Browsing by Issue Date, starting with "1997-09-08"

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Negative differential resistance in InGaAs/AlAsSb/InGaAs single-barrier heterostructure
(1997-09-08)
Chen, WK
;
Cheng, RH
;
Ou, J
;
交大名義發表
;
電子物理學系
We have observed negative differential resistance in InP lattice-matched InGaAs/AlAsSb/InGaAs single-barrier tunneling heterostructure. With a 10-nm-thick barrier, the diode exhibits a peak-to-valley current ratio of 4.2 (1.2) and peak current density of 54 (158) A/cm(2) at 100 K(300 K). (C) 1997 American Institute of Physics.
Neural-network-based call admission control in ATM networks with heterogeneous arrivals
(1997-09-08)
Hah, JM
;
Tien, PL
;
Yuang, MC
;
交大名義發表
;
資訊工程學系
Call Admission Control (CAC) has been accepted as a potential solution for supporting diverse, heterogeneous traffic sources demanding different Quality of Services (QOSs) in ATM networks. Also, CAC is required to consume a minimum of time and space to make call acceptance decisions. In this paper, we present an efficient neutral-network-based CAC (NNCAC) mechanism for ATM networks with heterogeneous arrivals. All heterogeneous traffic calls are initially categorized into various classes. Based an the number of calls in each class, NNCAC efficiently and accurately estimates the cell delay and cell loss ratio of each class in real time by means of a pre-trained neutral network. According to our decent study which exhibits the superiority of the employment of analysis-based training data over simulation-based data, we particularly construct the training data from a heterogeneous-arrival dual-class queueing model M-[N1] + I-[N2]/D/1/K, where M and I represent the Bernoulli and interrupted Bernoulli processes, and N-1 and N-2 represent the corresponding numbers of calls, respectively. Analytic results of the queueing model are confirmed by simulation results. Finally, we demonstrate the profound agreement of our neural-network-based estimated results with analytic results, justifying the viability of our NNCAC mechanism. (C) 1997 Elsevier Science B.V.
Single-layer organic electroluminescent devices by vapor deposition polymerization
(1997-09-08)
Wen, WK
;
Jou, JH
;
Chiou, JF
;
Chang, WP
;
Whang, WT
A novel method is presented herein regarding the preparation of a single-layer organic electroluminescent (EL) polymer thin film device. The device was made by simultaneously co-depositing two polymer thin film forming monomers, a hole-transporting molecule triphenyldiamine derivative (TPD) and an EL active molecule 1,1 4, 4-tetraphenyl-1,3-butadiene (TPB) on an indium-tin-oxide (ITO) glass substrate. The device structure was composed of the ITO glass, the TPD, and TPB co-doped polymer thin film, and a thermally evaporated aluminum electrode. Carrier injection from the two electrodes was significantly observed and the blue light in EL spectra, with an emission maximum at 435 nm, was triggered at a driving voltage of 17 V. (C) 1997 American Institute of Physics.
Deviations from quantized Hall conductivity and current density distribution in finite 2DEG samples
(1997-09-10)
Bartos, I
;
Rosenstein, B
;
電子物理學系
;
Department of Electrophysics
Simple expressions for local and total Hall conductivities in finite two dimensional electron systems under magnetic field are obtained from the Kubo formula. The deviations of the Hall conductivity from integer values are always negative and their magnitude is inversely proportional to the effective width of the sample and proportional to the slope of the Landau branch dispersion relation at the Fermi level, Eq. (22). We also calculate the local conductivity in finite samples. The conductivity density is constant in the bulk and sums up to an integer value. Its spatial distribution is terminated in the bulk in a universal manner. Illustrations for simple models of the confinement barrier, as well as relation to recent experimental data for quantum wires are given.
Deviations from quantized Hall conductivity and current density distribution in finite 2DEG samples
(1997-09-10)
Bartos, I
;
Rosenstein, B
;
電子物理學系
;
Department of Electrophysics
Simple expressions for local and total Hall conductivities in finite two dimensional electron systems under magnetic field are obtained from the Kubo formula. The deviations of the Hall conductivity from integer values are always negative and their magnitude is inversely proportional to the effective width of the sample and proportional to the slope of the Landau branch dispersion relation at the Fermi level, Eq. (22). We also calculate the local conductivity in finite samples. The conductivity density is constant in the bulk and sums up to an integer value. Its spatial distribution is terminated in the bulk in a universal manner. Illustrations for simple models of the confinement barrier, as well as relation to recent experimental data for quantum wires are given.
