Crossover from two-dimensional to three-dimensional superconducting states in bismuth-based cuprate superconductor

To decipher the mechanism of high-temperature superconductivity, it is important to know how the superconducting pairing emerges from the unusual normal states of cuprate superconductors 1 – 4 , including the pseudogap 5 , 6 , strange metal 7 , 8 and anomalous Fermi liquid 9 phases. A long-standing issue is how the superconducting pairing is formed and condensed in the strange metal phase, because this is where the superconducting transition temperature is highest. Here, we use state-of-the-art high-pressure measurements to report the experimental observation of a pressure-induced crossover from two- to three-dimensional (2D to 3D) superconducting states in optimally doped Bi 2 Sr 2 CaCu 2 O 8 + δ bulk superconductor. By analysing the temperature dependence of the resistance, we find that the 2D superconducting transition exhibits a Berezinskii–Kosterlitz–Thouless-like behaviour 10 . The emergence of this 2D superconducting transition provides direct evidence that the strange metal state is predominantly 2D-like. This is important for a thorough understanding of the phase diagram of cuprate superconductors. Applying pressure to a cuprate reveals that the strange metal phase has a two-dimensional character, as shown by emerging Berezinskii–Kosterlitz–Thouless behaviour.