We report the low-temperature specific-heat measurements on polydomain Yb 4 As 3 at magnetic fields up to 20 T. Taking into account the Bethe ansatz results, the zero-field data have been used for the estimation of the lattice specific heat, resulting in a value of the exchange integral for the Heisenberg model of the antiferromagnetic spin S = 1 2 chain of J / k B = −28 K. A quantitative agreement has been achieved between the experimental magnetic specific-heat data in magnetic field and the numerical results obtained by the quantum transfer-matrix ͑QTM͒ simulation technique. The finite-size QTM approximants have been analyzed and an extrapolation procedure recovering the known density matrix renormalization group ͑DMRG͒ results down to very low temperature has been proposed. On the basis of the data in magnetic field and using the earlier DMRG results, the energy-gap size ⌬ has been analyzed as a function of the applied magnetic field B, leading to an experimental verification of the scaling law ⌬ϰB 2/3 following from the sine-Gordon model.

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