title: Local spin relaxation within the random Heisenberg chain.
abstract: Finite-temperature local dynamical spin correlations S(nn)(ω) are studied numerically within the random spin-1/2 antiferromagnetic Heisenberg chain. The aim is to explain measured NMR spin-lattice relaxation times in BaCu(2)(Si(0.5)Ge(0.5))(2)O(7), which is the realization of a random spin chain. In agreement with experiments we find that the distribution of relaxation times within the model shows a very large span similar to the stretched-exponential form. The distribution is strongly reduced with increasing T, but stays finite also in the high-T limit. Anomalous dynamical correlations can be associated with the random singlet concept but not directly with static quantities. Our results also reveal the crucial role of the spin anisotropy (interaction), since the behavior is in contrast with the ones for the XX model, where we do not find any significant T dependence of the distribution.
authors.affiliation: J. Stefan Institute, SI-1000 Ljubljana, Slovenia.
authors.forename: J
authors.initials: J
authors.lastname: Herbrych
authors.forename: J
authors.initials: J
authors.lastname: Kokalj
authors.forename: P
authors.initials: P
authors.lastname: Prelovšek