Browsing by Author "Wu, T. C."

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Anisotropic pinning in Nb thin films with triangular pinning arrays
(2007-07-01)
Cao, R.
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Wu, T. C.
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Kang, P. C.
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Wu, J. C.
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Yang, T. J.
A superconducting thin film with regular triangular arrays has been fabricated to explore the anisotropic vortex pinning behavior. We found that the critical currents for these films depend strongly on the current directions. Some interesting temperature-dependent phenomena are observed and discussed. We also made molecular dynamic simulations to study the dynamics of the vortex motion. The simulation results confirm the anisotropic features we found in the experiments. (c) 2007 Elsevier Ltd. All rights reserved.
Matching effect and flux pinning force in a Nb superconducting thin film with a triangular lattice of artificial pinning centers
(2004-05-01)
Horng, Lance
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Wu, J. C.
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Kang, P. C.
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Wu, T. C.
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Yang, T. J.
A strong pinning force in a Nb superconducting thin film is formed by a triangular lattice of defects due to the structural corrugation that acts as pinning centers. The magnetoresistance measurement close to the transition temperature shows a set of minima of magnetoresistance at equal external field intervals. A crossover in current-voltage relations with matching field and half-integer matching field occurs for currents exceeding a certain threshold value. The phenomenon is related to the presence of pinning structures at matching fields. (C) 2003 Elsevier B.V. All rights reserved.
Optical characteristics of silver film on the moth-eye structure
(2007-10-01)
Ting, C. J.
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Tsai, H. Y.
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Chou, C. P.
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Lin, H. Y.
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Wu, T. C.
The development of coating optics to lower the reflected light and thereby to increase the optical efficiency of an optical system has been a very important issue for many years. Conventional solutions to this, such as multilayered alternation of high and low refractive index layers, often lead to an expensive coating process. Recently, the use of antireflection structured (ARS) surface, which is called "moth-eye structure", has been proposed as an applicable option based on both the theoretical and experimental studies. In the current study, the experimental results of the reflectance and transmittance of two different thicknesses of silver films deposited on the moth-eye structure were carried out. The moth-eye structure arrays were fabricated by holographic exposure and photolithographic processes on the polymer film. The structure arrays were consisted of periodic length of about 300 nm, with the diameter of about 250 nm and the height of 150 nm. Compared with the silver coating film on the flat PET substrate, the optical property of the silver coating film on the moth-eye structure showed a better result for the anti-reflection application. The 25 nm-thick silver film on the moth-eye structure is suggested to be applied for the car window glass of antireflective films to obtain the high performance of heat insulation with acceptable transparency in the visible range.
Properties of vortex propagation in a niobium film with spacing-graded density of pinning sites
(2007-05-01)
Horng, Lance
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Wu, T. C.
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Wu, J. C.
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Cao, R.
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Yang, T. J.
The vortex propagation has been investigated in Nb superconductors with one dimension spacing-graded density of submicrometer-scaled holes in this research. A rectification effect can be discerned because the applied ac is transformed into a net dc voltage. These results demonstrate that the rectification is mainly characterized by the asymmetric pinning landscape and the rectified voltage depends considerably on temperature and the amplitude of applied ac. The dc voltage increases steeply up to the maximum value and then decreases to zero dc voltage with the increase of ac. The shape of peaks in the dc voltage signal become steeper and move to higher applied ac regime with the decrease of temperature.
Vortex ratchet effect in a niobium film with spacing-graded density of pinning sites
(2007-08-01)
Wu, T. C.
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Horng, Lance
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Wu, J. C.
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Cao, R.
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Kolacek, Jan
The vortex propagation exhibits rectification effect in Nb superconductors with spacing-graded density of holes. A rectified dc voltage is obtained when the vortex lattice is driven by ac current. The asymmetric geometry of the pinning array produces a significant influence on the vortex motion. The rectified voltage depends considerably on the amplitude of the applied ac current and the magnetic field. The experimental results reveal a drastic change of the vortex rectification for magnetic field above/below the first matching field. The reason may be that the interstitial vortices are formed in the film above the first matching field. A reversible vortex motion is induced by the interstitial vortices for the field above the first matching field. (c) 2007 American Institute of Physics.