Browsing by Author "Juang, JY"

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A comparison between the Fehrenbacher-Rice and the Liechtenstein-Mazin models
(2000-11-01)
Liu, SJ
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Lin, JY
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Chen, JM
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Nachimuthu, P
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Luo, CW
Unoccupied electronic states of Y1-xPrxCu3O7-y (x = 0 - 1) thin films have been investigated using high-resolution polarization-dependent O K-edge x-ray absorption spectroscopy (XANES). After determining the hole distribution among different oxygen sites of the Pr-doped thin films, we compare the experimental data with the Fehrenbacher and Rice (FR) model and the Liechtenstein and Mazin (LM) model, both of which are based on the hybridization of Pr 4f(z(x2-y2)) and O 2p(pi) orbitals in PrBa2Cu3O7-y. Our experimental results are more consistent with the FR model.
Activated kinetics of room-temperature-deposited SrTiO(3) thin films investigated by reflection-high-energy-electron-diffraction-monitored annealing at different heating rates
(2005-02-01)
Wang, TC
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Lee, JY
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Hsieh, CC
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Juang, JY
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Wu, KH
A series of reflection-high-energy-electron-diffraction-monitored annealings with different heating rates have been conducted on Strontium Titanate films deposited by laser ablation at room temperature. All the films exhibited a steep RHEED intensity rise above 660 degrees C during annealing with increasing temperature. The peak temperatures of the intensity derivatives were found to shift at different heating rates, suggesting an activated surface state transition. A Kissinger type plot of the peak temperatures showed an effective activation energy of 4 eV.
Activated kinetics of room-temperature-deposited SrTiO3 thin films investigated by reflection-high-energy-electron-diffraction-monitored annealing at different heating rates
(2005-02-01)
Wang, TC
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Lee, JY
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Hsieh, CC
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Juang, JY
;
Wu, KH
A series of reflection-high-energy-electron-diffraction-monitored annealings with different heating rates have been conducted on Strontium Titanate films deposited by laser ablation at room temperature. All the films exhibited a steep RHEED intensity rise above 660 degrees C during annealing with increasing temperature. The peak temperatures of the intensity derivatives were found to shift at different heating rates, suggesting an activated surface state transition. A Kissinger type plot of the peak temperatures showed an effective activation energy of 4 eV.
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
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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
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Wu, KH
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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
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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.
Annealing characteristics of pulsed laser deposited homoepitaxial SrTiO3 thin films
(2001-08-10)
Lee, JY
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Juang, JY
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Wu, KH
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Uen, TM
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Gou, YS
The homoepitaxial growth of SrTiO3 (STO) films on as-polished STO(1 0 0) substrates by pulsed laser deposition has been exploited in detail. The intimate correlation between the surface step edge density and reflection high-energy electron diffraction (RHEED) intensity is clearly demonstrated by measuring the relation between the initial RHEED intensity drop and laser repetition rates. Systematic in situ annealing schemes were performed to investigate the film growth mechanisms. The results indicate that the two or three level growth May have occurred during some annealing schemes and can be interpreted by Stoyanov's step edge density model. Complementary atomic force microscopy investigations of the film surfaces further tend direct support to the RHEED intensity observations. (C) 2001 Elsevier Science B.V. All rights reserved.
Bipolar photogenerated terahertz radiation in biased photoconductive switches
(2003-05-01)
Lin, PI
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Luo, CW
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Liu, HS
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Wu, KH
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Juang, JY
The characteristics of optically induced bipolar THz radiation in biased photoconductive switches were investigated systematically by using free-space electro-optic sampling technique. The emitted radiation shows nearly symmetrical waveform with broadband frequency spectrum spanning over 0.1-3 THz. It was observed that the bipolar nature and the emitted frequency spectrum distribution remained unchanged on varying the optical excitation fluence, strength of biased field and the emitter gap spacing. The dynamics of emitted THz transient are in agreement with the optically induced ultrafast charge transport process driven by the biased field. (C) 2003 Elsevier Science B.V. All rights reserved.
Characteristics of photogenerated bipolar terahertz radiation in biased photoconductive switches
(2002-10-15)
Lin, PI
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Chen, SF
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Wu, KH
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Juang, JY
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Uhn, TM
The characteristics of the optically induced bipolar terahertz (THz) radiation from biased semi-insulating GaAs photoconductive switches were investigated using a free-space electrooptic sampling technique, The emitted radiation shows a nearly symmetrical waveform with a broad-band frequency spectrum spanning over 0.1-3 THz, which displays essentially no dependence on the optical excitation fluence or strength of biased field. However, it does slightly depend on the file emitter gap spacing. The dynamics of the emitted THz transient is in agreement with the optically induced ultrafast charge transport process driven by the biased field.
