Browsing by Author "Chou, F. C."

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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.
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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.
Disrupted long-range spin-spiral ordering and electric polarization in the Zn-substituted quantum helimagnet LiCu(2-x)Zn(x)O(2)
(2010-06-18)
Hsu, H. C.
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Lee, W. L.
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Lin, J. -Y.
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Liu, H. L.
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Chou, F. C.
Long-range spin-spiral ordering in quantum quasi-two-dimensional helimagnet LiCu(2)O(2) is destroyed by similar to 8% Zn(2+) substitution and a magnetic anomaly near similar to 14 K of different character emerges. Zn substitution may have introduced either a magnetic ground state of spin glass in nature, antiferromagnetic ordering with complex easy-plane anisotropy, or a gapped phase which is phase separated and proportional to the Zn substitution level on a short-range ordered background. The magnetic ground state is modified at the same time that the electric polarization disappears.
Disrupted long-range spin-spiral ordering and electric polarization in the Zn-substituted quantum helimagnet LiCu2-xZnxO2
(2010-06-18)
Hsu, H. C.
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Lee, W. L.
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Lin, J. -Y.
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Liu, H. L.
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Chou, F. C.
Long-range spin-spiral ordering in quantum quasi-two-dimensional helimagnet LiCu2O2 is destroyed by similar to 8% Zn2+ substitution and a magnetic anomaly near similar to 14 K of different character emerges. Zn substitution may have introduced either a magnetic ground state of spin glass in nature, antiferromagnetic ordering with complex easy-plane anisotropy, or a gapped phase which is phase separated and proportional to the Zn substitution level on a short-range ordered background. The magnetic ground state is modified at the same time that the electric polarization disappears.
Magnetic ground state and transition of a quantum multiferroic LiCu(2)O(2)
(2008-08-15)
Huang, S. W.
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Huang, D. J.
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Okamoto, J.
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Mou, C. Y.
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Wu, W. B.
Based on resonant soft x-ray magnetic scattering, we report that LiCu(2)O(2) exhibits a large interchain coupling which suppresses quantum fluctuations along spin chains, and a quasi-2D short-range magnetic order prevails at temperatures above the magnetic transition. These observations unravel the fact that the ground state of LiCu(2)O(2) possesses long-range 2D-like incommensurate magnetic order rather than being a gapped spin liquid as expected from the nature of quantum spin-1/2 chains. In addition, the spin coupling along the c axis is found to be essential for inducing electric polarization.
Magnetic order of LiCu2O2 studied by resonant soft x-ray magnetic scattering
(2008-08-01)
Huang, S. W.
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Huang, D. J.
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Okamoto, J.
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Wu, W. B.
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Chen, C. T.
We present results of resonant soft x-ray magnetic scattering measurements on single crystals of LiCu2O2 to address the magnetic order. LiCu2O2 exhibits a long-range magnetic order incommensurate with the lattice, and the modulation vector is (0.5, zeta, 0) in reciprocal lattice units with zeta similar to 0.174, depending upon the temperature. The spin-spin correlation length along the spin chain deduced from the width of momentum scan is larger than 2100 angstrom. The inter-chain correlation length that lies within the ab plane was discovered to be substantial, similar to 690 angstrom. (c) 2008 Elsevier Ltd. All rights reserved.
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.
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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.
Nonmagnetic impurity perturbation to the quasi-two-dimensional quantum helimagnet LiCu(2)O(2)
(2010-09-29)
Hsu, H. C.
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Lin, J. -Y.
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Lee, W. L.
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Chu, M. -W.
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Imai, T.
A complete phase diagram of Zn-substituted quantum quasi-two-dimensional helimagnet LiCu(2)O(2) has been presented. Helical ordering transition temperature (T(h)) of the original LiCu(2)O(2) follows finite size scaling for less than similar to 5.5% Zn substitution, which implies the existence of finite helimagnetic domains with domain boundaries formed with nearly isolated spins. Higher Zn substitution >= 5.5% quenches the long-range helical ordering and introduces an intriguing Zn-level-dependent magnetic phase transition with slight thermal hysteresis and a universal quadratic field dependence for T(c)(Zn > 0.055, H). The magnetic coupling constants of nearest-neighbor J(1) and next-nearest-neighbor J(2) (alpha = J(2)/J(1)) are extracted from high temperature series expansion fitting and N = 16 finite chain exact diagonalization simulation. We have also provided evidence of direct correlation between long-range helical spin ordering and the magnitude of electric polarization in this spin-driven multiferroic material.
Nonmagnetic impurity perturbation to the quasi-two-dimensional quantum helimagnet LiCu2O2
(2010-09-29)
Hsu, H. C.
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Lin, J. -Y.
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Lee, W. L.
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Chu, M. -W.
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Imai, T.
A complete phase diagram of Zn-substituted quantum quasi-two-dimensional helimagnet LiCu2O2 has been presented. Helical ordering transition temperature (T-h) of the original LiCu2O2 follows finite size scaling for less than similar to 5.5% Zn substitution, which implies the existence of finite helimagnetic domains with domain boundaries formed with nearly isolated spins. Higher Zn substitution >= 5.5% quenches the long-range helical ordering and introduces an intriguing Zn-level-dependent magnetic phase transition with slight thermal hysteresis and a universal quadratic field dependence for T-c(Zn > 0.055, H). The magnetic coupling constants of nearest-neighbor J(1) and next-nearest-neighbor J(2) (alpha = J(2)/J(1)) are extracted from high temperature series expansion fitting and N = 16 finite chain exact diagonalization simulation. We have also provided evidence of direct correlation between long-range helical spin ordering and the magnitude of electric polarization in this spin-driven multiferroic material.
