Browsing by Author "Luo, CW"

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A comparison between the Fehrenbacher-Rice and the Liechtenstein-Mazin models
(2000-11-01)
Liu, SJ
;
Lin, JY
;
Chen, JM
;
Nachimuthu, P
;
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.
Anisotropic characteristics of in-plane aligned a-axis YBa2Cu3O7-delta thin films
(2003-05-01)
Luo, CW
;
Chen, MH
;
Chin, CC
;
Wu, KH
;
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
;
Chen, MH
;
Liu, SJ
;
Wu, KH
;
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
;
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.
Bipolar photogenerated terahertz radiation in biased photoconductive switches
(2003-05-01)
Lin, PI
;
Luo, CW
;
Liu, HS
;
Wu, KH
;
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.
Comment on "Conductivity of underdoped YBa2Cu3O7-delta: Evidence for incoherent pair correlations in the pseudogap regime"
(2003-05-02)
Luo, CW
;
Juang, JY
;
Lin, JY
;
Wu, KH
;
Uen, TM
Femtosecond spectroscopy studies of ultrafast dynamics in PrxY1-xBa2Cu3O7 and YBa2Cu3O7-delta thin films at different temperatures
(2000-04-01)
Wu, KH
;
Luo, CW
;
Juang, JY
;
Uen, TM
;
Gou, YS
We have systematically measured the transient reflectivity (Delta R/R) in Pr-doped YBCO (PrxY1-xBa2Cu3O7) and partially oxygen deficient YBCO (YBa2Cu3O7-delta) thin films at temperatures ranging from 300 K to 12 K by using the femtosecond pump-probe technique. At room temperature, the electron-phonon coupling strength and the position of Fermi-level as a function of Pr doping concentration and oxygen content have been studied. Measurements performed at low temperatures indicate that the magnitude, polarity, and relaxation time of the transient reflectivity is strongly dependent on the photon energy and pumping intensity of the optical excitation, the ambient temperature, and the doping level of the superconducting thin films. Moreover, the temperature dependence of the Fermi-level position was also observed.
Fluctuation-enhanced conductivity in YBa2Cu2Ox (6.5 <= x <= 6.95) and Tl2Ba2Ca2Cu3O10 +/-delta
(2000-01-01)
Juang, JY
;
Hsieh, MC
;
Luo, CW
;
Uen, TM
;
Wu, KH
The fluctuation-induced conductivity (Delta sigma(T)) near T-c of a sole YBa2Cu3Ox (YBCO) film with various precisely controlled oxygen contents (6.5 less than or equal to x less than or equal to 6.95) was studied and compared with those obtained from its Tl-based counterparts. For YBCO films with x > 6.7, Delta sigma (T) displays a distinct 3D to 2D crossover as the temperature approaches T-c. On the other hand, as T -->T-c, Delta sigma (T) of both Tl-based and YBCO film with x less than or equal to 6.7 exhibits a reversed 2D to 3D crossover. It is suggestive that the coupling between the CuO2 layers may have changed significantly with reducing oxygen. (C) 2000 Elsevier Science B.V. All rights reserved.
On the origin of photogenerated terahertz radiation from current-biased superconducting YBa2Cu3O7-delta thin films
(2004-06-15)
Lin, PI
;
Luo, CW
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Liu, HS
;
Chen, SF
;
Wu, KH
The origin of photogenerated terahertz radiation pulse emitted from current-biased superconducting YBa2Cu3O7-delta thin films excited by femtosecond optical laser pulses is delineated. By investigating the performance of the transient terahertz radiation generated under different operating parameters, pulse reshaping in the measured terahertz electric field caused by the kinetic inductance of the superconducting charge carriers is identified. After recovering the original wave forms of the emitted terahertz pulses, the transient supercurrent density directly correlated to the optically excited quasiparticle dynamics is obtained. A fast decreasing component of about 1.0 ps and a slower recovery process with a value of 2.5 ps are unambiguously delineated in the optically induced supercurrent modulation. The radiation mechanism of the transient terahertz pulse related to nonequilibrium superconductivity is discussed. (C) 2004 American Institute of Physics.
Phase-resolved nonlinear response of a two-dimensional electron gas under femtosecond intersubband excitation
(2004-01-30)
Luo, CW
;
Reimann, K
;
Woerner, M
;
Elsaesser, T
;
Hey, R
Strong electric-field transients resonant to intersubband transitions in n-type modulation-doped GaAs/AlGaAs quantum wells induce coherent Rabi oscillations, which are demonstrated by a phase-resolved measurement of the light emitted by the sample. The time evolution of the intersubband polarization is influenced by Coulomb-mediated many-body effects. The subpicosecond period and the phase of the Rabi oscillations are controlled by the properties of the midinfrared driving pulse.
