  We predict that superconducting particles will show an apparent increase in
thickness at low temperatures when measured by electron holography. This will
result not from a real thickness increase, rather from an increase in the mean
inner potential sensed by the electron wave traveling through the particle,
originating in expansion of the electronic wavefunction and resulting negative
charge expulsion from the interior to the surface of the superconductor, giving
rise to an increase in the phase shift of the electron wavefront going through
the sample relative to the wavefront going through vacuum. The temperature
dependence of the observed phase shifts will yield valuable new information on
the physics of the superconducting state of metals.

Superconductivity