  It was recently reported that, in the highly overdoped side of single-crystal
$La_{2-x}Sr_xCuO_4$ films, the transition temperature $T_c$ and
zero-temperature superfluid phase stiffness $\rho_s(0)$ will obey a parabolic
scaling $T_c=\gamma \cdot \sqrt{\rho_s(0)}$. Parabolic scaling indicates a
quantum phase transition from a superconductor to a normal metal, for which
there has been scant understanding [Nature 536, 309-311 (2016)]. The current
study shows that, using the quantum critical model for zero-temperature Cooper
pairs [EPL 118, 57007 (2017)], parabolic scaling can be exactly derived, where
$\gamma=\gamma(\varepsilon_F,a)$ is uniquely determined by the Fermi energy
$\varepsilon_F$ and the minimal lattice constant $a$ of superconducting
materials. For single-crystal $La_{2-x}Sr_xCuO_4$ films, we calculate the
theoretical value of $\gamma$, which yields $4.29 K^{1/2}$ and is in accordance
with an experimental measure value $(4.2 \pm 0.5) K^{1/2} $ with high accuracy.
Our formula for $\gamma$ can be further tested by investigating other BCS-like
materials.
