Thermal expansion of CeCu_5.8Ag_0.2

We present low-temperature thermal expansion measurements on the heavy fermion system CeCu5.8Ag0.2CeCu5.8Ag0.2, which is located at an antiferromagnetic (AF) quantum critical point (QCP). At zero magnetic field, the volume expansion coefficient divided by temperature shows a logarithmic divergence upon cooling below 1 K. This temperature dependence is incompatible with the predictions of the itinerant spin-density wave theory for an AF QCP. The application of magnetic fields leads to a cross-over to Landau Fermi liquid behavior as expected for a zero-field QCP