  We report femtosecond resonant soft X-ray diffraction measurements of the
dynamics of the charge order and of the crystal lattice in non-superconducting,
stripe-ordered La1.875Ba0.125CuO4. Excitation of the in-plane Cu-O stretching
phonon with a mid-infrared pulse has been previously shown to induce a
transient superconducting state in the closely related compound
La1.675Eu0.2Sr0.125CuO4. In La1.875Ba0.125CuO4, we find that the charge stripe
order melts promptly on a sub-picosecond time scale. Surprisingly, the low
temperature tetragonal distortion is only weakly reduced, reacting on
significantly longer time scales that do not correlate with light-induced
superconductivity. This experiment suggests that charge modulations alone, and
not the LTT distortion, prevent superconductivity in equilibrium.
