Thermal conductivity of insulating Bi2Sr2YCu2O8 and superconducting Bi2Sr2CaCu2O8: Failure of the phonon-gas picture

The ab-plane thermal conductivity κ(T) of insulating Bi2Sr2YCu2O8 and superconducting Bi2Sr2CaCu2O8 is measured from T=10 to 300 K on single-crystal samples. Metallic Bi2Sr2CaCu2O8 has a significantly higher κ than Bi2Sr2YCu2O8; the difference Δκ agrees well in magnitude with a Wiedemann-Franz estimate of the electronic contribution to κ in Bi2Sr2CaCu2O8. The shape of κ(T) in insulating Bi2Sr2YCu2O8 differs from normal insulators described by the Peierls-Boltzmann theory. Assuming that atomic vibrations are the main heat carrier, and noting that κ is more similar to that of silica glass than to a normal insulating crystal like CuO, we suggest that the ''phonon'' mean free path is sufficiently short that the Peierls-Boltzmann theory is not applicable. This is consistent with evidence from neutron scattering that phonons are poorly defined. Our data support the idea that the peak in κ(T) observed below Tc in superconducting samples originates from electronic rather than vibrational heat currents.