Browsing by Author "Choi, KY"

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    Properties of spin-1/2 triangular-lattice antiferromagnets CuY2Ge2O8 and CuLa2Ge2O8
    (American Physical Society, 2017-04-05) Cho, H; Kratochvílová, M; Sim, H; Choi, KY; Kim, CH; Paulsen, C; Avdeev, M; Peets, DC; Jo, Y; Lee, S; Noda, Y; Lawler, MJ; Park, JG
    We found new two-dimensional (2D) quantum (S=1/2) antiferromagnetic systems: CuRE2Ge2O8 (RE=Y and La). According to our analysis of high-resolution x-ray and neutron diffraction experiments, the Cu network of CuRE2Ge2O8 (RE=Y and La) exhibits a 2D triangular lattice linked via weak bonds along the perpendicular b axis. Our bulk characterizations from 0.08 to 400 K show that they undergo a long-range order at 0.51(1) and 1.09(4) K for the Y and La systems, respectively. Interestingly, they also exhibit field induced phase transitions. For theoretical understanding, we carried out the density functional theory (DFT) band calculations to find that they are typical charge-transfer-type insulators with a gap of Eg≅2eV. Taken together, our observations make CuRE2Ge2O8 (RE=Y and La) additional examples of low-dimensional quantum spin triangular antiferromagnets with the low-temperature magnetic ordering. ©2017 American Physical Society
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    Simulation of light C4+ ion irradiation and its enhancement to the critical current density in BaFe1.9Ni0.1As2 single crystals
    (American Scientific Publishers, 2014-07-01) Shahbazi, M; Wang, XL; Ionescu, M; Ghorbani, SR; Dou, SX; Choi, KY
    In this work, we analyse the influence of C4+ irradiation with ion flounce of 3 × 1012 up to 2.3 × 1015 ion·cm−2 on significant enhancement of the critical current density, Jc , in BaFe1.9Ni0.1As2 single crystals. Jc was increased from 0.61 × 105 up to 0.94 × 105 A/cm2 at T = 10 K and H = 0.5 T. BaFe1.9Ni0.1As2 single crystals with and without the C4+-irradiation were characterized by magneto-transport and magnetic measurements up to 13 T over a wide range of temperatures below and above the superconducting critical temperature, Tc . It is found that the C4+-irradiation causes little change in Tc , but it can greatly enhance the in-field critical current density by a factor of up to 1.5. Higher dose of C4+ ions, causes further Jc enhancement at T=10 K. furthermore, flux jumping completely disappeared at T=2 K after second C4+-irradiation. Our Monte Carlo simulation results show that all the C4+ ions end up in a well defined layer, causing extended defects and vacancies at the layer, but few defects elsewhere on the irradiation paths. Furthermore, the normal state resistivity is enhanced by the light C4+ irradiation, while the upper critical field, H c2, the irreversibility field, H irr, and Tc were affected very little. © 2014 American Scientific Publisher
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    Vortex-glass phase transition and enhanced flux pinning in C4+-irradiated BaFe1.9Ni0.1As2 superconducting single crystals
    (IOP Publishing, 2013-08-13) Shabhazi, M; Wang, XL; Ghorbani, SR; Ionescu, M; Shcherbakova, OV; Wells, FS; Pan, AV; Dou, SX; Choi, KY
    We report the effects of C4+-irradiation on the superconducting properties of BaFe1.9Ni0.1As2 single crystal. The BaFe1.9Ni0.1As2 single crystal before and after C4+-irradiation was characterized by magnetic, magneto-transport and magneto-optical techniques over a wide range of magnetic fields (0–13 T) and temperatures (2–200 K). We demonstrate that the C4+-irradiation significantly enhances the in-field critical current density (by a factor of up to 1.5 at 5 K) and induces enhanced flux jumping at 2 K, with only a small degradation (by 0.5 K) of the critical temperature, Tc. The vortex phase diagram describing the evolution of the vortex-glass transition temperature with magnetic field and the upper critical field has been resolved for the C4+-irradiated sample. For temperatures below Tc, the resistivity curves and the pinning potential were found to show good scaling, using a modified model for vortex-liquid resistivity. The vortex state is three dimensional at temperatures lower than a characteristic temperature. Good agreement between the thermally activated flux flow model, which is usually employed to account for the resistivity in the vortex-liquid region, and the modified vortex-liquid model, has been observed. © 2013 IOP Publishing