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Discovery
The drop of electrical resistance to zero in Hg was observed by Heinz Kamerlingh Onnes in 1911 when he lowered the sample temperature below {$T_c=4.2$} K.
After this discovery many more metals were shown to display the same property, termed superconductivity, with a few notable exceptions such as Cu, Ag, Au. The metal superconductor transition for elemental metals is invariably low or very low: the highest is 9 K in Nb.A few metals, like Cu, Ag and Au, never become superconductors.
Initially this phenomenon was described simply as the metal becoming a perfect conductor. A perfect conductor must also be a perfect diamagnet, that prevents the penetration of magnetic induction inside its volume by establishing persistent eddy currents around its surface, the same way a metal does for a transient initial time after the induction field has been switched on. This phenomenon is explained by the Faraday-Lenz law, stating that the time derivative of the magnetic flux, {$d\Phi_{\boldsymbol B}/dt$}, across a given surface equals the electromotive force {$\epsilon$} around the closed contour of the surface, and that the sign of {$\epsilon$} must drive the current, if it may flow, to oppose the flux variation.
True eddy currents rapidly decay, due to Joule dissipation, {$IR^2$}. In contrast, for a perfect conductor {$R=0$} and the shielding currents never decay. This means that {$d\Phi_{\boldsymbol B}/dt=0$} for any surface totally contained in the perfect conductor.
Meissner effect
However, this simple explanation is not exhaustive of a superconductor magnetic properties, as it was shown by Meissner and Ochsenfeld in 1933. A conductor is eventually penetrated by a uniform induction and lowering the temperature below {$T_c$} does not produce any
{$d\Phi_{\boldsymbol B}/dt\ne0$}. Therefore a perfect conductor would not expell {$\Phi_{\boldsymbol B}$}, while a superconductor, surprisingly, does.
These peculiar magnetic properties imply that the induction always vanishes inside a superconductor, irrespective if whether the sample is cooled in field (Field Cooling, FC) or cooled in a vanishing field (Zero Field Cooling, ZFC) across the critical temperature {$T_c$}. Since {$\boldsymbol B = \mu_0 (\boldsymbol H + \boldsymbol M)$} the magnetic field {$\boldsymbol H$} must be equal and opposite to the sample magnetisation, hence its susceptibility must be {$\chi=-1$}.
Critical field
Another important experimental fact is that zero electrical resistivity and diamagnetism, including field expulsion below {$T_c$}, survive up to a maximum, critical field value {$H_c$}, above which normal metallic properties are re-established.
For an extensive description of superconductivity refer e.g. to Tinkham.
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