M. H. Chen

National Chiao Tung University, Hsinchu, Taiwan, Taiwan

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Publications (10)15.78 Total impact

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    ABSTRACT: The spatially resolved relaxation characteristics of photoinduced quasiparticles (QPs) in Cu O2 planes of underdoped YBCO are disclosed by polarized fs time-resolved spectroscopy. The relaxation time (tau) along b axis diverges at Tc , and appears to be governed by a temperature-dependent gap Delta (T) at T< Tc . Furthermore, for T> Tc , 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 Deltap . 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.
    Physical Review B 09/2005; 72. DOI:10.1103/PhysRevB.72.092506 · 3.66 Impact Factor
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    ABSTRACT: Highly in-plane aligned (100) Y_1-xCa_xBa_2Cu_3O_7-delta thin films have been deposited on (100) LaSrGaO4 substrates by pulsed laser. Before the Y_1-xCa_xBa_2Cu_3O_7-delta deposition, the (010) PrBa_2Cu_3O_7-delta was grown as a template layer. The electric transport properties were measured by a standard four-probe method. The crystallinity of the films was analyzed by measuring the x-ray theta -2theta diffraction pattern and phi scanning. The surface morphology of the films was examined by atomic force microscopy (AFM) and scanning electron microscopy (SEM). Furthermore, the electronic structure and ultrafast response of the (100) Y_1-xCa_xBa_2Cu_3O_7-delta thin films have been investigated by the polarization-dependent x-ray absorption spectroscopy and femtosecond pump-probe technique, respectively. The anisotropic characterizations of these in-plane aligned (100) Y_1-xCa_xBa_2Cu_3O_7-delta thin films are discussed.
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    ABSTRACT: The polarized femtosecond spectroscopies obtained from well characterized (100) and (110) YBa2Cu3O7-δ 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.
    Physical Review B 11/2003; 68(22). DOI:10.1103/PhysRevB.68.220508 · 3.66 Impact Factor
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    ABSTRACT: 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.
    Journal of Applied Physics 09/2003; 94(5):3648-3650. DOI:10.1063/1.1590057 · 2.19 Impact Factor
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    ABSTRACT: We report, the time-resolved ultrafast optical spectroscopy results obtained from a series of well characterized (001)-, (100)- and (110)-oriented YBa2Cu3O7– (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.
    Journal of Low Temperature Physics 05/2003; 131(5):767-774. DOI:10.1023/A:1023410522965 · 1.04 Impact Factor
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    ABSTRACT: Highly in-plane aligned a-axis oriented YBa2Cu3O7-d (YBCO) thin films with unprecedented transition temperature (Tc~91.3 K) have been deposited on (100) LaSrGaO4 substrates using a continuously grown PrBa2Cu3O7-d (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 Tc'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.
    Journal of Low Temperature Physics 05/2003; 131(3):545-549. DOI:10.1023/A:1022963605952 · 1.04 Impact Factor
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    ABSTRACT: 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 (E||b or E||c) are significantly different. The opening of superconducting gap in CuO3 chain was obviously observed from the measurements of the DeltaR/R curves for E||b at temperatures below Tc. However, the DeltaR/R curves for E||c did not exhibit the similar characteristics.
    Physica C Superconductivity 05/2003; 388:477-478. DOI:10.1016/S0921-4534(02)02587-X · 1.11 Impact Factor
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    ABSTRACT: Polarization-dependent X-ray absorption spectra (XAS) on the O 1s 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 (E||b or E||c), were obtained in a normal-incidence alignment. Furthermore, the XAS for E||a 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.
    Physica C Superconductivity 05/2003; 388:435-436. DOI:10.1016/S0921-4534(02)02561-3 · 1.11 Impact Factor
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    ABSTRACT: Two–parametric model for a channel in 2DEG is proposed which allows for the explicit analytical solution for the problem of quantum electron transport through the constriction. Conductance steps smearing appear naturally and a simple criterion for the steps occurrence in terms of channel parameters is given.
    Superlattices and Microstructures 05/2002; 31(5):207-217. DOI:10.1006/spmi.2001.1013 · 1.98 Impact Factor
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    ABSTRACT: The spatial symmetry of temperature-dependent gap and temperature-independent gap, which are generally associated with the superconducting gap Delta(T) and pseudogap Delta_p, respectively, in near optimally doped and underdoped YBCO are clearly delineated by the orientation- and time-resolved femtosecond spectroscopy. For the slightly oxygen-deficient (100) YBCO thin films, the coexistence of superconducting gap and pseudogap were evidently observed along b-axis, which is extremely different from the near optimal doping cases, where only superconducting gap is observed. Combing the well-textured samples, (110) and (100) YBCO thin films, the intrinsic symmetries (d_xy in near optimal doping and d_x^2_-y^2 in underdoped region) of pseudogap in YBCO have been disclosed.