Transition from surface-spin to bulk-spin relaxation in anS=1/2 nearest-neighborXYchain at infinite temperature

The time evolution of a spin operator ${\mathit{S}}_{\mathit{j}}^{\mathit{x}}$(t) in a nearest-neighbor S=1/2 semi-infinite XY chain is considered at T=∞. It is shown that the surface spin relaxes very differently compared to the subsurface spin, which already behaves like a bulk spin. The transition from surface to bulk dynamics is discussed using the recurrence-relations method of solving the Heisenberg equation of motion.