Browsing by Author "Volkova, Olga S."

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Spin-State Transition, Magnetism and Local Crystal Structure in Eu1-xCaxCoO3-delta
(2013-04-01)
Vasiliev, Alexander N.
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Vasilchikova, Tatyana M.
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Volkova, Olga S.
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Kamenev, Anton A.
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Kaul, Andrey R.
The doping series Eu1-xCaxCoO3-delta provides a rather peculiar way to study the spin-state transitions in cobalt-based complex oxides since partial substitution of Eu3+ ions by Ca2+ ions does not increase the mean valence of cobalt but is accompanied by appearance of oxygen vacancies in the ratio delta similar to x/2. In the parent compound EuCoO3, the low spin (LS)-intermediate spin (IS) or high spin (HS) transition takes place at temperatures so high that the chemical decomposition prevents its direct observation. The substitution of Eu3+ by Ca2+ shifts the spin state transition to lower temperatures and the associated energy gap Delta in octahedrally-coordinated Co3+ ions changes from 1940K in EuCoO3 to 1540K in Eu0.9Ca0.1CoO2.95 and 1050K in Eu0.8Ca0.2CoO2.9. Besides, each O2- vacancy reduces the local coordination of two neighboring Co3+ ions from octahedral to pyramidal thereby locally creating magnetically active sites which couple to dimers. These dimers form another gapped magnetic system with a very different energy scale, D-dim similar to 3 K, on the background of the intrinsically non-magnetic lattice of octahedrally-coordinated low-spin Co3+ ions.
Unveiling the hidden nematicity and spin subsystem in FeSe
(2017-06-28)
Luo, Chih-Wei
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Cheng, Po Chung
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Wang, Shun-Hung
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Chiang, Jen-Che
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Lin, Jiunn-Yuan
The nematic order (nematicity) is considered as one of the essential ingredients to understand the mechanism of Fe-based superconductivity. In most Fe-based superconductors (pnictides), nematic order is reasonably close to the antiferromagnetic order. In FeSe, in contrast, a nematic order emerges below the structure phase transition at T-s = 90 K with no magnetic order. The case of FeSe is of paramount importance to a universal picture of Fe-based superconductors. The polarized ultrafast spectroscopy provides a tool to probe simultaneously the electronic structure and the magnetic interactions through quasiparticle dynamics. Here we show that this approach reveals both the electronic and magnetic nematicity below and, surprisingly, its fluctuations far above Ts to at least 200 K. The quantitative pump-probe data clearly identify a correlation between the topology of the Fermi surface and the magnetism in all temperature regimes, thus providing profound insight into the driving factors of nematicity in FeSe and the origin of its uniqueness.
Wide-Range Tuning of the Mo Oxidation State in La1-xSrxFe2/3Mo1/3O3 Perovskites
(2016-06)
Istomin, Sergey Ya.
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Chernova, Varvara V.
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Antipov, Evgeny V.
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Lobanov, Maxim V.
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Bobrikov, Ivan A.
La1-xSrxFe2/3Mo1/3O3 (0x1) perovskites allow exceptionally wide tuning of the Mo oxidation state from +3 (x = 0) to +6 (x = 1) with the Fe-III oxidation state virtually unchanged. The end members of this series show antiferromagnetic ordering in LaFe2/3Mo1/3O3 at T-N = 520 K and ferrimagnetic ordering in SrFe2/3Mo1/3O3 at T-C = 420 K. The crucial role of electron correlations in the Mo 4d shell suggests that LaFe2/3Mo1/3O3 with a half-filled band is a Mott insulator, whereas SrFe2/3Mo1/3O3 is a band insulator. In both cases, the magnetic structure is dictated by antiferromagnetic superexchange between localized magnetic moments. At intermediate compositions, an interplay of antiferromagnetic-superexchange and double-exchange interactions results in nonmonotonous variations of both the magnetic-ordering temperature and saturation magnetization.