Browsing by Author "Chen, Hsueh-Ju"

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Manifestation of a Second Dirac Surface State and Bulk Bands in THz Radiation from Topological Insulators
(2015-09-15)
Tu, Chien-Ming
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Yeh, Tien-Tien
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Tzeng, Wen-Yen
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Chen, Yi-Ru
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Chen, Hsueh-Ju
Topological insulators (TIs) are interesting quantum matters that have a narrow bandgap for bulk and a Dirac-cone-like conducting surface state (SS). The recent discovered second Dirac surface state (SS) and bulk bands (BBs) located similar to 1.5 eV above the first SS are important for optical coupling in TIs. Here, we report on the time-domain measurements of THz radiation generated from TIs n-type Cu0.02Bi2Se3 and p-type Bi2Te3 single crystals by ultrafast optical pulse excitation. The observed polarity-reversal of the THz pulse originated from transient current is unusual, and cannot be reconciled with the photo-Dember effect. The second SS and BBs are found to be indispensable for the explanation of the unusual phenomenon. Thanks to the existence of the second SS and BBs, TIs manifest an effective wide band gap in THz generation. The present study demonstrates that time-domain THz spectroscopy provide rich information of the optical coupling and the electronic structure of TIs.
Nanomechanical properties of Bi2Te3 thin films by nanoindentation
(2015-01-15)
Tasi, Cheng-Hsun
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Tseng, Yu-Chin
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Jian, Sheng-Rui
;
Liao, Ying-Yen
;
Lin, Chih-Ming
In this study, the microstructural, morphological and nanomechanical properties of Bi2Te3 thin films are investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), atomic force microscopy (AFM) and nanoindentation techniques. The Bi2Te3 thin films were deposited on c-plane sapphire substrates using pulsed laser deposition (PLD) under the various helium gas pressures. The XRD results indicated that the Bi2Te3 thin films are textured with the c-axis preferentially oriented normal to the films surface. Both the grain size and surface roughness of the Bi2Te3 thin films exhibit an increasing trend with increasing the helium gas pressure. Furthermore, the hardness and Young\'s modulus of the Bi2Te3 thin films are measured by a Berkovich nanoindenter operated with the continuous contact stiffness measurements (CSM) mode were found to range from 2.92 +/- 0.12 to 4.02 +/- 0.14 GPa and from 106.31 +/- 0.63 to 127.46 +/- 9.21 GPa, respectively, when the helium gas pressure was increased from 2 x 10(-5) to 2 x 10(-3) Torr. (C) 2014 Elsevier B.V. All rights reserved.
Superconductivity in textured Bi clusters/Bi2Te3 films
(2014-09-01)
Le, Phuoc Huu
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Tzeng, Wen-Yen
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Chen, Hsueh-Ju
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Luo, Chih Wei
;
Lin, Jiunn-Yuan
We report superconductivity at an onset critical temperature below 3.1 K in topological insulator similar to 200-nm-thick Bi2Te3 thin films grown by pulsed laser deposition. Using energy-dispersive X-ray spectroscopy elemental mapping and Auger electron spectroscopy elemental depth profiling, we clearly identified bismuth (Bi) precipitation and Bi cluster signatures. Superconductivity in the Bi2Te3 films was attributed to the proximity effect of Bi clusters precipitated on the surface of the Bi2Te3 films. (C) 2014 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution 3.0 Unported License.
Temperature-dependent carrier-phonon coupling in topological insulator Bi2Se3
(2014-12-08)
Lai, Yi-Ping
;
Chen, Hsueh-Ju
;
Wu, Kuang-Hsiung
;
Liu, Jia-Ming
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電子物理學系
Temperature-dependent (11.0K +/- 294.5 K) carrier-phonon coupling in Bi2Se3 is investigated by ultrafast pump-probe spectroscopy. The rise time of the differential reflectivity is interpreted by a combined effect of electron temperature relaxation and hot-phonon lifetime. The electron-phonon coupling constant of the bulk state (lambda = 0.63 +/- 0.05) is deduced from theoretical fitting. Increasing hot-phonon lifetime with decreasing temperature is attributed to a decreasing phonon-phonon collision rate. A complete analysis of the thermalization process is presented. Understanding carrier and phonon dynamics is essential for future optoelectronic and spintronic applications of topological insulators. (C) 2014 AIP Publishing LLC.
THz Generation and Detection on Dirac Fermions in Topological Insulators
(2013-11-01)
Luo, Chih Wei
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Chen, Hsueh-Ju
;
Tu, Chien Ming
;
Lee, Chia Ching
;
Ku, Shin An
Ultrafast carrier dynamics in Cu(In,Ga)Se-2 thin films probed by femtosecond pump-probe spectroscopy
(2012-06-04)
Chen, Shih-Chen
;
Liao, Yu-Kuang
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Chen, Hsueh-Ju
;
Chen, Chia-Hsiang
;
Lai, Chih-Huang
Ultrafast carrier dynamics in Cu(In,Ga)Se-2 films are investigated using femtosecond pump-probe spectroscopy. Samples prepared by direct sputtering and co-evaporation processes, which exhibited remarkably different crystalline structures and free carrier densities, were found to result in substantially different carrier relaxation and recombination mechanisms. For the sputtered CIGS films, electron-electron scattering and Auger recombination was observed, whereas for the co-evaporated CIGS films, bandgap renormalization accompanied by band filling effect and hot phonon relaxation was observed. The lifetime of defect-related recombination in the co-evaporated CIGS films is much longer than that in the direct-sputtered CIGS films, reflecting a better quality with higher energy conversion efficiency of the former. (c) 2012 Optical Society of America