Pre-formed Cooper pairs in copper oxides and LaAlO3-SrTiO3heterostructures

Pairs of electrons can form above the superconducting critical temperature. The authors review the similarities and differences in this phenomenology in copper-based superconductors and oxide heterostructures. The Bardeen-Cooper-Schrieffer theory of superconductivity and the Landau-Fermi liquid theory form the basis of our current understanding of conventional superconductors and their parent non-superconducting phases. However, some exotic superconductors do not conform to this physical picture but instead feature an unusual 'normal' state that is not a Fermi liquid. One explanation of this unusual behaviour is that pre-formed pairs of electrons are established above the superconducting temperatureT(c). Here, we highlight recent experiments that show the likely existence of these pre-formed pairs in two rather different materials-a high-temperature cuprate superconductor and strontium titanate. Moreover, in both materials the normal state from which superconductivity emerges has other shared properties, including a pseudogap and electronic nematicity-rotational symmetry breaking in the electron fluid that is not expected in Fermi liquid theory nor more generally from the crystal lattice symmetry. These experimental findings should provoke more interaction between the communities working on these materials and new insights into the underlying mechanism of the creation of pre-formed pairs.