A linearized spin-wave theory for thermodynamics of quantum Heisenberg antiferromagnet on a square lattice

The thermodynamics of the quantum Heisenberg antiferromagnet on a square lattice is revisited through a linearized spin-wave theory which is well defined at any finite temperature. We re-examine in details the temperature dependence of the free energy, the internal energy, the entropy and the specific heat. Most conclusions of the thermodynamics in previous studies can be reproduced in our linearized spin-wave theory. Specially, our calculation at low temperature $T<J$ agrees quantitatively with the numerical Quantum Monte Carlo simulation and high temperature series expansions.