Browsing by Author "Young, Ben-Li"

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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.
Evidence for s-wave superconductivity in noncentrosymmetric Re(24)Nb(5) from (93)Nb NMR measurements
(2011-08-25)
Lue, C. S.
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Su, T. H.
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Liu, H. F.
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Young, Ben-Li
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電子物理學系
We report a (93)Nb nuclear magnetic resonance (NMR) study on the noncentrosymmetric superconductor Re(24)Nb(5). Below the superconducting temperature T(c)(H), the spin susceptibility probed by the (93)Nb NMR Knight shift gradually decreases with lowering temperature, accompanied by the broadening of the resonance spectrum. Such behavior is commonly observed in the BCS-type superconductors. The (93)Nb NMR spin-lattice relaxation rate (1/T(1)) shows a well-defined coherence peak just below T(c)(H), followed by a marked decrease with further decreasing temperature. Moreover, the 1/T(1) data in the superconducting state were found to obey a single exponential expression, yielding a nodeless gap Delta/kB = 10.3 K. This value gives the ratio of 2 Delta/kBT(c)(H) = 3.55, that is almost identical with the value of 3.5 predicted from BCS theory. On these bases, we conclude that the noncentrosymmetric Re(24)Nb(5) compound can be characterized as a weakly coupled BCS-type superconductor.
Evidence for s-wave superconductivity in noncentrosymmetric Re24Nb5 from Nb-93 NMR measurements
(2011-08-25)
Lue, C. S.
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Su, T. H.
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Liu, H. F.
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Young, Ben-Li
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電子物理學系
We report a Nb-93 nuclear magnetic resonance (NMR) study on the noncentrosymmetric superconductor Re24Nb5. Below the superconducting temperature T-c(H), the spin susceptibility probed by the Nb-93 NMR Knight shift gradually decreases with lowering temperature, accompanied by the broadening of the resonance spectrum. Such behavior is commonly observed in the BCS-type superconductors. The Nb-93 NMR spin-lattice relaxation rate (1/T-1) shows a well-defined coherence peak just below T-c(H), followed by a marked decrease with further decreasing temperature. Moreover, the 1/T-1 data in the superconducting state were found to obey a single exponential expression, yielding a nodeless gap Delta/kB = 10.3 K. This value gives the ratio of 2 Delta/kBT(c)(H) = 3.55, that is almost identical with the value of 3.5 predicted from BCS theory. On these bases, we conclude that the noncentrosymmetric Re24Nb5 compound can be characterized as a weakly coupled BCS-type superconductor.
Evolution of the pseudogap in Sr(1-x)Y(x)Al(2)Si(2) (x=0, 0.05, 0.1, and 0.15) probed via (27)Al nuclear magnetic resonance
(2011-03-28)
Fang, C. P.
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Lue, C. S.
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Young, Ben-Li
;
電子物理學系
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Department of Electrophysics
With the aim of providing microscopic information about the electronic characteristics of SrAl(2)Si(2) and effects of chemical substitution on its pseudogap features, we carried out a study on Sr(1-x)Y(x)Al(2)Si(2) (x = 0, 0.05, 0.1, and 0.15) by means of (27)Al nuclear magnetic resonance (NMR) spectroscopy. For stoichiometric SrAl(2)Si(2), the temperature-dependent NMR Knight shift and spin-lattice relaxation rate are associated with a sharp feature in the electronic density of state within a pseudogap at around the Fermi level. On the other hand, the NMR observations for Y-substituted compounds exhibit ordinary metallic behavior, suggesting that the Fermi level has moved out of the pseudogap for these materials, resulting in the dominant Korringa process responsible for the observed relaxation rates.
Evolution of the pseudogap in Sr1-xYxAl2Si2 (x=0, 0.05, 0.1, and 0.15) probed via Al-27 nuclear magnetic resonance
(2011-03-28)
Fang, C. P.
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Lue, C. S.
