Magnon specific heat of high-TCTC parent compounds La2CuO4 and Y Ba2Cu3O6 : Green’s function approach

We calculate the temperature dependence of the magnon contribution to specific heat of the high-TCTC superconducting parent compounds La2CuO4 and Y Ba2Cu3O6 using the Green’s function (GF) method within the Tyablikov’s approximation (RPA). By making use of quantum Heisenberg S=12 three-dimensional antiferromagnetic model, we obtain the specific heat temperature dependence in the whole temperature range from absolute zero to the Néel temperature. The analysis performed for La2CuO4 suggests that the magnon contribution to the total specific heat measured in the experiment is small, but not negligible as predicted by the simplified two-dimensional model often used within the linear spin-wave (LSW) theory. This discrepancy originates from the renormalization of the magnon energies due to the RPA in GF approach. Further, we discuss the influence of the in-plane frustration and the three-dimensionality of the system on the magnon specific heat in La2CuO4. We study the low-temperature behavior of the specific heat as well as the behavior in the vicinity of the Néel temperature which gives for the critical exponent αα value α=−12. Finally, we tend to improve the LSW theory results found in the literature by applying that theory to the three-dimensional orthorhombic model with second- and third-neighbor exchange interaction.

Research highlights
► Magnon specific heat is not vanishingly small in Green’s function approach.
► Planar frustration in La2CuO4 has stronger influence than three-dimensionality.
► Linear spin-wave theory results improved by taking a 3D model with planar frustration.