Thermal conductivity in the stripe-ordered phase of cuprates and nickelates

The temperature dependence of the thermal conductivity κ and the resistivity ρ have been measured on well-characterized crystals La_1.60-xNd_0.4Sr_xCuO₄ (x=0.12, 0.15, and 0.20), La₂CuO₄+δ, and La_2-xSr_xNiO₄ (x=0, 0.33, 0.50). In La₂CuO₄+δ, the phonon component of κ(T) decreases sharply with increasing δ. The charge ordering at T_CO in La_1.67Sr_0.33NiO₄ enhances κ(T). Similarly in the La_1.60-xNd_0.4Sr_xCuO₄ system, ordering of the mobile holes into stripes within the CuO₂ sheets on cooling through a low-temperature tetragonal/orthorhombic (LTT/LTO) transition at T_t gives a jump Δκ in κ(T). In contrast to a previous report on ceramic samples, the superconductive crystals x=0.15 and 0.20 of the La_1.60-xNd_0.4Sr_xCuO₄ system show a clear low-temperature peak in the normal state κ(T) below T_t that shows phonons are the dominant carriers of heat in the LTT phase. Restoring phonons in the LTT phase is thus seen to be compatible with superconductivity in the copper oxides, albeit with a lower T_c than is found in the LTO phase where the phonon component of κ(T) is suppressed in the normal state by the bond-length fluctuations.