;
Young, Ben-Li
;
電子物理學系
;
Department of Electrophysics
With the aim of providing microscopic information about the electronic characteristics of SrAl2Si2 and effects of chemical substitution on its pseudogap features, we carried out a study on Sr1-xYxAl2Si2 (x = 0, 0.05, 0.1, and 0.15) by means of Al-27 nuclear magnetic resonance (NMR) spectroscopy. For stoichiometric SrAl2Si2, the temperature-dependent NMR Knight shift and spin-lattice relaxation rate are associated with a sharp feature in the electronic density of state within a pseudogap at around the Fermi level. On the other hand, the NMR observations for Y-substituted compounds exhibit ordinary metallic behavior, suggesting that the Fermi level has moved out of the pseudogap for these materials, resulting in the dominant Korringa process responsible for the observed relaxation rates.
Hyperfine Fields and Magnetic Structure in the B Phase of CeCoIn(5)
(2010-02-01)
Curro, Nicholas J.
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Young, Ben-Li
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Urbano, Ricardo R.
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Graf, Matthias J.
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電子物理學系
We re-analyze Nuclear Magnetic Resonance (NMR) spectra observed at low temperatures and high magnetic fields in the field-induced B phase of CeCoIn(5). The NMR spectra are consistent with incommensurate antiferromagnetic order of the Ce magnetic moments. However, we find that the spectra of the In(2) sites depend critically on the direction of the ordered moments, the ordering wavevector and the symmetry of the hyperfine coupling to the Ce spins. Assuming isotropic hyperfine coupling, the NMR spectra observed for H parallel to [100] are consistent with magnetic order with wavevector Q = pi (1+delta/a, 1/a, 1/c) and Ce moments ordered antiferromagnetically along the [100] direction in real space. If the hyperfine coupling has dipolar symmetry, then the NMR spectra require Ce moments along the [ 001] direction. The dipolar scenario is also consistent with recent neutron scattering measurements that find an ordered moment of 0.15 mu(B) along [001] and Q(n) = pi(1+delta/a, 1+delta/a, 1/c) with in-commensuration delta = 0.12 for field H parallel to [1 (1) over bar0]. Using these parameters, we find that a hyperfine field with dipolar contribution is consistent with findings from both experiments. We speculate that the B phase of CeCoIn(5) represents an intrinsic phase of modulated superconductivity and antiferromagnetism that can only emerge in a highly clean system.
Hyperfine Fields and Magnetic Structure in the B Phase of CeCoIn5
(2010-02-01)
Curro, Nicholas J.
;
Young, Ben-Li
;
Urbano, Ricardo R.
;
Graf, Matthias J.
;
電子物理學系
We re-analyze Nuclear Magnetic Resonance (NMR) spectra observed at low temperatures and high magnetic fields in the field-induced B phase of CeCoIn5. The NMR spectra are consistent with incommensurate antiferromagnetic order of the Ce magnetic moments. However, we find that the spectra of the In(2) sites depend critically on the direction of the ordered moments, the ordering wavevector and the symmetry of the hyperfine coupling to the Ce spins. Assuming isotropic hyperfine coupling, the NMR spectra observed for H parallel to [100] are consistent with magnetic order with wavevector Q = pi (1+delta/a, 1/a, 1/c) and Ce moments ordered antiferromagnetically along the [100] direction in real space. If the hyperfine coupling has dipolar symmetry, then the NMR spectra require Ce moments along the [ 001] direction. The dipolar scenario is also consistent with recent neutron scattering measurements that find an ordered moment of 0.15 mu(B) along [001] and Q(n) = pi(1+delta/a, 1+delta/a, 1/c) with in-commensuration delta = 0.12 for field H parallel to [1 (1) over bar0]. Using these parameters, we find that a hyperfine field with dipolar contribution is consistent with findings from both experiments. We speculate that the B phase of CeCoIn5 represents an intrinsic phase of modulated superconductivity and antiferromagnetism that can only emerge in a highly clean system.
