Browsing by Author "Chen, Y. -J."

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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.
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.
Estimations of tidal characteristics and aquifer parameters via tide-induced head changes in coastal observation wells
(2011)
Chen, Y. -J.
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Chen, G. -Y.
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Yeh, H. -D.
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Jeng, D. -S.
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環境工程研究所
The groundwater fluctuations due to tidal variations at an observation well in a coastal aquifer can be used to determine the tidal characteristics and aquifer parameters without conducting an aquifer test. In this study, a method, comprised of Jeng et al.'s solution (2005) and simulated annealing (SA) algorithm, is developed to determine the coastal aquifer parameters (hydraulic diffusivity, beach slope, and aquifer thickness) as well as the tidal characteristics (bichromatic-tide amplitudes, bichromatic-tide wave frequencies, and tidal phase lag) from the analysis of the tide-induced well-water-level (WWL) data. The synthetic WWL data generated from Jeng et al.'s solution (2005) with assumed parameter values and field data obtained from Barrenjoey beach, Australia, are analyzed. The estimated parameter values obtained from analyzing synthetic WWL data by the present method show good agreements with the previously assumed parameter values. The parameter estimation procedure may however fail in the case of a large shallow water parameter which in fact violates the constraint on the use of Jeng et al.'s solution (2005). In the analysis of field WWL data, the results indicate that the aquifer parameters estimated from the present method with single or multiple well data are significantly different from those given in Nielsen (1990). Inspecting the observed WWL data and the WWL data predicted from Jeng et al.'s solution (2005) reveals that the present method may provide better estimations for the aquifer parameters than those given in Nielsen (1990).
Ordered YBCO sub-micron array structures induced by pulsed femtosecond laser irradiation
(2008-12-08)
Luo, C. W.
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Lee, C. C.
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Li, C. H.
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Shih, H. C.
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Chen, Y. -J.
We report on the formation of organized sub-micron YBa(2)Cu(3)O(7) (YBCO) dots induced by irradiating femtosecond laser pulses on YBCO films prepared by pulse laser deposition with fluence in the range of 0.21 similar to 0.53 J/cm(2). The morphology of the YBCO film surface depends strongly on the laser fluences irradiated. At lower laser fluence (similar to 0.21 J/cm(2)) the morphology was pattern of periodic ripples with sub-micrometer spacing. Slightly increasing the laser fluence to 0.26 J/cm(2) changes the pattern into organized sub-micron dots with diameters ranging from 100 nm to 800 nm and height of 150 nm. Further increase of the laser fluence to over 0.32 J/cm(2), however, appeared to result in massive melting and led to irregular morphology. The mechanism and the implications of the current findings will be discussed. Arrays of YBCO sub-micron dots with T(c) = 89.7 K were obtained. (C) 2008 Optical Society of America
Polarization-insensitive, 40 Gb/s wavelength and RZ-OOK-to-RZ-BPSK modulation format conversion by XPM in a highly nonlinear PCF
(2008-08-04)
Astar, W.
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Wei, C. -C.
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Chen, Y. -J.
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Chen, J.
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Carter, G. M.
Polarization-insensitive wavelength conversion, as well as the conversion of return-to-zero (RZ) ON-OFF keying (RZ-OOK) to RZ binary phase-shift keying (RZ-BPSK), has been simultaneously achieved at 40 Gb/s for the first time by cross-phase modulation (XPM) in a highly birefringent, nonlinear photonic crystal fiber (PCF). A 10(-9)-BER receiver sensitivity conversion penalty of < 3 dB was achieved for a polarization scrambled, 40 Gb/s 25%-RZ-OOK pump, when the 40 Gb/s RZ probe was launched at 45 degrees with respect to the birefringence axes of the PCF and when the pump-probe detuning was greater than about 6 nm. (C) 2008 Optical Society of America.
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.
The unconventional doping in YBa2Cu3O7-x/La0.7Ca0.3MnO3 heterostructures by termination control
(2017-01-16)
Tra, V. T.
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Huang, R.
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Gao, X.
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Chen, Y. -J.
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Liu, Y. T.
