Transport properties and low temperature structural phase transition around x=0.125 in La_2-x-yRE_ySr_xCuO₄ (RENd, Sm, Gd, Tb)

In La_1.9-itxGd_0.1Sr_xCuO₄, a prominent change of the thermoelectric power S below ∼80K is observed in a narrow region of x around 0.12 although the structural phase transition to the tetragonal low-temperature (TLT) phase is observed at ∼ 115K in a wide region of x. These results suggest that the anomalous behaviors of the transport properties below ∼80K are responsible for the change of the electronic state itself. In La_1.875-yGd_ySr_0.125CuO₄, T^u_ittdβ, which is the structural phase transition temperature to the TLT pahse, increases with increasing y. The temperature where S begins to decrease rapidly is close to T^u_dβ in y≤0.04. In y≥0.05, a prominent change of S is observed below a lower temperature than T^u_dβ and an obvious sign reversal of S appears appears at ∼40K. These results indicate that the change of the electronic state is enhanced in the TLT phase.