Browsing by Author "Chen, MH"

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An explicit model for a quantum channel in 2DEG
(2002-05-01)
Fedirko, VA
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Chen, MH
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Chiu, CC
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Chen, SF
;
Wu, KH
A two-parametric model for a channel in a two-dimensional electron gas (2DEG) is proposed which allows for the explicit analytical solution for the problem of quantum electron transport through the constriction. Conductance step smearing appears naturally and a simple criterion for the step occurrence in terms of channel parameters is given. (C) 2002 Elsevier Science Ltd. All rights reserved.
Anisotropic characteristics of in-plane aligned a-axis YBa2Cu3O7-delta thin films
(2003-05-01)
Luo, CW
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Chen, MH
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Chin, CC
;
Wu, KH
;
Juang, JY
Highly in-plane aligned a-axis oriented YBa2Cu3O7-delta (YBCO) thin films with unprecedented transition temperature (T-c similar to 91.3 K) have been deposited on (100) LaSrGaO4 substrates using a continuously grown PrBa2Cu3O7-delta (PBCO) template layer by pulsed laser. The anisotropies between b- and c-axis were investigated by transport measurements and atomic force microscopy (AFM). Although, the onset T-c's along b- and c-axis appeared approximately the same, a significant filing was observed in the resistive transition along c-axis. The temperature-dependent of the tailing effect, however, cannot be explained by thermally activated phase-slippage (TAPS) commonly encountered in grain-boundary weak links.
Anisotropic electronic structure of in-plane aligned a-axis YBa2Cu3O7-delta thin films
(2003-09-01)
Luo, CW
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Chen, MH
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Liu, SJ
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Wu, KH
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Juang, JY
Polarization-dependent x-ray absorption near-edge spectra (XANES) of the O 1s has been measured on a highly in-plane aligned a-axis YBa2Cu3O7-delta (YBCO) thin film. The in-plane XANES, with the electric field E of the linearly polarized synchrotron light being parallel to the b or c axis of YBCO films (E//b or E//c) were obtained in a normal-incidence alignment. The XANES for E//a was then calculated from the data obtained by varying the angle. The results lend strong support to those obtained by using detwinned YBCO single crystals in all crystalline orientations, including the extrapolated c-axis spectra. (C) 2003 American Institute of Physics.
Anisotropic photoexcited carrier dynamics in (100)-, (001)-, and (110)-oriented YBCO films by polarized ultrafast optical spectroscopy
(2003-06-01)
Luo, CW
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Chen, MH
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Chiu, CC
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Chen, SP
;
Wu, KH
We report the time-resolved ultrafast optical spectroscopy results obtained from a series of well characterized (001)-, (100)- and (110)-oriented YBa2Cu3O7-delta (YBCO) superconducting films prepared by pulsed laser deposition. The well-controlled film orientation has allowed us to probe the relaxation dynamics along various crystal orientations using respectively polarized pump and probe laser beams. The significant anisotropies in both the magnitude of the characteristic relaxation time and the temperature dependence of the photo-induced transient reflectance change indicate the nature of relaxation channel might be intrinsically different along different orientations. The implications of the results on the fundamental characteristics concerning superconducting gaps are discussed.
Effect of thermal cycles on ferrite content of austenitic stainless steel
(1999)
Chen, MH
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Chou, CP
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機械工程學系
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Department of Mechanical Engineering
The properties of austenitic stainless steel weld metals are strongly influenced by the duplex structure of austenite and ferrite. The objective of the present study was to investigate the effect of thermal cycles on the ferrite content in austenitic stainless steel weld metal, and to explore the ferrite-austenite solid state transformation of austenitic stainless steel weld metal. The results indicate that the peak temperature, cooling rate, and number of thermal cycles have a significant effect on ferrite content in the austenitic stainless steel weld. As the cooling rate increases, or the number of thermal cycles decreases, the amount of retained ferrite is increased at room temperature. New delta ferrite phase can be formed if the reheating peak temperature is above the optimum gamma solvus temperature of austenitic stainless steel, nod this new delta ferrite can be retained under rapid cooling conditions.
Hot ductility behaviour of 18%Ni maraging steel
(1999)
Chen, MH
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Chou, CP
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機械工程學系
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Department of Mechanical Engineering
The hat ductility behaviour of a material exhibits a good correlation with its heat affected zone (HAZ) hot cracking sensitivity. This study was aimed at evaluating the HAZ hot cracking susceptibility of 18% Ni maraging steel and exploring the relationship between hot ductility and microstructure of the HAZ in an 18% Ni maraging steel. optical metallography and scanning electron microscopy (SEM) investigations revealed that the ductility loss was accompanied by fracture transition from ductile transgranular mode to brittle intergranular made. Fracture specimens were examined using SEM with an attached energy dispersive X-ray spectrometer. The SEM fractographs indicate that the ductile transgranular dimple fracture (i.e. coarse) surface was the titanium rich inclusion precipitated at the grain boundary, which promotes microvoid nucleation, growth, and coalescence and leads to localisation deformation until final fracture. Results of hot ductility tests also indicate that the titanium I ich inclusion can reduce the ultimate tensile strength without reducing the tensile ductility. The appearance of brittle intergranular fracture is caused by the constitutional liquation of titanium sulphide inclusions that induces formation of a liquid film penetrating the grain boundaries. The results show that titanium sulphide inclusion nor only reduces the HAZ tensile ductility, but also reduces the ultimate tensile strength.
