Browsing by Author "Luo, C. W."

Now showing 1 - 20 of 77

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.
;
Tzeng, W. Y.
;
Chu, Y. J.
;
Luo, C. W.
;
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 gasket-type standard leak element using femtosecond laser micromachining
(2020-10-01)
Chan, C. K.
;
Tu, C. Y.
;
Yeh, S. D.
;
Chang, C. C.
;
Lin, T. C.
A gasket-type standard leak element with a microfluidic channel was developed and measured. The microfluidic channel with a diameter of several micrometers and a length of one hundred micrometers was fabricated by using femtosecond laser micromachining. The conductances (<1 x 10(-6) L s(-1)) of the micmfluidic channel for various non-condensable gases - H-2, He, CH4, N-2 and CO2 were determined by a measuring system based on conductance modulation (CM) method. The experimental results show that the flow characteristics of our devices exhibit molecular flow at a pressure up to 100 mbar and the conductance values of H-2, He, CH4, N-2 and CO2 are inversely proportional to the square root of their molecular mass.
A wide range THz pulses generation in ZnTe crystals
(2012)
Ku, S. A.
;
Tu, C. -M.
;
Luo, C. W.
;
Wu, K. H.
;
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.
;
Yang, Y. Q.
;
Mak, I. T.
;
Chang, Y. H.
;
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.
;
Gou, I. C.
;
Luo, C. W.
;
Uen, T. M.
;
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.
;
Gou, I. C.
;
Luo, C. W.
;
Uen, T. M.
;
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 ultrafast dynamics in doped Y1-xCaxBa2Cu3O7-delta superconducting thin films
(2008)
Luo, C. W.
;
Wu, K. H.
;
Lin, J. -Y.
;
Uen, T. M.
;
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.
;
Huang, T. C.
;
Chiang, M. T.
;
Wu, K. H.
;
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.
;
Huang, T. C.
;
Chiang, M. T.
;
Wu, K. H.
;
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.
;
Hsieh, C. C.
;
Shih, H. C.
;
Luo, C. W.
;
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.
Carrier Dynamics in Self-Assembled CdTe Stranski-Krastanow Quantum Dots Grown on ZnSe by Molecular Beam Epitaxy
(2008-11-01)
Yang, C. S.
;
Chin, K. F.
;
Lai, J. Y.
;
Luo, C. W.
;
Chou, W. C.
Self-assembled CdTe/ZnSe Stranski-Krastanow quantum-dot (QD) structures, which have a CdSe-like precursor-layer (PCL) between CdTe quantum dots and ZnSe matrix, were grown by molecular beam epitaxy. The carrier dynamics of the structures was studied by photoluminescence (PL) and time-resolved photoluminescence (TRPL) measurements. The temperature-dependent PL spectra revealed that there is a carrier transformation from small dots to larger dots via the precursor layer. Temperature-dependent PL measurements verify the existence of the QD excited excitonic state in 5.0 mono-layer (ML) coverage. In 0.6 ML coverage, the TRPL spectra exhibited a double-exponential decay process, as the detection energy at PCL emission energy. The faster decay time is corresponded to the carrier lifetime in PCL, the slower decay time exhibits the carrier in small QD. However, in 5.0 ML coverage, a faster and slower decay time represents the carrier lifetime in QD excited state for larger QD and the carrier lifetime in QD ground state for smaller QD.
Characterization of the charge density wave transition and observation of the amplitude mode in LaAuSb2
(2019-06-10)
Kuo, C. N.
;
Shen, D.
;
Li, B. S.
;
Nguyen Nhat Quyen
;
Tzeng, W. Y.
In order to elucidate the charge density wave (CDW) nature in LaAuSb2, we have carried out the electrical resistivity, Hall coefficient, Seebeck coefficient, thermal conductivity, as well as ultrafast pump-probe spectroscopy measurements on single crystalline LaAuSb2. The CDW phase transition has been characterized by marked features near T-C similar or equal to 88 K. In particular, the obtained Hall carrier concentration and electronic thermal conductivity exhibit significant reduction as T-C drops, providing strong evidence for partial gapping of the Fermi surfaces during the CDW phase transition. Furthermore, the ultrafast pump-probe spectroscopy reveals pronounced oscillatory features in the photoinduced transient reflectivity in the CDW ordered state. A characteristic oscillation mode with frequency of about 0.32 THz was extracted. The comparison of the obtained oscillation frequency with the frequencies of the amplitude mode in the well-studied CDW analogue LaAgSb2 allows us to ascribe the observation to the amplitude mode of CDW collective excitations in LaAuSb2.
Coherent acoustic phonons in hexagonal HoMnO(3) probed by femtosecond spectroscopy
(2010)
Luo, C. W.
;
Shih, H. C.
;
Wu, K. H.
;
Lin, J-Y
;
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.
Comment on "GaSe1-xSx and GaSe1-xTex thick crystals for broadband terahertz pulses generation" [Appl. Phys. Lett. 99, 081105 (2011)]
(2012-03-26)
Ku, S. A.
;
Luo, C. W.
;
Andreev, Yu M.
;
Lanskii, Grigory
;
電子物理學系
Controllable subwavelength-ripple and -dot structures on YBa2Cu3O7 induced by ultrashort laser pulses
(2012-11-01)
Luo, C. W.
;
Tang, W. T.
;
Wang, H. I.
;
Liao, L. W.
;
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.
Dichotomy of photoinduced quasiparticle on CuO2 planes of YBa2Cu3O7 directly revealed by femtosecond polarization spectroscopy
(2007-08-01)
Luo, C. W.
;
Chen, L. Y.
;
Lee, Y. H.
;
Wu, K. H.
;
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.
;
Tseng, P. S.
;
Chen, H. -J.
;
Wu, K. H.
;
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.
;
Lo, H. P.
;
Su, C. H.
;
Wu, I. H.
;
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.
;
Lo, H. P.
;
Su, C. H.
;
Wu, I. H.
;
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.
;
Jiang, M. G.
;
Luo, C. W.
;
Lin, J. -Y.
;
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.