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 Bi${}_2$Sr${}_2$CaCu${}_2$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.