FIELD-INDUCED TOMONAGA–LUTTINGER LIQUID OF A QUASI-ONE-DIMENSIONAL S = 1 ANTIFERROMAGNET

We review the results of specific-heat experiments on the S = 1 quasi-one-dimensional (quasi-1D) bond-alternating antiferromagnet Ni(C9H24N4)(NO2)ClO4, 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.