Strong electron correlation behind the superconductivity in Ce-free and Ce-underdoped high-Tc T'-cuprates

To investigate the electronic state of Ce-free and Ce-underdoped high-Tc cuprates with the so-called T′ structure, we have performed muon spin relaxation (μSR) and specific heat measurements of Ce-free T′-La1.8Eu0.2CuO4+δ (T′-LECO) polycrystals and Ce-underdoped T′-Pr1.3−xLa0.7CexCuO4+δ (T′-PLCCO) single crystals with x = 0.10. The μSR spectra of the reduced superconducting samples of both T′-LECO with Tc = 15 K and T′-PLCCO with x = 0.10 and Tc = 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.