Time evolution of a one-dimensional periodic lattice of spins, starting from a uniform z state at τ e =0, under a linearly varying external field along the lattice. Each time slice represents an ensemble nuclear spin state with a well-defined k vector. The x, y, z coordinate axes on the left mark the Bloch sphere directions for the spins represented by the cones. For τ e >0, the spins dephase and the expectation value of the z-axis magnetization ?M z ? drops to zero. Once the spins on...