Abstract
We review the results of specific-heat experiments on the S = 1 quasi-one-dimensional (quasi-1D) bond-alternating antiferromagnet Ni(C 9 H 24 N 4)( NO 2) ClO 4, alias NTENP. At low temperatures above the transition temperature of a field-induced long-range order, the magnetic specific heat (C mag ) of this compound becomes proportional to temperature (T), when a magnetic field along the spin chains exceeds the critical field H c at which the energy gap vanishes. The ratio C mag /T, which increases as the magnetic field approaches H c from above, is in good quantitative agreement with a prediction of conformal field theory combined with the field-dependent velocity of the excitations calculated by the Lanczos method. This result is the first conclusive evidence for a Tomonaga–Luttinger liquid in a gapped quasi-1D antiferromagnet.
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