Browsing by Author "Wu, K. H."

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3D visualizations of nanoscale phase separation and ultrafast dynamic correlation between phases in (Na0.32K0.68)(0.95)Fe1.75Se2
(2019-03-25)
Cheng, P. C.
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Tzeng, W. Y.
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Chu, Y. J.
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Luo, C. W.
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Zhukov, A. A.
Phase separation of metallic and antiferromagnetic (AFM) insulating phases in alkaline iron selenides (A(x)Fe(2)-ySe(2)) continues to attract intense interest because the relationship between two peculiar features probably is a key to clarifying the pairing mechanism of A(x)Fe(2-y)Se(2) superconductors. Here we report that the three-dimensional (3D) visualizations of nanoscale phase separation in (Na0.32K0.68)(0.95)Fe1.75Se2 single crystals are revealed by hybrid focused-ion-beam scanning electron microscopy and the superconducting paths are fully percolative in 3D. Moreover, the phase-related ultrafast dynamics in (Na0.32K0.68)(0.95)Fe1.75Se2 is studied by dual-color pump-probe spectroscopy. The anomalous changes in the electron and acoustic phonon components of transient reflectivity change (Delta R/R) identify two characteristic temperatures T* similar to 100K (the onset temperature of coupling between the nanometallic and AFM phases) and T-H similar to 230K (the onset temperature of the metallic-interface phase). An energy-transfer channel between the nanometallic and AFM phases is inferred. This proposed channel provides insight into the pairing mechanism of alkaline iron selenide superconductors.
A wide range THz pulses generation in ZnTe crystals
(2012)
Ku, S. A.
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Tu, C. -M.
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Luo, C. W.
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Wu, K. H.
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Yabushita, A.
The THz generation in ZnTe crystals by widely varying pumping fluences is reported. Some abnormal nonlinear effects were observed including the evolution of THz spectra and the phase shift of temporal waveforms. Moreover, the THz efficiency can enhance again at high pumping fluence due to the saturation of free carrier.
A widely tunable dual-wavelength CWTi : sapphire laser with collinear output
(2008-03-03)
Luo, C. W.
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Yang, Y. Q.
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Mak, I. T.
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Chang, Y. H.
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Wu, K. H.
We report a collinear output and tunable dual-wavelength CW Ti:sapphire laser with a simple cavity configuration. The wavelength splitting range is easily tuned from 10 nm to 110 nm, which provides 56 THz bandwidth for terahertz generation. The total output power of two colors with the spatial mode of TEM00 is between 700 mW and 300 mW, for small and large wavelength splittings, respectively, under 5 W argon-ion laser pumping. (C) 2008 Optical Society of America.
Anisotropic electronic structure in single crystalline orthorhombic TbMnO(3) thin films
(2010-02-01)
Wu, K. H.
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Gou, I. C.
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Luo, C. W.
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Uen, T. M.
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Lin, J. -Y.
We have deposited the c-axis-oriented orthorhombic TbMnO(3) (o-TMO) films with well-aligned in-plane orientations on NdGaO(3) (001) substrates by using pulsed laser deposition. The distinctive orientation alignments between the film and substrate allow the study of the X-ray absorption spectroscopy (XAS) with the electric field along three major crystallographic directions, respectively. Polarization-dependent XAS spectra show significant anisotropy in the electronic Structure of o-TMO. The correlation between the electronic structure, the bonding anisotropy, and the magnetoelectric effect in the multiferroic materials is revealed. (C) 2009 Elsevier B.V. All rights reserved.
Anisotropic electronic structure in single crystalline orthorhombic TbMnO3 thin films
(2010-02-01)
Wu, K. H.
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Gou, I. C.
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Luo, C. W.
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Uen, T. M.
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Lin, J. -Y.
We have deposited the c-axis-oriented orthorhombic TbMnO3 (o-TMO) films with well-aligned in-plane orientations on NdGaO3 (001) substrates by using pulsed laser deposition. The distinctive orientation alignments between the film and substrate allow the study of the X-ray absorption spectroscopy (XAS) with the electric field along three major crystallographic directions, respectively. Polarization-dependent XAS spectra show significant anisotropy in the electronic Structure of o-TMO. The correlation between the electronic structure, the bonding anisotropy, and the magnetoelectric effect in the multiferroic materials is revealed. (C) 2009 Elsevier B.V. All rights reserved.
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.
Anisotropic Ultrafast Dynamics of Quasiparticles on CuO(2) Planes of Y(0.7)Ca(0.3)Ba(2)Cu(3)O(7-delta)
(2010-07-01)
Luo, C. W.
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Huang, T. C.
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Chiang, M. T.
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Wu, K. H.
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Juang, J. Y.
