X-ray diffraction observation of charge density wave order at zero   magnetic field in ortho-II YBa$_2$Cu$_3$O$_{6.54}$
  X-ray diffraction measurements show that the high-temperature superconductor YBa$_2$Cu$_3$O$_{6.54}$, with ortho-II oxygen order, has charge density wave order (CDW) in the absence of an applied magnetic field. The dominant wavevector of the CDW is $\mathbf{q}_{\mathrm{CDW}} = (0, 0.328(2), 0.5)$, with the in-plane component parallel to the $\mathbf{b}$-axis (chain direction). It has a similar incommensurability to that observed in ortho-VIII and ortho-III samples, which have different dopings and oxygen orderings. Our results for ortho-II contrast with recent high-field NMR measurements, which suggest a commensurate wavevector along the $\mathbf{a}$-axis.   We discuss the relationship between spin and charge correlations in YBa$_2$Cu$_3$O$_{y}$, and recent high-field quantum oscillation, NMR and ultrasound experiments. 
E. Blackburn, J. Chang, M. Hucker, A. T. Holmes, N. B. Christensen,   Ruixing Liang, D. A. Bonn, W. N. Hardy, U. Rutt, O. Gu...
cond-mat.supr-con cond-mat.str-el