Systematic study of the Grüneisen ratio near quantum critical points

At any pressure-sensitive quantum critical point (QCP), the volume thermal expansion coefficient is more singular than the specific heat. Consequently, the resulting critical Grüneisen ratio Γcr∼βcr/Ccr, where βcr and Ccr denote the thermal expansion and specific heat after subtraction of non-critical background contributions, diverges. The related critical exponent ε in Γcr∼T−ε can be used to characterize the nature of the underlying quantum critical fluctuations. We have performed a comparative study on various heavy fermion (HF) systems close to antiferromagnetic QCPs. In particular, we have studied (i) CeIn3−xSnx, (ii) CeNi2Ge2, (iii) YbRh2(Si0.95Ge0.05)2, as well as (iv) CeCu5.8Ag0.2, all of which show a divergent Grüneisen ratio. For the two former systems the critical exponent ε=1 is compatible with the predictions of the well-established Hertz-Millis-Moriya theory for three-dimensional extended quantum critical fluctuations. By contrast, for the two latter systems ε<1 is found to be incompatible with “conventional” quantum criticality. Our results thus suggest the existence of at least two different classes of QCPs in HF systems.