Browsing by Author "Gou, Y. S."

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Anisotropic magnetoresistance of La0.7Ca0.3MnO3 thin film biepitaxial step junctions
(2006-12-01)
Chen, S. F.
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Chang, W. J.
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Hsieh, C. C.
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Liu, S. J.
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Juang, J. Y.
The angular dependence of magnetoresistance (MR) of the La0.7Ca0.3MnO3 thin film biepitaxial step junction (BSJ) shows a simple sin(2)(theta) dependence in the in-plane high-field magnetoresistance, with theta being the angle between the applied field and current. This behavior is similar to the spin-orbit coupling-induced anisotropic magnetoresistance (AMR) commonly observed in transition ferromagnetic metals, except for two salient features. First, the maximum MR in the present case occurs at an oblique angle between the applied field (H) and electric current (I), while it is usually observed to occur when H parallel to I. Second, the AMR in the plane perpendicular to the film surface displays a remarkable value (Delta rho/rho similar to 8%), which is about an order of magnitude larger than that of the in-plane AMR. Such a large AMR cannot be solely explained by spin-orbit coupling effect. We suggest instead that the metallic and ferromagnetic inhomogeneous granules existing in the BSJ region might have acted as the source of spin-polarized scattering giving rise to the enhanced AMR when the colossal magnetoresistance was measured across the biepitaxial step boundaries. (c) 2006 American Institute of Physics.
Anisotropic ultrafast dynamics in doped Y1-xCaxBa2Cu3O7-delta superconducting thin films
(2008)
Luo, C. W.
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Wu, K. H.
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Lin, J. -Y.
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Uen, T. M.
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Gou, Y. S.
The anisotropic dynamics of photoinduced quasiparticle in the Y1-xCaxBa2Cu3O7-delta (YCBCO) is revealed by using the orientation-resolved femtosecond reflection spectroscopy. This bulk-sensitive spectroscopy, combining with the well-textured (110)- and (100)- YCBCO thin films, serves as an effective probe to quasiparticle relaxation dynamics along different crystallographic orientations. Since the temperature at which the peak of temperature-dependent relaxation time (tau) manifested in the transient reflectance (Delta R/R) measurements is believed to intimately relate to the opening of superconducting gap, the results reported here should shed some light on the issue of the competing order scenarios for over-doped high-T-c superconductors.
CRITICAL CURRENT ANISOTROPY AND FLUX PINNING IN YBCO(001) AND YBCO(110) SUPERCONDUCTING THIN FILMS PREPARED BY KrF PULSED LASER ABLATION
(1993-03-01)
Juang, J. Y.
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Wu, K. H.
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Wang, S. J.
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Lai, T. C.
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Kao, R. T.
Direct transport critical current density (J(C)) measurements were performed on laser ablated high quality Y1Ba2Cu3O7 superconducting thin films with T-CO >= 90 K, J(C) approximate to 5x10(6) A/cm(2) at 77 K, and intentionally controlled film orientations to investigate the J(C) anisotropy and predominant pinning mechanisms in these films. It was found that, for films with (001) oriented normal to the substrate the field dependencies of the critical current densities J(C)(H) showed very different behaviors for different field orientations, indicating that the active pinning mechanisms may be different in two field directions. For the (110) oriented films with field applied normal to film surface, the pinning mechanism, as depicted from the scaling behavior of the global pinning force density, was found to follow the manifestation of surface core pinning. At zero field, however, the temperature dependence of J(C) revealed that, in all cases, the critical current was limited by flux creep with essentially the same creep kinetic factor expected for high-T-C cuprates. Detailed studies of the scaling behaviors of the pinning force density as functions of both field and temperatures for fields up to 7 Tesla will be presented and compared for different films and field orientations.
Dichotomy of photoinduced quasiparticle on CuO2 planes of YBa2Cu3O7 directly revealed by femtosecond polarization spectroscopy
(2007-08-01)
Luo, C. W.
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Chen, L. Y.
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Lee, Y. H.
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Wu, K. H.
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Juang, J. Y.
The dichotomy of photoinduced quasiparticle dynamics in the anisotropic high T, superconductors (YBa2CU3O7) is revealed by using the orientation-resolved ferntosecond reflection spectroscopy in the specially designed thin films. The transient reflectivity Delta R/R along the ab diagonal and b axis are totally different in the slow relaxation process which relates to the opening of the superconducting gap. In addition, the distribution of the temperature evolution of Delta R/R on the CuO2 planes of YBa2Cu3O7 may indicate the d-wave symmetry of the superconducting gap in high T, superconductors. (c) 2007 American Institute of Physics.
