Browsing by Author "Shu, G. J."

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Cobalt magnetism in a superstructured metallic antiferromagnet Na0.825CoO2
(2013-08-22)
Young, Ben-Li
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Chu, P-Y.
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Juang, J. Y.
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Shu, G. J.
;
Chou, F. C.
The Na atomic ordering and the local Co magnetism, in a metallic antiferromagnet made of Na0.825CoO2, are investigated by nuclear magnetic-resonance (NMR) techniques. Multiple well-resolved NMR peaks of Na-23 and Co-59 are observed, which suggests a superlattice structure forming in this Na-vacant cobaltate. Examining clues from published x-ray data, we refine the lattice superstructure so as to obtain the best description for the peaks seen in our NMR data. Magnetic Co ions are found to form a striped structure, which produces nontrivial magnetism. Analyzing the NMR frequency shift for each peak, we observe a frequency-shift anomaly, i.e., the breakdown of linearity between the NMR shift (that is, the local susceptibility) and the bulk susceptibility. We find that the shift anomaly can be described by an additional susceptibility component emerging at similar to 60 K. Slow spin dynamics are observed near a metamagnetic transition where the spins flip between the antiferromagnetic and ferromagnetic orders. Unlike in ordinary spin glasses, this glassy behavior is induced by a magnetic field, which causes competition between the in-plane ferromagnetic and out-of-plane antiferromagnetic interactions, in a magnetic-frustrated triangular lattice.
Mn vacancy defects, grain boundaries, and A-phase stability of helimagnet MnSi
(2016-01-20)
Ou-Yang, T. Y.
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Shu, G. J.
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Lin, J-Y
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Hu, C. D.
;
Chou, F. C.
Mn vacancy defect and grain size are shown to modify the magnetic phase diagram of MnSi significantly, especially near the critical regime of A-phase (skyrmion lattice) formation and the helimagnetic phase transition. Crystals grown using controlled nonstoichiometric initial precursors creates both grain boundaries and intrinsic Mn vacancy defect of various levels in MnSi. The results of combined transport, specific heat, and AC spin susceptibility measurements are compared for MnSi single crystal samples of various manganese deficiency levels and grain sizes. The finite-size effect and Mn vacancy level dependent helical phase transition temperature T-c have been identified and verified. The stability of A-phase in H-T phase space has been examined through AC spin susceptibility data analysis.
Probing the bulk electronic states of Bi2Se3 using nuclear magnetic resonance
(2012-08-21)
Young, Ben-Li
;
Lai, Zong-Yo
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Xu, Zhijun
;
Yang, Alina
;
Gu, G. D.
We report a nuclear magnetic resonance (NMR) study of Bi2Se3 single crystals grown by three different methods. All the crystals show nine well-resolved peaks in their Bi-209 NMR spectra of the nuclear quadrupolar splitting, albeit with an intensity anomaly. Spectra at different crystal orientations confirm that all the peaks are purely from the nuclear quadrupolar effect, with no other hidden peaks. We identify the short nuclear transverse relaxation time (T-2) effect as the main cause of the intensity anomaly. We also show that the Bi-209 signal originates exclusively from bulk, while the contribution from the topological surface states is too weak to be detected by NMR. However, the bulk electronic structure in these single crystals is not the same, as identified by the NMR frequency shift and nuclear spin-lattice relaxation rate (1/T-1). The difference is caused by the different structural defect levels. We find that the frequency shift and 1/T-1 are smaller in samples with fewer defects and a lower carrier concentration. Also, the low-temperature power law of the temperature-dependent 1/T-1 (proportional to T-alpha) changes from the Korringa behavior alpha = 1 in a highly degenerate semiconductor (where the electrons obey Fermi statistics) to alpha < 1 in a less degenerate semiconductor (where the electrons obey Boltzmann statistics).