Distribution of NMR relaxations in a random Heisenberg chain

TL;DR

This study uses NMR to investigate how randomness affects magnetic relaxation in a spin-1/2 Heisenberg chain, revealing a broadening distribution of relaxations and evidence for random-singlet states.

Contribution

First experimental observation of a broadening distribution of NMR relaxations in a random Heisenberg chain, supporting the random-singlet state theory.

Findings

Randomness causes a distribution of local magnetic relaxations.

Distribution broadens as temperature decreases.

Evidence from NMR, magnetization, and Monte Carlo simulations.

Abstract

Nuclear magnetic resonance (NMR) measurements of the 29Si spin-lattice relaxation time T1 were used to probe the spin-1/2 random Heisenberg chain compound BaCu2(Si(1-x)Gex)2O7. Remarkable differences between the pure (x = 0) and the fully random (x = 0.5) case 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.