151Eu Moessbauer effect study of T'- and T*-phase superconductors

The (La,Eu)2CuO4 system shows superconductivity by doping Ce or Sr. Carriers in T'-phase doped with Ce are electrons and those in T*-phase doped with Sr are holes. In this work, 151Eu Moessbauer analysis is applied for the T'-phase (La1-xEux)2-yCeyCuO4 and the T*-phase (La1-xEux)2-ySryCuO4 in order to compare the electronic state and the lattice vibration of Eu in these superconductors. In addition, correlations between Tc and Moessbauer parameters are examined. The isomer shift of 151Eu is 0.784 - 0.840 mm/s in the T*-phase and 0.689 - 0.733 mm/s in the T'-phase, which shows that the lanthanide in these superconductors is tri-valent. The Debye temperature of 151Eu is 180 - 208 K in the T*-phase and 160 - 192 K in the T'-phase. The difference of isomer shift between these two phases is explained by the Eu-O distance. For (La1-xEux)2-yCeyCuO4, a tight correlation between Tc and the Debye temperature is observed, which means the importance of the lattice vibration in high-Tc superconductivity. (orig.)