Dependence of T$_{c}$ of YBa$_{2}$ Cu$_{3}$ O$_{6.67}$ on in-plane uniaxial stress

We probe the effect on $T_c$ of in-plane uniaxial stress applied to the high-temperature superconductor $YBa_2Cu_3O_{6.67}$. We find a highly anisotropic response. Under compression along the $b$ axis, which reduces the orthorhombicity of the $CuO_2$ planes, $T_c$ is broadly flat for stresses up to at least 1.7 GPa. Under compression along the $a$ axis, $T_c$ decreases steeply. For stresses beyond $≈1$ GPa the decrease is quasilinear. We hypothesize that superconductivity is suppressed by competition with uniaxial charge density wave order, which has been found to onset at $≈1$ GPa.