Microstructure and corrosion properties of AA2091-T3 in weld heat affected zone
(1999)
Chen, MH
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Hwang, RY
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Chou, CP
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機械工程學系
;
Department of Mechanical Engineering
The microstructure and corrosion behaviour of at? AA2091-T3 alloy weld hear affected zone (HAZ) were investigated using differential scanning calorimetry and electrochemical measurements iii 3.5%NaCl solution. The delta' phase was precipitated during the welding cooling process. The ai amount of delta' precipitates depends on the peak temperature of the,veld thermal cycle. The quantity of Guinier-Preston (GP) zones,vas related to the duration of natural aging. Variation of pitting potential of the weld HAZ was due to the lithium content in the matrix. The increasing dissolution Of GP zones and delta' phase was accompanied by increasing lithium contents in the matrix and caused a negative shifting of pitting potential. The legion containing the lowest quantity of GP zones and delta' phase had the lowest pitting potential. The higher the peak temperature of rite weld thermal cycles the mole precipitation of T-2 phase on gl ain boundaries occurs. These T-2 phases can cause severe intergranular exfoliation corrosion.
Photoexcited carrier relaxation in a-axis oriented YBa2Cu3O7-delta thin films measured by femtosecond time-resolved spectroscopy
(2003-05-01)
Luo, CW
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Chen, MH
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Wu, KH
;
Lin, JY
;
Juang, JY
The photoexcited carrier relaxation dynamics in a-axis oriented YBa2Cu3O7-delta (YBCO) thin films has been investigated by femtosecond time-resolved spectroscopy. Distinct responses along b-axis (CuO3 chain) and c-axis of YBCO have been separated by the polarization-dependent femtosecond pump-probe measurements. It is found that the transient reflectivity (DeltaR/R) curves for the electric field E of the polarized light parallel to the b- or c-axes of the a-axis oriented YBCO film (Eparallel tob or Eparallel toc) are significantly different. The opening of superconducting gap in CuO3 chain was obviously observed from the measurements of the DeltaR/R curves for Eparallel tob at temperatures below T-c. However, the DeltaR/R curves for Eparallel toc did not exhibit the similar characteristics. (C) 2003 Elsevier Science B.V. All rights reserved.
Polarization-dependent X-ray absorption spectroscopy of in-plane aligned (100) YBa2Cu3O7-delta thin films
(2003-05-01)
Luo, CW
;
Liu, SJ
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Chen, MH
;
Wu, KH
;
Lin, JY
Polarization-dependent X-ray absorption spectra (XAS) on the 0 Is edge has been measured on a highly in-plane aligned (100) a-axis YBa2Cu3O7-delta (YBCO) thin films. The in-plane XAS, that is, the electric field E of the linearly polarized synchrotron light parallels to b- or c-axes of YBCO films (Eparallel tob or Eparallel toc), were obtained in a normal-incidence alignment. Furthermore, the XAS for Eparallel toa was calculated from the oblique incidence to the sample with different angles. The present results are consistent with those obtained by using detwinned YBCO single crystals. (C) 2003 Elsevier Science B.V. All rights reserved.
Spatial symmetry of the superconducting gap of YBa2Cu3O7-delta obtained from femtosecond spectroscopy
(2003-12-01)
Luo, CW
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Chen, MH
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Chen, SP
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Wu, KH
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Juang, JY
The polarized femtosecond spectroscopies obtained from well characterized (100) and (110) YBa2Cu3O7-delta thin films are reported. This bulk-sensitive spectroscopy, combined with the well-textured samples, serves as an effective probe to quasiparticle relaxation dynamics in different crystalline orientations. The significant anisotropy in both the magnitude of the photoinduced transient reflectivity change and the characteristic relaxation time indicates that the nature of the relaxation channel is intrinsically different in various axes and planes. By an orientation-dependent analysis, d-wave symmetry of the bulk-superconducting gap in cuprate superconductors emerges naturally.
Spatially resolved relaxation dynamics of photoinduced quasiparticles in underdoped YBa2Cu3O7-delta
(2005-09-01)
Luo, CW
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Shih, PT
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Chen, YJ
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Chen, MH
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Wu, KH
The spatially resolved relaxation characteristics of photoinduced quasiparticles (QPs) in CuO2 planes of underdoped YBCO are disclosed by polarized fs time-resolved spectroscopy. The relaxation time (tau) along b axis diverges at T-c, and appears to be governed by a temperature-dependent gap Delta(T) at T < T-c. Furthermore, for T>T-c, a monotonic increase of tau with decreasing T along the b axis and ab diagonal was observed and can be attributed to a temperature-independent gap Delta(p). The results lend support to the recombination dominant scenario of QP dynamics. However, the QP thermalization may take part along the nodal direction in the highly underdoped samples.