Orbital physics in transition metal compounds: new trends

Streltsov, S. V.; Khomskiî, D. I.

doi:10.3367/ufne.2017.08.038196

Cited by 166 publications

(136 citation statements)

References 170 publications

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“…As a result, by changing Me 1 and Me 2 one may vary the valency of Mo ions. Furthermore, the Mo ions form isolated trimers (the third important aspect), which makes these materials interesting testbed also for studying of the cluster-Mott physics [6,7]. Fe 2 Mo 3 O 8 (the kamiokite [8]) is one of such materials, whose properties were under intensive investigation during last years.…”

Section: Introductionmentioning

confidence: 99%

Microscopic toy model for magnetoelectric effect in polar Fe2Mo3O8

Solovyev

Streltsov

2019

Phys. Rev. Materials

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The kamiokite, Fe2Mo3O8, is regarded as a promising material exhibiting giant magnetoelectric (ME) effect at the relatively high temperature T . Here, we explore this phenomenon on the basis of first-principles electronic structure calculations. For this purpose we construct a realistic model describing the behavior of magnetic Fe 3d electrons and further map it onto the isotropic spin model. Our analysis suggests two possible scenaria for Fe2Mo3O8. The first one is based on the homogeneous charge distribution of the Fe 2+ ions amongst tetrahedral (t) and octahedral (o) sites, which tends to low the crystallographic P63mc symmetry through the formation of an orbitally ordered state. Nevertheless, the effect of the orbital ordering on interatomic exchange interactions does not seem to be strong, so that the magnetic properties can be described reasonably well by averaged interactions obeying the P63mc symmetry. The second scenario, which is supported by obtained parameters of on-site Coulomb repulsion and respects the P63mc symmetry, implies the charge disproportionation involving somewhat exotic 1+ ionization state of the t-Fe sites (and 3+ state of the o-Fe sites). Somewhat surprisingly, these scenarios are practically indistinguishable from the viewpoint of exchange interactions, which are practically identical in these two cases. However, the spin-dependent properties of the electric polarization are expected to be different due to the strong difference in the polarity of the Fe 2+ -Fe 2+ and Fe 1+ -Fe 3+ bonds. Our analysis uncovers the basic aspects of the ME effect in Fe2Mo3O8. Nevertheless, the quantitative description should involve other ingredients, apparently related to the lattice and orbitals degrees of freedom. I.J t J ô J 1 J ij (meV) J || J 2 3 4 5 6 7 8 9 10 11 -6 -4 -2 0 J t J ô J || J

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Section: Introductionmentioning

confidence: 99%

Microscopic toy model for magnetoelectric effect in polar Fe2Mo3O8

Solovyev

Streltsov

2019

Phys. Rev. Materials

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“…The 5d Ir ions have a strong spin-orbit coupling and form face-sharing Ir 4+ trimers, which renders the usual local J eff =1/2 moment picture [28,29] inapplicable due to enhanced covalency. Instead, molecular orbitals at each Ir trimer are expected to form [24,27,30]. Finally, the material contains intrinsic disorder due to site mixing between Cu and Ir, as well as Cu displacement from the prism center [25,26] [see Fig.…”