Characteristics of YBa2Cu3O7 thin films deposited on substrates buffered by various TiO2 layers
(2001-04-15)
Lin, PL
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Liu, CW
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Hsieh, CC
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Wu, KH
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Juang, JY
Titanium nitride (TiN) and superconducting YBa2Cu3O7 (YBCO) thin films have been deposited sequentially on SrTiO3(STO)(100) substrates by in situ pulsed laser ablation. The TiN films were originally intended to serve as the lower contact electrode of the c-axis YBCO thin films. It was found that, although high-quality YBCO films could be obtained with the YBCO/TiN/STO(100) bilayer structure, the TiN(100) layer was oxidized which changed the structure into YBCO/TiO2/STO(100) during YBCO deposition. Comparative studies of depositing YBCO films directly onto a dc-sputtered TiO2/STO(100) template conventionally used in the selective epitaxial growth (SEG) process have, however, resulted in formation of a nonsuperconducting YBCO top layer. The characteristics of the resultant TiO2 layers obtained using various processes were analyzed to delineate the apparent discrepancies.
Charge transfer process in Tl2Ba2Ca2Cu3O10 and Tl2Ba2CaCu2O8 thin films probed by polarized X-ray absorption spectroscopy
(1997-09-26)
Chen, JM
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Liu, RG
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Liu, RS
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Lin, HC
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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.
Chemical vapor deposition of zinc oxide thin films on Y2O3/Si substrates
(2004)
Lin, CW
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Cheng, TY
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Chang, L
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Juang, JY
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材料科學與工程學系
Highly-oriented ZnO films were deposited on Y2O3/Si(111) and Y2O3/Si(100) substrates by atmospheric pressure metal-organic chemical vapor deposition (MOCVD) using zinc acetylacetonate (Zn(C5H7O2)(2)) and oxygen as sources. The substrate temperature and O-2 gas temperature were all kept at 500 degreesC. Y2O3, thermodynamically more stable than SiO2 and epitaxially formed on Si, is used as buffer layer. Epitaxial Y2O3 of 200 nm thickness were first grown by pulsed laser deposition on Si(111) and Si(100) substrates at 800 degreesC for 30 min. X-ray diffraction patterns of the samples only show sharp diffraction peaks of Y2O3{222} and ZnO{0002}. The photoluminescence spectra of ZnO show strong peaks of near the band edge and very weak peaks of deep-level emission at 7 K and room temperature. Cross-sectional transmission electron microscopy shows that the highly-oriented ZnO films has columnar structure with a grain size distribution of 600-850 Angstrom. Most of ZnO grains exhibit orientation relationship with Y2O3 as {0002}(ZnO)//{222}(Y2O3), <0110>(ZnO)//(112)(Y2O3), and <1120>(ZnO)//<110>(Y2O3). (C) 2004 WILEY-VCH Verlag GmbH & Co. KGaA. Weinheim.
Comment on "Conductivity of underdoped YBa2Cu3O7-delta: Evidence for incoherent pair correlations in the pseudogap regime"
(2003-05-02)
Luo, CW
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Juang, JY
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Lin, JY
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Wu, KH
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Uen, TM
Critical current anisotropy and pinning in (103)YBCO superconducting thin films on SrTiO3(110) substrates
(1996)
Wu, KH
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Huang, SJ
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Wang, SJ
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Chen, SP
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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.
Critical current densities of submillimeter single-grain TL-2223 superconducting thin film
(1996-02-15)
Lin, HC
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Juang, JY
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Wu, KH
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Gou, YS
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Uen, TM
Large single-crystalline superconducting grains of Tl2Ba2Ca2Cu3O10+x (Tl-2223) with sizes up to 150 mu m x 150 mu m x 1 mu m are obtained on both MgO(100) and LaAlO3 (100) substrates by a single-target de-sputtering process followed by appropriate postannealing at temperatures around 910 degrees C. Such superconducting grains were isolated and patterned into 10-mu m-wide bridges by reactive ion etching. The etching process was found to only minimally affect the transition temperature, T-c, typically around 110 K, of the films. Although there was evidence that these films were indeed two-dimensional in nature, the critical current densities, J(c), were greater than 10(6) A/cm(2) at temperatures below 90 K. At lower temperatures, e.g., T = 30 K, the J(c) value of these films remains well above 10(6) A/cm(2) even with magnetic fields as large as 7 T applied parallel to the c-axis of the grains. The results are in sharp contrast to the low J(c) values expected from the low-dimensionality nature of this system.
