  We use femtosecond resonant soft x-ray diffraction to measure the optically
stimulated ultrafast changes of charge density wave correlations in underdoped
YBa2Cu3O6.6. We find that when coherent interlayer transport is enhanced by
optical excitation of the apical oxygen distortions, at least 50% of the
in-plane charge density wave order is melted. These results indicate that
charge ordering and superconductivity may be competing up to the charge
ordering transition temperature, with the latter becoming a hidden phase that
is accessible only by nonlinear phonon excitation.
