  In a solid, transport of electricity can occur via electrons or via holes. In
the normal state no experiment can determine unambiguously whether the
elementary mobile carriers have positive or negative charge. This is no longer
true in the superconducting state: superconductors know the difference between
electrons and holes. This indicates that electron-hole asymmetry plays a
fundamental role in superconductivity, as proposed by the theory of hole
superconductivity.
