Browsing by Author "Horng, L"

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Anisotropic pinning effect on a Nb thin film with triangular arrays of pinning sites
(2004-06-01)
Wu, TC
;
Kang, PC
;
Horng, L
;
Wu, JC
;
Yang, TJ
A superconducting Nb thin film with triangular arrays of pinning sites has been fabricated to investigate the commensurate vortex lattices. Vortex-vortex interactions in this pattern film make the observed anisotropy in the pinning properties. Periodic minima appear in the magnetoresistance curves at equal field intervals that can be associated with matching effect. The transport properties at near critical temperatures show anisotropic pinning properties related to the configuration of pinning centers. The critical current is depressed when the driving force is applied along the short diagonal in the rhombic unit cell of pinning array. It implies that the pinning potential along the short diagonal creates a moving channel of vortices in triangular arrays of defects. The pinned and interstitial vortices can be movable in an incommensurate vortex row and a straight unhindered one-dimensional path. (C) 2004 American Institute of Physics.
Formation of multiple-quanta vortices at circular defects in a superconducting thin film
(1997-08-01)
Wei, JC
;
Horng, L
;
Yang, TJ
;
電子物理學系
;
Department of Electrophysics
The possibility of the formation of multiple-quanta vortices in a superconducting thin film with circular defects is discussed. It is shown that the condition for creating two-quanta vortices on a circular defect depends on the radius of defect, the separation between defects and the configuration of defects.
Investigation of hysteresis and transport critical current under field cycle for Bi-based multifilamentary tapes
(1996-04-01)
Horng, L
;
Chen, K
;
Lee, WD
;
Yang, TJ
;
電子物理學系
The transport properties of Hi-based multifilamentary tapes on magnetic field have been studied. The tapes were fabricated by powder-in-silver tubing technique followed by appropriate sintering. The hysteretic behavior of transport critical current of the tapes with bending strain has been investigated. The experimental data are obtained at 77 K under field cycling from 0 to 1000 G to 0. The critical current density under a field does not vary when bending strain is smaller than a critical bending strain, and then decreases gradually with increasing bending strain. The normalized values are unaffected by bending strain up to 1.31%. The remnant magnetic filed H-m associated with the flux pinning force is obtained by an empirical expression, which is similar to generalized critical state model. The H-m is almost a constant for all bending strain. This observation indicates that the intrinsic pinning properties are not varied with bending strain.
Magnetic force acting on a magnetic dipole over a superconducting thin film
(1996-12-01)
Wei, JC
;
Chen, JL
;
Horng, L
;
Yang, TJ
;
電子物理學系
The magnetostatic interaction energy and corresponding magnetic force acting on a magnetic point dipole placed above a type-II thin superconducting film in the mixed state with a single vortex are calculated using electromagnetics coupled with the London theory of superconductivity. If a vortex is trapped by a circular defect of radius b much less than Lambda, the magnetic forces, caused by the vortex, differ from the results of free from defect pinning by the factor (1 - b/Lambda), where Lambda is the effective penetration depth. The possibility of formation of the vortex in the thin film only in the field of the magnetic point dipole is investigated. The critical position of the dipole for creating the first vortex under the electromagnetic pinning of a circular defect and that position in the absence of defect pinning are obtained for comparison. In particular, in the limit of a/Lambda much greater than 1, where a is the separation between the dipole and the thin film, the only difference between two results is in the cutoff length, i.e., in the case of a circular defect the only difference in the critical position calculation is the cutoff at radius b rather than at coherence length xi. The pinning force of a single vortex by a circular defect is also calculated. Further, we investigate the conditions of the vortex creation for various cases (including the first, second, and third vortices) for a free of pinning center in the examining region. It is found that the creation of a new single vortex in the thin film causes an abrupt change in vertical levitation force: the force changed discontinuously.
