Specific-heat measurements on two high-transition-temperature superconducting oxides: La1.85Ba0.15CuO4 and La1.8Sr0.2CuO4

Abstract

We have measured the specific heat c of ${\mathrm{La}}_{1.85}$ ${\mathrm{Ba}}_{0.15}$ ${\mathrm{CuO}}_4$ and ${\mathrm{La}}_{1.8}$ ${\mathrm{Sr}}_{0.2}$ ${\mathrm{CuO}}_4$, as well as their resistance and diamagnetic susceptibility. The results for these two materials are very similar. Below 10 K, c is a linear function of $T^3$. It is therefore tempting to identify the $T^3$ part of the low-temperature data as the lattice specific heat, which can be extrapolated to higher temperatures. Subtracting it from c would yield the electronic specific heat throughout the temperature range investigated. A thermodynamic analysis could then be made to find the normal-state value of the electron specific-heat constant γ. The values of γ which come out of this analysis are, however, far too large compared with the results obtained by Finnemore et al. from measurements of the upper critical field and the normal-state magnetic susceptibility performed on a similar material. We believe the most likely explanation for this discrepancy is that the phonon spectrum has soft modes. These soft modes might help explain the very high superconducting transition temperatures of these materials.