Phys Rev. B. 85, 195206 (2012)

In a superconductor, current is carried by bound pairs of like charges. For these 'Cooper pairs' to form there must be some mechanism to oppose their Coulombic repulsion: in conventional superconductors, it's mediated by phonons. Under certain conditions, however, Cooper pairs of unlike charges — an electron and a hole — might also form, provided their Coulomb attraction is counteracted to impede recombination.

Marijn Versteegh and colleagues report evidence that electron–hole Cooper-pair formation could occur in a photoexcited electron–hole gas in a semiconductor. At low density, Coulomb attraction binds electron–hole pairs tightly. But as the density is increased, this attraction is screened and becomes gradually weaker. And at high densities it could become weak enough to be balanced by Pauli repulsion, enabling Cooper pairs to form.

Versteegh et al. subjected a ZnO crystal to an intense infrared laser light. As they increased the laser intensity and decreased the temperature, they observed the emergence of a peak in the resulting emission spectra consistent with the recombination of preformed electron–hole Cooper pairs.