The orientation-resolved femtosecond spectroscopy, combined with the well-textured (110)- and (001)Y(0.7)Ca(0.3)Ba(2)Cu(3)O(7-delta) thin films, serves as an effective probe to quasiparticle relaxation dynamics on the ab planes and along the diagonal orientations. The significant divergences in the temperature-dependent relaxation time (tau) associated with the opening of superconducting gap were observed along the nodal directions and on the CuO(2) planes which are dominated by the a axis and b axis in the over-doped region. Moreover, the divergence in the temperature-dependent served along the nodal direction disappears around optimal doped region. This implies that the superconducting gap evolves from the dominant s-wave symmetry in over-doped region into the dominant d-wave symmetry in optimal doped region.
Anisotropic Ultrafast Dynamics of Quasiparticles on CuO2 Planes of Y0.7Ca0.3Ba2Cu3O7-delta
(2010-07-01)
Luo, C. W.
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Huang, T. C.
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Chiang, M. T.
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Wu, K. H.
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Juang, J. Y.
The orientation-resolved femtosecond spectroscopy, combined with the well-textured (110)- and (001)Y0.7Ca0.3Ba2Cu3O7-delta thin films, serves as an effective probe to quasiparticle relaxation dynamics on the ab planes and along the diagonal orientations. The significant divergences in the temperature-dependent relaxation time (tau) associated with the opening of superconducting gap were observed along the nodal directions and on the CuO2 planes which are dominated by the a axis and b axis in the over-doped region. Moreover, the divergence in the temperature-dependent served along the nodal direction disappears around optimal doped region. This implies that the superconducting gap evolves from the dominant s-wave symmetry in over-doped region into the dominant d-wave symmetry in optimal doped region.
Anomalous magnetic ordering in b-axis-oriented orthorhombic HoMnO(3) thin films
(2008-03-31)
Lin, T. H.
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Hsieh, C. C.
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Shih, H. C.
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Luo, C. W.
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Uen, T. M.
Orthorhombic HoMnO(3) films with well-aligned crystallographic orientations were deposited on LaAlO(3)(110) single crystal substrates by using pulsed laser deposition. The nearly perfect b-axis-oriented films provide the opportunity of investigating the orientation-dependent physical property of this material. The temperature dependent magnetization evidently displays an antiferromagnetic ordering near 42 K, irrespective to the direction of applied field. Furthermore, the theoretically expected lock-in transition was clearly observed at around 30 K when field was applied along the c axis and was undetectable along the a and b axes. The 30 K transition was suppressed to 26 K when the applied field increased from 100 to 500 Oe. (C) 2008 American Institute of Physics.
Broadband Microwave Measurements of Overdoped Y0.9Ca0.1Ba2Cu3O7-delta Films Using Corbino Geometry
(2013-04-01)
Farber, E.
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Bachar, N.
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Lavi, D.
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Castro, H.
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Chen, Y. -J.
The surface impedance of Y0.9Ca0.1Ba2Cu3O7-delta thin films was measured using the Corbino spectroscopy method. This special geometry, in which the sample dimensions are well defined by a ring pattern, is ideal for broadband high frequency reflection measurements. Using the complex reflected signal, S (11), measured by a vector network analyzer, one can find the surface impedance of the thin film, from which the complex conductivity can be deduced. In the current work we present the three-standard approach for calibration of the Corbino method and demonstrate the benefits of this approach in measuring superconducting Y0.9Ca0.1Ba2Cu3O7-delta thin films up to 20 GHz and down to 6 K. For the data analysis the well-known generalized two-fluid model was implemented, taking into account a film thickness which is much smaller than the normal state skin depth and superconducting penetration depth.
Coherent acoustic phonons in hexagonal HoMnO(3) probed by femtosecond spectroscopy
(2010)
Luo, C. W.
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Shih, H. C.
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Wu, K. H.
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Lin, J-Y
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Juang, J. Y.
We have observed the coherent acoustic phonons in the hexagonal manganite HoMnO(3) single crystals using wavelength-dependent femtosecond pump-probe spectroscopy. The oscillatory component of the photoinduced transient reflectivity change (Delta R/R) at various pump-probe wavelengths is consistent with the model of strain pulse propagation. Furthermore, the oscillation period of the oscillation is strong temperature- and wavelength-dependent. In particular, the period of all oscillations in Delta R/R for various wavelengths are very similar in the vicinity of the Neel temperature (T(N)), which associates with the appearance of the antiferromagnetic (AFM) long-range ordering.
Controllable subwavelength-ripple and -dot structures on YBa2Cu3O7 induced by ultrashort laser pulses
(2012-11-01)
Luo, C. W.
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Tang, W. T.
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Wang, H. I.
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Liao, L. W.
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Lo, H. P.
The evolution of periodic ripples and dots on an YBa2Cu3O7 surface irradiated by single-beam and dual-beam femtosecond laser pulses is investigated. The density and size of the dots can be controlled by the number of pulses. The spatial period of these ripples does not show a close relation to the pulse number. However, the orientation and geometrical period of the ripples are strongly dependent on the polarization and incident angle of the laser pulses, respectively. The formation processes are also discussed based on the dependence.