Magnetic fluctuations in FeSe(1-delta) and Cu-doped FeSe(1-delta): (77)Se NMR experiments
(2010-04-01)
Young, Ben-Li
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Wu, Jack
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Huang, Tzu-Wen
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Yeh, Kuo-Wei
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Wu, Maw-Kuen
The roles of Se deficiency and Fe atoms, for the superconductivity of FeSe, have been, respectively, investigated in FeSe(1-delta) and (Fe(1-x)Cu(x))Se(1-delta), by our NMR experiments. The data, for nuclear spin-lattice relaxation rate (1/T(1)), show that the spin fluctuations are weakened at a larger delta, and are correlated with the superconductivity in FeSe. The superconducting volume fraction, estimated by our ac susceptibility experiments, is found to vary inversely with delta. Our findings suggest that the Se-deficient FeSe has an inhomogeneous phase, where the superconductivity is associated with the regions having few or no Se vacancies. As for (Fe(1-x)Cu(x))Se(1-delta), T(c) is rapidly suppressed by Cu doping and vanishes around x=0.03. The (77)Se and the (63)Cu NMR linewidths suggest that a local moment is induced at the Fe sites and not at the Cu sites. However, 1/T(1) shows no obvious change with the Cu doping. We suspect that other effects, such as disorder or change in the density of states, have more influence on T(c) suppression since a metal-insulator transition, induced by Cu substitution, occurs in the resistivity measurements.
Magnetic fluctuations in FeSe1-delta and Cu-doped FeSe1-delta: Se-77 NMR experiments
(2010-04-01)
Young, Ben-Li
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Wu, Jack
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Huang, Tzu-Wen
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Yeh, Kuo-Wei
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Wu, Maw-Kuen
The roles of Se deficiency and Fe atoms, for the superconductivity of FeSe, have been, respectively, investigated in FeSe1-delta and (Fe1-xCux)Se1-delta, by our NMR experiments. The data, for nuclear spin-lattice relaxation rate (1/T-1), show that the spin fluctuations are weakened at a larger delta, and are correlated with the superconductivity in FeSe. The superconducting volume fraction, estimated by our ac susceptibility experiments, is found to vary inversely with delta. Our findings suggest that the Se-deficient FeSe has an inhomogeneous phase, where the superconductivity is associated with the regions having few or no Se vacancies. As for (Fe1-xCux)Se1-delta, T-c is rapidly suppressed by Cu doping and vanishes around x=0.03. The Se-77 and the Cu-63 NMR linewidths suggest that a local moment is induced at the Fe sites and not at the Cu sites. However, 1/T-1 shows no obvious change with the Cu doping. We suspect that other effects, such as disorder or change in the density of states, have more influence on T-c suppression since a metal-insulator transition, induced by Cu substitution, occurs in the resistivity measurements.
NMR evidence for the partially gapped state in CeOs(2)Al(10)
(2010-11-22)
Lue, C. S.
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Yang, S. H.
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Su, T. H.
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Young, Ben-Li
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電子物理學系
We report the results of a (27)Al nuclear magnetic resonance (NMR) study of CeOs(2)Al(10) at temperatures between 4 and 300 K. This material has been of current interest due to indications of hybridization gap behavior below the transition temperature T(o)similar or equal to 29 K. Five (27)Al NMR resonance lines that are associated with five nonequivalent crystallographic aluminum sites have been resolved. For each individual aluminum site, the low-temperature NMR Knight shift goes over a thermally activated response. The temperature-dependent spin-lattice-relaxation rate exhibits a rapid drop below T(o), indicative of the formation of an energy gap in this material. We interpret the Knight shift and the relaxation-rate data in light of the presence of a pseudogap with residual electronic density of states at the Fermi level. Moreover, the magnitude of the pseudogap of 120 K is extracted from NMR results, in agreement with the value obtained from the inelastic neutron-scattering experiment.
NMR evidence for the partially gapped state in CeOs2Al10
(2010-11-22)
Lue, C. S.
;
Yang, S. H.
;
Su, T. H.
