Pressure Effects Show that the Pseudogap Phase of Cuprates is Confined by Fermi Surface Topology

The pseudogap phase is the central puzzle of cuprate high-temperature superconductors, and why it onsets below a critical doping p* that is much lower in La_2−xSr_xCuO₄ than in La_1.6−xNd_ySr_xCuO₄ or Bi₂Sr₂CuO_6+δ, for instance, is unknown. We recently performed high-magnetic-field transport measurements in the cuprate La_1.6−xNd_ySr_xCuO₄ and observed a large and unexpected shift of the pseudogap critical point p* as a function of applied pressure. We find that the shift in p* is driven by a corresponding shift in p_FS, the doping at which the large Fermi surface goes from hole-like to electron-like, so that p* ≤ p_FS must be obeyed. Consequently, the pseudogap cannot open on an electron-like Fermi surface. This necessary condition for pseudogap formation, imposed by details of the Fermi surface, is a strong constraint for theories of the pseudogap phase. Our finding that p* can be tuned with a modest pressure opens a new route for experimental studies of the pseudogap.