Contribution of one-site two-magnon bound states to the specific heat of S=1 Heisenberg chain with easy-axis anisotropy

In this work we discuss the possibility of contribution of one-site two-mag non bound states (OSTMBS's) to the thermodynamic equilibrium properties in S = 1 Heisenberg chain with easy-axis anisotropy. We found that for suffici ently strong anisotropy \D/J\ = 5 a good agreement between OSTMBS's theoret ical contribution to the specific heat using dilute approximation and numer ical calculations for the aforementioned system was obtained for temperatur es below T = 0.12\D/k(B)\. The obtained result confirms that OSTMBS's are t he lowest energy excitations in S=1 Heisenberg chains with sufficiently str ong easy-axis anisotropy. Further, the compound Ni(N2H5)(2)(SO4)(2) previou sly identified as S = 1 antiferromagnetic Heisenberg chain with easy-axis a nisotropy with D/k(B) = -15.6 K, D/J = 5, is proposed as an appropriate sys tem for experimental studies of OSTMBS's contribution to thermodynamic prop erties.