Evolution of exchange interactions in XXYY spin tetramers

The antiferromagnetic spin XXYY tetramer system consisting of two coupled dimers with exchange interactions α₁ and α₁' and dimer-dimer exchange interaction α₂ is studied. Using exact diagonalization, we examine the evolution of the quantum energy levels and thermodynamic properties for various spin configurations and exchange interactions. Furthermore, we show the development of quantum phase transitions that are produced by the convergence of the Schottky anomaly with both exchange variance and external magnetic field.