Browsing by Author "Tzeng, W. 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.
;
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.
Characterization of the charge density wave transition and observation of the amplitude mode in LaAuSb2
(2019-06-10)
Kuo, C. N.
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Shen, D.
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Li, B. S.
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Nguyen Nhat Quyen
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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.
Flexible Heteroepitaxy Photoelectrode for Photo-electrochemical Water Splitting
(2018-08-01)
Le Thi Quynh
;
Chien Nguyen Van
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Tzeng, W. Y.
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Huang, Chun-Wei
;
Lai, Yu-Hong
MICAtronics represents a new research direction by taking the advantages of good mechanical flexibility and optical transparency of muscovite mica, leading to a new platform to use oxide heteroepitaxy for novel transparent soft technology. Here we report a model flexible photoelectrode for photo-electrochemical (PEC) water splitting based on Fe2O3/ZnO/mica heteroepitaxy. The heteroepitaxy was confirmed by a combination of X-ray diffraction and transmission electron microscopy. The PEC performance of this flexible and semitransparent photoelectrode under various bending states was investigated. We found that the photocurrent of the heteroepitaxial system was enhanced by three times relative to pure ZnO and Fe2O3 under visible-light irradiation, and the heteroepitaxial photoelectrodes retain their photocurrent after continuous bending in cycling (>3000 cycles) with a smallest bending radius of 3.5 mm. The energy band alignment and charge dynamics under light excitation were characterized to understand the mechanism of the enhanced PEC performance. This study provides a new platform for design and fabrication of a flexible transparent photoelectrode in the harvest of solar energy based on oxide heteroepitaxy.
Microstructure and magnetic properties of oxidized titanium nitride thin films in situ grown by pulsed laser deposition
(2013-02-20)
Chen, S. C.
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Sung, K. Y.
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Tzeng, W. Y.
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Wu, K. H.
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Juang, J. Y.
Different oxidation states of titanium nitride thin films, including pure TiN(h 0 0), TiN1-xOx(h 0 0), Ti2O3(0 0 l) and pure anatase TiO2(0 0 l), were prepared by pulsed laser deposition with various oxygen pressures (P-O2) using a TiN target. Elaborative evolutions of the crystal and electronic structures of the obtained films were examined systematically by x-ray diffraction and x-ray absorption spectroscopy. We found that the Ti2O3(0 0 l) film, which was prepared at oxygen pressures P-O2 = 10(-4) Torr, exhibited the maximum room temperature ferromagnetism (RTFM) behaviour. The bound magnetic polaron model is used to clarify the origin of RTFM in these films.
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
Tuning Electronic Transport in a Self-Assembled Nanocomposite
(2014-06-01)
Chang, Wei Sea
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Liu, Heng-Jui
;
Vu Thanh Tra
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Chen, Jhih-Wei
;
Wei, Tzu-Chiao
Self-assembled nanocomposites with a high interface-to-volume ratio offer an opportunity to overcome limitations in current technology, where intriguing transport behaviors can be tailored by the choice of proper interactions of constituents. Here we integrated metallic perovskite oxide SrRuO3 - wurzite semiconductor ZnO nanocomposites to investigate the room-temperature metal - insulator transition and its effect on photoresponse. We demonstrate that the band structure at the interface can be tuned by controlling the interface-to-volume ratio of the nanocomposites. Photoinduced carrier injection driven by visible light was detected across the nanocomposites. This work shows the charge interaction of the vertically integrated multiheterostructures by incorporating a controllable interface-to-volume ratio, which is essential for optimization of the design and functionality of electronic devices.