  The thermal conductivity of Zn-doped YBCO crystals was studied at low
temperature (0.15 < T < 0.8 K) for different concentrations of Zn impurities. A
small amount of Zn induces a dramatic decrease in the non-linear component of
the low-temperature thermal conductivity. Moreover, the magnitude of the linear
component (obtained by extrapolating the data to T=0) is found to depend on Zn
concentration. After an initial decrease, this linear term, associated with the
electronic contribution to the conductivity, increases with increasing Zn
dopage. Such an increase is consistent with the introduction of low-energy
excitations by Zn impurities as expected for a $d_{x^2-y^2}$ superconducting
state in contrast to an anisotropic s-wave gap. The results are compared to
quantitative predictions of available theoretical models.
