Low temperature thermal conductivity of YBa2Cu3O6+x (x = 0.1 → 0.9) compounds

Thermal conductivity measurements of YBa 2 Cu 3 O 6+x have been performed on three superconducting samples (x ⩾, 0.55) and an insulating one (x0.1) in the temperature range 100 mK-150 K. A quasi-linear temperature dependent contribution was evidenced in the superconducting samples which was missing in the insulating one. The origin of this contribution, discussed with reference to specific heat data carried out on the same samples, is attributed to scattering of phonons by twins rather than to normal charge carriers.