Determination of the doping dependence of the penetration depth using YBCO microstrip ring resonators
(2001-11-01)
Gou, YS
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Zeng, HK
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Juang, JY
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Lin, JY
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Wu, KH
We present a method to determine the temperature dependent London penetration depth measured by a microstrip ring resonator for YBCO thin films with various oxygen contents. It allows us to get the following important results of the superconducting properties in the underdoped cuprate superconductors. With increasing delta (e.g. delta = 0.05, 0.2, and 0.4), the superfluid density, lambda (2)(5 K) /lambda (2)(T), is linear in the low temperature range (T < T-c/3), and the slope changes with increasing oxygen deficiency. After the temperature normalization with respect to the critical temperature, the temperature dependence of the superfluid density is shown to be independent of doping concentration. Moreover the maximum energy gap can be estimated from the model of the d-wave pairing. We found that the ratio Delta /T-c is nearly fixed value, 3.0 +/- 0.4. (C) 2001 Elsevier Science B.V. All rights reserved.
Dimensionality and Kosterlitz-Thouless transition in single grain Tl-2223 superconducting thin films
(1996-04-01)
Juang, JY
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Lin, HC
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Liu, CK
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Wang, SJ
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Yang, TH
Tl2Ba2Ca2Cu3O10+x (Tl-2223) superconducting films with microstructure consisted of submillimeter size grains combining with reactive ion etching technique, has enabled us to investigate the transport properties of this material in a more controllable manner. Both conductivity fluctuations based on the Aslamazov-Larkin theory and the Kosterlitz-Thouless transitions were studied to delineate the two dimensional nature of the material. It was found that, depending on the substrate used, the effective thickness of critical fluctuation and the detailed features of the K-T transition were very different. For films deposited on LaAlO3(100) substrates, the effective thickness of critical fluctuation is about 35 Angstrom compared to a value of 17.5 Angstrom obtained for films deposited on MgO(100) substrate, roughly equal to the c-axis lattice constant and the distance between the trilayer CuO2 planes, respectively. The effective vortex dielectric constant measuring the correlations between vortex pairs near K-T transition were estimated to be 3.0 and 1.6 for films on LaAlO3 and on MgO, repectively. Possible mechanisms based on the defect structures are proposed to account for the observed results.
Effects of compressive epitaxial strain on the magnetotransport properties of single-phase electron-doped La0.7Ce0.3MnO3 films
(2004-10-15)
Chang, WJ
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Hsieh, CC
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Juang, JY
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Wu, KH
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Uen, TM
Single-phase electron-doped manganite thin films with nominal composition of La0.7Ce0.3MnO3 (LCeMO) have been prepared on SrTiO3 (100) substrates by pulsed laser deposition. The conditions for obtaining purely single-phase LCeMO films lie within a very narrow window of substrate temperature (T(s)similar to720 degreesC) and laser energy density (E(D)similar to2 J/cm(2)) during deposition. In situ postdeposition annealing, mainly to relax the possible epitaxial in-plane tensile strain between the film and the substrate, leads to an increasing c-axis lattice constant accompanied by the formation of secondary CeO2 phase and higher metal-insulator transition temperature. This is indicative of a strong coupling between the electron and lattice degree of freedom. (C) 2004 American Institute of Physics.
Effects of oxygen content on the penetration depth in YBCO thin films
(1996)
Hsieh, MC
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Tseng, TY
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Fu, CM
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Wu, KH
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Juang, JY
By varying oxygen content of the microstrip line resonator of YBa2Cu3O7-x, the effective penetration depth was studied. Based on the two-fluid model, we fit the data of the resonant frequency by means of the inductance formula. It allows us to determine the zero-temperature penetration depth of the fully oxygenated,(O approximate to 7) and oxygen deficient (O = 6.8) states in the same strip to be 483.2 nm and 789.8 nm, respectively. The ratio of the superconducting carrier densities with different oxygen contents is then derived and found to be consistent with the result of Hall measurement. A correlation between oxygen contents and charge carriers of the samples will be obtained in our work as accurately as possible.