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.