Fermi-surface induced modulation in an optimally doped YBCO superconductor

We have observed a Fermi-surface (FS) induced lattice modulation in a YBCO superconductor with a wavevector along CuO chains, {\it i.e.} ${\bf q}_1$=(0,$\delta$,0)\mathrm{.}Thevalueof$\delta\sim0.21$ is twice the Fermi wavevector ($2{\mathbf{k}}_F$) along {\bf b*} connecting nearly nested FS `ridges’. The ${\mathbf{q}}_1$ modulation exists only within O-vacancy-ordered islands (characterized by ${\mathbf{q}}_0$=$(\frac{1}{4},0,0))$ and persists well above and below $T_c$. Our results are consistent with the presence of a FS-induced charge-density wave.