  In order to separate electronic and phonon mechanisms of thermal transport in
high temperature superconducting Bi2212, we measured thermal conductivity of
single crystals of Y-doped Bi2212, an insulating analogue of pure Bi2212, in
the temperature range between 40 mK and 2 K. Temperature dependence of the
thermal conductivity of insulating samples changes from T$^{1.7}$ between 1 and
2 K to $T^{2.6}$ below 200 mK, indicating saturation of the phonon mean free
path with decreasing temperature. Overall, thermal conductivity of the
insulating samples (which is due entirely to phonons) is strongly suppressed in
comparison to that of the superconducting samples. Therefore, a large (and at
the lowest temperatures, predominant) part of the total thermal transport in
superconducting Bi2212 is due to electrons.
