Interaction-generated frustration in the ferromagnetic spin system on the kagome lattice: Exact analysis on the star...

Frustration effects caused by the presence of the six-site interaction in the ferromagnetic spin-1/2 Ising system on the kagome lattice are investigated in detail using the star kagomelike recursive lattice approximation. It is shown that although the model always exhibits the existence of only two standard phases (the ferromagnetic phase and the paramagnetic one) separated by the curve of the second-order phase transitions, depending on the value of the multisite interaction, the ferromagnetic phase splits into three different ground states in the zero-temperature limit with different magnetization and thermodynamic properties. The free energy of the model is derived, the residual entropies of all ground states are determined, and it is shown that the presence of the six-site multisite interaction leads to the formation of two highly macroscopically degenerated ground states in the studied ferromagnetic system, one of which is realized only for unique ratio of the six-site interaction to the ferromagnetic interaction. It is demonstrated that the existence of this highly macroscopically degenerated single-point-like ground state leads to appearance of the Schottky anomaly in the low-temperature behavior of the specific heat capacity in the vicinity of this ground state. It is also shown that the simultaneous presence of the frustration and of the second-order phase transitions in the studied model is responsible for the existence of even three local maxima in the temperature behavior of the specific heat capacity.