Low-temperature in-plane thermal conductivity of untwinned YBa2Cu3O6.9 crystals

The thermal conductivity of high-quality untwinned single crystals of YBa2Cu3O6.9 was studied as a function of temperature (0.1 K < T < 1 K) and with heat current injected along two different orientations (a and b crystalline axes). We detect the presence of a linear term with comparable magnitude for the two orientations. This is in contrast with the high-temperature data, where an anisotropic behaviour is reported. The possible electronic origin of the linear term and its implication for the gap symmetry are discussed.