The connection between superconducting phase correlations and spin excitations in YBa$_2$Cu$_3$O$_{6.6}$: A magnetic field study

K ( < T c ) and T = 70 K ( T c + 7 . 3 K) at wave vector Q = (0 . 5 , 1 . 5 , 1 . 8) for zero and 6.8 T field in the B ∼ || c -axis geometry. e , f , Similar data in the B || ab -plane geometry. Because the differences in sample geometries, the actual intensity gain of the resonance in the B || ab -plane experiment is slightly higher than that of the B ∼ || c -axis case at zero field for the same incident beam monitor (300 counts which correspond to ∼ 8 min per point at ¯ hω = 34 meV). We normalized the intensity gain of the resonance in these two experiments at zero field and multiplied the same scale factor to the finite field case.