Lattice softening effects at the Mott critical point of Cr-doped V$_2$O$_3$

We have performed sound velocity measurements in (V$_{1-x}$Cr$_x$)$_2$O$_3$ in the vicinity of the critical point of the first order Mott transition line. The pressure sweeps at constant temperature reveal a large dip in the $c_{33}$ compression modulus, this dip sharpens as the critical point is approached. We do not observe signs of criticality on the shear modulus $c_{44}$ which is consistent with a transition governed by a scalar order parameter, in accordance with the DMFT description of the transition. However, the amplitude of the effect is an order of magnitude smaller than the one obtained from DMFT calculations for a single band Hubbard model. We analyze our results using a simple model with the electronic response function obtained from the scaling relations for the conductivity.