Yellow luminescence in n-type GaN epitaxial films
(1997-09-15)
Chen, HM
;
Chen, YF
;
Lee, MC
;
Feng, MS
;
材料科學與工程學系
Photoluminescence, resonant Raman scattering, and photoconductivity measurements have been employed to study the yellow; emission in undoped n-type and a set of Se-doped GaN epitaxial films. It is best described by a transition from the conduction-band edge to a deep acceptor. Unlike the donors and accepters used in most previous studies that substitute Ga sires, Se atoms can replace N sites. With this unique fact, we identify that the origin of the yellow emission involves the nitrogen antisite. In addition, it is found that persistent photoconductivity can be observed after the yellow; band excitation, We further suggest that the nitrogen antisite exhibits a metastable behavior similar to the arsenic antisite in GaAs.
Precision side-polished fibers with a long interaction length
(1997-09-15)
Liaw, SK
;
Chiu, JY
;
Tseng, SM
;
光電工程學系
;
Department of Photonics
We succeeded in fabricating precision side-polished fibers embedded in silicon V-grooves and then polished six fibers using a home-made polisher. The control of the polishing depth of the side-polished fibers was as accurate as +/-0.25 mu m. Another merit of our polished samples was their negligible losses even though the effective interaction lengths were long. Experimental calibrations of our samples agreed with theoretical results.
Precision side-polished fibers with a long interaction length
(1997-09-15)
Liaw, SK
;
Chiu, JY
;
Tseng, SM
;
光電工程研究所
;
Institute of EO Enginerring
We succeeded in fabricating precision side-polished fibers embedded in silicon V-grooves and then polished six fibers using a home-made polisher. The control of the polishing depth of the side-polished fibers was as accurate as +/-0.25 mu m. Another merit of our polished samples was their negligible losses even though the effective interaction lengths were long. Experimental calibrations of our samples agreed with theoretical results.
Electroreflectance of surface-intrinsic- n(+)-type doped GaAs
(1997-09-15)
Wang, DP
;
Huang, KM
;
Shen, TL
;
Huang, KF
;
Huang, TC
The electroreflectance spectra of surface-intrinsic-n(+)-type-doped GaAs were measured at various bias voltages (V-bias). Results revealed many Franz-Keldysh oscillations (FKOs) above the band-gap energy, which have been attributed to a uniform electric field (F) in the undoped layer below the surface. However, there has been no other evidence for the uniformity of F in the undoped layer. Since it is known that F can be deduced from the periods of the FKOs, the relations between F and V-bias can, thereby, be obtained. The nearly linear relation, thus found, confirms the existence of a nearly uniform field in the undoped layer. From the plot of F against V-bias,the Values of the thickness of the undoped layer and the barrier height can also be evaluated. (C) 1997 American Institute of Physics.
Electroreflectance of surface-intrinsic- n(+)-type doped GaAs
(1997-09-15)
Wang, DP
;
Huang, KM
;
Shen, TL
;
Huang, KF
;
Huang, TC
The electroreflectance spectra of surface-intrinsic-n(+)-type-doped GaAs were measured at various bias voltages (V-bias). Results revealed many Franz-Keldysh oscillations (FKOs) above the band-gap energy, which have been attributed to a uniform electric field (F) in the undoped layer below the surface. However, there has been no other evidence for the uniformity of F in the undoped layer. Since it is known that F can be deduced from the periods of the FKOs, the relations between F and V-bias can, thereby, be obtained. The nearly linear relation, thus found, confirms the existence of a nearly uniform field in the undoped layer. From the plot of F against V-bias,the Values of the thickness of the undoped layer and the barrier height can also be evaluated. (C) 1997 American Institute of Physics.
Probing flavor changing neutral Higgs couplings at LHC
(1997-09-18)
Hou, WS
;
Lin, GL
;
Ma, CY
;
Yuan, CP
;
電子物理學系
Uniquely heavy with mass at the weak scale, the top quark may have large flavor changing couplings to Higgs bosons that are as yet unexplored. We show that such couplings could be directly probed at the LHC through the parton subprocess c((c) over bar)g --> t((t) over bar)A(0), where the pseudoscalar A(0) subsequently decays into t (c) over bar or (t) over bar c, giving rise to the intriguing final state of like sign top quark pairs. After demanding l(+/-)l(+/-), missing energy and two b-jets, the major background turns out to be q (q) over bar' --> Wt (t) over bar, which can be partially suppressed by jet counting. The signal can then manifest itself in the asymmetry of numbers of l(+)l(+) and l(-)l(-) events. To further improve the signal over background, efficient t vs. (t) over bar tagging methods should be developed. (C) 1997 Elsevier Science B.V.
