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Low-temperature thermodynamics of spin-1/2 orthogonal-dimer chain with Ising and Heisenberg interactions

We consider an exactly solvable version of the quantum spin-1/2 orthogonal-dimer chain with the Heisenberg intra-dimer and Ising inter-dimer couplings. The investigated quantum spin system exhibits at zero temperature fractional plateaux at 1/4 and 1/2 of the saturation magnetization and it has a highly degenerate ground state at critical fields where the magnetization jumps. We study the field dependence of the specific heat at low temperature. The lattice-gas description is formulated in a vicinity of critical fields to explain the low-temperature behaviour of specific heat.