Abstract
The thermodynamic lower critical field ${H}_{c1}$ in ${\text{Bi}}_{2}{\text{Sr}}_{2}{\text{CaCu}}_{2}{\text{O}}_{8+y}$ crystals, in the presence of columnar defects, was determined using magneto-optical measurements combined with the shaking technique. We observe a downward shift in ${H}_{c1}$ due to columnar defects. Using partially irradiated samples, we show that small local variations in ${H}_{c1}$ 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.
Published Version
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