Photoexcited carrier relaxation in a-axis oriented YBa2Cu3O7-delta thin films measured by femtosecond time-resolved spectroscopy
(2003-05-01)
Luo, CW
;
Chen, MH
;
Wu, KH
;
Lin, JY
;
Juang, JY
The photoexcited carrier relaxation dynamics in a-axis oriented YBa2Cu3O7-delta (YBCO) thin films has been investigated by femtosecond time-resolved spectroscopy. Distinct responses along b-axis (CuO3 chain) and c-axis of YBCO have been separated by the polarization-dependent femtosecond pump-probe measurements. It is found that the transient reflectivity (DeltaR/R) curves for the electric field E of the polarized light parallel to the b- or c-axes of the a-axis oriented YBCO film (Eparallel tob or Eparallel toc) are significantly different. The opening of superconducting gap in CuO3 chain was obviously observed from the measurements of the DeltaR/R curves for Eparallel tob at temperatures below T-c. However, the DeltaR/R curves for Eparallel toc did not exhibit the similar characteristics. (C) 2003 Elsevier Science B.V. All rights reserved.
Polarization-dependent X-ray absorption spectroscopy of in-plane aligned (100) YBa2Cu3O7-delta thin films
(2003-05-01)
Luo, CW
;
Liu, SJ
;
Chen, MH
;
Wu, KH
;
Lin, JY
Polarization-dependent X-ray absorption spectra (XAS) on the 0 Is edge has been measured on a highly in-plane aligned (100) a-axis YBa2Cu3O7-delta (YBCO) thin films. The in-plane XAS, that is, the electric field E of the linearly polarized synchrotron light parallels to b- or c-axes of YBCO films (Eparallel tob or Eparallel toc), were obtained in a normal-incidence alignment. Furthermore, the XAS for Eparallel toa was calculated from the oblique incidence to the sample with different angles. The present results are consistent with those obtained by using detwinned YBCO single crystals. (C) 2003 Elsevier Science B.V. All rights reserved.
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.
Spatially resolved relaxation dynamics of photoinduced quasiparticles in underdoped YBa2Cu3O7-delta
(2005-09-01)
Luo, CW
;
Shih, PT
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Chen, YJ
;
Chen, MH
;
Wu, KH
The spatially resolved relaxation characteristics of photoinduced quasiparticles (QPs) in CuO2 planes of underdoped YBCO are disclosed by polarized fs time-resolved spectroscopy. The relaxation time (tau) along b axis diverges at T-c, and appears to be governed by a temperature-dependent gap Delta(T) at T < T-c. Furthermore, for T>T-c, a monotonic increase of tau with decreasing T along the b axis and ab diagonal was observed and can be attributed to a temperature-independent gap Delta(p). The results lend support to the recombination dominant scenario of QP dynamics. However, the QP thermalization may take part along the nodal direction in the highly underdoped samples.
Ultrafast dynamics in Pr-doped and oxygen-deficient YBCO films by femtosecond spectroscopy
(2000-07-01)
Wu, KH
;
Luo, CW
;
Juang, JY
;
Uen, TM
;
Gou, YS
We have systematically measured the transient reflectivity (Delta R/R) in Pr-doped YBCO (PrxY1-xBa2Cu3O7) and partially oxygen deficient YBCO (YBa2Cu3O7-delta) thin films at temperatures ranging from 300 to 12 K by using the femtosecond pump-probe technique. Measurements performed at low temperatures indicate that the magnitude, polarity, and relaxation time of the transient reflectivity is strongly dependent on the photon energy and pumping intensity of the optical excitation, the ambient temperature, and the doping level of the superconducting thin films. Moreover, the temperature dependence of the Fermi-level position was also observed. (C) 2000 Elsevier Science B.V. All rights reserved.
W state generation and effect of cavity photons on the purification of dot-like single quantum well excitons
(2005-11-01)
Li, CM
;
Chen, YN
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Luo, CW
;
Hsieh, JY
;
Chuu, DS
A scheme of three-particle entanglement purification is presented in this work. The physical system undertaken for investigation is dot-like single quantum well excitons independently coupled through a single microcavity mode. The theoretical framework for the proposed scheme is based on the quantum jump approach for analyzing the progress of the trible-exciton entanglement as a series of conditional measurement has been taken on the cavity field state. We first investigate how cavity photon affects the purity of the double-exciton state and the purification efficiency in two-particle protocol. Then we extend the two-particle case and conclude that the three-exciton state can be purified into W state, which involves the one-photon-trapping phenomenon, with a high yield. Finally, an achievable setup for purification using only modest and presently feasible technologies is also proposed.