Browsing by Author "Liu, RS"

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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.
Comparison of irreversibility lines of silver-sheathed Bi-2223, Tl-1223 and Tl-1234 superconducting tapes
(1996-01-01)
Wei, JC
;
Yang, TJ
;
Liu, RS
;
光電工程學系
;
Department of Photonics
The irreversibility lines of silver-sheathed (Bi,Pb)(2)Sr2Ca2Cu3O10 (Bi-2223), (Tl,Pb,Bi)(Sr,Ba)(2)Ca2Cu2O9 (T1-1223) and (Tl,Pb)Sr2Ca3Cu4O11 (T1-1234) superconducting tapes were investigated and compared with those of other superconducting systems. We have demonstrated that the shape of the irreversibility lines of T1-1223 and T1-1234 superconducting tapes are similar in behavior to those of YBa2Cu3O7 (Y-123) and (Tl,Pb)Sr-2(Ca,Y)Cu2O7 (T1-1212) systems and better than those of Tl2Ba2Ca2Cu3O10 (T1-2223) and (Bi,Pb)(2)Sr2Ca2Cu5O10 (Bi-2223) superconductors. This means that the flux pinning ability of T1-1223 and T1-1234 superconducting tapes was stronger than that of Bi-2223 and T1-2223 superconductors. The possible reason for controlling flux pinning can be attributed to the intrinsic crystal structure difference in the materials.
Comparison of irreversibility lines of silver-sheathed Bi-2223, Tl-1223 and Tl-1234 superconducting tapes (vol 43, pg 83, 1996)
(1996-10-01)
Wei, JC
;
Yang, TJ
;
Liu, RS
;
光電工程學系
;
Department of Photonics
HMRSVP: A hierarchical mobile RSVP protocol
(2003)
Tseng, CC
;
Lee, GC
;
Liu, RS
;
Wang, TP
;
資訊工程學系
In this paper, we propose a hierarchical Mobile RSVP (HMRSVP) that can achieve mobility independent QoS-guaranteed services in mobile computing environments. The HMRSVP integrates RSVP with Mobile IP regional registration and makes advance resource reservations only when an inter-region movement may possibly occur. We first show that, by NS simulator, our HMRSVP can achieve the same QoS guarantees as MRSVP does with fewer resource reservations. Then, we show that HMRSVP outperforms MRSVP in terms of reservation blocking, forced termination and session completion probabilities.
HMRSVP: A hierarchical Mobile RSVP Protocol
(2001-01-01)
Tseng, CC
;
Lee, GC
;
Liu, RS
;
資訊工程學系
;
Department of Computer Science
In this paper we propose a hierarchical Mobile RSVP (HMRSVP) that can achieve mobility, independent QoS guaranteed services in mobile computing environments. The HMRSVP integrates RSVP with Mobile IP regional registration and makes advance resource reservations only when an inter-region movement may possibly happen. We first show that, by NS simulation, our HMRSVP can achieve the same QoS guarantees as MRSVP with fewer resource reservations. Then, we show that HMRSVP outperforms MRSVP in terms of reservation blocking, forced termination and session completion probabilities.
HMRSVP: A hierarchical Mobile RSVP Protocol
(2001-01-01)
Tseng, CC
;
Lee, GC
;
Liu, RS
In this paper we propose a hierarchical Mobile RSVP (HMRSVP) that can achieve mobility, independent QoS guaranteed services in mobile computing environments. The HMRSVP integrates RSVP with Mobile IP regional registration and makes advance resource reservations only when an inter-region movement may possibly happen. We first show that, by NS simulation, our HMRSVP can achieve the same QoS guarantees as MRSVP with fewer resource reservations. Then, we show that HMRSVP outperforms MRSVP in terms of reservation blocking, forced termination and session completion probabilities.
Hole states of X-Bi2Sr2CaCu2O8 [X=I, HgI2, and (Py-CH3)(2)HgI4] probed by OK-edge X-ray absorption spectroscopy
(2006-01-01)
Chen, JM
;
Chang, SC
;
Liu, RS
;
Lee, JM
;
Park, M
Hole states of X-Bi2Sr2CaCU2O8[X-Bi2212, X=I, HgI2, and (Py-CH3)(2)-HgI4, Py = pyridinel have been probed by O K-edge X-ray absorption near-edge structure (XANES) spectra. In the O 1s absorption edge of X-Bi2212, the pre-edge peak at similar to 528.3 eV is attributed to transitions into the 0 2p hole states located in the CuO2 planes. The hole concentration in the CuO2 planes of X-Bi2212 increases for X =I and HgI2, but decreases for X= (Py-CH3)(2)HgI4 as coMPared to pristine Bi2212. The variation of CuO2-plane hole concentration of Bi2212 is primarily responsible for the T-c variation upon intercalation. (c) 2005 Elsevier Ltd. All rights reserved.
