and charge order transition temperature from sound velocity measurements in YBCO*

We study the uniaxial strain dependence of superconducting T_c in YBCO as a function of doping using ultrasounds. We focus on the strain dependence along the a-axis dT_c/dε₁ and along the b-axis dT_c/dε₂. For dopings 0.08<p<0.12 we find an anisotropic strain dependence with dT_c/dε₂ >> dT_c/dε₁. We attribute this anisotropy to a strain-tuned competition between superconductivity and 3D uniaxial CDW. This is supported by sound velocity measurement of the 3D CDW for longitudinal modes along a and b-axis. The elastic behaviour through this CDW transition is anisotropic with |dT_CDW/dε₂|>>|dT_CDW/dε₁|. The uniaxial CDW appears as an intrinsic instability even in zero magnetic field and in the zero strain limit probed here.
For 0.12<p<0.15 where the uniaxial strain dependencies are maximum, we observe dT_c/dε₂ ≈ dT_c/dε₁.
An isotropic dT_c/dε_i is difficult to explain as being due to a competition to a uniaxial electronic order or as being due to the orthorhombic structure. It indicates that other mechanisms must be at play. This maximum in the strain dependence of T_c with in-plane isotropy could reflect an extrema in the electronic compressibility.