energy Excitation Spectra in the Excitonic Phase of Cobalt Oxides

2017

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We study the excitonic phase and low-energy excitation spectra of perovskite cobalt oxides. Constructing the five-orbital Hubbard model defined on the three-dimensional cubic lattice for the 3d bands of Pr0.5Ca0.5CoO3, we calculate the excitonic susceptibility in the normal state in the random-phase approximation (RPA) to show the presence of the instability toward excitonic condensation. On the basis of the excitonic ground state with a magnetic multipole obtained in the mean-field approximation, we calculate the dynamical susceptibility of the excitonic phase in the RPA and find that there appear a gapless collective excitation in the spin-transverse mode (Goldstone mode) and a gapful collective excitation in the spin-longitudinal mode (Higgs mode). The experimental relevance of our results is discussed.The Bose-Einstein condensation of fermion pairs is one of the most intriguing phenomena in condensed matter physics. The excitonic phase (EP) is representative of such a pair condensation, [1][2][3][4] where holes in valence bands and electrons in conduction bands spontaneously form pairs owing to attractive Coulomb interaction. After Mott's prediction of the EP half a century ago, 5) a number of candidate materials for this phase have come to our attention. Among them are the transition-metal chalcogenides 1T -TiSe 2 6-8) and Ta 2 NiSe 5 , 9-11) where the electrons and holes on different atoms are considered to form spin-singlet pairs to condense into the EP, which is accompanied by lattice distortion. 12)Another class of materials includes the perovskite cobalt oxides, [13][14][15] where the valence-band holes and conduction-band electrons form spin-triplet pairs in different orbitals on the same atoms. Pr 0.5 Ca 0.5 CoO 3 (PCCO) is an example in which the "metal-insulator" phase transition is observed at T c ≃ 80 K, which is associated with a sharp peak in the temperature dependence of the specific heat and a drop in the magnetic susceptibility below T c , 16) together with a valence transition of Pr ions.17, 18) Some results of experiments indicate that the resistivity is in fact small and nearly temperature independent below T c , 19) suggesting that the bands may not be fully gapped. Note that no local magnetic moments are observed, but the exchange splitting of the Pr 4+ Kramers doublet occurs, 19) the result of which may therefore be termed as a hidden order, and also that no clear signatures of the spin-state transition are observed in the X-ray absorption spectra. 19, 20)Kuneš and Augustinsky argued that the anomalies of PCCO can be attributed to the EP transition, 13)whereby they applied the dynamical-mean-field-theory calculation to the two-orbital Hubbard model defined on * t.yamaguchi@chiba-u.jp a two-dimensional square lattice and claimed that the anomalous behaviors of the specific heat, dc conductivity, and spin susceptibility can be explained. They also performed the LDA+U band-structure calculation and showed that the magnetic multipole ordering occurs in PCCO as a result of the excitonic con...

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“…6(c) using magnetization measurements [13]. Later it is argued by several theoretical groups that the B1 and B2 phases correspond to the LS-HS solid phase and the excitonic condensation phase, respectively [44].…”

Section: Spin Crossover Oxide: Lacoomentioning

confidence: 99%

Magnetostriction studies up to megagauss fields using fiber Bragg grating technique

Ikeda

Matsuda

Nakamura

et al. 2018

2018 16th International Conference on Megagauss Magnetic Field Generation and Related Topics (MEGAGAUSS)

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We here report magnetostriction measurements under pulsed megagauss fields using a high-speed 100 MHz strain monitoring system devised using fiber Bragg grating (FBG) technique with optical filter method. The optical filter method is a detection scheme of the strain of FBG, where the changing Bragg wavelength of the FBG reflection is converted to the intensity of reflected light to enable the 100 MHz measurement. In order to show the usefulness and reliability of the method, we report the measurements for solid oxygen, spin-controlled crystal, and volborthite, a deformed Kagomé quantum spin lattice, using static magnetic fields up to 7 T and non-destructive millisecond pulse magnets up to 50 T. Then, we show the application of the method for the magnetostriction measurements of CaV4O9, a two-dimensional antiferromagnet with spin-halves, and LaCoO3, an anomalous spin-crossover oxide, in the megagauss fields.

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Magnetic Field Effects in a Correlated Electron System with Spin-State Degree of Freedom — Implications for an Excitonic Insulator —

Cited by 42 publications

References 38 publications

Optical response in the excitonic insulating state: Variational cluster approach

Optical response in the excitonic insulating state: Variational cluster approach

Low-Energy Excitation Spectra in the Excitonic Phase of Cobalt Oxides

Magnetostriction studies up to megagauss fields using fiber Bragg grating technique

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