Thermal conductivity of underdoped YBa2Cu3Oy

We present experimental results on the thermal conductivity κ of an untwinned, underdoped YBa2Cu3Oy monocrystal along the 'a direction' with a critical temperature of Tc=61 K. We observe a peak at low temperatures as it has been previously found for the optimally doped material, for which it is known that the peak originates from heat transport by electronic quasiparticles. However, unlike for this latter case we do not observe a sharp onset of the peak at Tc but find a continuous increase already below T∼150 K. This increase becomes steeper at Tc and κ eventually peaks around 30 K. A magnetic field perpendicular to the CuO2 planes significantly suppresses κ in the superconducting phase. (orig.)