Influence of spatial variations in the lower critical field on the equilibrium field penetration into superconductors

Abstract

International audienceThe thermodynamic lower critical field Hc1 in Bi2Sr2CaCu2O8+y crystals, in the presence of columnar defects, was determined using magneto-optical measurements combined with the shaking technique. We observe a downward shift in Hc1 due to columnar defects. Using partially irradiated samples, we show that small local variations in Hc1 dramatically affect the way the field penetrates into superconductors. We present a theoretical model which provides a quantitative fit to the experimental results and explains how the influence of small sample inhomogeneities is enhanced by geometrical effects and leads to significant variations in the local-field distribution and penetration

Suppression of geometrical barrier in $Bi_2Sr_2CaCu_2O_{8+δ}$ crystals by Josephson vortex stacks

“…The paramagnetic response of the grooves, B z > H z , is similar to the situation described in Ref. 48, where such a phenomenon was described in terms of demagnetization effect. Our numerical calculations also provide a qualitative understanding of the behavior on the descending field as follows.…”

“…It was recently shown that small local inhomogeneities in H c1 could lead to very large variations in the equilibrium local induction, a phenomenon attributed to demagnetization effects. 48 In order to see whether demagnetization can explain the phenomenon at hand we studied the GB mechanism in sample A after two shallow grooves were etched on its surface across the width of the sample as described above. We found that the grooves have a surprisingly large effect: a 160 nm deep groove, just 2% of the sample thickness, reduces the GB hysteresis comparable to the suppression by the in-plane field of about 10 Oe, and vortex whiskers are formed along the groove.…”

Upper and lower critical fields in NdFeAs(O,F) single crystals : a study by Hall probe magnetization and specific heat

“…1a, the transmittivity measured in the center of the sample first presents a small paramagnetic increase starting at ∼ 37K followed by a sharp diamagnetic jump at T ∼ 34K (sample 1). This paramagnetic bump (also observed in zero field cooled DC measurements) reflects a non homogeneous distribution of the field between 34 and 37 K [7]. However the presence of a clear anomaly in the specific heat emphasizes the overall good quality of the platelets.…”

Critical current in thin flat superconductors with Bean-Livingston and geometrical barriers

“…It is known [60] that the effect of the elasticity of the curved lattice on the balance of the forces can be taken into account if the current density in the condition j = 0 is considered as j = rotH rather than j = (1/µ 0 ) rotB where H is the thermodynamic magnetic field, H = ∂F/∂B, and F is the free-energy density of the superconductor. In isotropic superconductors, the field H is parallel to B, and the dependence H(B) is well approximated by the function [61,62],…”