Electron-hole asymmetry is the key to superconductivity

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.