Entropy properties of antiferromagnetic model on kagome lattice: Effective-field theory approach

Abstract The entropy properties of the antiferromagnetic spin- 1 ∕ 2 Ising model in the presence of the external magnetic field on the kagome lattice are studied in the framework of various effective-field theory cluster approximations up to the size of the cluster consisting of 12 connected sites, which form typical basic star-like geometrical structure of the kagome lattice. The dependence of the entropy on the reduced temperature is studied for various specific values of the external magnetic field and the corresponding residual entropies are found. It is shown that the effective-field theory cluster approximation technique is suitable for description of the entropy properties of the model for values of the magnetic field in which real single-point ground states are formed. In this case, obtained values of the residual entropies are even in very good quantitative accordance with the known exact result for the model in the zero external magnetic field. On the other hand, in the intervals of the magnetic field, in which the artificial plateau and single-point ground states are formed, the entropy demonstrates strong low-temperature dependence on the used cluster approximation and, moreover, it can also exhibit the unphysical reentrant behavior, i.e., the existence of temperature intervals in which the entropy decreases with increasing temperature. It is shown, however, that this nonstandard low-temperature behavior of the entropy naturally explains the formation of artificial low-temperature inverse Schottky peaks in the temperature behavior of the specific heat capacity observed and discussed recently in Jurcisinova and Jurcisin (2019) and is reduced with increasing the size of the used cluster approximation towards the physically acceptable behavior.