Structure of flux line lattices with weak disorder at large length scales

Dislocation-free decoration images containing up to 80 000 vortices have been obtained on high quality ${\mathrm{Bi}}_{2}{\mathrm{Sr}}_{2}{\mathrm{CaCu}}_{2}{\mathrm{O}}_{8+x}$ superconducting single crystals. The observed flux line lattices are in the random manifold regime with a roughening exponent of 0.44 for length scales up to 80--100 lattice constants. At larger length scales, the data exhibit nonequilibrium features that persist for different cooling rates and field histories.