Electronic phase diagram of La${}_{1.875}$Ba${}_{0.125-x}$Sr${}_x$CuO${}_4$

We performed systematic measurements of magnetic susceptibility on single crystals of La${}_{1.875}$Ba${}_{0.125-x}$Sr${}_x$CuO${}_4$. The dependence of the superconducting transition temperature on Sr-concentration demonstrates a step-like pattern upon doping at {\it x}$\sim$0.08 as the crystal structure changes from low-temperature tetragonal (LTT) to low-temperature orthorhombic (LTO) phase at low temperature. Upon cooling, an anomalous upturn in the susceptibility was observed at the structural phase transition between the LTT-LTO phases under the magnetic field parallel to {\it c}-axis.