In strongly correlated oxides, heterostructures provide a powerful route to manipulate the charge, spin, orbital, and lattice degrees of freedom to create distinctive functionalities. In this work, we have achieved atomically precise interface control in YBa2Cu3O7-x/La0.7Ca0.3MnO3 (YBCO/LCMO) heterostructures and find a hidden effective doping. This mechanism is responsible for higher Tc in the sample with the MnO2-terminated interface than in that with the La0.7Ca0.3O-terminated interface. The MnO2-terminated sample also shows a larger magnetic moment of Mn together with a lower valence state. For more than a decade, the control of T-c in these heterostructures prior to this work has been solely via the variation of YBCO or LCMO thickness. This work hints at an alternative way of exploiting and exploring the interactions between superconductivity and magnetism in this system. Published by AIP Publishing.
Theoretical investigation of polarization-insensitive data format conversion of RZ-OOK to RZ-BPSK in a nonlinear birefringent fiber
(2009-03-16)
Wei, C. C.
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Astar, W.
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Chen, J.
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Chen, Y. -J.
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Carter, G. M.
Polarization-insensitive conversion of return-to-zero (RZ) ON-OFF keying (RZ-OOK) to RZ binary phase-shift keying (RZ-BPSK) has been achieved by cross-phase modulation (XPM) in a nonlinear birefringent fiber. This work presents a theoretical analysis of the dependence of format conversion on pump-probe detuning, and the pump state-of-polarization (SOP) that can fluctuate unpredictably in a realistic system. An investigation of the impact of pump polarization fluctuation on receiver sensitivity and receiver optimal threshold for the converted RZ-BPSK probe is also carried out. It was found that although the desired XPM-induced pi phase shift can be achieved by launching both the RZ-OOK pump and the probe along the same birefringent axis of the fiber, the phase shift degrades to pi/3 the SOP of the RZ-OOK pump unpredictably switches to the other axis of the fiber, resulting in a large receiver sensitivity penalty fluctuation of 14 dB. By contrast, launching the probe at 45 degrees relative to the birefringent axes can reduce the polarization-dependent receiver sensitivity penalty fluctuation to about 2 dB as the SOP of the RZ-OOK pump is swept over the Poincare sphere. These conclusions are in good agreement with recently published experimental results. (C) 2009 Optical Society of America
Ultrafast Dynamics in Optimally Doped YBa(2)Cu(3)O(7-delta) Observed with White Light Pump-probe Spectroscopy
(2011-01-01)
Wang, Y. T.
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Lee, Y. H.
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Chen, Y. -J.
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Tang, W. T.
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Luo, C. W.
The quasiparticle relaxation dynamics in optimally doped (001) YBa(2)Cu(3)O(7-delta) thin films were measured at various temperatures by white light pump-probe technique. The 2D ultrafast spectroscopy with broadband (similar to 200 nm) and high time-resolution (similar to 9 fs) was performed by the non-collinear optical parametric amplifier (NOPA). The relaxation time and the amplitude of the transient transmissivity changes (Delta T/T) are wavelength-dependent. The maximum Delta T/T appears at around 625 nm (similar to 1.98 eV) which corresponds to the transition between the Fermi level and the UHB (upper Hubbard band). Furthermore, the oscillations with similar to 150, similar to 320, similar to 500 cm(-1) have been clearly observed in the Fourier transformed graph of the 2D ultrafast spectroscopy, which are consistent with the phonon modes in Raman-scattering spectra.
Ultrafast Dynamics in Optimally Doped YBa2Cu3O7-delta Observed with White Light Pump-probe Spectroscopy
(2011-01-01)
Wang, Y. T.
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Lee, Y. H.
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Chen, Y. -J.
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Tang, W. T.
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Luo, C. W.
The quasiparticle relaxation dynamics in optimally doped (001) YBa2Cu3O7-delta thin films were measured at various temperatures by white light pump-probe technique. The 2D ultrafast spectroscopy with broadband (similar to 200 nm) and high time-resolution (similar to 9 fs) was performed by the non-collinear optical parametric amplifier (NOPA). The relaxation time and the amplitude of the transient transmissivity changes (Delta T/T) are wavelength-dependent. The maximum Delta T/T appears at around 625 nm (similar to 1.98 eV) which corresponds to the transition between the Fermi level and the UHB (upper Hubbard band). Furthermore, the oscillations with similar to 150, similar to 320, similar to 500 cm(-1) have been clearly observed in the Fourier transformed graph of the 2D ultrafast spectroscopy, which are consistent with the phonon modes in Raman-scattering spectra.