Browsing by Author "Chen, SP"

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A new type of perfect-reconstruction QMF banks
(1997)
Chen, SG
;
Kao, MC
;
Chen, SP
;
交大名義發表
;
電子工程學系及電子研究所
A new type of perfect-reconstruction QMF system is proposed. The filter system is based on half-band filters so that it has lower complexity and group delay than the known QMF systems, with a comparable performance. It is shown that the new PR QMF system achieves zero phase distortion and zero amplitude distortion, in addition to producing aliase-free outputs. A filter optimization technique is also proposed for its prototype low pass filter. For realization, its polyphase architecture is derived, which has very simple structure.
Anisotropic photoexcited carrier dynamics in (100)-, (001)-, and (110)-oriented YBCO films by polarized ultrafast optical spectroscopy
(2003-06-01)
Luo, CW
;
Chen, MH
;
Chiu, CC
;
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.
Critical current anisotropy and pinning in (103)YBCO superconducting thin films on SrTiO3(110) substrates
(1996)
Wu, KH
;
Huang, SJ
;
Wang, SJ
;
Chen, SP
;
Juang, JY
High quality (103) oriented YBCO superconducting thin films with T-c approximate to 89 K have been prepared by pulsed laser deposition on SrTiO3 (110) substrates. The grain morphology of the films, as revealed by both scanning electron microscopy and atomic force microscopy, shows distinct brick wall structure. The critical current densities measured along (001) direction of the substrates are about an order of magnitude higher than that measured on the other in-plane direction with no apparent weak-link behavior observed. The critical current densities in both directions, however, show a similar linear temperature dependence over a wide temperature range, indicative of a flux-creep limited mechanism. The creep rate in both cases, as fit to the Anderson-Kim creep model, is of order of 1 which is about an order of magnitude higher than that of conventional type-II superconductors. Samples co-ablated with silver were also studied. Detail mechanisms of pinning as wall as grain morphology evolutions are discussed.
Investigation of the evolution of YBa2Cu3O7-delta films deposited by scanning pulsed laser deposition on different substrates
(1997-10-01)
Wu, KH
;
Chen, SP
;
Juang, JY
;
Uen, TM
;
Gou, YS
The evolution of surface morphology of YBa2Cu3O7-delta (YBCO) thin films with sequential thickness ranging from 2 to 250 nm on (110) NdGaO3, as-polished and annealed (110) SrTiO3 and (100) MgO substrates has been systematically investigated with atomic force microscopy (AFM) to elucidate the effects of lattice mismatch, pre-annealing treatment and substrate orientation on the growth mechanism of YBCO films. A scanning pulsed laser deposition system, which has been used to produce 50-mm-diameter thin films, allows us to deposit (001) or (103) YBCO films with various thicknesses on separated substrates in a single run under essentially identical deposition conditions. The AFM images show that (001) YBCO films grown on NdGaO3 follow the Stranski-Krastanov (layer then island growth) mode, while films grown on as-polished MgO and on annealed MgO follow the Volmer-Weber (island without layer growth) mode and step-flow mode, respectively. On the other hand, the evolution of (103) YBCO films grown on both as-polished and annealed (110) SrTiO3 substrates reveals the similar step-flow growth along tilt steps, i.e. the growth proceeds with the c-axis of YBCO along [100] or [010] directions of substrate. The formations of distinct elongated grains and microcracks in planar view of such films are also discussed. (C) 1997 Elsevier Science B.V.
Morphology evolution and crack formation of YBa2Cu3O7 on (110)SrTiO3 substrates
(1997-08-01)
Wu, KH
;
Chen, SP
;
Juang, JY
;
Uen, TM
;
Gou, YS
The detailed grain evolution of (103)-oriented YBa2Cu3O7 (YBCO) films grown on as-polished and preannealed (110)SrTiO3 was studied using atomic force microscopy (AFM). The scanning laser deposition system used in preparing the films allows us to deposit films with various thicknesses in a single deposition run. The AFM images revealed that although more regular alignment of grains at the initial nucleation for films grown on the pre-annealed substrates, there were no apparent differences in surface structure for the thicker films grown on both as-polished and annealed substrates. It was also found that as the films increased to a critical thickness, microcracks were formed in both cases.
Parallel-coupled microstrip filters with over-coupled end stages for suppression of spurious responses
(2003-10-01)
Kuo, JT
;
Chen, SP
;
Jiang, M
;
交大名義發表
;
電信工程研究所
In a parallel-coupled microstrip filter, end stages with over-coupling are designed to suppress the unwanted responses at twice the passband frequency (2f(o)). The inherent transmission zero of an over-coupled input/output stage is shown tunable. It is found that increasing the image impedance of the filter sections can further enhance the suppression. The designed bandpass filters thus have a wide upper stopband and improved passband response symmetry. Measured results of fabricated circuits show that the idea works very well.
