Resistivity of ${\mathrm{La}}_{1.825}$${\mathrm{Sr}}_{0.175}$${\mathrm{CuO}}_4$ and ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_7$ to 1100 K: Absence of saturation and its implications

Resistivity was measured from $T_c$ to 1100 K for ${\mathrm{La}}_{1.825}$${\mathrm{Sr}}_{0.175}$${\mathrm{CuO}}_4$, where it is linear to 1100 K, and for ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_7$, where it is linear to 600 K, increases faster than linear at T>600 K because of loss of oxygen, and changes slope at the 950-K structural transition. The observed absence of saturation in the linear parts of the data implies weak electron-phonon coupling in both superconductors, thus excluding the possibility of phonon-mediated superconductivity. Values of the plasma energy are derived in combination with other experimental data.