Thermodynamics and the pairon model for cuprates

Two recent papers explore the idea of "pairons" (incoherent bound pairs above Tc) as a model for the pseudogap in cuprate high-Tc superconductors. From this model the authors calculate the resultant electronic specific heat and static magnetic susceptibility. At elevated temperature the pairons thermally unbind causing a broad peak in the specific heat, additional to the second-order peak at Tc where the pairons coherently condense. With this unbinding the electronic entropy recovers to its bare quasiparticle linear-in -T value. We show that this is inconsistent with the measured specific heat which reveals an entropy which never recovers to the highest temperature investigated (about 400 K). In the case of Bi2Sr2CaCu2O8 and La2-xSrxCuO4 there is a broad peak above Tc in the specific heat coefficient, gamma, but this arises from the nearby van Hove singularity (vHs) which is more distant in the case of Y0.8Ca0.2Ba2Cu3Oy and therefore not discernible. We propose a number of further critical tests. In the pairon model the BCS ratios are not satisfied until vanishingly near where superconductivity disappears in the heavily overdoped region whereas, experimentally, these mean-field ratios are sustained across the overdoped regime once the pseudogap has closed at critical doping.