Thermal expansion and Grüneisen ratio near quantum critical points

Recently it has been shown that at any quantum critical point (QCP) thermal expansion is more singular than specific heat and that the resulting Grüneisen ratio divergence characterizes the nature of the critical fluctuations. Here, we report a comparative study on heavy fermion and d-metal systems. In Ce0.98Ni2+xGe2-xCe0.98Ni2+xGe2-x polycrystals (0⩽x⩽0.0150⩽x⩽0.015) an extreme sensitivity of the ground-state behavior on small changes of composition is observed. For YbRh2(Si1-xGex)2YbRh2(Si1-xGex)2 the Grüneisen ratio changes sign between the antiferromagnetic and paramagnetic state and diverges with a fractional exponent in the quantum critical regime. In the alloy NixPd1-xNixPd1-x the absence of a Grüneisen ratio divergence is incompatible with a ferromagnetic QCP.