Effect of temperature and magnetic field on the thickness dependence of the critical current density ofYBa2Cu3O7−x

Abstract

In order to shed light on the puzzling thickness ͑t͒ dependence of the critical current density ͑J c ͒ in YBa 2 Cu 3 O 7−x ͑YBCO͒ films at 77 K and self-field, we have investigated J c -t behavior as a function of temperature ͑T͒ and applied magnetic field ͑H͒ in the thickness range that is within the modified Larkin length L c . It has been found that the J c -t behavior can be well described by the collective pinning ͑CP͒ model as monotonically decreasing J c ϳ t −1/2 when thermal assisted flux motion ͑TAFM͒ is suppressed at low T close to zero. TAFM becomes increasingly important at elevated temperatures and causes deviation of the J c -t from the CP model in the thickness range below the magnetic penetration depth. The deviation is more pronounced at smaller thickness and increases with T and H. Consequently, a completely reversed J c -t from monotonically decreasing to monotonically increasing occurs when TAFM dominates in high T and H ranges. This result suggests that the monotonic decreasing J c -t behavior is an intrinsic property of uniform YBCO films, while extrinsic effects such as material nonuniformity can greatly complicate it.The strong dependence of critical current density ͑J c ͒ on the thickness ͑t͒ of high-T c superconducting ͑HTS͒ films has presented a tough question that challenges our understanding of basic physics governing electric transport properties in HTS films. 1-5 Basically, J c measured at 77 K and self-field ͑SF͒ on films of HTS materials, such as YBa 2 Cu 3 O 7−x ͑YBCO͒, decreases monotonically with increasing film thickness. A basic question is whether this J c -t behavior is simply caused by material inhomogeneity through thickness or related to an intrinsic property of superconductors. It should be realized that the major J c reduction occurs in the thickness range below ϳ0.6 m, where microstructure degradation is not a major concern. Interestingly, the J c data reported on a large number of YBCO film samples 5-7 made with different techniques fall more or less on the curve of J c ϳ t −1/2 at 77 K and SF, which agrees with the prediction of weak collective pinning model ͑CP͒ for superconducting films thinner than the longitudinal collective pinning length L c ͑Larkin length͒. 8 In the CP model, a quantized magnetic flux line ͑FL͒ with a length of L Ͻ L c is rigid and cannot bend to accommodate different pinning centers nearby. When the dominant pinning centers are randomly distributed point defects, the pinning force ͑F p ͒ exerted on a FL scales as L 1/2 while the Lorentz force scales as L. This leads to the J c ϳ t −1/2 behavior. As L exceeds L c , the FL resumes its elasticity and bends itself to accommodate the pinning center, resulting in a thickness-independent J c .Although the CP model fits the SF J c -t relatively well at 77 K, some questions remain. The first question is about the order of magnitude of the L c . The original L c for a weak collective pinning system is calculated from L c Ϸ r p ͑J 0 / J c ͒ 1/2 , where r p is the pinning center interaction radius and J...

Eliminating thickness dependence of critical current density in YBa2Cu3O7−x films with aligned BaZrO3 nanorods

“…This seems to agree with the observed higher threshold value of H for the TAFM to be effective in the high T group. As we reported earlier on YBCO films without BZO doping, 6 a reversed J c − t trend is observed when TAFM dominates, which is the case for both low T and high T group at H Ͼ 0.5 T. 6 The observation of negligible thickness dependence of J c in low H fields in BZO doped YBCO films supports the previous argument that the collective pinning mechanism is responsible for the observed J c − t in standard YBCO films. This observation is also consistent with our previous report on reduced thickness dependence of J c in YBCO/NTP samples.…”

“…Interestingly, the lower J c values of YBCO/BZO films in SF and lower H fields can not be attributed completely to the T c difference and moreover, high J c s are observed for thicker YBCO/BZO films. In addition, unlike for the standard films, on which temperature has a pronounced effect on J c − t through TAFM, 6 the J c of YBCO/BZO shows nearly no thickness dependence in a large range of temperatures ranging from = 0.11 to = 0.86. This suggests that the TAFM is greatly suppressed in the SF by the strong correlated pinning from aligned BZO nanorods along the c-axis.…”

“…4,5 Our previous study of the J c − t behavior on pulsed laser deposited ͑PLD͒ YBCO films suggests that the J c − t evolves with changing temperature ͑T͒ and magnetic field ͑H͒ in the way consistent with the modified collective pinning ͑CP͒ model, despite complications due to thermally assisted flux motion ͑TAFM͒ in an elevated T and high H range. 6,7 The modified CP model considers largedimension, random-distributed point defects as the dominant pinning centers in typical YBCO films and therefore predicts a thickness dependent F p ϰ t −1/2 . 8 In order to eliminate this undesired thickness dependence in HTS coated conductors, a key strategy is to introduce pinning centers that can maintain a constant F p regardless of changing thickness.…”

Critical Role Played by Interface Engineering in Weakening Thickness Dependence of Superconducting and Structural Properties of FeSe0.5Te0.5-Coated Conductors

“…The thickness effect refers to the phenomenon where the J c of a superconducting thin film depends strongly on its thickness, resulting in a crossover thickness that separates the thickness-dependent and thickness-independent regimes. This effect is predicted by the collective pinning theory, 33,53,54 which proposes that each vortex is collectively pinned by point defects when subjected to a magnetic field. At small thicknesses, the short segments of the vortex are independently pinned parallel to the field, resulting in high J c values.…”

Thickness dependence of structural and superconducting properties of Co-doped BaFe2As2 coated conductors

“…The thickness dependence of J c for our IBS CCs is quite different from that in cuprate superconductors. It has been found that the J c - t behavior in cuprate superconductors can be addressed by collective pinning theory, by which the J c reduction with increasing thickness can be well described as J c ∼ t −1/2 , followed by a crossover thickness t c , over which J c becomes almost thickness independent ( Wang and Wu, 2007 ; Tran et al., 2013 ; Gurevich, 2007 ). It is interesting to note that the thinnest films with the lowest T c showed the highest J c up to 5 T for H//c and 9 T for H//ab .…”