Phonon effects at the metamagnetic transition inCeRu2Si2

We present experiments and model calculations concerning the metamagnetic transition in ${\mathrm{CeRu}}_2$ ${\mathrm{Si}}_2$. Our sound velocity, sound attenuation (down to 60 mK), and ac magnetic susceptibility measurements (down to 20 mK) all show strong anomalies near the critical magnetic field of this transition. We can account for this behavior using scaling arguments and a more microscopic model based on the Kondo volume collapse.