Incoherent Strange Metal Sharply Bounded by a Critical Doping in Bi2212*

In normal metals, macroscopic properties are understood using the concept of quasiparticles. In the cuprate high-temperature superconductors, the metallic state above the highest Tc is found to be very different and called the “strange metal”. To study this state, we use angle-resolved photoemission spectroscopy to directly measure its spectral function. With increasing doping across a temperature-independent critical value pc ~ 0.19, we observe a dramatic change near the Brillouin zone boundary where the strange metal characterized by incoherent spectral function abruptly reconstructs into a more conventional metal with quasiparticle-like excitations. This sharp reconstruction signals the incoherent strange metal as a distinct state of matter. Furthermore, above the temperature scale of superconducting fluctuations, we find that the pseudogap — the anomalous suppression of low-energy spectral intensity with decreasing temperature — also sharply collapses at the very same pc. This suggests that the pseudogap is a low-temperature phenomenon associated with the incoherent strange metal.