Spin gap in S=1/2 1D Heisenberg antiferromagnetic chain, Cu(C₆H₈N₂)ClBr

We report a randomness induced spin gap in Cu²⁺ (S=1/2) Heisenberg antiferromagnetic (HAF) chain, Cu(C₆H₈N₂)ClBr. Zero field muon spin relaxation (mSR)data suggest that there is no magnetic ordering down to 50 mK in spite of an antiferromagnetic Curie-Weiss temperature, K, extracted from bulk magnetic susceptibility. The mSR rate increases around 1 K from a nuclear dominated value to a constant below 1 K indicating persistent dynamics in the system. Longitudinal field-μSR measurements indicate the diffusive motion of spinons, potentially resulting from chain breaks. ¹H-spin lattice relaxation rate, 1/T₁ shows an exponential decay below about 3 K, indicating gapped excitations. The magnetic heat capacity also shows an exponential decay with temperature in zero field with gaps of about 0.5 K and 3 K indicating gapped excitations, which is consistent with ¹H-NMR-1/T₁. On application of a 3 Tesla magnetic field, the gap closes and a power law variation has been observed in the magnetic heat capacity. All these results suggest that a spin gap opens up at low-T in the titled 1 D HAF chain material.