Browsing by Author "Lee, WD"

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Analysis of average transport critical current density of oxide superconductors deposited on a silver base
(1996-01-01)
Lee, WD
;
Yang, TJ
;
Chiou, BS
;
電子物理學系
;
電控工程研究所
Considering the resistivity of a silver base and the contact resistance of a silver base with a bulk superconductor, the dependence of average critical current density J(c)(D,H-a) of a high-T-c, oxide superconductor deposited oil a silver base on the sample dimension D and the applied magnetic field H-a is calculated using a generalized critical-state model, which was recently extended to low field by Chen and Yang [Physica C 224 (1994) 345]. The self-field effect of the average critical current was also taken into account in the model. Based on the generalized critical state model, new behaviors of J(c)(D, H-a) in the low-field range of H-c1 less than or equal to H-a much less than H-c2 are obtained. In addition, a constant J(c)(D, H-a) for 0 less than or equal to H-a less than or equal to H-c1 is obtained, which is different from the results derived from a model that does not include the self-field effect. However, these theoretical results can be fitted to the experimental data to obtain some important macroscopic material parameters and may be used to interpret the dimension effect of transport J(c) of a high-T-c oxide superconductor deposited on a silver base.
Coupling technique for efficient interfacing between silica waveguides and planar photonic crystal circuits
(2004-12-20)
Chau, YF
;
Yang, TJ
;
Lee, WD
;
電子物理學系
;
Department of Electrophysics
We describe a two-step-size tapered structure with one defect pair that can markedly enhance the coupling efficiency at the entrance and exit terminals of a planar photonic crystal (PPC) waveguide. PPC waveguides are composed of circular dielectric rods set in two-dimensional square lattices. On the basis of our simulations, we found that the optimized scheme maximizes the power transmission above 90% at a wavelength of 1.55 mum. Besides, one can control the central frequency for optical communications by determining this defect configuration in an optimization procedure. Moreover, by properly adjusting the defect radii in PPC tapers, one can use the PPC circuit as a good reflector. (C) 2004 Optical Society of America.
Efficient mode coupling technique between photonic crystal heterostructure waveguide and silica waveguides
(2005-09-15)
Chau, YF
;
Yang, TJ
;
Gu, BY
;
Lee, WD
;
電子物理學系
We report an efficient mode coupling technique between silica waveguides (SWGs) and a planar photonic crystal heterostructure waveguide (PPCHWG) composed of two semi-infinite two-dimensional photonic crystals (PCs) with different filling fractions. By calculating the transmittance at output end of SWG, we find that the designed structures possess the merits of high coupling efficiency and relaxing fabricating tolerance. The maximal transmittance exceeds 90% at a wavelength near 1.65 mu m. The high transmission efficiency of PPCHWGs can be achieved without introducing any defect structures for optimal mode matching. Besides, the width of PPCHWGs can be flexibly adjusted via changing the separation of two component PCs in PC heterostructures. (c) 2005 Elsevier B.V. All rights reserved.
Influence of an external electric field on high-T-c superconductivity
(1996-04-20)
Lee, WD
;
Chen, JL
;
Yang, TJ
;
Chiou, BS
;
電子物理學系
The surface superconductivity induced by applying an electric field perpendicular to the surface of a high-T-c superconductor covered by an insulator and a gate electrode is studied theoretically. Taking the idea into account that the change of the surface carrier density confined in a thin screening layer modulates the transport properties of superconductors, we develop a simple method based on the Chen and Yang model to study the T-c shift dependence on the applied voltage in the frameworks of a Thomas-Fermi approximation and BCS theory. In such a case a novel boundary condition is introduced to modify the G-L equations. Solving the modified one, the T-c shift derived by the present model is similar in qualitative behavior to the Shapiro result, but much simpler in form. In addition, it is shown to be proportional to the applied field (the square of applied field) for thin film (thick film) for a given material. Based on our simple results, the critical current density J(c) and nucleation field H-c3 as functions of the applied voltage are easily derived.
Investigation of hysteresis and transport critical current under field cycle for Bi-based multifilamentary tapes
(1996-04-01)
Horng, L
;
Chen, K
;
Lee, WD
;
Yang, TJ
;
電子物理學系
The transport properties of Hi-based multifilamentary tapes on magnetic field have been studied. The tapes were fabricated by powder-in-silver tubing technique followed by appropriate sintering. The hysteretic behavior of transport critical current of the tapes with bending strain has been investigated. The experimental data are obtained at 77 K under field cycling from 0 to 1000 G to 0. The critical current density under a field does not vary when bending strain is smaller than a critical bending strain, and then decreases gradually with increasing bending strain. The normalized values are unaffected by bending strain up to 1.31%. The remnant magnetic filed H-m associated with the flux pinning force is obtained by an empirical expression, which is similar to generalized critical state model. The H-m is almost a constant for all bending strain. This observation indicates that the intrinsic pinning properties are not varied with bending strain.
The electric field effect in g(3) model of the layer superconductors
(2001-11-01)
Yang, TJ
;
Lee, WD
;
電子物理學系
;
Department of Electrophysics
High-T-c superconductor is considered as alternately stacked metal and insulating layers. The electric field is applied along the stacking direction and investigated in the frame of Ginzburg-Landau theory with the coupling g(3) between metallic layers for the transition temperature T-c. Then the system of equations for T-c can be solved by the transfer matrix method. The shift of the transition temperature is analytically derived and we find the criterion for the enhancement of T-c while the electric field is applied. (C) 2001 Elsevier Science B.V. All rights reserved.
The variation of the transition temperature of high T-c superconductors by electric field effect
(2000-11-01)
Yang, TJ
;
Lee, WD
;
電子物理學系
;
Department of Electrophysics
High Te superconductor is considered as an alternately stacked metal and insulating layers. The electric field effect in this layer superconductor is investigated by the Thomas-Fermi(TF) approximation for the screening of an applied electric field and in the frame of Ginzburg-Landau(CL) theory with the coupling between metallic layers for the transition temperature. The change of the transition temperature related to the coupling constant and applied voltage is derived for two and three CuO2 layers. The transition temperature for more than three layers is also briefly discussed.