Quasi-static magnetism at the pseudogap critical point in a high-Tc cuprate superconductor

Hole-doped cuprates are highly correlated materials well-known for their structural, charge, magnetic and superconducting properties. Their temperature vs. hole-doping phase diagrams have been scrutinized but much remains unknown, especially at low temperature where superconductivity hides the normal (i.e. non-superconducting) ground state. The application of large magnetic fields has proven to be an efficient way to overcome this problem. Here we use ultrasound measurements up to 80 T, in order to probe the doping dependence of the low temperature ground state of the lanthanum-based cuprate La2-xSrxCuO4 (LSCO). The phase diagram of LSCO hosts a glassy antiferromagnetic (AFM) phase, which, in zero magnetic field, exists from a doping level p = 0.02 up to p ≈ 0.135. We show that, once superconductivity is sufficiently suppressed by a magnetic field, this AFM glass actually exists up to a much higher doping level. We find that the AFM glass disappears at the critical doping level p* ≈ 0.19 where the pseudogap ends. This means that the AFM-glass phase exists from the doped Mott insulator at p = 0.02 up to p* ≈ 0.19, which provides a connection between the pseudogap and the physics of the Mott insulator.