Browsing by Author "UEN, TM"

Now showing 1 - 20 of 32

CRITICAL SLOWING DOWN OF THE SPIN RELAXATION AND CENTER SHIFT ANOMALY IN NIFE2O4 OBSERVED WITH MOSSBAUER-SPECTROSCOPY
(1988-06-01)
UEN, TM
;
YANG, FH
;
電子物理學系
;
Department of Electrophysics
CRITICAL-CURRENT ANISOTROPY AND FLUX-PINNING IN YBCO(001) AND YBCO(110) SUPERCONDUCTING THIN-FILMS PREPARED BY KRF PULSED-LASER ABLATION
(1993)
JUANG, JY
;
WU, KH
;
WANG, SJ
;
LAI, TC
;
KAO, RT
EFFECT OF SN ON THE SUPERCONDUCTIVITY OF THE DYBA2CU3O7-DELTA SYSTEM
(1989-12-01)
WANG, C
;
CHEN, LT
;
CHANG, HL
;
CHU, ML
;
HSU, FW
EFFECTS OF ANNEALING CONDITIONS ON THE PHASE-STABILITY AND SUPERCONDUCTING PROPERTIES OF DC SPUTTERING TLBACACUO THIN-FILMS
(1993-12-01)
LIN, HC
;
LI, MH
;
LI, JJ
;
JANG, FI
;
FU, CM
We have studied the influence of heat treatment on the properties of the superconducting transition and critical current on the TiBaCaCuO thin films prepared by DC sputtering. The superconducting transition temperature is increasing as the maximum annealing temperature is raised. The XRD analysis reveals the evolution of the major phase of the films from 2212 phase to 2223 phase, when the films were annealed at maximum temperature varied from 895-degrees-C to 922-degrees-C. The surface morphology of the sample reveals the formation of the intergrowth of grains by partial melting, which may associated with the evolution of cntical current for the annealed films.
EFFECTS OF MAGNETIC-FIELD AND TEMPERATURE ON THE CRITICAL CURRENT HYSTERESIS IN TL-CA-BA-CU-O THIN-FILMS
(1990-11-01)
CHANG, HL
;
JUANG, JY
;
LIOU, SH
;
UEN, TM
;
GOU, YS
EFFECTS OF SUBSTRATE TREATMENTS ON THE GROWTH OF YBCO SUPERCONDUCTING THIN-FILMS ON MGO BY PULSED-LASER ABLATION
(1993-12-01)
WU, KH
;
LI, MH
;
JUANG, JY
;
UEN, TM
;
GOU, YS
Different treatments on single crystalline MgO(100) substrates have been performed to study the effects of the substrate conditions on the growth mechanisms and superconducting properties of YBCO thin films deposited in-situ by pulsed laser ablation process. The nucleation and growth of YBCO thin films on these substrates during deposition have been studied by using the in-situ resistance measurement method. The characteristics of as-deposited films have also been investigated by X-ray diffraction analysis (XRD), temperature dependent resitivity measurement (R - T), temperature dependent critical current density measurement (J(c) - T), scanning electron mircoscopy (SEM), and scanning tunneling mircoscopy (STM). It has been found that, while the zero-resistance transition temperature (T(co)) and the overall crystallographic orientation of the films are only affected slightly, the surface morphology and grain structure of the films have shown strong dependencies on the substrate conditions. Recognizing that for YBCO grown on MgO, due to the large lattice mismatch between the film and substrate, the film growth is semicoherent in nature, the observed results are consistently explained by attributing to the interplays between nucleation and grain coalescence processes. The dramatic differences in superconducting transport properties exhibited by films obtained with the conditions studied are also reported to reinforce the arguments proposed.