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confidence: 99%

Random singlet state in Ba5CuIr3O12 single crystals

et al. 2020

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We study the thermodynamic and high-magnetic-field properties of the magnetic insulator Ba5CuIr3O12, which shows no magnetic order down to 2 K consistent with a spin liquid ground state. While the temperature dependence of the magnetic susceptibility and the specific heat shows only weak antiferromagnetic correlations, we find that the magnetization does not saturate up to a field of 59 Tesla, leading to an apparent contradiction. We demonstrate that the paradox can be resolved, and all of the experimental data can be consistently described within the framework of random singlet states. We demonstrate a generic procedure to derive the exchange coupling distribution P (J) from the magnetization measurements and use it to show that the experimental data is consistent with the power-law form P (J) ∼ J −α with α ≈ 0.6. Thus, we reveal that high-magnetic-field measurements can be essential to discern quantum spin liquid candidates from disorder dominated states that do not exhibit long-range order.Strong quantum fluctuations in insulating magnetic compounds can give rise to quantum spin liquid (QSL) ground states, where the interaction-driven ordering tendencies are thwarted completely. Devoid of long-range order, QSLs lie beyond the Landau symmetry-based classification, and are characterized instead by their unconventional entanglement properties and the presence of exotic fractionalized excitations [1,2]. However, identifying the elusive QSL behavior in real materials has proven to be a formidable task [2][3][4]. The search for QSL candidate materials represents a major challenge of modern condensed matter physics.Disorder is one of the major hindrances to identify QSL materials [5][6][7], as it can drive the formation of random singlet states (RSS) [8] or disordered stripe states [9] instead of a QSL. Importantly, this includes single-crystal samples due to intrinsic disorder [10,11]. A convenient reference point can be found in one-dimensional (1D) systems, where the quantum fluctuations are dominant [12] and the effect of disorder was clarified some time ago [13,14]. In 1D it converts the spin liquid ground state into a RSS, where the effective exchange coupling follows a broad probability distribution that has a universal form [15] at low energies. In 2D and 3D, on the contrary, the fate of disordered spin systems is still an open question. While a random singlet state with a power-law distribution has been conjectured [16], the true ground state of such systems is still under debate and might not be universal [17][18][19]. In particular, enhanced suppression of QSL states by disorder has been found in model calculations [19,20]. However, mechanisms for the stabilization of QSL states by disorder have also been proposed [21]. Additionally, a strong spin-orbit coupling (SOC) is an important ingredient in many QSL candidates. While its effects on clean QSLs have been studied [1, 2] and particularly emphasized for the so-called Kitaev materials [22,23], the interplay of SOC with disorder still remains to be underst...

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“…This disagreement is probably related to the dependence of the magnetic ordering on the type of orbital ordering in the oxide material with Jahn -Teller cations Mn 3+ . 49,50 With increasing pressure, the spin crossover in is accompanied by the transition of the cation Mn 3+ from the HS Jahn -Teller state 5 E to the state 3 T . Therefore, the orbital ordering with increasing pressure should disappear, and the FM nature of the superexchange will manifest itself (see Tab.…”

Section: Superexchange In Oxides With Cations In Other Electron Comentioning

confidence: 99%

Cation spin and superexchange interaction in oxide materials below and above spin crossover under high pressure

2020

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We derived simple rules for the sign of superexchange interaction based on the multielectron calculations of the superexchange interaction in the transition metal oxides that are valid both below and above spin crossover under high pressure. The superexchange interaction between two cations in d n configurations is given by a sum of individual contributions related to the electron-hole virtual excitations to the different states of the d n+1 and d n−1 configurations. Using these rules, we have analyzed the sign of the superexchange interaction of a number of oxides with magnetic cations in electron configurations from d 2 till d 8 : the iron, cobalt, chromium, nickel, copper and manganese oxides with increasing pressure. The most interesting result concerns the magnetic state of cobalt and nickel oxides CoO, Ni2O3 and also La2CoO4, LaNiO3 isostructural to well-known high-TC and colossal magnetoresistance materials. These oxides have a spin 1 2 at the high pressure. Change of the interaction from antiferromagnetic below spin crossover to ferromagnetic above spin crossover is predicted for oxide materials with cations in d 5 (FeBO3) and d 7 (CoO) configurations, while for materials with the other d n configurations spin crossover under high pressure does not change the sign of the superexchange interaction.

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Orbital physics in transition metal compounds: new trends

Cited by 166 publications

References 170 publications

Microscopic toy model for magnetoelectric effect in polar Fe2Mo3O8

Microscopic toy model for magnetoelectric effect in polar Fe2Mo3O8

Random singlet state in Ba5CuIr3O12 single crystals

Cation spin and superexchange interaction in oxide materials below and above spin crossover under high pressure

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