Critical point and the nature of the pseudogap of single-layered copper-oxide Bi₂Sr_2-xLa_xCuO_6+δ superconductors

We apply strong magnetic fields of H = 28.5 to 43 T to suppress superconductivity (SC) in the cuprates Bi₂Sr_2-xLa _xCuO_6+δ (x = 0.65, 0.40, 0.25, 0.15, and 0), and investigate the low temperature (T) normal state by ⁶³Cu nuclear spin-lattice relaxation rate (l/T₁) measurements. We find that the pseudogap (PG) phase persists deep inside the overdoped region but terminates at x ∼ 0.05, which corresponds to the hole doping concentration of approximately 0.21. Beyond this critical point, the normal state is a Fermi liquid that persists as the ground state when superconductivity is removed by the magnetic field. A comparison of the superconducting state with the H-induced normal state in the x = 0.40 (T_c = 32 K) sample indicates that there remains substantial part of the Fermi surface even in the fully developed PG state, which suggests that the PG and SC are coexisting matters.