# Pre-formed Cooper pairs in copper oxides and LaAlO3—SrTiO3 heterostructures

## Abstract

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 temperature Tc. 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.

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## Acknowledgements

The research at Brookhaven National Laboratory was supported by the US Department of Energy, Basic Energy Sciences, Materials Sciences and Engineering Division. The work at Yale was supported by the Gordon and Betty Moore Foundation’s EPiQS Initiative through grant no. GBMF4410. The work at Pittsburgh was supported by a Vannevar Bush Faculty Fellowship program sponsored by the Basic Research Office of the Assistant Secretary of Defense for Research and Engineering and funded by the Office of Naval Research through grant no. N00014-15-1-2847, and NSF grant no. PHY-1913034.

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Correspondence to Jeremy Levy.

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Božović, I., Levy, J. Pre-formed Cooper pairs in copper oxides and LaAlO3—SrTiO3 heterostructures. Nat. Phys. 16, 712–717 (2020). https://doi.org/10.1038/s41567-020-0915-8