Browsing by Author "Matysiak, R."

We report the low-temperature specific-heat measurements on polydomain Yb4As3 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 (QTM) simulation technique. The finite-size QTM approximants have been analyzed and an extrapolation procedure recovering the known density matrix renormalization group (DMRG) 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 Delta has been analyzed as a function of the applied magnetic field B, leading to an experimental verification of the scaling law Delta alpha B-2/3 following from the sine-Gordon model.

We report low-temperature specific heat, C(T), measurements on (Yb1-xLux)(4)As-3, with x = 0.01 and x = 0.03, where nonmagnetic Lu atoms are randomly distributed on antiferromagnetic S = 1/2 Heisenberg chains with J/k(B) = 28 K. The observed reduction of C below 15 K with increasing x is accurately described by quantum transfer matrix simulations without any adjustable parameter, implying that the system is an excellent experimental realization of segmented quantum spin chains. Finite-size effects consistent with conformal-field theory predictions are leading to the formation of an effective low-energy gap. The size of the gap increases with Lu content and accounts for the impurity-driven reduction of the specific heat. For both concentrations our results verify experimentally the low-temperature scaling behavior established theoretically and also confirm the value of J determined from pure Yb4As3.