Randomness in 1D spin systems
external pageNMR measurements of the 29Si spin-lattice relaxation time T1 were used to probe the spin-1/2 random Heisenberg chain compound BaCu2(Si1−xGex)2O7. Remarkable differences between the pure (x=0) and the fully random (x=0.5) cases are observed, indicating that randomness generates a distribution of local magnetic relaxations. This distribution, which is reflected in a stretched exponential NMR relaxation, exhibits a progressive broadening with decreasing temperature, caused by a growing inequivalence of magnetic sites. Compelling independent evidence for the influence of randomness is also obtained from magnetization data and Monte Carlo calculations. These results suggest the formation of random-singlet states in this class of materials, as previously predicted by theory.