Low-Temperature Charge Ordering in the Superconducting State ofYBa2Cu3O7−δ

Measurements of the nuclear quadrupole resonance (NQR) transverse relaxation rate of ${}^{63,65}\mathrm{Cu}$ and of the NQR linewidths in ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{7-\delta }$ in the superconducting state reveal unusual features below $T=35\mathrm{}\mathrm{K}$. A narrow peak in the transverse relaxation rate and an increased quadrupolar line broadening with decreasing temperature are attributed to a charge density wave ordered state below $T=35\mathrm{}\mathrm{K}$.