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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.