Probing flavor changing neutral Higgs couplings at LHC
(1997-09-18)
Hou, WS
;
Lin, GL
;
Ma, CY
;
Yuan, CP
;
電子物理學系
Uniquely heavy with mass at the weak scale, the top quark may have large flavor changing couplings to Higgs bosons that are as yet unexplored. We show that such couplings could be directly probed at the LHC through the parton subprocess c((c) over bar)g --> t((t) over bar)A(0), where the pseudoscalar A(0) subsequently decays into t (c) over bar or (t) over bar c, giving rise to the intriguing final state of like sign top quark pairs. After demanding l(+/-)l(+/-), missing energy and two b-jets, the major background turns out to be q (q) over bar' --> Wt (t) over bar, which can be partially suppressed by jet counting. The signal can then manifest itself in the asymmetry of numbers of l(+)l(+) and l(-)l(-) events. To further improve the signal over background, efficient t vs. (t) over bar tagging methods should be developed. (C) 1997 Elsevier Science B.V.
Charge transfer process in Tl2Ba2Ca2Cu3O10 and Tl2Ba2CaCu2O8 thin films probed by polarized X-ray absorption spectroscopy
(1997-09-26)
Chen, JM
;
Liu, RG
;
Liu, RS
;
Lin, HC
;
Uen, TM
We report high-resolution polarized O 1s X-ray absorption spectra of highly c-axis-oriented Tl2Ba2Ca2Cu3O10 (Tl-2223) and Tl2Ba2CaCu2O8 (Tl-2212) superconducting thin films. Near the O Is edge, the low-energy pre-edge peak at 528.3 eV has mainly O 2p(xy) symmetry, while the pre-edge peak at 529.4 eV has predominantly O 2p, character. The spectral weight of the pre-edge peak at 528.3 eV, originating from the CuO2 planes, increases similar to 60% from the Tl-2212 to Tl-2223 thin films. Conversely, the high-energy pre-edge peak at 530.3 eV, originating from the TlO planes, is lower in spectral intensity by similar to 30% in Tl-2223 as compared to the Tl-2212 thin film. The experimental results clearly demonstrate the pictures of the self doping due to the charge transfer from the CuO2 layers to the TlO layers. (C) 1997 Published by Elsevier Science B.V.
Effect of lateral substituents on the mesomorphic properties of side-chain liquid crystalline polysiloxanes containing 4-[(S)-2-methyl-1-butoxy]phenyl 4-(alkenyloxy)benzoate side groups
(1997-09-30)
Hsu, CS
;
Chu, PH
;
Chang, HL
;
Hsieh, TH
;
交大名義發表
The synthesis of side-chain liquid crystalline polysiloxanes containing either 4-[(S)-2-methyl-1-butoxy] phenyl 4-(alkenyloxy)benzoate or laterally fluoro-, chloro-, bromo-, and methoxy-substituted 4-[(S)-2-methyl-1-butoxy] phenyl 4-(alkenyl oxy)benzoate mesogenic side groups is presented. The mesomorphic properties of the synthesized polymers have been characterized by optical polarizing microscopy, differential scanning calorimetry, and X-ray diffraction measurements. The effects of spacer length and lateral substituent on the mesomorphic properties of the obtained polymers are examined. The five polymers which contain three methylene units in the spacers show no mesophase, while the five polymers which contain eleven methylene units in the spacer display smectic mesomorphism. Among the other fifteen polymers which contain respectively four, five, or six methylene units in the spacers, those with small fluoro and chloro substituents reveal respectively an Sd phase, while those with bulky bromo and methoxy substituents show no liquid crystalline behavior. The experimental results demonstrate that introducing a bulky lateral substituent into the mesogenic core of a polymer depresses the tendency to form a mesophase. Furthermore, the technique of thermally stimulated current has been used to study the dipolar relaxation mechanisms in a side-chain liquid crystalline polysiloxane. (C) 1997 John Wiley & Sons Inc.
