Browsing by Author "Wei, JC"

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Comparison of irreversibility lines of silver-sheathed Bi-2223, Tl-1223 and Tl-1234 superconducting tapes
(1996-01-01)
Wei, JC
;
Yang, TJ
;
Liu, RS
;
光電工程學系
;
Department of Photonics
The irreversibility lines of silver-sheathed (Bi,Pb)(2)Sr2Ca2Cu3O10 (Bi-2223), (Tl,Pb,Bi)(Sr,Ba)(2)Ca2Cu2O9 (T1-1223) and (Tl,Pb)Sr2Ca3Cu4O11 (T1-1234) superconducting tapes were investigated and compared with those of other superconducting systems. We have demonstrated that the shape of the irreversibility lines of T1-1223 and T1-1234 superconducting tapes are similar in behavior to those of YBa2Cu3O7 (Y-123) and (Tl,Pb)Sr-2(Ca,Y)Cu2O7 (T1-1212) systems and better than those of Tl2Ba2Ca2Cu3O10 (T1-2223) and (Bi,Pb)(2)Sr2Ca2Cu5O10 (Bi-2223) superconductors. This means that the flux pinning ability of T1-1223 and T1-1234 superconducting tapes was stronger than that of Bi-2223 and T1-2223 superconductors. The possible reason for controlling flux pinning can be attributed to the intrinsic crystal structure difference in the materials.
Comparison of irreversibility lines of silver-sheathed Bi-2223, Tl-1223 and Tl-1234 superconducting tapes (vol 43, pg 83, 1996)
(1996-10-01)
Wei, JC
;
Yang, TJ
;
Liu, RS
;
光電工程學系
;
Department of Photonics
Current distribution and vortex-vortex interaction in a superconducting film of finite thickness
(1996-11-01)
Wei, JC
;
Yang, TJ
;
電子物理學系
;
光電工程學系
;
Department of Electrophysics
The current distribution due to a single vortex in a superconducting film of finite thickness is calculated, based on the London model. At large distances from the vortex core, the current density on the surface of the superconductor was found to follow the 1/r(2) law which has a simple expression, J(s)(r, 0) approximate to phi(0)[coth(d/lambda) + csch (d/lambda)]/2 pi mu(0) lambda r(2). We also find that the interaction energy between two vortices has the form, U-int(r/(12)) phi(0)(2)[H-0(r(12)/lambda') - Y-0(r(12)/lambda l)], when the film is not too thick and at large distances between two vortices. This potential differs from Pearl's result by the new effective penetration depth lambda' = 2 lambda coth (d/lambda).
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.
Interaction between a cylindrical superconducting impurity and a vortex in a type-II superconductor
(1997-02-10)
Wei, JC
;
Chen, JL
;
Liu, RS
;
Yang, TJ
;
電子物理學系
We examined the effectiveness of an infinite cylindrical superconducting impurity of radius a in an infinite type-II superconductor as a pinning center. The impurity and the superconducting background have the penetration depth lambda(i) and lambda(s), respectively. With regard to a single-quantum vortex, the free energy of the system and the pinning potential for a vortex were calculated, based upon the London model of the vortex line. Here, the pinning potential U-pin is defined by the difference between the free energy of a vortex being at the center of the impurity and that being infinitely distant from the impurity. U-pin is negative for lambda(i) > lambda(s). On the other hand, U-pin is positive for lambda(i) < lambda(s). In the limit of a much greater than lambda(i) and lambda(s), the pinning potential is found to be U-pin = phi(0)(2)(1/lambda(i)(2) - 1/lambda(s)(2)) ln kappa/4 pi mu(0). In the limit of a much less than lambda(i) and lambda(s), the pinning potential is found to be U-pin = phi(0)(2)[(1/lambda(i)(2) - 1/lambda(ss)(2)) ln(lambda(s)/a) - (1/lambda(i)(2)) ln(lambda(i)/a)]/4 pi mu(0). There is a clear correlation between the intrinsic property of these two superconducting materials and the pinning potential.
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.
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.