Magnetic force acting on a magnetic dipole over a superconducting thin film
(1996-12-01)
Wei, JC
;
Chen, JL
;
Horng, L
;
Yang, TJ
;
電子物理學系
The magnetostatic interaction energy and corresponding magnetic force acting on a magnetic point dipole placed above a type-II thin superconducting film in the mixed state with a single vortex are calculated using electromagnetics coupled with the London theory of superconductivity. If a vortex is trapped by a circular defect of radius b much less than Lambda, the magnetic forces, caused by the vortex, differ from the results of free from defect pinning by the factor (1 - b/Lambda), where Lambda is the effective penetration depth. The possibility of formation of the vortex in the thin film only in the field of the magnetic point dipole is investigated. The critical position of the dipole for creating the first vortex under the electromagnetic pinning of a circular defect and that position in the absence of defect pinning are obtained for comparison. In particular, in the limit of a/Lambda much greater than 1, where a is the separation between the dipole and the thin film, the only difference between two results is in the cutoff length, i.e., in the case of a circular defect the only difference in the critical position calculation is the cutoff at radius b rather than at coherence length xi. The pinning force of a single vortex by a circular defect is also calculated. Further, we investigate the conditions of the vortex creation for various cases (including the first, second, and third vortices) for a free of pinning center in the examining region. It is found that the creation of a new single vortex in the thin film causes an abrupt change in vertical levitation force: the force changed discontinuously.
Magnetic force acting on a magnetic point dipole over a superconducting thin film with a circular defect
(1996)
Yang, TJ
;
Wei, JC
;
Chen, JL
;
Horng, L
;
電子物理學系
London theory is used to calculate the magnetic levitation force acting on the magnetic point dipole placed above the type-II superconducting thin film with a circular defect. The condition of the position of point dipole to create the first vortex in the thin film is found for b much less than boolean AND. The difference of the lateral magnetic force Delta F-r acting on the point dipole between with and without a single vortex created in the thin film is also derived and used to estimate the force to overcome the pinning force of the vertex due to the circular defect.
Magnetic force signal of vortex creation in type-II superconducting thin films
(1997-07-15)
Wei, JC
;
Horng, L
;
Yang, TJ
;
電子物理學系
;
Department of Electrophysics
The possibility of the formation of a vortex in thin film due to the field of magnetic dipole moment when a point dipole is vertically approaching the surface of a thin film is discussed. Calculations are performed for certain discrete values of the critical position of the point dipole for creating the vortex in the thin film and the equilibrium positions of the vortex, both of which depend on the strength of magnetic dipole moment. The creation of a new single vortex in the thin film causes an abrupt change in the vertical force. (C) 1997 Elsevier Science B.V.
Magnetic force signal of vortex creation in type-II superconducting thin films
(1997-07-15)
Wei, JC
;
Horng, L
;
Yang, TJ
;
電子物理學系
;
Department of Electrophysics
The possibility of the formation of a vortex in thin film due to the field of magnetic dipole moment when a point dipole is vertically approaching the surface of a thin film is discussed. Calculations are performed for certain discrete values of the critical position of the point dipole for creating the vortex in the thin film and the equilibrium positions of the vortex, both of which depend on the strength of magnetic dipole moment. The creation of a new single vortex in the thin film causes an abrupt change in the vertical force. (C) 1997 Elsevier Science B.V.
Pinning effects induced by periodic arrays of structural corrugations in Nb thin films
(2003-05-01)
Horng, L
;
Wu, JC
;
Kang, PC
;
Lin, PH
;
Wu, TC
Artificial vortex pinning arrays have been fabricated by electron-beam lithography in conjunction with reactive ion etching in Nb superconducting thin films in order to investigate flux pinning. Based on the structural corrugation in the Nb thin film, the square lattice of defects that act as pinning centers was created. A strong periodic pinning effect, as the vortex lattice matches the array of defects, can be observed in the evolution of resistivity and critical current with the applied magnetic fields in the mixed state of the Nb film, thus, revealing a synchronized pinning effect due to structural corrugation. This result is in agreement with the resistivity-temperature curves at matching fields and half-integer matching fields in which a crossover occurs for temperatures exceeding a certain threshold value. The phenomenon is related to the presence of the pinning structure at matching fields.
Temperature dependence of vortex configuration by honeycomb hole arrays in a superconducting Nb film
(2005-05-15)
Wu, TC
;
Wang, JC
;
Horng, L
;
Wu, JC
;
Yang, TJ
A honeycomb array of submicrometer holes in a Nb superconducting thin film has been fabricated to investigate the flux pinning effect. It is found that the minima positions reveal two regimes characterized by the matching fields and the fractional ones. It is believed that the complex behavior may come from more than one vortex being captured per pinning site. Furthermore, the temperature dependence of the saturation number of vortices per pinning site together with vortex-vortex interaction gives the complex vortex configurations. (c) 2005 American Institute of Physics.
