  In a solid, transport of electricity can occur via negative electrons or via
positive holes. In the normal state of superconducting materials experiments
show that transport is usually dominated by $dressed$ $positive$ $hole$
$carriers$. Instead, in the superconducting state experiments show that the
supercurrent is always carried by $undressed$ $negative$ $electron$ $carriers$.
These experimental facts indicate that electron-hole asymmetry plays a
fundamental role in superconductivity, as proposed by the theory of hole
superconductivity.