EFFECTS OF ZRO2 ON THE TEXTURING AND PROPERTIES OF MELT-PROCESSED Y1BA2CU3O7-DELTA
(1994-04-25)
JUANG, JY
;
WU, CL
;
WANG, SJ
;
CHU, ML
;
WU, KH
The addition of ZrO2 is found to be very effective as the nucleation sites to promote the oriented growth of Y1Ba2Cu3O7-delta (YBCO) during peritectic solidification. Extended regions of well-textured YBCO grains with superior superconducting properties of the YBCO bulk are readily obtained by a single-step melt process at much lower process temperatures. At 77 K, the platelike Josephson weak-link behaviors observed in pure melt-processed YBCO have been effectively eliminated, indicating that the coherency between the spherulitic superconducting YBCO grains may have been greatly improved by ZrO2 additions.
ENHANCEMENT OF CRITICAL-CURRENT DENSITY IN DIRECT-CURRENT-SPUTTERED TLBA2CA2CU3O9-/+DELTA SUPERCONDUCTING THIN-FILMS
(1995-02-13)
JUANG, JY
;
HORNG, JH
;
CHEN, SP
;
FU, CM
;
WU, KH
FABRICATION AND PROPERTIES OF DC-SPUTTERED TL-1223 SUPERCONDUCTING THIN-FILMS
(1995-06-01)
JUANG, JY
;
HORNG, JH
;
LIN, HC
;
WANG, SJ
;
FU, CM
A multi-step deposition and post annealing scheme has been developed for preparing nearly single phase, c-axis oriented TlBa2Ca2Cu3O9 (Tl-1223) superconducting thin films by dc-sputtering. The effects of the compensates used during post annealing on final phase formation suggest the crucial role played by thermodynamic conditions and further validates the viability of the present process scheme. In particular, films obtained by the present process have shown, for the first time in this system, a critical current density (Jc) above 10(6) A/cm(2) at 17 K with a zero- resistance transition temperature (Tco) above 110 K. The results are attributed to the improvement of film morphology, specifically a decreased density of weak-linking.
FLUX LINE DYNAMICS CROSSOVER IN THE TL-BA-CA-CU-O THIN-FILM
(1990-11-01)
WANG, CY
;
CHANG, HL
;
CHU, ML
;
JUANG, JY
;
GOU, YS
By extending the observation of the current-voltage characteristics to a much higher dissipative state, the flux-line motion experiences at least two stages of crossover, namely, from glassy hopping to creep-dominating, and finally an abrupt transition into the flow regime.
GROWTH AND PROPERTIES OF SUBMILLIMETER SINGLE GRAIN TL2BA2CA2CU3O10+X SUPERCONDUCTING THIN-FILMS
(1995-10-02)
LIN, HC
;
UEN, TM
;
LIU, CK
;
JUANG, JY
;
WU, KH
Distinct crystalline superconducting grains of Tl2Ba2Ca2Cu3O10+x(Tl-2223) with sizes up to 150 mu mx150 mu mX1 mu m were prepared by a single target dc-sputtering process followed by proper postannealing at temperatures around 910 degrees C. A reactive ion etching process was developed to pattern an similar to 10 mu m wide bridge with four contact leads directly on a single grain to study its superconducting transport properties. The etching process was found to have only minimal effect on the superconducting transition temperature T-c, which was typically around 110 K. Grains grown on MgO(100) and LaAlO3(100) substrates, although, displaying somewhat different in-plane orientations, all had critical current densities J(c), well above 10(6) A/cm2 at 77 K. The existence of only one grain boundary, however, can reduce the J(c) by orders of magnitude. (C) 1995 American Institute of Physics.
GROWTH AND PROPERTIES OF SUBMILLIMETER SINGLE GRAIN TL2BA2CA2CU3O10+X SUPERCONDUCTING THIN-FILMS
(1995-10-02)
LIN, HC
;
UEN, TM
;
LIU, CK
;
JUANG, JY
;
WU, KH
Distinct crystalline superconducting grains of Tl2Ba2Ca2Cu3O10+x(Tl-2223) with sizes up to 150 mu mx150 mu mX1 mu m were prepared by a single target dc-sputtering process followed by proper postannealing at temperatures around 910 degrees C. A reactive ion etching process was developed to pattern an similar to 10 mu m wide bridge with four contact leads directly on a single grain to study its superconducting transport properties. The etching process was found to have only minimal effect on the superconducting transition temperature T-c, which was typically around 110 K. Grains grown on MgO(100) and LaAlO3(100) substrates, although, displaying somewhat different in-plane orientations, all had critical current densities J(c), well above 10(6) A/cm2 at 77 K. The existence of only one grain boundary, however, can reduce the J(c) by orders of magnitude. (C) 1995 American Institute of Physics.
