Fig. 5.

Field dependence of neutron scattering at higher temperatures. Field-cooled vortex ensembles are probed in perpendicular field geometry (Fig. 2b) with horizontal applied field. Backgrounds measured at zero field are subtracted. a Rocking curves at 3.0 K showing counts summed over an area on the 2D multidetector encompassing the top Bragg spot. Error bars are determined by Poisson statistics. b Top and bottom Bragg spot rocking curves are integrated numerically and the sum I plotted versus applied field. I vanishes much before the peak effect field H_pp anticipated from the phase diagram (Fig. 1). Each 2D image shows the SANS multidetector averaged over the rocking scan. Noise from subtraction at the centre of each image is masked. Red boxes indicate areas summed in the rocking curves. c Semilogarithmic plots at 2.7 and 3.0 K of I versus flux density B calculated from observed Bragg spot position g using the flux quantisation relation $g=2\pi {\left(2B{\mathrm{∕\Phi }}_0\sqrt{3}\right)}^{1∕2}$ for a triangular lattice. Dotted lines are guides to the eye illustrating I(B) forms expected if vortices were arranged in perfect 2D lattices. Solid lines are guides to the eye showing the steeper descents expected in the Bragg glass picture