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RISS 인기검색어
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- 저자 : B. Ndiaye (검색결과 4 건)
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Proton dynamics in antiferroelectric CsH3(SeO3)2 single crystal investigated by 1H NMR measurements
B. Ndiaye,김용환,이무희,임애란 한국물리학회 2020 Current Applied Physics Vol.20 No.5
1H nuclear magnetic resonance (NMR) measurements have been performed to study the proton dynamics associated with the antiferroelectric transition of a hydrogen-bonded single crystal of CsH3(SeO3)2. Herein, 1H NMR spectrum, shift, linewidth, and spin-lattice relaxation rate 1/T1 are measured in the temperature range of 80–296 K with the c-axis parallel to a magnetic field of ~4.85 T. The spectrum exhibits a composite structure with two narrow peaks at 296 K; at a low temperature, this structure evolves into a single broad shape with three humps. This complex shape and evolution are deconvoluted into five or six components based on the number of inequivalent and disordered hydrogen sites. By estimating the chemical shift and linewidth for each proton site, we identify all peaks. The spin-lattice relaxation recovery exhibits a double-exponential behavior with two relaxation times, short T1S and extremely large T1L. Both T1S and T1L follow Arrhenius behavior. From the respective 1/T1(T), the activation energies for proton motion are measured to be small: 1.16 ± 0.1 and 0.83 ± 0.06 kJ/mol for T1S and T1L, respectively. While the static NMR data, chemical shift and linewidth, show no evidence for the transition, the dynamic data 1/T1L highlights a clear increase across TN = 145 K, which is possibly a signature of the transition.
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Orientation of a MgB2 Powder in a Magnetic Field
강기혁,B. J. Mean,김성훈,B. Ndiaye,조병기,J. Y. Kim,이무희 한국물리학회 2010 THE JOURNAL OF THE KOREAN PHYSICAL SOCIETY Vol.57 No.61
We have attempted magnetic orientation of MgB2 powder grains in an epoxy resin under a 7-T horizontal magnetic field. The horizontal field was crucial to get rid of gravitational sedimentation by rotating the mixture of MgB2 powder grains and the epoxy resin along the horizontal axis. Since we did not know the principal axes and the magnitude of the magnetic anisotropy tensor for MgB2, we devised different alignment schemes assuming two cases: case I for the easy axis along the c-axis and case II for the two easy axes perpendicular to the c-axis. For each case, we built different rotation devices: namely, for case I, the rotation axis was parallel to the magnetic field whereas, for case II, it was perpendicular to the magnetic field. For both cases, the powder grains should be aligned along the rotation axis. For assessment of the c-axis alignment, X-ray diffraction (XRD) and 11B nuclear magnetic resonance (NMR) measurements were utilized. However, the XRD and 11B NMR data for both cases were the same as those for the powder sample and showed no evidence of orientation. For possible scenarios, we suggest two possibilities: negligible anisotropy of the magnetic susceptibility tensor and the large size of the ground MgB2 powder particles still in multi-grains.
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김성훈,강기혁,민병진,이무희,김준성,Kim, Sung-Hoon,Kang, Ki-Hyeok,Mean, B.J.,Ndiaye, B.,Lee, Moo-Hee,Kim, Jun-Sung The Korean Superconductivity Society 2010 Progress in superconductivity Vol.12 No.1
$^{13}C$ NMR (nuclear magnetic resonance) measurements have been performed to investigate the local electronic structure of a superconducting graphite intercalation compound $CaC_6$ ($T_c$ = 11.4 K). A large number of single crystals were stacked and sealed in a quartz tube for naturally abundant $^{13}C$ NMR. The spectrum, Knight shift, linewidth, and spin-lattice relaxation time $T_1$ were measured in the normal state as a function of temperature down to 80 K at 8.0 T perpendicular to the c-axis. The $^{13}C$ NMR spectrum shows a single narrow peak with a very small Knight shift. The Knight shift and the linewidth of the $^{13}C$ NMR are temperature-independent around, respectively, +0.012% and 1.2 kHz. The spin-lattice relaxation rate, $1/T_1$, is proportional to temperature confirming a Korringa behavior as for non-magnetic metals. The Korringa product is measured to be $T_1T\;=\;210\;s{\cdot}K$. From this value, the Korringa ratio is deduced to be $\xi$ = 0.73, close to unity, which suggests that the independent-electron description works well for $CaC_6$, without complications arising from correlation and many-body effects.
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