Interaction between a cylindrical superconducting impurity and a vortex in a type-II superconductor
(1997-02-10)
Wei, JC
;
Chen, JL
;
Liu, RS
;
Yang, TJ
;
電子物理學系
We examined the effectiveness of an infinite cylindrical superconducting impurity of radius a in an infinite type-II superconductor as a pinning center. The impurity and the superconducting background have the penetration depth lambda(i) and lambda(s), respectively. With regard to a single-quantum vortex, the free energy of the system and the pinning potential for a vortex were calculated, based upon the London model of the vortex line. Here, the pinning potential U-pin is defined by the difference between the free energy of a vortex being at the center of the impurity and that being infinitely distant from the impurity. U-pin is negative for lambda(i) > lambda(s). On the other hand, U-pin is positive for lambda(i) < lambda(s). In the limit of a much greater than lambda(i) and lambda(s), the pinning potential is found to be U-pin = phi(0)(2)(1/lambda(i)(2) - 1/lambda(s)(2)) ln kappa/4 pi mu(0). In the limit of a much less than lambda(i) and lambda(s), the pinning potential is found to be U-pin = phi(0)(2)[(1/lambda(i)(2) - 1/lambda(ss)(2)) ln(lambda(s)/a) - (1/lambda(i)(2)) ln(lambda(i)/a)]/4 pi mu(0). There is a clear correlation between the intrinsic property of these two superconducting materials and the pinning potential.
Polarized X-ray absorption studies in double-thallium-layer superconducting
(1998-04-01)
Chen, JM
;
Liu, RG
;
Liu, RS
;
Lin, HC
;
Uen, TM
Polarization-dependent O Is X-ray absorption spectra of highly c-axis-oriented Tl2Ba2Ca2Cu3O10 (Tl-2223) and TI2Ba2CaCu2O8 (Tl-2212) superconducting thin films have been measured by using synchrotron radiation. Near the O Is absorption edge, three distinct pre-edge peaks for both systems were clearly revealed. 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(z) character. The spectral weight of the pre-edge peak at 528.3 eV, originating from the CuO2 planes, increase about 60% from the Tl-2212 to Tl-2223 thin films. Conversely, the high-energy pre-edge peak at 530.3 eV, originating from the TIO 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 demonstrates the pictures of the self doping due to the charge transfer from the CuO2 layers to the TlO layers.
Random-coincidence corrections using iterative reconstruction for PET images
(2000)
Chen, JC
;
Liu, RS
;
Tu, KY
;
Lu, HHS
;
Chen, TB
Iterative reconstruction (IR) algorithms can reduce artifacts caused by filtered backprojection (FBP) or convolution backprojection (CBP). Recently, the computational effects required for IR of positron emission tomography (PET) studies have been reduced to make it practically appealing. We have made an implementation of the improved Maximum Likelihood-Expectation Maximization (ML-EM) algorithm. The transition matrix was generated based on the geometry of the instrument. Phantoms of 6 line sources and 19 line sources were used to test our accelerated ML-EM algorithms against the FBP method. The singles were used to calculate the random coincidence rates by a well known formula and were compared to the randoms obtained by another geometric method. We also designed a new model using two line sources to determine the ratio of random events to true events. The artifacts near those line sources were eliminated with the ML-EM method. With decay correction, the RC events were uniformly distributed in whole field after 10 iterations. The ML-EM reconstructed images are superior to those obtained with FBP. The patterns of randoms provide insightful information for random correction, which the hardware correction by the delay window can not provide. This information is particular valuable when the delay window correction is not available in the old fashion PET scanner.
Soft-x-ray absorption spectroscopy of heterostructured high-T-c superconducting nanohybrids: X-Bi2Sr2CaCu2O8 [X=I, HgI2, and (Py-CH3)(2)HgI4]
(2005-03-01)
Chen, JM
;
Chang, SC
;
Liu, RS
;
Lee, JM
;
Park, M
Unoccupied electronic states of X-Bi2Sr2CaCu2O8 [X-Bi2212, X=1, HgI2, and '(Py-CH3)(2)-HgI4, Py =pyridine] have been probed by O K-edge and Cu L-edge x-ray absorption near-edge structure (XANES) spectra using a bulk-sensitive x-ray-fluorescence-yield technique. In the 0 Is absorption edge of X-Bi2212, the pre-edge feature at similar to 528.3 eV is attributed to transitions into O 2p hole states located in the CuO2 planes. As deduced from 0 K-edge and Cu L-edge x-ray absorption spectra, the hole concentration in the CuO2 planes of X-Bi2212 increases for X=I and HgI2, but decreases for X= (Py-CH3)(2)HgI4, relative to pristine Bi2212. The present XANES results clearly demonstrate that the hole density within the CuO2 planes of intercalated Bi2Sr2CaCu2O8 can be not only decreased but also increased, depending on the chemical character of the intercalants.