μSR study of Cu benzoate at very low temperature -: Existence or nonexistence of long range order in coupled chains

Zero-, longitudinal- and trans verse-field muSR experiments are per-formed for a linear chain antiferromagnet Cu benzoate, Cu(C6H5COO)(3H2O)-H-., and no long range order (LRO) is found down to 20 mK. The upper limit of the ratio of the Neel temperature to the exchange coupling, k(B)T(N)/J < 0.002, is even better than that of Sr2CuO3, which is known as the best candidate of S = 1/2 antiferromagnetic Heisenberg quantum spin chain. Significance of our findings is discussed in view of whether there is a non-zero critical coupling ratio necessary for LRO in coupled Heisenberg chains.