Browsing by Author "Tsuei, Ku-Ding"

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Anti-site defect effect on the electronic structure of a Bi2Te3 topological insulator
(2018-01-01)
Chuang, Pei-Yu
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Su, Shu-Hsuan
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Chong, Cheong-Wei
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Chen, Yi-Fan
;
Chou, Yu-Heng
Tuning the Fermi level (EF) in Bi2Te3 topological-insulator (TI) films is demonstrated on controlling the temperature of growth with molecular-beam epitaxy (MBE). Angle-resolved photoemission spectra (ARPES) reveal that EF of Bi2Te3 thin films shifts systematically with the growth temperature (Tg). The key role that a Bi-on-Te(1) (BiTe1) antisite defect plays in the electronic structure is identified through extended X-ray-absorption fine-structure (EXAFS) spectra at the Bi L-3-edge. Calculations of electronic structure support the results of fitting the EXAFS, indicating that the variation of EF is due to the formation and suppression of BiTe1 that is tunable with the growth temperature. Our findings provide not only insight into the correlation between the defect structure and electronic properties but also a simple route to control the intrinsic topological surface states, which could be useful for applications in TIbased advanced electronic and spintronic devices.
Dependence of the final-state effect on the coupling between a CdSe nanoparticle and its neighbors studied with photoemission spectroscopy
(2007-03-01)
Wu, Pin-Jiun
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Tsuei, Ku-Ding
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Hsieh, Meng-Ting
;
Wei, Kung-Hwa
;
Liang, Keng S.
A simple model based on an electrostatic calculation of the photoemission effect in the final state is used to characterize the energy shift, relative to bulk material, which is dependent on size in the photoemission spectra of organically passivated CdSe nanoparticles (NPs). For trioctylphosphine oxide/hexadecylamine-passivated NPs, the core-level shifts are well described with a model involving a static effect in the final state. After the NPs are treated with pyridine, the energy shifts are smaller; this decrease is ascribed to a dynamic effect in the final state describing a finite lifetime, on a femtosecond scale, of the photohole residing in the NP. This condition is intimately related to the coupling between the NP and metallic substrate and among NPs themselves. For the valence-band edge, an additional effect of the initial state due to quantum confinement is required to elucidate the energy shift. We find that this initial-state shift is smaller than a prediction according to a semiempirical pseudopotential calculation.
Energy shift of photoemission spectra for organics-passivated CdSe nanoparticles: The final-state effect
(2007-01-01)
Wu, Pin-Jiun
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Tsuei, Ku-Ding
;
Wei, Kung-Hwa
;
Liang, Keng S.
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材料科學與工程學系
Size-dependent energy shift of photoemission spectra with respect to bulk sample has been examined for colloidally prepared CdSe nanoparticles with a series of particle sizes. The core-level shifts are well described by a theoretical calculation based on a final-state effect model, whereas an additional initial-state effect due to quantum confinement is required to elucidate the valence-band edge shifts. The results indicate that the interaction between the photohole and the dielectric background in the final state has to be considered in photoemission measurements for organics-passivated nanoparticles. The calculated results in the literature appear to overestimate the initial-state effect compared to our experimental observation. (c) 2006 Elsevier Ltd. All rights reserved.
Enhancement of subband effective mass in Ag/Ge(111) thin film quantum wells
(2008-12-01)
Tang, S. -J.
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Chang, Wen-Kai
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Chiu, Yu-Mei
;
Chen, Hsin-Yi
;
Cheng, Cheng-Maw
Subband dispersions of quantum-well states in Ag films on Ge(111) have been determined by angle-resolved photoemission. The effective masses of the subbands at the zone center increase substantially for decreasing film thicknesses. This peculiar behavior is attributed to a kinetic constraint for standing-wave formation governed by a momentum-dependent phase-shift function. No evidence is found for in-plane electron localization within the confined geometry.
Intermediate-Valence Ytterbium Compound Yb4Ga24Pt9: Synthesis, Crystal Structure, and Physical Properties
(2017-08-07)
Sichevych, Olga
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Prots, Yurii
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Utsumi, Yuki
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Akselrud, Lev
;
Schmidt, Marcus
The title compound was synthesized by a reaction of the elemental educts in a corundum crucible at 1200 degrees C under an Ar atmosphere. The excess of Ga used in the initial mixture served as a flux for the subsequent crystal growth at 600 degrees C. The crystal structure of Yb4Ga24Pt9 was determined from single-crystal X-ray diffraction data: new prototype of crystal structure, space group C2/m, Pearson symbol mS74, a = 7.4809(1) angstrom, b = 12.9546(2) angstrom, c = 13.2479(2) angstrom, beta = 100.879(1)degrees, V = 1260.82(6) angstrom(3), R-F = 0.039 for 1781 observed reflections and 107 variable parameters. The structure is described as an ABABB stacking of two slabs with trigonal symmetry and compositions Yb4Ga6 (A) and Ga12Pt6 (B). The hard. X-ray photoelectron spectrum (HAXPES) of Yb4Ga24Pt9 shows both Yb2+ and Yb3+ contributions as evidence of an intermediate valence state of ytterbium The evaluated Yb valence of similar to 2.5 is in good agreement with the results obtained from the magnetic susceptibility measurements. The compound is, a bad metallic conductor.
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
;
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.
Robustness of a Topologically Protected Surface State in a Sb2Te2Se Single Crystal
(2016-11-18)
Lee, Chao-Kuei
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Cheng, Cheng-Maw
;
Weng, Shih-Chang
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Chen, Wei-Chuan
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Tsuei, Ku-Ding
A topological insulator (TI) is a quantum material in a new class with attractive properties for physical and technological applications. Here we derive the electronic structure of highly crystalline Sb2Te2Se single crystals studied with angle-resolved photoemission spectra. The result of band mapping reveals that the Sb2Te2Se compound behaves as a p-type semiconductor and has an isolated Dirac cone of a topological surface state, which is highly favored for spintronic and thermoelectric devices because of the dissipation-less surface state and the decreased scattering from bulk bands. More importantly, the topological surface state and doping level in Sb2Te2Se are difficult to alter for a cleaved surface exposed to air; the robustness of the topological surface state defined in our data indicates that this Sb2Te2Se compound has a great potential for future atmospheric applications.
Size dependence of tetrahedral bond lengths in CdSe nanocrystals
(2007-04-16)
Wu, Pin-Jiun
;
Stetsko, Yuri P.
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Tsuei, Ku-Ding
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Dronyak, Roman
;
Liang, Keng S.
The structural characteristics of organically passivated CdSe nanocrystals (NCs) were investigated with x-ray diffraction and extended x-ray absorption fine structure. As the NC size decreases, the axial bond length R-(1) for an atomic tetrahedron extends but the equatorial bond length R-(2) contracts, with a similar tendency of distortion for the lattice parameters of the wurtzite structure. The authors suggest that the observed hexagonal distortion is attributed to the surface stress of the NCs related to the organic passivation effect and the relaxation of atomic positions at the stacking fault interface. (c) 2007 American Institute of Physics.
THz Generation and Detection on Dirac Fermions in Topological Insulators
(2013-11-01)
Luo, Chih Wei
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Chen, Hsueh-Ju
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Tu, Chien Ming
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Lee, Chia Ching
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Ku, Shin An