Fabrication and low temperature thermoelectric properties of NaxCoO2 (x=0.68 and 0.75) epitaxial films by the reactive solid-phase epitaxy
(2007-02-05)
Chang, W. J.
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Hsieh, C. C.
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Chung, T. Y.
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Hsu, S. Y.
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Wu, K. H.
The authors have fabricated NaxCoO2 thin films via lateral diffusion of sodium into Co3O4 (111) epitaxial films (reactive solid-phase epitaxy [Ohta , Cryst. Growth Des. 5, 25 (2005)]). The environment of thermal diffusion is key to the control of the sodium content in thin films. From the results of x-ray diffraction and in-plane rho(ab), the epitaxial growth and the sodium contents of these films were identified. The thermoelectric measurements show a large thermoelectric power similar to that observed in single crystals. The quasiparticle scattering rate is found to approach zero at low temperatures, consistent with the small residual resistivity, indicating high quality of the NaxCoO2 thin films. (c) 2007 American Institute of Physics.
Fabrication and X-ray absorption NaxCoO2 spectroscopy in layered cobaltate thin films
(2007-03-01)
Chang, W. J.
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Lin, J.-Y.
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Chung, T. Y.
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Chen, J. M.
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Hsu, C.-H.
NaxCoO2 (x similar to 0.68) thin films were fabricated on sapphire (0 0 0 1) substrates via the lateral diffusion of sodium into Co3O4 (1 1 1) films, which were grown by pulsed-laser deposition. From the results of X-ray diffraction and in-plane resistivity rho(ab), the single phase and the metallic behaviors of these NaxCoO2 films can be identified. For the same sodium content x, rho(ab) is consistent with that of single crystals. In addition, the O 1s X-ray absorption near edge spectra of thin films are measured and compared with those of single crystals. (c) 2006 Elsevier B.V. All rights reserved.
Magnetotransport properties, electronic structure, and microstructure of La0.7Sn0.3MnO3 thin films
(2006-10-01)
Cheng, T. Y.
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Lin, C. W.
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Chang, L.
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Hsu, C. H.
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Lee, J. M.
Single-phase La0.7Sn0.3MnO3 (LSnMO) thin films were fabricated on LaAlO3 (LAO) substrates by pulsed laser deposition (PLD). The as-deposited films, though showed insulating characteristics with no sign of insulator-metal transition (IMT) down to very low temperatures, did display a paramagnetic-ferromagnetic transition (PFT) around 180 K. The x-ray absorption spectroscopy (XAS) of the as-deposited LSnMO films shows signature of Mn3+/Mn2+ mixed valence indicating that tetravalent Sn ions may have resulted in electron doping into the e(g) band of Mn. The transmission electron microscopy (TEM) analyses on the as-deposited LSnMO films further confirmed that the films are epitaxial with uniform composition distributions. It is suggestive that the doping level of x=0.3 in La1-xSnxMnO3 can be achieved without disrupting the perovskite structure or any composition inhomogeneity. On the other hand, ex situ annealing in oxygen as well as in argon atmosphere, though both drive the films to display IMT and a marked enhancement in the transition temperature, the preservation of LSnMO phase is somewhat doubtful. In the oxygen-annealing case, the evidence from the XAS measurements on Sn ions though showed the existence of tetravalent characteristics, the Hall measurements indicated that the obtained LSnMO films are p type in nature. Furthermore, the TEM analyses also revealed the emergence of the Sn compounds, which may ultimately drive the obtained films into La-deficient La1-xMnO3 phases.
Nonlinear current-voltage characteristics of oxygen-deficient La(0.67)Ca(0.33)MnO(3-y) films
(2008-01-15)
Liu, S. J.
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Juang, J. Y.
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Lin, J. -Y.
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Wu, K. H.
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Uen, T. M.
Two different types of nonlinear current-voltage characteristics are observed in oxygen-deficient La(0.67)Ca(0.33)MnO(3-y) (LCMO) films at temperatures below the insulator-metal transition. The paraboliclike dynamic conductance G(V), defined as dI/dV, curves near the zero bias observed in highly oxygen-deficient LCMO films implies the contribution from the spin-dependent tunneling transport between ferromagnetic clusters, with magnetic-disordered regions serving as tunneling barriers. On the other hand, for the slightly oxygen-deficient LCMO films, dips around the zero bias were observed in nonlinear G(V) curves and have been attributed to spin-flip scattering, with oxygen vacancies serving as scattering centers. (C) 2008 American Institute of Physics.
Physical properties of YBa2Cu3O7-delta thin films using microstrip ring resonators technique
(2006-09-01)
Lai, L. S.
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Zeng, H. K.
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Juang, J. Y.
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Wu, K. H.
