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Durán, C., P. Esquinazi, J. Luzuriaga, and E. H. Brandt. "Bc1 of high-Tc La1.8Sr0.2CuO4 and amorphous Zr70Cu30 superconductors measured by a vibrating reed." Physics Letters A 123, no. 9 (1987): 485–488.
Abstract: The lower critical field Bc1 of the high-Tc superconductor La1.8Sr0.2CuO4 (Tc=40 K, Bcless-than-or-equals, slant7.7 mT) and of amorphous Zr70Cu30 (Tc=2.6K, Bc1less-than-or-equals, slant3.8 mT) is obtained from the resonance frequency ?/2? of a vibrating reed made of these materials. At applied magnetic fields Ba<Bc1, ?(Ba)=?i(Ba) as predicted for an ideally diamagnetic reed. Above Bc1 we observe a reversible linear law ?2??2inot, vert, similarB2a?B2c1. This allows a simple definition of Bc1 even in ceramic superconductors into which flux starts to penetrate at Bamuch less-thanBc1 due to their granular structure.
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