ВЛИЯНИЕ РАЗРЫВОВ ЦЕПОЧКИ НА ТЕПЛОЕМКОСТЬ ХАЛДЕЙНОВСКОЙ СИСТЕМЫ
Disordered ground state with a gap in the spectrum of the magnetic excitation spectrum is realized in the compounds that contain the chains of the magnetic ions with S = 1 spins (Haldane chains). In framework of VBS model, each S = 1 spin can be represented as two pseudo-particle with S = 1/2 spins connected by antiferromagnetic interaction with the pseudo-particle in a neighboring ion. Nonmagnetic impurities within the chain can lead to the formation of the chain segments, and unpaired S = 1/2 spins appear at each end of the chain segments.
In this paper, the influence of the interaction of S=1/2 spins appearing at the ends of the segments of the Haldane chain on the temperature dependence of the specific heat C(T) is studied. The interaction of the end S = 1/2 spins leads to a splitting of the four-fold degenerated ground state of the chain segment into the singlet and the triplet states separated by a gap.The temperature-driven repopulation of the ground state sublevels causes the Schottky anomaly in the C (T) dependence. Using the energy spectrum of the chain segment, which is obtained by taking into account the single-ion anisotropy, we obtain an expression for the specific heat. The analysis shows that in the absence of an external magnetic field, when one of the parameters or is fixed the specific heat peak shifts to higher temperatures with increasing another parameter. For the case of the singlet ground state, the maximum of the specific heat shifts to higher temperatures with increasing magnetic field H. If the ground state is triplet, maximum shifts to lower temperatures at H<3T, and it shifts to higher temperatures at H>7T. In the intermediate field range, the C (T) dependence is more complex. The calculation results are compared with experimental data for compounds Gd2BaNiO5 and (Y1-xNdx)2BaNiO5 published previously.