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Uen, T. M.
Microstrip ring resonators with quality factor (Q) over 10(4) at temperature 5 K, were fabricated using the double-side YBa2CU3O7-delta (YBCO) epitaxial films deposited on LaAO(3) (LAO) substrates. By placing a narrow gap in the ring resonator, we observed that the original fundamental resonating mode (resonance frequency f = 3.61 GHz) splits into a dual-mode with different resonating frequencies (f = 1.80 GHz and f = 5.33 GHz). These two kinds of the resonator allow us to determine the temperature and frequency dependences of the magnetic penetration depth lambda(T,f) and the surface loss. Several salient features of the above findings related to the nature of low-lying excitations for high-T-c superconductivity as a function of oxygen content will be elucidated. In particular, the current models, suggested by Wen and Lee, will be examined in a quantitative manner. It allows us to give a justification of quasiparticle as Fermi-liquid in the superconducting state. In addition, an equivalent inductance circuit model is suggested to account for the occurrence of the dual-mode resonance. (c) 2006 Published by Elsevier B.V.
Spatial dichotomy of quasiparticle dynamics in underdoped thin-film YBa2Cu3O7-delta superconductors
(2006-11-01)
Luo, C. W.
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Hsieh, C. C.
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Chen, Y. -J.
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Shih, P. T.
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Chen, M. H.
By the spatial-temporal-resolved femtosecond spectroscopy on well-textured (110)- and (100)-YBCO thin films, two distinctly temperature-dependent characteristics of quasiparticle relaxation in the nodal and antinodal directions are clearly identified. One temperature dependence associated with a high-energy gap has been observed along the ab diagonal for the whole hole-doping region and along the b axis except near optimal doping, while the other temperature dependence related to the opening of the superconducting gap appears along b axis regardless of the hole concentration. This spatial dichotomy between the nodal and antinodal quasiparticle dynamics and the evolution of gap symmetry with hole doping are discussed for enlightenment on the nature of the phase diagram of hole-doped cuprates. These strongly suggest that the two characteristics may have different physical origins and compete with each other.
Temperature-dependent X-ray absorption near edge spectroscopy of doped LaMnO(3): Ion-size and the Jahn-Teller distortion effects
(2009-05-01)
Tsai, Y. T.
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Chang, W. J.
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Huang, S. W.
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Lin, J. -Y.
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Lee, J. Y.
The temperature-dependent X-ray absorption near-edge spectroscopy (XANES) of LaMnO(3) thin films doped with various divalent ions was systematically carried out to investigate the effects of the ion size and associated Jahn-Teller distortions (JTDs) on the electronic structure modifications. Doped LaMnO(3) thin films were grown on (100) SrTiO(3) and (110) NdGaO(3) substrates by pulsed-laser deposition with typical thickness of 250-300 nm. Temperature-dependent resistivity p(T) and magnetization M(T) were measured. The results indicate that the drastic changes in magneto-transport properties induced by the substitution of A-site ion and the splitting of the O K-edge pre-edge peak associated with the e(g)(up arrow) and t(2g)(down arrow) energy levels at low temperatures in La(1-x)Ca(x)MnO(3) and La(1-x)Sr(x)MnO(3) (x = 0.3, 0.375) may both relate to the lattice disorder and the associated JTD effects on modifying the electronic structure of doped-LaMnO(3). (C) 2009 Elsevier B.V. All rights reserved.
Temperature-dependent X-ray absorption near edge spectroscopy of doped LaMnO3: Ion-size and the Jahn-Teller distortion effects
(2009-05-01)
Tsai, Y. T.
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Chang, W. J.
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Huang, S. W.
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Lin, J. -Y.
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Lee, J. Y.
The temperature-dependent X-ray absorption near-edge spectroscopy (XANES) of LaMnO3 thin films doped with various divalent ions was systematically carried out to investigate the effects of the ion size and associated Jahn-Teller distortions (JTDs) on the electronic structure modifications. Doped LaMnO3 thin films were grown on (100) SrTiO3 and (110) NdGaO3 substrates by pulsed-laser deposition with typical thickness of 250-300 nm. Temperature-dependent resistivity p(T) and magnetization M(T) were measured. The results indicate that the drastic changes in magneto-transport properties induced by the substitution of A-site ion and the splitting of the O K-edge pre-edge peak associated with the e(g)(up arrow) and t(2g)(down arrow) energy levels at low temperatures in La1-xCaxMnO3 and La1-xSrxMnO3 (x = 0.3, 0.375) may both relate to the lattice disorder and the associated JTD effects on modifying the electronic structure of doped-LaMnO3. (C) 2009 Elsevier B.V. All rights reserved.