On the darkening of secondary electron images of YBa2Cu3O7-xsuperconductors at the onset of the superconducting phase transition

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Abstract
Quantitative observations have been made on the darkening of secondary electron images of YBa2Cu3O7-x superconductors as the temperature is lowered below Tc. A cooling stage with a special electron detector has been used to obtain information about the mechanism responsible for the effect. Evidence is given that this is not related either to an overlaying non-superconducting film or to a change in the secondary electron emission, as proposed by some authors. It is demonstrated that the darkening can be understood on the basis of the Meissner effect, which produces a change in the collection efficiency of the Everhart-Thornley secondary electron detector.