GROWTH OF PULSED-LASER DEPOSITED YBCO THIN-FILMS ON YSZ AND YSZ/SI SUBSTRATES
(1994-02-01)
JUANG, JY
;
KAO, RT
;
HSIEH, MC
;
LI, MH
;
CHU, ML
The growth mechanism of superconducting YBCO thin films deposited in - situ on both single crystalline YSZ and YSZ-buffered Si substrates as revealed by scanning tunneling microscopy (STM) are found to be very different. The results are attributed to the initial surface condition of the substrates.
INSITU DEPOSITION OF SUPERCONDUCTING YBCO FILMS ON SI WITH CERAMIC BUFFER LAYERS
(1993-03-01)
JUANG, JY
;
WU, KH
;
LAI, TC
;
LEE, MH
;
HSEIH, MC
Systematic studies on the optimization of depositing superconducting YBCO thin films on Si substrates with YSZ buffer layers by pulsed laser ablation have been carried out. Nearly c-axis oriented YBCO films with T(co) almost-equal-to 85 K were obtained routinely on Si(100) substrates with in-situ deposition of YSZ layers on top. Preliminary measurements on the critical current density Jc of these films showed Jc > 2.0 x 10(4) A/cm2 at 77K and Jc > 1.4 x 10(6) A/CM2 at 20K. The significant improvements on Jc are attributed to the great reduction of microcracks and the crystallinity of YSZ layer, achieved by the in-situ annealing of the buffer layer prior to the YBCO deposition. The effects of oxygen partial pressure and substrate temperature during YSZ deposition on the crystallinity of YSZ layer as well as on the quality of subsequently deposited YBCO films will be discussed. In our most recent attempt the microcracks has been sucessfully eliminated by a process combining the in-situ annealing and slow cooling process.
INSITU GROWTH OF Y1BA2CU3O7-X SUPERCONDUCTING THIN-FILMS USING A PULSED NEODYMIUM YTTRIUM-ALUMINUM-GARNET LASER WITH CO2-LASER HEATED SUBSTRATES
(1991-03-11)
WU, KH
;
LEE, CL
;
JUANG, JY
;
UEN, TM
;
GOU, YS
By replacing the resistive heater with a CO2 laser to heat the substrates together with concentered oxygen stream blowing onto the substrate during deposition, high quality Y1Ba2Cu3O7-x thin films with nearly perfect c-axis orientation and T(c0) = 87 K were grown in situ without subsequent slow cooling in oxygen atmosphere or any further heat treatment. The rapid quenching, from typically 600-degrees-C to room temperature in less than 50 s of the as-deposited films, excludes the possibility of further oxygen incorporation during the cooling process and indicates that the in situ growth may have completed during deposition under proper deposition conditions.
LOW-FIELD SCALING BEHAVIORS OF GLOBAL FLUX PINNING IN TL-BA-CA-CU-O THIN-FILMS
(1992-07-01)
JUANG, JY
;
WANG, SJ
;
UEN, TM
;
GOU, YS
;
CHANG, HL
The scaling behaviors of global pinning force density (F(p) = J(c) X B) in dc sputtered TI-Ba-Ca-Cu-0 superconducting thin films as functions of both the reduced magnetic field and temperature were investigated by direct transport measurements for fields up to 4 kOe. It was found that, depending on the film granularities (originating from different deposition and annealing conditions and characterized by the temperature and magnetic-field dependencies of critical currents), vastly different scaling behaviors were manifested. For films with very high critical current densities, where the intragranular behaviors prevailed, the conventional surface core pinning by some high-density normal pinning centers combined with a significant flux creep appeared to be the predominant limiting mechanisms for the critical current densities, whereas in more granular films, suggestive collective-pinning effects arising from the weakly pinned fluxoids in the intergranular networks were observed.
