Which phonons induce non-equilibrium superconductivity in YBa$_2$Cu$_3$O$_{6.5}$?.

Terahertz-frequency excitation in the cuprates has been shown to induce non-equilibrium superconducting correlations above the thermodynamic transition temperature, $T_C$. In YBa$_2$Cu$_3$O$_{6+x}$, this phenomenon has been associated with the nonlinear excitation of certain lattice modes. However, to date it has not been possible to tune the pump wavelength widely to systematically compare the effect of different modes. Aided by a newly developed optical device, we measured the response of YBa$_2$Cu$_3$O$_{6.5}$ to tuneable driving between 3 and 24 THz, covering all phonon resonances. We show that superconductivity is enhanced only for excitation of the 16.4 THz and 19.2 THz vibrational modes that modulate the position of apical oxygen atoms along the $c$ axis. Other phonons only enhance the dissipation. We argue here that not only the average deformation of the lattice, but also a direct coupling between lattice vibrations and the electronic structure of the CuO$_2$ planes should contribute to enhanced non-equilibrium superconductivity.