Browsing by Author "Kuo, C. N."

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Characterization of the charge density wave transition and observation of the amplitude mode in LaAuSb2
(2019-06-10)
Kuo, C. N.
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Shen, D.
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Li, B. S.
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Nguyen Nhat Quyen
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Tzeng, W. Y.
In order to elucidate the charge density wave (CDW) nature in LaAuSb2, we have carried out the electrical resistivity, Hall coefficient, Seebeck coefficient, thermal conductivity, as well as ultrafast pump-probe spectroscopy measurements on single crystalline LaAuSb2. The CDW phase transition has been characterized by marked features near T-C similar or equal to 88 K. In particular, the obtained Hall carrier concentration and electronic thermal conductivity exhibit significant reduction as T-C drops, providing strong evidence for partial gapping of the Fermi surfaces during the CDW phase transition. Furthermore, the ultrafast pump-probe spectroscopy reveals pronounced oscillatory features in the photoinduced transient reflectivity in the CDW ordered state. A characteristic oscillation mode with frequency of about 0.32 THz was extracted. The comparison of the obtained oscillation frequency with the frequencies of the amplitude mode in the well-studied CDW analogue LaAgSb2 allows us to ascribe the observation to the amplitude mode of CDW collective excitations in LaAuSb2.
Crystal structure and magnetic properties of intermetallic semiconductor FeGa3 lightly doped by Co and Ni
(2018-05-15)
Likhanov, M. S.
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Verchenko, V. Yu.
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Nasonova, D. I.
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Gippius, A. A.
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Zhurenko, S. V.
We have explored the crystal structure and magnetic properties of two intermetallic solid solutions of the IrIn3 structure type. Although Fe0.95Co0.05Ga3 and Fe0.975Ni0.025Ga3 are isomorphous and possess exactly the same concentration of valence electrons, they display noticeable differences in their crystal structure associated with a different nature of the Fe-Co and Fe-Ni dumbbells. The former contains an odd number of electrons compared to an even number of the latter dumbbell. Responding to localized spins of the Fe-Co dumbbells, the Ga-69 NQR relaxation rate in Fe0.95Co0.05Ga3 is governed by temperature independent spin-lattice relaxation typical for diluted paramagnetic centers at low temperatures. In Fe0.975Ni0.025Ga3 with zero magnetic moment of the dumbbells the Ga-69 spin-lattice relaxation rate is strongly reduced and in-gap states in the energy gap show up as a broad maximum near 70 K. (C) 2018 Elsevier B.V. All rights reserved.
Dynamics of phonons in Sr3Ir4Sn13: an experimental study by ultrafast spectroscopy measurements
(2016-07-26)
Luo, C. W.
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Cheng, P. C.
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Tu, C. M.
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Kuo, C. N.
;
Wang, C. M.
We report a study of ultrafast dynamics of photoexcited electrons and phonons in Sr3Ir4Sn13 using dual-color transient reflectivity change (Delta R/R) measurements. Time resolved optical spectroscopy of collective excitations reveal the marked features near its structural phase transition temperature T* similar or equal to 147 K. Two distinctive oscillatory timescales in Delta R/R have been clearly resolved. The rapid THz-range oscillations are attributed to the dynamics of the optical phonons which strongly correlate to the structural phase transition. The slow GHz-range oscillatory phenomenon which only occurs below about 150 K is associated with the dynamic response of the longitudinal-acoustic phonons. These low-energy phonons show a softening feature on approaching the transition temperature, also indicating a strong relevance to the structural phase transition. The information that we demonstrated would provide a deeper understanding of the structural phase transition in Sr3Ir4Sn13.
Electronic structure investigation of a charge density wave coupled to a metal-to-metal transition in Ce3Co4Sn13
(2018-12-18)
Singh, A.
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Huang, H. Y.
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Chin, Y. Y.
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Liao, Y. F.
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Huang, T. C.
We study the electronic structure of the skutterudite Ce3Co4Sn13, which is known to exhibit a charge density wave (CDW) transition, at temperature T-CDW similar to 160 K, coupled to a metal-to-metal transition. We use temperature dependent hard x-ray photoemission spectroscopy (HAXPES) and x-ray absorption spectroscopy (XAS) to investigate the occupied and unoccupied electronic states of Ce3Co4Sn13. The Co 2p and Sn 3p core level spectra show small but finite shifts in binding energy positions across T-CDW while Ce 3d core level spectra do not show any change across the transition. The Ce M-4,M-5-edge XAS spectrum compared with calculations indicate a typical trivalent ionic Ce3+ spectral shape, ruling out Kondo screening in Ce3Co4Sn13. In contrast, the Co L-2,L-3-edge XAS spectrum compared with a calculated spectrum shows evidence for hybridization with neighboring Sn atoms in a trigonal prismatic co-ordination. Temperature dependent XAS across the Co L-2,L-3-edge shows a small shift across T-CDW, consistent with HAXPES results. Detailed XAS measurements as a function of temperature show that the spectral shifts occur with a hysteresis across T-CDW , indicative of a first-order transition. Valence band spectra show a normal Fermi edge above and below T-CDW. The Co 3d states are observed at a binding energy of similar to 2 eV while the Ce 4f states occur as a weak feature within 0.5 eV of the Fermi level. The results suggest an unusual CDW transition coupled to a metal-to-metal transition in Ce3Co4Sn13.
Investigation of normal and superconducting states in noncentrosymmetric Re24Ti5
(2013-05-01)
Lue, C. S.
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Liu, H. F.
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Kuo, C. N.
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Shih, P. S.
;
Lin, J-Y
We report a detailed characterization of the noncentrosymmetric superconductor Re24Ti5 using powder x-ray diffraction (XRD), magnetic susceptibility, electrical resistivity, thermal conductivity, Seebeck coefficient, and specific heat measurements. Rietveld refinement of powder XRD data confirms that Re24Ti5 crystallizes in the alpha-Mn structure. All measured quantities demonstrate a bulk superconducting transition at T-c = 5.8 K. Our low-temperature specific heat data measured down to 0.5 K yield a Sommerfeld coefficient gamma = 111.8 mJ mol(-1) K-2, which implies a high density of states at the Fermi level. Moreover, the electronic specific heat in the superconducting state was found to obey a typical s-wave expression, revealing a single gap Delta/k(B) = 10.6 K. This value gives a ratio of 2D/k(B)T(c) = 3.68, higher than the value of 3.5 predicted from BCS theory. On this basis, we conclude that the noncentrosymmetric Re24Ti5 compound can be characterized as a moderately coupled BCS-type superconductor. Furthermore, the obtained parameters from the present study of Re24Ti5 were compared to those of the isostructural compound Re23.8Nb5.2, indicating the similarity between both systems.
Study of Enhancement-Mode Tri-Gate InAs HEMTs for Low Noise Application
(2019-01-01)
Wang, C.
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Lin, Y. C.
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Kuo, C. N.
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Lee, M. W.
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Yao, J. N.
An enhancement-mode tri-gate InAs HEMT is investigated for low noise application in this paper. The 3-D tri-gate structure is sidewall-gate-metal connected to InAlAs layers, the gate connection to the InAlAs layers increases their potential to the positive direction with increasing the gate bias, resulting gate control ability enhancement. Compared with planar device, the tri-gate device shows high transconductance and low noise figure. The enhancement-mode tri-gate device exhibits excellent low noise figure with less than 3.5 dB when the device operation frequency range of 18 GHz to 50 GHz.