Pseudogap ground state and its doping evolution in Bi ₂Sr _2-xLa _xCuO _6+δ: Removing the superconducting state by the application of very high magnetic fields

We present results for the pseudogap ground state and its doping evolution in single-layered copper-oxide Bi ₂Sr _2-xLa _xCuO _6+δ. We apply very high magnetic fields up to 44 T to remove the superconducting state and reveal the hidden low temperature (T ) normal state. Through 63Cu- NMR Knight shift and spin-lattice relaxation rate measurements, we find that there remains a finite density of states (DOS) at the Fermi level in the zero-T limit when the superconductivity is removed, which indicates that the pseudogap ground state is a metallic state with a finite volume of Fermi surface. The residual DOS in the pseudogap ground state decreases with decreasing doping (increasing x), but remains quite large even at the vicinity of the magnetically ordered phase of x ≥ 0.8. The result indicates that the superconductivity emerges from the remaining Fermi surface and coexists with the pseudogap.