Low-temperature structural transitions and ${\mathit{T}}_{\mathit{c}}$ suppression in ${\mathrm{La}}_{2\mathrm{-}\mathit{x}}$${\mathit{M}}_{\mathit{x}}$${\mathrm{CuO}}_4$ (M=Ba, Sr)

Features of low-temperature structural transitions in ${\mathrm{La}}_{2\mathrm{-}\mathit{x}}$ ${\mathit{M}}_{\mathit{x}}$ ${\mathrm{CuO}}_4$ (M=Ba, Sr) and ${\mathrm{La}}_{1.880\mathrm{-}\mathit{y}}$ ${\mathrm{Nd}}_{\mathit{y}}$ ${\mathrm{Sr}}_{0.120}$ ${\mathrm{CuO}}_4$ have been investigated by means of electron diffraction in order to understand a physical origin of a ${\mathit{T}}_{\mathit{c}}$ suppression observed in these oxides. A low-temperature orthorhombic (LTO)-to-Pccn structural transition was experimentally found in ${\mathrm{La}}_{1.885}$ ${\mathrm{Sr}}_{0.115}$ ${\mathrm{CuO}}_4$, in addition to both an LTO-to-low-temperature tetragonal (LTT) one in ${\mathrm{La}}_{1.875}$ ${\mathrm{Ba}}_{0.125}$ ${\mathrm{CuO}}_4$ and LTO-Pccn-LTT ones in ${\mathrm{La}}_{1.880\mathrm{-}\mathit{y}}$ ${\mathrm{Nd}}_{\mathit{y}}$ ${\mathrm{Sr}}_{0.120}$ ${\mathrm{CuO}}_4$, which have already been reported. An important feature of these low-temperature transitions is that the transitions are characterized by the appearance of 1/21/20-type superlattice spots in electron-diffraction patterns. On the basis of other experimental data such as the Seebeck coefficient as well as the present result, the superlattice spots are suggested to be due to the appearance of charge-density waves, which are regarded as an ordered polaron state. The ${\mathit{T}}_{\mathit{c}}$ suppression in ${\mathrm{La}}_{2\mathrm{-}\mathit{x}}$ ${\mathit{M}}_{\mathit{x}}$ ${\mathrm{CuO}}_4$ can therefore be explained in terms of a charge localization due to the charge-density waves.