The epoxy-polycarbonate blends cured with aliphatic amine .2. Thermal and mechanical properties mechanisms
(1997-09-30)
Lin, ML
;
Chang, KH
;
Chang, FC
;
Li, MS
;
Ma, CCM
The transamidation reaction converts the carbonate and amine into N-aliphatic aromatic carbamate and urea, causing the equivalent of oxirane and amine nonstoichiometric in the primary cure stage. After postcure, the substitution reaction takes place and results in a more tightly crosslinked structure. Such a higher crosslinking density is responsible for higher T-g, tensile strength, and tensile modulus in the postcure stage than that in the primary and secondary cure stages. This trend is more pronounced in those PC-epoxy blends containing higher molecular weight aliphatic amine or a higher content of PC. This phenomenon is due to the difference in the fraction of amino group of aliphatic amine consumed in the transamidation. PC-epoxy/aliphatic amine blends show minor improvement in the high strain rate Izod impact tests, while toughness improvement for some blends is substantial at low strain rate tensile tests. (C) 1997 John Wiley & Sons, Inc.
The epoxy-polycarbonate blends cured with aliphatic amine .1. Mechanism and kinetics
(1997-09-30)
Lin, ML
;
Chang, KH
;
Chang, FC
;
Li, MS
;
Ma, CCM
Reaction mechanism of the PC-epoxy blends cured by aliphatic amine has been investigated by varying PC contents in the blends. The transamidation reaction tends to convert nearly all the carbonates into N-aliphatic aromatic carbamates even at ambient temperature before normal curing. The remaining amine proceeds the normal curing with epoxy at a higher temperature (80 degrees C). For the PC-epoxy/aliphatic amine blend containing 6 wt % PC, the yielded N-aliphatic aromatic carbamate further reacts with amine to produce the urea structure. The urea undergoes substitution reaction with the hydroxyl formed from the normal curing to give the N-aliphatic aliphatic carbamate. For the blend containing 12 wt % PC, the N-aliphatic aromatic carbamate converts into the N-aliphatic aliphatic carbamate via two different routes. For the blend containing lower molecular weight of the aliphatic amine, the N-aliphatic aromatic carbamate reacts with hydroxyl to form the N-aliphatic aliphatic carbamate directly. For the blend containing higher molecular weight of aliphatic amine, the N-aliphatic aromatic carbamate decomposes into the aliphatic isocyanate accelerated by the presence of the residual oxirane. The isocyanate formed then reacts with hydroxyl to yield the N-aliphatic aliphatic carbamate. The activation energy (E-alpha) and preexponential factor (A) of the PC-epoxy/POPDA blends decrease with the increase of the PC content. Kinetic study by thermal analysis by the method of autocatalyzed model is able to correctly predict oxirane conversion vs. time relationship for the neat epoxy/aliphatic amine and the PC-epoxy/aromatic amine systems because the dominant reaction is the normal curing reaction between amine and oxirane. The model fails to predict the PC-epoxy/aliphatic amine system because the system is complicated by several other reactions besides the normal curing reaction. (C) 1997 John Wiley & Sons, Inc.
The epoxy-polycarbonate blends cured with aliphatic amine .2. Thermal and mechanical properties mechanisms
(1997-09-30)
Lin, ML
;
Chang, KH
;
Chang, FC
;
Li, MS
;
Ma, CCM
The transamidation reaction converts the carbonate and amine into N-aliphatic aromatic carbamate and urea, causing the equivalent of oxirane and amine nonstoichiometric in the primary cure stage. After postcure, the substitution reaction takes place and results in a more tightly crosslinked structure. Such a higher crosslinking density is responsible for higher T-g, tensile strength, and tensile modulus in the postcure stage than that in the primary and secondary cure stages. This trend is more pronounced in those PC-epoxy blends containing higher molecular weight aliphatic amine or a higher content of PC. This phenomenon is due to the difference in the fraction of amino group of aliphatic amine consumed in the transamidation. PC-epoxy/aliphatic amine blends show minor improvement in the high strain rate Izod impact tests, while toughness improvement for some blends is substantial at low strain rate tensile tests. (C) 1997 John Wiley & Sons, Inc.
