Strong electron correlation behind the superconductivity in Ce-free and Ce-underdoped high-T_c T′-cuprates

To investigate the electronic state of Ce-free and Ce-underdoped high-T_c cuprates with the so-called T′ structure, we have performed muon spin relaxation (μSR) and specific heat measurements of Ce-free T′-La_1.8Eu_0.2CuO_4+δ (T′-LECO) polycrystals and Ce-underdoped T′-Pr_1.3-xLa_0.7Ce_xCuO_4+δ (T′-PLCCO) single crystals with x = 0.10. The μSR spectra of the reduced superconducting samples of both T′-LECO with T_c = 15 K and T′-PLCCO with x = 0.10 and T_c =27 K have revealed that a short-range magnetic order coexists with the superconductivity in the ground state. The formation of a short-range magnetic order due to a very small amount of excess oxygen in the reduced superconducting samples strongly suggests that the Ce-free and Ce-underdoped T′-cuprates are strongly correlated electron systems.