Antiferromagnetic order in weakly coupled random spin chains

The ordering of weakly coupled random antiferromagnetic $S=1/2$ chains, as relevant for recent experimentally investigated spin chain materials, is considered theoretically. The one-dimensional isotropic Heisenberg model with random exchange interactions is treated numerically on finite chains with the density-matrix renormalization-group approach as well as with the standard renormalization analysis, both within the mean-field approximation for interchain coupling $J_{\perp}$. Results for the ordering temperature $T_N$ and for the ordered moment $m_0$ are presented and are both reduced with the increasing disorder agreeing with experimental observations. The most pronounced effect of the random singlet concept appears to be a very large span of local ordered moments, becoming wider with decreasing $J_{\perp}$, consistent with $μ$SR experimental findings.

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