Phonon dynamics in CuxBi2Se3 (x=0, 0.1, 0.125) and Bi2Se2 crystals studied using femtosecond spectroscopy
(2012-09-17)
Chen, H. -J.
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Wu, K. H.
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Luo, C. W.
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Uen, T. M.
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Juang, J. Y.
Phonon dynamics in Bi, Bi2Se2, and CuxBi2Se3 (x=0, 0.1, 0.125) single crystals was investigated using femtosecond pump-probe spectroscopy. Two damped oscillations in measured transient reflectivity changes were observed, respectively, attributed to coherent optical and acoustic phonons. The frequency shift of the phonon modes in Bi-rich Bi2Se2 crystals indicates the intercalation of the additional Bi atoms into the Bi2Se3 matrix and formation of the Bi-2 layer between the quintuple layers (QLs). The deformation of QLs in CuxBi2Se3 crystals was also found. These phonon dynamics results suggest that Cu atoms in CuxBi2Se3 crystals are predominantly intercalated between pair of QLs. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4754005]
Probing the bulk electronic states of Bi2Se3 using nuclear magnetic resonance
(2012-08-21)
Young, Ben-Li
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Lai, Zong-Yo
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Xu, Zhijun
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Yang, Alina
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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).
Snapshots of Dirac Fermions near the Dirac Point in Topological Insulators
(2013-12-01)
Luo, C. W.
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Wang, H. J.
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Ku, S. A.
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Chen, H. -J.
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Yeh, T. T.
The recent focus on topological insulators is due to the scientific interest in the new state of quantum matter as well as the technology potential for a new generation of THz optoelectronics, spintronics and quantum computations. It is important to elucidate the dynamics of the Dirac fermions in the topologically protected surface state. Hence we utilized a novel ultrafast optical pump mid-infrared probe to 4 explore the dynamics of Dirac fermions near the Dirac point. The femtosecond snapshots of the relaxation process were revealed by the ultrafast optics. Specifically, the Dirac fermion-phonon coupling strength in the Dirac cone was found to increase from 0.08 to 0.19 while Dirac fermions were away from the Dirac point into higher energy states. Further, the energy-resolved transient reflectivity spectra disclosed the energy loss rate of Dirac fermions at room temperature was about 1 meV/ps. These results are crucial to the design of Dirac fermion devices.
Superconducting to spin-glass state transformation in beta-pyrochlore K(x)Os(2)O(6)
(2011-03-14)
Lee, C. C.
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Lee, W. L.
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Lin, J. -Y.
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Tsuei, C. C.
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Lin, J. G.
beta-pyrochore KOs(2)O(6), which shows superconductivity below similar to 9.7 K, has been converted into K(x)Os(2)O(6) (x less than or similar to 2/3-1/2) electrochemically to show spin-glass-like behavior below similar to 6.1 K. A room-temperature sample surface potential versus charge transfer scan indicates that there are at least two two-phase regions for x between 1 and 0.5. A rattling model of superconductivity for the title compound has been examined using electrochemical potassium de-intercalation. The significant reduction of superconducting volume fraction due to minor potassium reduction suggests the importance of defect and phase coherence in the rattling model. Magnetic susceptibility, resistivity, and specific heat measurement results have been compared between the superconducting and spin-glass-like samples.
Superconducting to spin-glass state transformation in beta-pyrochlore KxOs2O6
(2011-03-14)
Lee, C. C.
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Lee, W. L.
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Lin, J. -Y.
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Tsuei, C. C.
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Lin, J. G.
beta-pyrochore KOs2O6, which shows superconductivity below similar to 9.7 K, has been converted into KxOs2O6 (x less than or similar to 2/3-1/2) electrochemically to show spin-glass-like behavior below similar to 6.1 K. A room-temperature sample surface potential versus charge transfer scan indicates that there are at least two two-phase regions for x between 1 and 0.5. A rattling model of superconductivity for the title compound has been examined using electrochemical potassium de-intercalation. The significant reduction of superconducting volume fraction due to minor potassium reduction suggests the importance of defect and phase coherence in the rattling model. Magnetic susceptibility, resistivity, and specific heat measurement results have been compared between the superconducting and spin-glass-like samples.
Ultrafast dynamics in topological insulators
(2013)
Luo, C. W.
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Chen, H. -J.
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Wang, H. J.
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Ku, S. A.
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Wu, K. H.
Ultrafast dynamics of carriers and phonons in topological insulators CuxBi2Se3-y (x= 0, 0.1, 0.125, y= 0, 1) was studied using femtosecond optical pump-probe spectroscopy. One damped oscillation was clearly observed in the transient reflectivity changes (Delta R/R), which is assigned to the coherent optical phonon (A(1g)(1)). According to the red shift of A(1g)(1) phonon frequency, the Cu atoms in CuxBi2Se3 crystals may predominantly intercalated between pair of the quintuple layers. Moreover, the carrier dynamics in the Dirac-cone surface state is significantly different from that in bulk state, which was investigated using optical pump mid-infrared (mid-IR) probe spectroscopy. The rising time and decay time of the negative component in Delta R/R, which is assigned to carrier relaxation in Dirac cone, is 1.62 ps and 20.5 ps, respectively.