MICROWAVE-ENHANCED SUPERCONDUCTIVITY AND TRANSPORT-PROPERTIES OF Y-BA-CU-O BICRYSTAL WEAK-LINKS
(1995-06-01)
FU, CM
;
JUANG, JY
;
CHEN, MF
;
WU, KH
;
UEN, TM
The effects of the tilt angle on microwave-enhanced transition temperatures and critical currents of YBCO bicrystal weak-links have been investigated. It was found that, with smaller grain boundary tilt angles (e.g. 24 degrees), the enhancement was small, displaying essentially the same behavior as that manifested in intragranular regions. By contrast, when the tilt angle was increased from 36.8 degrees to 45 degrees, noticeable Tc enhancement was found to occur even at low input microwave powers. The results were compared to the temperature dependent critical current, in order to delineate possible correlations between the two physical parameters.
MOSSBAUER SPECTROSCOPIC STUDIES OF K2SNCL6 AND SNCL4.5H2O IN FROZEN AQUEOUS-SOLUTIONS AND IN ION-EXCHANGE RESINS
(1984)
CHENG, HS
;
HSU, CM
;
UEN, TM
;
TSENG, PK
;
光電工程學系
OBSERVATION OF THE VORTEX-GLASS TRANSITION IN THE TL-BASED SUPERCONDUCTING THIN-FILMS
(1992-08-01)
WANG, CY
;
CHANG, HL
;
JUANG, JY
;
GOU, YS
;
UEN, TM
The current-voltage characteristics (IVC) of Tl-based superconducting thin films as well as the magnetization of the Tl-based bulk sample were investigated. A crossover similar to the prediction of the vortex-glass to vortex-liquid phase transition in the IVC of Tl-based superconducting thin films was clearly demonstrated. More significantly, striking scaling behavior was found in the IVC which is only sample dependent and is irrelevant to the applied magnetic fields. By comparison to dc magnetization measurements on similar bulk samples, it is found that the phase boundary is not the irreversibility line commonly encountered in the flux-creep scenario.
OPTIMIZATION OF DEPOSITING Y1BA2CU3O7-DELTA SUPERCONDUCTING THIN-FILMS BY EXCIMER LASER ABLATION WITH CO2 LASER-HEATED SUBSTRATES
(1992-06-01)
WU, KH
;
JUANG, JY
;
LEE, CL
;
LAI, TC
;
UEN, TM
The optimum deposition conditions for growing Y1Ba2Cu3O7-delta superconducting thin films in situ by using xenon, Nd:YAG, and KrF excimer lasers with CO2 laser-heated substrates were studied. The dependences of the T(c0) and microstructure of the films on the deposition parameters such as substrate temperature, oxygen partial pressure, laser energy density and repetition rates have been investigated systematically. It was found that both the surface mobility on the substrate and the reaction temperatures for the particulates arriving at the substrates are the key factors for obtaining high-quality films. High-quality Y1Ba2Cu3O7-delta thin films with mirror-like surface morphology, T(c0) almost-equal-to 90 K, and nearly perfect c-axis oriented normal to the film surface were routinely grown in situ on SrTiO3 substrates with the following deposition conditions: substrate temperature at 670-degrees-C, oxygen partial pressure of 0.1 torr, laser energy density of 4 J/cm2 per pulse, and repetition rate of 10 Hz. More significantly, it took only about 3 min to grow a 0.3-mu-m thick film and less than 50 s to cool down the film to room temperature after deposition by the present process. Subsequent processes, such as slow cooling in oxygen atmosphere or any further heat treatments, were unnecessary for the present process. The unique advantage of the present process further offered us good opportunities to delineate the effects of each deposition parameter on the detailed film growth mechanisms without having to deal with the complications introduced by prolonged post-deposition treatments. The details of all the interesting features observed will be presented and their physical implications will be discussed.