The ac effect of vortex pinning in the arrays of defect sites on Nb films
(2006-04-15)
Wu, TC
;
Horng, L
;
Wu, JC
;
Hsiao, CW
;
Kolacek, J
Niobium thin films with spacing-graded array of submicrometer-scaled holes had been fabricated using electron beam lithography through a lift-off technique. The magnetoresistance measurements and current-voltage characteristics were carried out with the external magnetic field applied perpendicular to the film plane, in which commensurable effects were observed in both experiments. The magnetoresistance with positive/negative directions of dc current revealed identical curves except the dips at matching fields separated. Two distinct current-voltage curves, which resulted from the positive and negative applied current directions, respectively, were discerned when the external magnetic field was fixed at the matching field, which is believed to be due to asymmetry pinning potential in the spacing-graded array of holes. In addition, ac current-voltage curve measured at matching field showed a ratchet bump along with another extra peak associated with incommensurable effect. (C) 2006 American Institute of Physics.
Thickness dependence of magnetic force acting on a magnetic dipole over a type-II superconducting thin film
(1997-08-01)
Yang, TJ
;
Wei, JC
;
Chen, JL
;
Horng, L
;
電子物理學系
The thickness dependence of the magnetic force acting on a point magnetic dipole over a superconducting thin film in the mixed state with a single vortex line is calculated by London theory. The magnetic force is decomposed into two parts, vertical and lateral components. Both vertical and lateral force components approach to the saturated value with increasing thickness when the thickness is much larger than the London penetration depth. A single vortex created in the thin film by the field of the magnetic dipole is also considered. The thickness dependence of the critical position of the point dipole for creating the first vortex line in the thin film is exploited and derived.
Thickness dependence of magnetic force and vortex creation in type-II superconducting thin film
(1996-08-20)
Wei, JC
;
Chen, JL
;
Horng, L
;
Yang, TJ
;
電子物理學系
We examined a single vortex line in an infinite type-II superconducting thin film with finite thickness. The thickness dependence of the magnetic force acting on a point dipole placed above a superconducting film is calculated as the negative of the gradient of the interaction energy, coupled to the London theory of superconductivity with electromagnetics. We also considered the situation of a single vortex creation in the thin film, caused by the field of the magnetic dipole tip itself. The thickness dependence of the critical position of the point dipole for creating the first vortex line in the thin film is calculated.
Vortex dynamics in Nb thin films with triangular potential
(2005-06-01)
Wu, TC
;
Kang, PC
;
Wu, JC
;
Horng, L
;
Yang, TJ
Triangular artificial antidot arrays in Nb superconducting thin films have been fabricated to investigate the flux pinning effect when the magnetic field is applied perpendicular to the film. The spacing of the defect is around 545 nm and the diameter of the defect is around 300 nm. Arrays of triangular lattice show anisotropic properties of pinning effect induced by the anisotropic structure. The vortex-vortex interaction force is stronger when the vortices move along the short axis in the rhombic unit cell and cause an overall higher normalized critical current.
Vortex dynamics in spacing-graded array of defects on a niobium film
(2006-05-15)
Wu, TC
;
Horng, L
;
Wu, JC
;
Hsiao, CW
;
Kolacek, J
We have investigated the vortex dynamics in the niobium films having a spacing-graded array of pinning sites. The samples were fabricated by using electron beam lithography through a lift-off technique. The pinning sites of 200 nm in diameter were arranged with a constant hole-defect separation in x-axis direction and graded separation in y-axis direction from 392 nm to 408 nm. The magneto-resistance measurements and current-voltage characteristics were explored with the external magnetic field applied perpendicular to the film plane. Dc current-voltage measured at matching field revealed two distinct curves resulted from the positive and negative applied current directions, respectively. This is believed to be due to an asymmetry pinning potential formed in the spacing-graded array of holes, giving rise to asymmetry Lorentz forces. De voltage drop measured with respect to the ac current applied along the x-axis of the sample showed two separated maxima, which can be explained using the dynamics of pinned vortex lattice and interstitial vortices in the asymmetry pinning landscape. (c) 2006 Elsevier B.V. All rights reserved.