Stabilization of Long-Range Order by Additional Anisotropic Spins in Two-Dimensional Isotropic Heisenberg Antiferromagnets —A Possible Model of an Organic Compound with Magnetic Anions—

Seki, Hiroshi; Ito, Kazuhiro

doi:10.7566/jpsj.83.114702

Cited by 16 publications

(6 citation statements)

References 33 publications

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“…The experimental 22,[31][32][33] and theoretical 25) studies for the λ-Fe x Ga 1−x system indicate that the influence of the d-spins is indispensable for the transition to the antiferromagnetic longrange order. This may appear inconsistent with the fact that the d-spins are passive; however, it can be explained on the basis of a stabilization effect 34,35) by the anisotropy in the spin space and/or the enhanced three dimensionality introduced by the d-spins. These factors are not explicitly incorporated in the present model for the pure π-electron system; however, the stabilization of the long-range order by these factors is implicitly assumed in the present mean-field approximation.…”

Section: Introductionmentioning

confidence: 90%

Magnetic structures of electron systems on the extended spatially completely anisotropic triangular lattice near quantum critical points

Kono,

Shimahara

2021

Preprint

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We examine magnetic structures of electron systems on an extended triangular lattice that consists of two types of bond triangles with electron transfer energies t ℓ and t ′ ℓ (ℓ = 1, 2, and 3), respectively. We examine the ground state in the mean-field theory when t 1 = t ′ 1 , focusing on collinear states with two sublattices. It is shown that when the imbalance of the spatial anisotropies of the two triangles is large, up-up-down-down (uudd) phases are stable, and the most likely ground states of the λ-(BETS) 2 FeCl 4 system are the Néel state with the modulation vector (π/c, π/a) and a uudd state, where c = a 1 = a ′ 1 and a = (a 2 + a ′ 2 )/2, with a 1 , a ′ 1 , a 2 , and a ′ 2 being the lattice constants of the bonds with t 1 , t ′ 1 , t 2 , and t ′ 2 , respectively. These results are consistent with those from the classical spin system. In addition, this study reveals behaviors near the quantum critical point, which cannot be reproduced in the localized spin model. As the imbalance of the spatial anisotropies increases, the U c of the Néel state increases, and that of the uudd state decreases. In the phase diagrams containing areas of the paramagnetic state, the Néel state with (π/c, π/a), and a uudd state, their boundaries terminate at a triple point, near which all the transitions are of the first order. The phase boundary between the antiferromagnetic phases does not depend on U, and the transition is of the first order everywhere on the boundary. By contrast, the transitions from the two antiferromagnetic phases to the paramagnetic phase are of the second order, unless the system is close to the triple point.

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Section: Introductionmentioning

confidence: 90%

Magnetic structures of electron systems on the extended spatially completely anisotropic triangular lattice near quantum critical points

Kono,

Shimahara

2021

Preprint

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“…which results in β 2 = β I . Because it is reasonable to assume that d = 2 in the present system, [1][2][3][4][5][6][7][8][9][10][11][12][13] we can use the result β I = 1/8 obtained in a previous theory; 31) hence, we obtain…”

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confidence: 99%

“…The phase transition and critical phenomena in coupled magnetic systems have been examined experimentally [1][2][3][4][5] and theoretically [6][7][8][9] in connection with the organic πd antiferromagnet λ-(BETS) 2 FeCl 4 , [10][11][12][13][14][15][16][17] where BETS stands for bis(ethylenedithio)tetraselenafulvalene. In this compound, the long-range order is principally sustained in a π-electron system 2) despite the large length of d spins.…”

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confidence: 99%

“…Despite its large value, J 1 does not induce the transition by itself, because the π-electron system is two-dimensional and isotropic in the spin space. 2,6,18,19) Instead, an effective interaction between π-electrons via d-spins [6][7][8][9] introduces anisotropy in the spin space and induces the transition at a temperature T c . Consequently, J 1 ≫ T c ≫ J 2 is satisfied, where we use units in which k B = 1.…”

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confidence: 99%

“…Application to the πd antiferromagnet λ-(BETS) 2 FeCl 4 -For this compound below T c , because the system is insulating, simplification by localized spin models would be useful. According to previous studies, [1][2][3][4][5][6][7][8][9][10][11][12][13] we can assume that the π spin system is isotropic in the spin space, and the π-d interaction is anisotropic. Assuming that subsystems 1 and 2 correspond to the πand d-spin systems, respectively, we consider the model of coupled antiferromagnets 8,13) expressed by the…”

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Scaling Theory for Critical Phenomena in Coupled Magnetic Systems —Critical Exponents of πd Antiferromagnets—

Seki

2020

J. Phys. Soc. Jpn.

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The scaling theory for critical phenomena is extended to coupled magnetic systems that consist of two subsystems, and some extended relations of critical exponents are derived. It is shown that the extended theory practically reduces to the conventional scaling theory in ferromagnets and in non-ferromagnetic systems with β 1 = β 2 ; however, the extended form of the theory can be relevant otherwise, where β 1 and β 2 are exponents of the order parameters in subsystems 1 and 2, respectively. The theory is applied to a model of the organic πd antiferromagnet λ-(BETS) 2 FeCl 4 , which contains πand d-spin subsystems, where BETS stands for bis(ethylenedithio)tetraselenafulvalene. It is shown that an effective Hamiltonian for the π spins is reduced to the two-dimensional Ising model in the vicinity of the critical temperature T c . This supports a conjecture from a recent experimental observation. Consequently, β 1 = 1/8 is obtained, where subsystems 1 and 2 correspond to the πand d-spin systems, respectively. Additional relations α = 1 − β 1 − β 2 and β 1 = β 2 ≡ β are derived from specific features of the λ-(BETS) 2 FeCl 4 system. These relations result in α = 1 − 2β, which was previously obtained in a free-energy functional model. Critical exponents below T c are obtained as α = 3/4, β = 1/8, γ = 1, δ = 9, ψ = 1/5, and ν = 5/8. The value of α is close to a recent experimental result of α = 0.77 in λ-(BETS) 2 FeCl 4 .

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Selective observation of sublattice magnetization in the molecular π−d system λ−(BEDT−STF)2FeCl<

et al. 2020

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Stabilization of Long-Range Order by Additional Anisotropic Spins in Two-Dimensional Isotropic Heisenberg Antiferromagnets —A Possible Model of an Organic Compound with Magnetic Anions—

Cited by 16 publications

References 33 publications

Magnetic structures of electron systems on the extended spatially completely anisotropic triangular lattice near quantum critical points

Magnetic structures of electron systems on the extended spatially completely anisotropic triangular lattice near quantum critical points

Scaling Theory for Critical Phenomena in Coupled Magnetic Systems —Critical Exponents of πd Antiferromagnets—

Selective observation of sublattice magnetization in the molecular π−d system λ−(BEDT−STF)2FeCl<

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