First-Order Type Effects in YBa$_2$Cu$_3$O$_{6+x}$ at the Onset of Superconductivity

We present results of Raman scattering experiments on tetragonal ${rm (Y_{1-y}Ca_{y})Ba_{2}Cu_{3}O_{6+x}}$ for doping levels $p(x,y)$ between 0 and 0.07 holes/CuO$_2$. Below the onset of superconductivity at $p_{rm sc1} approx 0.06$, we find evidence of a diagonal superstructure. At $p_{rm sc1}$, lattice and electron dynamics change discontinuously with the charge and spin properties being renormalized at all energy scales. The results indicate that charge ordering is intimately related to the transition at $p_{rm sc1}$ and that the maximal transition temperature to superconductivity at optimal doping $T_{c}^{rm max}$ depends on the type of ordering at $p>p_{rm sc1}$.