Browsing by Author "Kuo, Y. K."

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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
;
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.
Design and fabrication of a InGaN vertical-cavity surface-emitting laser with a composition-graded electron-blocking layer
(2014-08-01)
Lin, B. C.
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Chang, Y. A.
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Chen, K. J.
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Chiu, C. H.
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Li, Z. Y.
In this study, the design and fabrication of a InGaN vertical-cavity surface-emitting laser (VCSEL) with a composition-graded electron blocking layer (GEBL) are revealed experimentally and theoretically. It has been demonstrated that laser output performance is improved by using a GEBL when compared to the typical VCSEL structure of a rectangular EBL. The output power obtained at 20 kA cm(-2) is enhanced by a factor of 3.8 by the successful reduction of threshold current density from 12.6 to 9.2 kA cm(-2) and the enlarged slope efficiency. Numerical simulation results also suggest that the improved laser output performances are due mainly to the reduction of electron leakage current and the enhanced hole injection efficiency in the multiple-quantum-well (MQW) active region.
Doping evolution of Zhang-Rice singlet spectral weight: A comprehensive examination by x-ray absorption spectroscopy
(2013-10-31)
Chen, Y. -J.
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Jiang, M. G.
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Luo, C. W.
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Lin, J. -Y.
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Wu, K. H.
The total spectral weight S of the emergent low-energy quasiparticles in high-temperature superconductors (HTSCs) is explored by x-ray absorption spectroscopy. In order to examine the applicability of the Hubbard model, regimes that cover from zero doping to overdoping are investigated. In contrast to results of mean-field theory, we found that S deviates from linear dependence on the doping level p. The slope of S versus p changes continuously throughout the whole doping range with no sign of saturation up to p = 0.23. Therefore, the picture of the Zhang-Rice singlet remains intact within the most prominent doping regimes of HTSCs.
Effects of Fe substitution on the transport properties of the superconductor MgB2
(2007-05-01)
Gahtori, Bhasker
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Lal, Ratan
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Agarwal, S. K.
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Kuo, Y. K.
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Sivakumar, K. M.
A systematic study has been made on the effect of Fe substitution by means of resistivity, thermal conductivity, and Seebeck coefficient of the Mg1-xFexB2 superconductor involving 0%, 0.3%, 0.6%, 1.2%, and 3.0% Fe content. The superconducting transition has been found to be very sharp (similar to 0.2 K) for a pristine sample and substitution of Fe results in the decrease of T-C with the increase in the transition width. Thermal conductivity is found to decrease with Fe content in general, such that the shoulder present in the pristine sample tends to fade away with increasing Fe. An analysis has been made on the normal state resistivity in terms of a two-band model, and of the thermal conductivity in terms of the Wiedemann-Franz law and the lattice thermal conductivity, and the information obtained on the basis of this analysis has been discussed. Besides, the electronic density of states (DOS) near the Fermi level remains nearly unaffected upon Fe substitution, as evidenced by the Seebeck coefficient measurements. When compared with Mn, Fe behaves like a nonmagnetic element with a modest variation in T-C and on the other hand, the T-C depression is much stronger when compared with other elements like Al, Cu, etc. Therefore, the observed variation in T-C for the presently investigated concentrations of Fe is attributed to the specific nature of the given substituent element (Fe) in altering the phonon frequency and/or electron-phonon coupling strength rather than spin-flip scattering or change in DOS or disorder.
Fabrication and low temperature thermoelectric properties of NaxCoO2 (x=0.68 and 0.75) epitaxial films by the reactive solid-phase epitaxy
(2007-02-05)
Chang, W. J.
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Hsieh, C. C.
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Chung, T. Y.
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Hsu, S. Y.
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Wu, K. H.
The authors have fabricated NaxCoO2 thin films via lateral diffusion of sodium into Co3O4 (111) epitaxial films (reactive solid-phase epitaxy [Ohta , Cryst. Growth Des. 5, 25 (2005)]). The environment of thermal diffusion is key to the control of the sodium content in thin films. From the results of x-ray diffraction and in-plane rho(ab), the epitaxial growth and the sodium contents of these films were identified. The thermoelectric measurements show a large thermoelectric power similar to that observed in single crystals. The quasiparticle scattering rate is found to approach zero at low temperatures, consistent with the small residual resistivity, indicating high quality of the NaxCoO2 thin films. (c) 2007 American Institute of Physics.
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.
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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.
Optical properties of In0.3Ga0.7N/GaN green emission nanorods fabricated by plasma etching
(2007-08-22)
Chiu, C. H.
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Lo, M. H.
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Lai, C. F.
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Lu, T. C.
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Huang, H. W.
In this study, we have fabricated In0.3Ga0.7N/GaN green emission nanorods and demonstrated optical enhancement by photoluminescence (PL) measurements. An enhancement factor of 3.5 and an emission peak blue-shift of 6.6 nm were observed at 300 K for the green emission nanorods structure in comparison to the as-grown flat surface structure. The blue-shift phenomenon from the nanorod structure could be caused by a partial reduction of the internal piezoelectric field. However, the similar carrier decay time for the green emission nanorod structure and the as-grown flat surface structure observed in low-temperature time-resolved PL measurements indicates that the dominant optical enhancement mechanism of the green emission nanorod structure could be mainly resulting from the large emission surface areas and the multiple scattering paths between the nanorods.