Density of normal carriers below [ital T][sub [ital c]] and thermal resistance of twin boundaries in YBa[sub 2]Cu[sub 3]O[sub 7[minus][ital x]] single crystals

Photothermal measurements of diffusivity in YBa[sub 2]Cu[sub 3]O[sub 7[minus][ital x]] have been made in regions only 10 [mu]m in extent. The measured diffusivity within a single crystallite is much larger than that across a twin boundary. The results obtained are shown to be consistent with the microwave penetration depth measurements of normal electron density over a wide temperature range below the critical temperature. The results are not in good agreement with the BCS theory. It is shown that phonons are reflected at the twin boundaries, and thermal resistance of the boundary is approximately a linear function of temperature in the range 20[lt][ital T][lt]300 K.