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.
Dirac fermion relaxation and energy loss rate near the Fermi surface in monolayer and multilayer graphene
(2014-08-07)
Luo, C. W.
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Tseng, P. S.
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Chen, H. -J.
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Wu, K. H.
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Li, L. J.
Ultrafast dynamics of Dirac fermions near the Fermi surface in monolayer and multilayer graphene are revealed using optical pump mid-infrared probe spectroscopy. The energy loss rate of Dirac fermions is also determined via energy-resolved transient transmissivity spectra, which is significantly suppressed as the number of layers in graphene increases.
Doping dependence of the ultrafast electronic dynamics of Y(1-x)Pr(x)Ba(2)Cu(3)O(7-delta) thin-film superconductors from femtosecond optical spectroscopy
(2010-09-17)
Luo, C. W.
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Lo, H. P.
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Su, C. H.
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Wu, I. H.
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Chen, Y. -J.
Time-domain spectroscopy, which probes the dynamics of the electronic states near Fermi surface that are associated with superconductivity, has proven to be a powerful method for providing insights into the fundamental nature of both pseudogap and superconducting gap. This study shows that the unique T-x phase diagram with the time-evolving ultrafast dynamics of Y(1-x)Pr(x)Ba(2)Cu(3)O(7-delta) can be used to identify clearly the Nernst, pseudogap, and superconducting regions. All of the orders appear together at a specific delay time, such as 1.2-3 ps, after pumping by an ultrashort pulse. These discoveries suggest that the Nernst effect, the pseudogap and even superconductivity may have the same physical origin.
Doping dependence of the ultrafast electronic dynamics of Y1-xPrxBa2Cu3O7-delta thin-film superconductors from femtosecond optical spectroscopy
(2010-09-17)
Luo, C. W.
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Lo, H. P.
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Su, C. H.
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Wu, I. H.
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Chen, Y. -J.
Time-domain spectroscopy, which probes the dynamics of the electronic states near Fermi surface that are associated with superconductivity, has proven to be a powerful method for providing insights into the fundamental nature of both pseudogap and superconducting gap. This study shows that the unique T-x phase diagram with the time-evolving ultrafast dynamics of Y1-xPrxBa2Cu3O7-delta can be used to identify clearly the Nernst, pseudogap, and superconducting regions. All of the orders appear together at a specific delay time, such as 1.2-3 ps, after pumping by an ultrashort pulse. These discoveries suggest that the Nernst effect, the pseudogap and even superconductivity may have the same physical origin.
Doping evolution of Zhang-Rice singlet spectral weight: A comprehensive examination by x-ray absorption spectroscopy
(2013-10-31)
Chen, Y. -J.
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Jiang, M. G.
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Luo, C. W.
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Lin, J. -Y.
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Wu, K. H.
The total spectral weight S of the emergent low-energy quasiparticles in high-temperature superconductors (HTSCs) is explored by x-ray absorption spectroscopy. In order to examine the applicability of the Hubbard model, regimes that cover from zero doping to overdoping are investigated. In contrast to results of mean-field theory, we found that S deviates from linear dependence on the doping level p. The slope of S versus p changes continuously throughout the whole doping range with no sign of saturation up to p = 0.23. Therefore, the picture of the Zhang-Rice singlet remains intact within the most prominent doping regimes of HTSCs.
Doping-Dependent Phase Diagram of Ca-Doped YBCO Observed by Femtosecond Spectroscopy
(2011-01-01)
Chen, Y. -J.
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Wu, K. H.
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Luo, C. W.
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Lin, J. -Y.
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Juang, J. Y.
The quasiparticle relaxation dynamics in a single (001) Ca-doped YBCO (Y(0.7)Ca(0.3)Ba(2)Cu(3)O(7-delta) ) superconducting thin film was probed by using the femtosecond time-resolved spectroscopy. The (001) Y(0.7)Ca(0.3)Ba(2)Cu(3)O(7-delta) thin film was prepared on a (100)STO substrate by pulsed laser deposition. Through controlling the oxygen pressure and the annealing temperature within quartz tube, the doping level (0.08 < p < 0.22) in the phase diagram could be widely tuned from the overdoped region to the underdoped region with varying the hole concentration (p) in a single sample. In the overdoped region, two different components (positive/negative) were obviously identified in the transient reflectivity curves (Delta R/R). The negative component in Delta R/R emerges at T < T (c) and quickly defeated by the development of the positive component. The dramatic change in the positive component of Delta R/R arises well below T (c). However, this anomaly change does not appear in the underdoped region. Only the positive Delta R/R was clearly observed at whole temperatures. These results indicate that the scenario of high-T (c) superconductivity in the overdoped region would be different from the underdoped region.