Observation of electronic bound states in charge-ordered   YBa$_2$Cu$_3$O$_y$
  Observing how electronic states in solids react to a local symmetry breaking provides insight into their microscopic nature. A striking example is the formation of bound states when quasiparticles are scattered off defects. This is known to occur, under specific circumstances, in some metals and superconductors but not, in general, in the charge-density-wave (CDW) state. Here, we report the unforeseen observation of bound states when a magnetic field quenches superconductivity and induces long-range CDW order in YBa$_2$Cu$_3$O$_y$. Bound states indeed produce an inhomogeneous pattern of the local density of states $N(E_F)$ that leads to a skewed distribution of Knight shifts which is detected here through an asymmetric profile of $^{17}$O NMR lines. We argue that the effect arises most likely from scattering off defects in the CDW state, which provides a novel case of disorder-induced bound states in a condensed-matter system and an insightful window into charge ordering in the cuprates. 
R. Zhou, M. Hirata, T. Wu, I. Vinograd, H. Mayaffre, S. Kr\"amer, M.   Horvati\'c, C. Berthier, A.P. Reyes, P.L. Kuhns, R. L...
cond-mat.supr-con