Competition between Charge- and Spin-Density-Wave Order and Superconductivity in ${\mathrm{La}}_{1.875}{\mathrm{Ba}}_{0.125-\mathit{x}}{\mathrm{Sr}}_{\mathit{x}}{\mathrm{CuO}}_4$

We have performed a series of elastic neutron scattering measurements on $1/8$-hole doped ${\mathrm{La}}_{1.875}-{\mathrm{Ba}}_{0.125-x}{\mathrm{Sr}}_x{\mathrm{CuO}}_4$ single crystals with $x=0.05\mathrm{}$, 0.06, 0.075, and 0.085. Both charge-density-wave (CDW) and spin-density-wave orders are found to develop simultaneously below the structural transition temperature between the low-temperature orthorhombic (LTO) and low-temperature tetragonal (LTT) or low-temperature less-orthorhombic (LTLO) phases. In the ground state the CDW order is observed only in the LTT/LTLO phase and drastically degrades towards the LTO boundary. The $x$ dependence of $T_c$ strongly suggests a direct effect of the CDW order on the suppression of superconductivity.