In-plane anisotropy of spin excitations in the normal and   superconducting states of underdoped YBa$_2$Cu$_3$O$_{6+x}$
  A detailed inelastic neutron scattering study of the in-plane anisotropy of magnetic excitations in twin-free YBa$_2$Cu$_3$O$_{6.6}$ ($T_c =$ 61 K) reveals that the spin excitation spectra in the superconducting and normal states are qualitatively different. Below $T_c$, the spectrum consists of upward- and downward-dispersing branches with modest in-plane anisotropy merging at an energy $E_{res} =$ 37.5 meV. In the normal state, the singularity at $E_{res}$ disappears, and the spectrum exhibits a steep dispersion with a strongly anisotropic in-plane geometry. These data have important implications for models based on static or dynamic "stripe" order of spins and charges. 
V. Hinkov, P. Bourges, S. Pailhes, Y. Sidis, A. Ivanov, C.T. Lin, D.P.   Chen, B. Keimer