The epoxy-polycarbonate blends cured with aliphatic amine .1. Mechanism and kinetics
(1997-09-30)
Lin, ML
;
Chang, KH
;
Chang, FC
;
Li, MS
;
Ma, CCM
Reaction mechanism of the PC-epoxy blends cured by aliphatic amine has been investigated by varying PC contents in the blends. The transamidation reaction tends to convert nearly all the carbonates into N-aliphatic aromatic carbamates even at ambient temperature before normal curing. The remaining amine proceeds the normal curing with epoxy at a higher temperature (80 degrees C). For the PC-epoxy/aliphatic amine blend containing 6 wt % PC, the yielded N-aliphatic aromatic carbamate further reacts with amine to produce the urea structure. The urea undergoes substitution reaction with the hydroxyl formed from the normal curing to give the N-aliphatic aliphatic carbamate. For the blend containing 12 wt % PC, the N-aliphatic aromatic carbamate converts into the N-aliphatic aliphatic carbamate via two different routes. For the blend containing lower molecular weight of the aliphatic amine, the N-aliphatic aromatic carbamate reacts with hydroxyl to form the N-aliphatic aliphatic carbamate directly. For the blend containing higher molecular weight of aliphatic amine, the N-aliphatic aromatic carbamate decomposes into the aliphatic isocyanate accelerated by the presence of the residual oxirane. The isocyanate formed then reacts with hydroxyl to yield the N-aliphatic aliphatic carbamate. The activation energy (E-alpha) and preexponential factor (A) of the PC-epoxy/POPDA blends decrease with the increase of the PC content. Kinetic study by thermal analysis by the method of autocatalyzed model is able to correctly predict oxirane conversion vs. time relationship for the neat epoxy/aliphatic amine and the PC-epoxy/aromatic amine systems because the dominant reaction is the normal curing reaction between amine and oxirane. The model fails to predict the PC-epoxy/aliphatic amine system because the system is complicated by several other reactions besides the normal curing reaction. (C) 1997 John Wiley & Sons, Inc.
Effect of lateral substituents on the mesomorphic properties of side-chain liquid crystalline polysiloxanes containing 4-[(S)-2-methyl-1-butoxy]phenyl 4-(alkenyloxy)benzoate side groups
(1997-09-30)
Hsu, CS
;
Chu, PH
;
Chang, HL
;
Hsieh, TH
;
應用化學系
The synthesis of side-chain liquid crystalline polysiloxanes containing either 4-[(S)-2-methyl-1-butoxy] phenyl 4-(alkenyloxy)benzoate or laterally fluoro-, chloro-, bromo-, and methoxy-substituted 4-[(S)-2-methyl-1-butoxy] phenyl 4-(alkenyl oxy)benzoate mesogenic side groups is presented. The mesomorphic properties of the synthesized polymers have been characterized by optical polarizing microscopy, differential scanning calorimetry, and X-ray diffraction measurements. The effects of spacer length and lateral substituent on the mesomorphic properties of the obtained polymers are examined. The five polymers which contain three methylene units in the spacers show no mesophase, while the five polymers which contain eleven methylene units in the spacer display smectic mesomorphism. Among the other fifteen polymers which contain respectively four, five, or six methylene units in the spacers, those with small fluoro and chloro substituents reveal respectively an Sd phase, while those with bulky bromo and methoxy substituents show no liquid crystalline behavior. The experimental results demonstrate that introducing a bulky lateral substituent into the mesogenic core of a polymer depresses the tendency to form a mesophase. Furthermore, the technique of thermally stimulated current has been used to study the dipolar relaxation mechanisms in a side-chain liquid crystalline polysiloxane. (C) 1997 John Wiley & Sons Inc.
Determination of puerarin, daidzein, paeoniflorin, cinnamic acid, glycyrrhizin, ephedrine, and [6]-gingerol in Ge-gen-tang by micellar electrokinetic chromatography
(1997-09-30)
Huang, HY
;
Hsieh, YZ
;
應用化學系
;
Department of Applied Chemistry
This paper presents a micellar electrokinetic chromatography (MEKC) method with diode-array detection to analyze for seven major components in the traditional Chinese medicinal preparation of Ge-gen-tang. Those analytes were successfully separated within 9 min using a pH 9.0 berate buffer containing 20 mM Sodium dodecyl sulfate at 30 degrees C. Experimental results indicated that the relative standard deviations of the analytes' migration times were <0.43% and the correlation coefficients of the analytes' linear calibration graphs exceeded 0.996. The quantities of the seven components in four different Ge-gen-tang samples were determined by the MEKC method coupled with a relatively simple extraction method. (C) 1997 Elsevier Science B.V.