Browsing by Author "Vasiliev, Alexander N."

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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.
Enhanced critical current density in the pressure-induced magnetic state of the high-temperature superconductor FeSe
(2015-11-09)
Jung, Soon-Gil
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Kang, Ji-Hoon
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Park, Eunsung
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Lee, Sangyun
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Lin, Jiunn-Yuan
We investigate the relation of the critical current density (J(c)) and the remarkably increased superconducting transition temperature (T-c) for the FeSe single crystals under pressures up to 2.43 GPa, where the T-c is increased by similar to 8 K/GPa. The critical current density corresponding to the free flux flow is monotonically enhanced by pressure which is due to the increase in T-c, whereas the depinning critical current density at which the vortex starts to move is more influenced by the pressure-induced magnetic state compared to the increase of T-c. Unlike other high-T-c superconductors, FeSe is not magnetic, but superconducting at ambient pressure. Above a critical pressure where magnetic state is induced and coexists with superconductivity, the depinning Jc abruptly increases even though the increase of the zero-resistivity T-c is negligible, directly indicating that the flux pinning property compared to the T-c enhancement is a more crucial factor for an achievement of a large J(c). In addition, the sharp increase in J(c) in the coexisting superconducting phase of FeSe demonstrates that vortices can be effectively trapped by the competing antiferromagnetic order, even though its antagonistic nature against superconductivity is well documented. These results provide new guidance toward technological applications of high-temperature superconductors.
Pressure dependence of upper critical fields in FeSe single crystals
(2016-03)
Kang, Ji-Hoon
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Jung, Soon-Gil
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Lee, Sangyun
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Park, Eunsung
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Lin, Jiunn-Yuan
We investigate the pressure dependence of the upper critical fields (mu H-0(c2)) for FeSe single crystals with pressure up to 2.57 GPa. The superconducting (SC) properties show a disparate behavior across a critical pressure where the pressure-induced antiferromagnetic phase coexists with superconductivity. The magnetoresistance for H//ab and H//c is very different: for H//c, magnetic field induces and enhances a hump in the resistivity close to the T-c for pressures higher than 1.2 GPa, while it is absent for H//ab. Since the measured mu H-0(c2) for FeSe samples is smaller than the orbital limited upper critical field (H-c2(orb)) estimated by the Werthamer, Helfand and Hohenberg model, the Maki parameter (alpha) related to Pauli spin-paramagnetic effects is additionally considered to describe the temperature dependence of mu H-0(c2)(T). Interestingly, the alpha value is hardly affected by pressure for H//ab, while it strongly increases with pressure for H//c. The pressure evolution of the mu H-0(c2)(0) for the FeSe single crystals is found to be almost similar to that of T-c(P), suggesting that the pressure-induced magnetic order adversely affects the upper critical fields as well as the SC transition temperature.
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.
The First Conducting Spin-Crossover Compound Combining a Mn-III Cation Complex with Electroactive TCNQ Demonstrating an Abrupt Spin Transition with a Hysteresis of 50 K
(1970-01-01)
Kazakova, Anna, V
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Tiunova, Aleksandra, V
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Korchagin, Denis, V
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Shilov, Gennady, V
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Yagubskii, Eduard B.
We present herein the synthesis, crystal structure, and electric and magnetic properties of the spin-crossover salt [Mn(5-Cl-sal-N-1,5,8,12)]TCNQ(1.5).2 CH3CN (I), where 5-Cl-sal-N-1,5,8,12=N,N '-bis(3-(2-oxy-5-chlorobenzylideneamino)propyl)-ethylenediamine, containing distinct conductive and magnetic blocks along with acetonitrile solvent molecules. The Mn-III complex with a Schiff-base ligand, [Mn(5-Cl-sal-N-1,5,8,12)](+), acts as the magnetic unit, and the pi-electron acceptor 7,7,8,8-tetracyanoquinodimethane (TCNQ(-)) is the conducting unit. The title compound (I) exhibits semiconducting behavior with room temperature conductivity sigma(RT)approximate to 1x10(-4) ohm(-1) cm(-1) and activation energy Delta approximate to 0.20 eV. In the temperature range 73-123 K, it experiences a hysteretic phase transition accompanied by a crossover between the low-spin S=1 and high-spin S=2 states of Mn-III and changes in bond lengths within the MnN4O2 octahedra. The pronounced shrinkage of the basal Mn-N bonds in I at the spin crossover suggests that the dx2-y2 orbital is occupied/deoccupied in this transition. Interestingly, the bromo isomorphic counterpart [Mn(5-Br-sal-N-1,5,8,12)]TCNQ(1.5).2 CH3CN (II) of the title compound evidences no spin-crossover phenomena and remains in the high-spin state in the temperature range 2-300 K. Comparison of the chloro and bromo compounds allows the thermal and spin-crossover contributions to the overall variation in bond lengths to be distinguished. The difference in magnetic behavior of these two salts has been ascribed to intermolecular supramolecular effects on the spin transition. Discrete hydrogen bonding exists between cations and cations and anions in both compounds. However, the hydrogen bonding in the crystals of II is much stronger than in I. The relatively close packing arrangement of the [Mn(5-Br-sal-N-1,5,8,12)](+) cations probably precludes their spin transformation.
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.