Preparation and electronic properties of YBa2Cu3Ox films with controlled oxygen stoichiometries
(1998-08-01)
Wu, KH
;
Hsieh, MC
;
Chen, SP
;
Chao, SC
;
Juang, JY
We describe a novel technique capable of controlling the oxygen content of YBa2Cu3Ox (YBCO) films in a precise and reversible manner. The temperature dependence of resistivity and the distinct two-plateau behavior in critical temperature T-CO versus oxygen content plot of these films are consistent with those observed in the bulk and single crystals of YBCO. The O 1s and Cu 2p absorption spectra of these films were measured by polarization-dependent X-ray absorption spectroscopy (XAS). The intensity variations of the pre-edge peaks as a function of oxygen content are discussed. We also used these films to systematically study the electron-phonon coupling strength and the position of Fermi level by using a femtosecond pump-probe technique. A clear sign-reversal of the transient reflectivity, which was consistently explained by the thermomodulation model, was observed. Both of these optical measurements support the idea that the electronic structure of YBCO cuprates is based on the charge transfer model with hybridization between the Cu and O sites.
Preparation of large area and investigation of initial film growth of YBa2Cu3O7 by scanning pulsed laser deposition
(1996-07-15)
Wu, KH
;
Wang, RC
;
Chen, SP
;
Lin, HC
;
Juang, JY
A pulsed laser deposition system with a rotating reflector, which can generate an extended plume with a diameter of about 40 mm, has been proven to be viable in producing large area YBa2Cu3O7 (YBCO) thin films. Films with uniformity of thickness and electrical properties to within a few percents were obtained. By making a further modification on this laser scanning system, YBCO films with various thicknesses can be deposited in situ on several pieces of substrates under essentially identical deposition conditions, and particularly, in a continuous fashion. The atomic force microscopy (AFM) images reveal that films grown on NdGaO3 follow the Stranski-Krastanov (layer then island growth) mode, while films grown on as-polished MgO and on annealed MgO follow the Volmer-Weber (islands without layer growth) mode and step-flow mode, respectively. (C) 1996 American Institute of Physics.
Preparation of large area and investigation of initial film growth of YBa2Cu3O7 by scanning pulsed laser deposition
(1996-07-15)
Wu, KH
;
Wang, RC
;
Chen, SP
;
Lin, HC
;
Juang, JY
A pulsed laser deposition system with a rotating reflector, which can generate an extended plume with a diameter of about 40 mm, has been proven to be viable in producing large area YBa2Cu3O7 (YBCO) thin films. Films with uniformity of thickness and electrical properties to within a few percents were obtained. By making a further modification on this laser scanning system, YBCO films with various thicknesses can be deposited in situ on several pieces of substrates under essentially identical deposition conditions, and particularly, in a continuous fashion. The atomic force microscopy (AFM) images reveal that films grown on NdGaO3 follow the Stranski-Krastanov (layer then island growth) mode, while films grown on as-polished MgO and on annealed MgO follow the Volmer-Weber (islands without layer growth) mode and step-flow mode, respectively. (C) 1996 American Institute of Physics.
Room-temperature single-electron charging and surface polarization in tunneling spectroscopy of high-T-c superconducting thin films
(1998-11-01)
Kao, RT
;
Wang, SJ
;
Chen, SP
;
Juang, JY
;
Wu, KH
The tunneling spectroscopy of both YBa2Cu3O7-delta (YBCO) and TlBa2Ca2Cu3O9+/-delta (Tl-1223) superconducting thin films was studied using scanning tunneling microspectroscopy. The marked staircase structure in the current-voltage characteristics showed the manifestation of single-electron charging of two series-coupled mesoscopic tunnel junctions. For YBCO films, the step width across the zero-bias voltage (Delta V-0) was larger than that of other steps (Delta V-n), with Delta V(0)congruent to 1.5L Delta V-n, as the bias voltage exceeded a threshold value. Such a step-widening anomaly, however, was not seen in Tl-1223 films. Preliminary surface polarization measurements showed that the polarization is much more hysteretic for YBCO than for Tl-1223 films, suggesting that the anomalous widening of Delta V-0 may originate from the polarization-induced residual charging effect. [S0163-1829(98)09841-5].
Single electron tunneling and superconducting energy gap in YBa2Cu3O7+delta and TlBa2Ca2Cu3O9+/-delta films
(1997-08-01)
Kao, RT
;
Wang, SJ
;
Chen, SP
;
Juang, JY
;
Wu, KH
The granularity of the high-Tc cuprates, though has been considered to be obnoxious for many applications, appeared to giving rise to the exotic incremental charging effects. This paper further demonstrates that it can also serve as an alternative for extracting the superconducting gap parameters of these materials. The results of 2 Delta(0)/KBTc approximate to 6-8(YBCO) and approximate to 9-11(Tl-1223), though are consistent with those obtained by other methods, the Delta(T)'s, however, are not BCS-like.
Spatial symmetry of the superconducting gap of YBa2Cu3O7-delta obtained from femtosecond spectroscopy
(2003-12-01)
Luo, CW
;
Chen, MH
;
Chen, SP
;
Wu, KH
;
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.