It is shown that the controversy on phonons or electrons being the most
influenced heat carriers below the critical temperature of high-Tc​
superconductors can be resolved. Electrical and thermal properties of the same
DyBa2​Cu3​O7−x​ monodomain have been measured for two highly different
oxygenation levels. While the oxygenated sample DyBa2​Cu3​O7​ has very
good superconducting properties (Tc​=90 K), the DyBa2​Cu3​O6.3​
sample exhibits an insulator behavior. A careful comparison between
measurements of the {\bf thermal diffusivity} of both samples allows us to
extract the electronic contribution. This contribution to the relaxation time
of heat carriers is shown to be large below Tc​ and more sensitive to the
superconducting state than the phonon contribution.Comment: 13 pages, 6 figure