  There are increasing indications that superconductivity competes with other
orders in cuprate superconductors, but obtaining direct evidence with
bulk-sensitive probes is challenging. We have used resonant soft x-ray
scattering to identify two-dimensional charge fluctuations with an
incommensurate periodicity of $\bf \sim 3.2$ lattice units in the copper-oxide
planes of the superconductors (Y,Nd)Ba$_2$Cu$_3$O$_{6+x}$ with hole
concentrations $0.09 \leq p \leq 0.13$ per planar Cu ion. The intensity and
correlation length of the fluctuation signal increase strongly upon cooling
down to the superconducting transition temperature, $T_c$; further cooling
below $T_c$ abruptly reverses the divergence of the charge correlations. In
combination with prior observations of a large gap in the spin excitation
spectrum, these data indicate an incipient charge-density-wave instability that
competes with superconductivity.
