Comparison of small-field behavior inMgB2, Low- and high-temperature superconductors

Abstract

Different types of superconductors have been investigated at small magnetic fields ͑B a ͒ over wide temperature ͑T͒ ranges at different B a orientations. It has been shown that the temperature dependence of the characteristic field ͑B * ͒, separating the B a -independent critical current density ͑J c ͒ plateau ͑single vortex pinning regime͒ and the region with J c ͑B a ͒ ͑collective pinning͒, can be attributed either to the temperature dependence of the magnetic penetration depth for Nb-film and MgB 2 bulk superconductors, or to thermally activated processes for Bi-based superconductors and YBa 2 Cu 3 O 7−␦ superconducting films. In both cases the vortex pinning influence appears to have a secondary role, affecting the effective vortex depinning radius. An exception in such B * ͑T͒ behavior is considered for Nb film when the magnetic field has its considerable component applied perpendicular to the main surface of the film.

Critical current density: Measurements vs. reality

“…4(a), the similar frequency dependent PPMS measurements are shown for MgB 2 bulk superconductor (being 1 × 2 × 3 mm 3 ) fabricated as described in our previous works [24,26,27]. MgB 2 is known to exhibit different vortex pinning behaviour to HTS [11,28], hence it would be expected to behave differently upon changing PPMS frequency. It does behave differently to the YBCO film (compare to Fig.…”

Dynamic magneto-optical imaging of superconducting thin films

“…This technique shows visually the position of cracks [5,8], weak links [4,6,7] and other micro-and macroscopic defects in the sample [2,9], however there may be other current-limiting mechanisms that are not so readily visible at the macroscopic scale nor directly dependent on critical current density [10,11].…”

Significant tunability of thin film functionalities enabled by manipulating magnetic and structural nano-domains

“…7] (note, this trend is independent of the B * definition). The B * increase indicates enhanced pinning and reduction of the individual depinning radius 77,78 . As B * is proportional to the dislocation density (dominating pinning in PLD YBCO films), it is natural to expect an enhancement of J c (B a ) consistent with the B * enhancement that is clearly the case, as can be seen in Fig.…”

“…The quantitative agreement between densities of dislocations and islands, as well as B * , J c (B a ) and J c (0) may be not easy to find 77 because while B * is determined by density of pinning centres in single vortex pinning regime, J c (0) is established by strength of individual pinning cites 28 . In addition, other effective pinning defects 77,78 , such as twin boundaries 26,81 , oxygen vacancies 82 and Y 2 O 3 inclusions 83,84 are present and can be affected by deposition frequency. Indeed, smoother transitions at the higher deposition frequencies from the plateau-like J c (B a ) dependence at B a → 0 to the powerlaw dependence at the larger fields [insert in Fig.…”

“…61 . This field, denoting transition from the single vortex pinning regime at low fields to a collective pinning regime at higher fields, can be used to accurately determine the pinning radius of the individual vortex pinning 77,78 . Note, B * can be defined as the field at which the critical current reaches the 90% 79 or 95% 77,78 of the value obtained for zero applied field.…”