Low-temperature specific heat of ${\mathrm{La}}_{1.85}$${\mathrm{Sr}}_{0.15}$Cu${\mathrm{O}}_4$ and ${\mathrm{La}}_2$Cu${\mathrm{O}}_4$

The specific heat of polycrystalline samples of the high-$T_c$ superconductor ${\mathrm{La}}_{1.85}$ ${\mathrm{Sr}}_{0.15}$Cu${\mathrm{O}}_4$ and the undoped insulator ${\mathrm{La}}_2$Cu${\mathrm{O}}_4$ have been measured from 80 mK to 4 K and from 7 to 55 K. The low-temperature limit of the specific heat of ${\mathrm{La}}_{1.85}$ ${\mathrm{Sr}}_{0.15}$Cu${\mathrm{O}}_4$ can be well accounted for by a sizable linear $T$ term ($\gamma T$) in addition to the Debye-like term ($B{T}^3$) with $\gamma =1.8\mathrm{}\pm 0.2$ mJ/${\mathrm{K}}^2$ mol. A contribution due to possible magnetic impurities is also present. In contrast, the coefficient of the linear $T$ term in ${\mathrm{La}}_2$Cu${\mathrm{O}}_4$ is found to be 0.2±0.2 mJ/${\mathrm{K}}^2$ mol. The large linear $T$ term in the superconducting sample suggests the presence of the Fermi-liquid-type excitations at low temperatures.