Browsing by Author "Chiang, Jen-Che"

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Crucial Role of Site Disorder and Frustration in Unusual Magnetic Properties of Quasi-2D Triangular Lattice Antimonate Na4FeSbO6
(2015-10-01)
Zvereva, Elena A.
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Presniakov, Igor A.
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Whangbo, Myung-Hwan
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Koo, Hyun-Joo
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Frantsuzenko, Tatyana V.
The effect of antisite disorder in the layered sodium iron antimonate Na4FeSbO6 was examined both experimentally and theoretically. The magnetic susceptibility and specific heat measurements show that Na4FeSbO6 does not undergo a long-range antiferromagnetic order, unlike its structural analog Li4FeSbO6. The electron spin resonance yields the complicated picture of coexistence of two magnetic subsystems corresponding to two different Fe cation positions in the lattice (regular and antisite) and driving the magnetic properties of the Na4FeSbO6. This conclusion found perfect confirmation from both the Mossbauer and X-ray absorption data which show the presence of two kinds of Fe3+ ions being in high-spin Fe3+ (S = 5/2) and low-spin Fe3+ (S = 1/2) states. These findings arise from the antisite disorder between the Fe3+ and Sb5+ ions in the (NaFeSbO6)(3-) layers of Na4FeSbO6. Our density functional calculations show that the Fe3+ ions located at the Sb5+ sites exist as low-spin Fe3+ ions, and that the spins of each Fe3+ (S = 5/2)-Fe3+ (S = 1/2)-Fe3+ (S = 5/2) trimer generated by the antisite disorder has a ferrimagnetic arrangement Fe(3+)a dagger aEuro"Fe(3+)a dagger"aEuro"Fe(3+)a dagger, which enhances the magnetization of Na4FeSbO6 and leads to an apparently positive Curie-Weiss temperature.
Synthesis and Characterization of Ba[CoSO]: Magnetic Complexity in the Presence of Chalcogen Ordering
(2015-07-20)
Valldor, Martin
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Roessler, Ulrich K.
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Prots, Yurii
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Kuo, Chang-Yang
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Chiang, Jen-Che
Barium thio-oxocobaltate(II), Ba[CoS2/2O2/2], was synthesized by the reaction of equimolar amounts of BaO, Co, and S in closed silica ampoules. The title compound (Cmcm, a=3.98808(3), b=12.75518(9), c=6.10697(4)angstrom) is isostructural to Ba[ZnSO]. The use of soft X-ray absorption spectroscopy confirmed that cobalt is in the oxidation state +2 and tetrahedrally coordinated. Its coordination consists of two sulfur and two oxygen atoms in an ordered fashion. High-temperature magnetic susceptibility data indicate strong low-dimensional spin-spin interactions, which are suggested to be closely related to the layer-type crystal structure and perhaps the ordered distribution of sulfur and oxygen. Antiferromagnetic ordering below T-N=222K is observed as an anomaly in the specific heat, coinciding with a significant lowering of the magnetic susceptibility. Density functional theory calculations within a generalized-gradient approximation (GGA)+U approach identify an antiferromagnetic ground state within the square-like two-dimensional layers of Co, and antiferromagnetic correlations for nearest and next nearest neighbors along bonds mediated by oxygen or sulfur. However, this magnetic state is subject to frustration by relatively strong interlayer couplings.
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
;
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.