Browsing by Author "Juang, J. Y."

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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 High Efficiency Field Emitters Pt/Si-nanocones Array with Vertically Aligned Structure
(2015-01-01)
Kao, P. H.
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Tai, H. M.
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Wang, H. W.
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Chiang, T. Y.
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Juang, J. Y.
An effective method of fabricating vertically aligned silicon nanocones (Si-NCs) was realized by using the self-assembled silver (Ag) nanoislands as natural metal-nanomask during dry etching process. The obtained Si-NCs were preferentially aligned along the c-axis direction. To enhance the field emission property, thin platinum (Pt) films (similar to 20 nm) were subsequently deposited on the Si-NCs by sputter coater to form Pt/Si-NCs heterostructures. The turn-on field defined by the 1 mu A/cm(2) current density criterion is similar to 2.85 V/mu m. As measurements of field emission, the low turn-on field was attributed to the vertical one-dimensional nanostructures, and perhaps more importantly, proper density distribution of the Pt/Si-NCs. The suggesting an interesting alternative route for producing Si-NCs that might be useful for optical and electronic applications.
Analysis of bump resistance and current distribution of ultra-fine-pitch microbumps
(2013-01-01)
Chang, Y. W.
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Peng, H. Y.
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Yang, R. W.
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Chen, Chih
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Chang, T. C.
To keep up with the demand of continuous increase in device densities, the integration of three-dimensional integrated circuits (3D-IC) has become the most probable solution, and the utilization of ultra-fine-pitch microbump has emerged as an essential component of 3D-IC technology. In this study, a Kelvin bump structure was fabricated and resistances measured at different angles on a 20.0 mu m microbump were investigated. The microbump resistance at 0 degrees, 60 degrees, 120 degrees, and 180 degrees are 74.7, 45.9, 14.6, and 13.7 m Omega, respectively. These high resistances in microbumps may result in high interconnect resistance and cause resistance/capacitance (RC) delay, and thus lower the electrical performance of 3D-IC. A series of finite-element-model (FEM) was built to analyze the distribution of electric field in microbump. The FEM results have shown that the current is distributed uniformly in the thin solder joint, but current crowding still occurs in the Cu under-bump-metallization (UBM). The finding of the current crowding in the Cu UBM is the main cause of high resistances in the microbump. Thickening the Al trace, for example, from 0.4 mu m to 1.5 mu m, is a direct solution to reduce the unexpected high microbump resistance. A numerical model which treated solder joints as a resistance network was also performed in this study. For comparison, both FEM and the numerical model show the same trend and agree with the measurement results from Kelvin bump structure. The results all point to one thing: thickening the Al trace turn out to be the most effective approach to reduce high microbump resistance. When the Al trace thickness is increased from 0.8 to 3.0 mu m, the microbump resistance is decreased to half of the original value, resulted from the alleviation of current crowding in the Cu UBM. (C) 2012 Elsevier Ltd. All rights reserved.
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 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.
Cobalt magnetism in a superstructured metallic antiferromagnet Na0.825CoO2
(2013-08-22)
Young, Ben-Li
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Chu, P-Y.
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Juang, J. Y.
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Shu, G. J.
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Chou, F. C.
The Na atomic ordering and the local Co magnetism, in a metallic antiferromagnet made of Na0.825CoO2, are investigated by nuclear magnetic-resonance (NMR) techniques. Multiple well-resolved NMR peaks of Na-23 and Co-59 are observed, which suggests a superlattice structure forming in this Na-vacant cobaltate. Examining clues from published x-ray data, we refine the lattice superstructure so as to obtain the best description for the peaks seen in our NMR data. Magnetic Co ions are found to form a striped structure, which produces nontrivial magnetism. Analyzing the NMR frequency shift for each peak, we observe a frequency-shift anomaly, i.e., the breakdown of linearity between the NMR shift (that is, the local susceptibility) and the bulk susceptibility. We find that the shift anomaly can be described by an additional susceptibility component emerging at similar to 60 K. Slow spin dynamics are observed near a metamagnetic transition where the spins flip between the antiferromagnetic and ferromagnetic orders. Unlike in ordinary spin glasses, this glassy behavior is induced by a magnetic field, which causes competition between the in-plane ferromagnetic and out-of-plane antiferromagnetic interactions, in a magnetic-frustrated triangular lattice.
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.
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.
Effects of compressive epitaxial strain in the b-axis on the magnetization response of orthorhombic HoMnO(3) thin films
(2009)
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.
The orthorhombic phase HoMnO(3) (o-HMO in Pbnm symmetry setting) thin films were prepared on LaAlO(3)(110) (LAO(110)) substrates by pulsed laser deposition. While for films grown on LAO(110) substrates, the compressive strain along the b-axis was resulted from the tensile strain in the a-c plane. The films provide the opportunity of investigating the effects of strain, hence lattice elasticity, on the physical properties of this material. For o-HMO films an antiferromagnetic ordering with T(N)similar to 42 K, irrespective to the direction of applied field was clearly observed. However, an additional magnetic ordering occurring around 26.4 K was observed when the field was applied along the c-axis of o-HMO. This transition, however, was absent when the field was applied along a-and b-axis. These results indicate that the second magnetic ordering observed along the c-axis could be more relevant to the Mn moments lying along the partially strained b direction of the o-HMO which has been theoretically expected to result in incommensurate-commensurate lock-in transition.