;
Young, Ben-Li
;
電子物理學系
We report the results of a Al-27 nuclear magnetic resonance (NMR) study of CeOs2Al10 at temperatures between 4 and 300 K. This material has been of current interest due to indications of hybridization gap behavior below the transition temperature T-o similar or equal to 29 K. Five Al-27 NMR resonance lines that are associated with five nonequivalent crystallographic aluminum sites have been resolved. For each individual aluminum site, the low-temperature NMR Knight shift goes over a thermally activated response. The temperature-dependent spin-lattice-relaxation rate exhibits a rapid drop below T-o, indicative of the formation of an energy gap in this material. We interpret the Knight shift and the relaxation-rate data in light of the presence of a pseudogap with residual electronic density of states at the Fermi level. Moreover, the magnitude of the pseudogap of 120 K is extracted from NMR results, in agreement with the value obtained from the inelastic neutron-scattering experiment.
NMR studies of field induced magnetism in CeCoIn(5)
(2010-12-01)
Curro, Nicholas J.
;
Young, Ben-Li
;
Urbano, Ricardo R.
;
Graf, Matthias J.
;
電子物理學系
Recent Nuclear Magnetic Resonance and elastic neutron scattering experiments have revealed conclusively the presence of static incommensurate magnetism in the field-induced B phase of CeCoIn(5). We analyze the NMR data assuming the hyperfine coupling to the In(2) nuclei is anisotropic and simulate the spectra for several different magnetic structures. The NMR data are consistent with ordered Ce moments along the [001] direction, but are relatively insensitive to the direction of the incommensurate wavevector. (C) 2009 Published by Elsevier B.V.
NMR studies of field induced magnetism in CeCoIn5
(2010-12-01)
Curro, Nicholas J.
;
Young, Ben-Li
;
Urbano, Ricardo R.
;
Graf, Matthias J.
;
電子物理學系
Recent Nuclear Magnetic Resonance and elastic neutron scattering experiments have revealed conclusively the presence of static incommensurate magnetism in the field-induced B phase of CeCoIn5. We analyze the NMR data assuming the hyperfine coupling to the In(2) nuclei is anisotropic and simulate the spectra for several different magnetic structures. The NMR data are consistent with ordered Ce moments along the [001] direction, but are relatively insensitive to the direction of the incommensurate wavevector. (C) 2009 Published by Elsevier B.V.
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
;
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).
Probing the electronic structure of pure and doped CeMIn5 (M=Co,Rh,Ir) crystals with nuclear quadrupolar resonance
(2008-06-01)
Rusz, Jan
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Oppeneer, Peter M.
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Curro, Nicholas J.
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Urbano, Ricardo R.
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Young, Ben-Li
We report calculations of the electric-field gradients (EFGs) in pure and doped CeMIn5 (M=Co, Rh, and Ir) compounds and compare with experiment. The degree to which the Ce 4f electron is localized is treated within various models: the local-density approximation, generalized gradient approximation (GGA), GGA+U, and 4f-core approaches. We find that there is a correlation between the observed EFG and whether the 4f electron participates in the band formation or not. We also find that the EFG evolves linearly with Sn doping in CeRhIn5, suggesting the electronic structure is modified by doping. In contrast, the observed EFG in CeCoIn5 doped with Cd changes little with doping. These results indicate that nuclear quadrupolar resonance is a sensitive probe of electronic structure.
Spin-Glass Transition and Giant Paramagnetism in Heavily Hole-Doped Bi2Sr2Co2Oy
(2014-02-01)
Hsu, Hung Chang
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Lee, Wei-Li
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Lin, Jiunn-Yuan
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Young, Ben-Li
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Kung, Hsiang-Hsi
Hole-doped single crystals of misfit-layered cobaltate Bi2-xPbxSr2-zCo2Oy (x = 0-0.61, y = 8.28-8.62, and z = 0.01-0.22) have been successfully grown using the optical floating-zone method. Heavier hole doping has been achieved through both Pb substitution in the Bi site and the more effective Sr vacancy formation. The Co4+ : Co3+ ratio can be raised significantly from its original similar to 1 : 1 to 4.5 : 1, as confirmed by iodometric titration. A spin-glass transition temperature of T-g similar to 70K is confirmed by ac susceptibility measurement when the Co4+ : Co3+ ratio becomes higher than 2 : 1, presumably owing to the significantly increased probability of triangular geometrical frustration among antiferromagnetically coupled localized Co4+ spins.