Structural transitions and related fluctuations inLa1.875−xSmxSr0.125CuO4

Abstract

Features of both structural transitions and related fluctuations in ${\mathrm{La}}_{1.875\mathrm{-}\mathit{x}}$ ${\mathrm{Sm}}_{\mathit{x}}$ ${\mathrm{Sr}}_{0.125}$ ${\mathrm{CuO}}_4$ have been examined by means of electron diffraction in a temperature range between about 600 K and about 20 K. As the temperature is lowered, a structural phase transition from a high-temperature tetragonal (HTT) structure to a low-temperature orthorhombic (LTO) one takes place in 0≤x≤0.2 and HTT→LTO→Pccn→LTT successive transitions in 0.3≤x≤0.4 where the Pccn and low-temperature tetragonal (LTT) structures are characterized as a rotation of an oxygen octahedron about two 〈110〉 directions. An enhancement of a structural fluctuation in the LTO phase, which indicates a soft-phonon mode in the LTO-to-Pccn transition, occurs on cooling even in 0≤x≤0.2 as well as in 0.3≤x≤0.4. Because an increase in the Sm content results in two effects, a lowering of ${\mathit{T}}_{\mathit{c}}$ and an enhancement of the structural fluctuation, the fluctuation is understood to suppress the superconductivity. Eventually, a dip in a composition dependence of ${\mathit{T}}_{\mathit{c}}$ in La-Sr-Cu-O, which was experimentally found by Oda et al. [J. Phys. Soc. Jpn. 58, 1137 (1989)], is concluded to be due to the structural fluctuation related to the LTO-to-Pccn transition.