Nobuhisa Takezawa

Optimal size of an insulating inclusion acting as a pinning center for magnetic flux in superconductors: calculation of pinning force

Abstract By numerically solving the two-dimensional Ginzburg-Landau equations, we have for the first time evaluated the optimal size of an insulating inclusion for pinning a single-quantum vortex in a superconductor. Although it has so far been believed that the optimal size of a pinning center is twice the coherence length, we found for the first time that, for the case of low magnetic fields and low pinning center density, a prismatic insulating inclusion with a square cross-section of the side length approximately equal to the penetration depth gives the strongest pinning force to the vortex.

Optimal size of a cylindrical insulating inclusion acting as a pinning center for magnetic flux in superconductors

Abstract We examined the effectiveness of an insulating inclusion in a superconductor as a pinning center. With regard to a single-quantum vortex, the dependence of the minimum pinning potential U p on a radius R of a cylindrical insulating inclusion was investigated by solving the Ginzburg-Landau equations numerically. Here, U p is defined by the free energy difference between the vortex being inside the inclusion and that being infinitely distant from the inclusion. U p is negative for all R s and is very shallow for R smaller than the coherence length ξ( T ). The depth of U p increases rapidly with increasing R up to the penetration depth λ( T ), and there U p is found to saturate. U p for R \ gl( T ) is much deeper than that for R ≲ξ( T ). In other words, even for a single vortex, the insulating inclusion with R ≳λ( T ) gives a stronger attractive interaction with the vortex and works as a more effective pinning center than that with R ≲ξ( T ).

Flux pinning and flux creep in La2−xSrxCuO4 with splayed columnar defects

The effects of columnar defects with splayed configurations on flux pinning and flux creep were investigated experimentally. Parallel columnar defects and splayed columnar defects were introduced into specimens of La2−xSrxCuO4 by high-energy heavy-ion irradiation. A large enhancement of the flux pinning and the critical current density Jc due to the splayed columnar defects was observed. At temperature T<15 K and a magnetic field H=0.1 T, the effective pinning potential U0 of a specimen with splayed columns was larger than that of a specimen with parallel columns. On the other hand, at 15 K≤T≤30 K and H=0.1 T, U0 of a specimen with splayed columns was smaller than that of a specimen with parallel columns.

Suppression of vortex motion in La1.85Sr0.15CuO4 with the splayed configuration of columnar defects

Abstract Remarkable enhancements of the critical current density J c were obtained in polycrystalline La 1.85 Sr 0.15 CuO 4 with the splayed configuration of the columnar defects which were introduced by 3.5 GeV Xe ion irradiation with splay angles θ = 0°, ±5° and ±10°. In addition J c increased with the irradiation dose from 1.0×10 11 to 3.0×10 11 ions/cm 2 . Larger upward shift of the irreversibility line for the splayed configuration, in comparison with conventional parallel configuration, was observed. The results suggest that the vortex motion in La 1.85 Sr 0.15 CuO 4 is suppressed effectively by the splayed irradiation.

Computational Analysis of Optimal Size of a Cylindrical Insulating Inclusion Acting as a Pinning Center

The dependence of minimum pinning potential, Up, on the radius, R, of a cylindrical insulating inclusion was investigated by numerically solving the Ginzburg-Landau equations with regard to a single-quantum vortex. Up is negative for R > 0. |Up| is very small for R smaller than the coherence length ξ(T). |Up| increases rapidly with increasing R in the region smaller than the penetration depth λ(T), and saturates for R ≳ λ(T). |Up| for R ≳ λ(T) is much larger than that for R ≲ ξ(T). These results indicate that an insulating inclusion with R ≳ λ(T) gives a stronger attractive interaction with the vortex and works as a more effective pinning center than that with R ≲ ξ(T).

Swift heavy-ion irradiation effects on the super-conducting material La1.85Sr0.15CuO4

The effects of columnar defects produced by swift heavy-ion irradiation on magnetic flux pinning were studied for the polycrystalline La1.85Sr0.15CuO4. The bulk specimens were irradiated with 3.5 GeV 136Xe31+ ions from a single direction or multi-directions in order to investigate the effects of columnar defect configuration. The magnetization hysteresis was measured with a super-conducting quantum interference device (SQUID) magnetometer, and the critical current density Jc and irreversibility line were observed. In the specimens irradiated from multi-directions, more enhancement of the critical current density and larger shifts of the irreversibility line to higher fields were observed than in the specimens irradiated from a single direction.