  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, including pseudogap, anomalous Fermi
liquid and strange metal (SM). A long-standing issue under debate is how the
superconducting pairing is formed and condensed in the SM phase because the
superconducting transition temperature is the highest in this phase. Here, we
report the first experimental observation of a pressure-induced crossover from
two- to three-dimensional superconducting states in the optimally-doped
Bi2Sr2CaCu2O8+delta bulk superconductor at a pressure above 2.8 GPa, through
state-of-the-art in-situ high-pressure measurements of resistance,
magnetoresistance and magnetic susceptibility. By analyzing the temperature
dependence of resistance, we find that the two-dimensional (2D) superconducting
transition exhibits a Berezinski-Kosterlitz-Thouless-like behavior. The
emergence of this 2D superconducting transition provides direct and strong
evidence that the SM state is predominantly 2D-like. This is important to a
thorough understanding of the phase diagram of cuprate superconductors.
