Systematic study of the Gruneisen ratio near quantum critical points

Abstract

At any pressure-sensitive quantum critical point (QCP), the volume thermal expansion coefficient is more singular than the specific heat. Consequently, the resulting critical Gruneisen ratio Gamma(cr)similar to beta(cr)/C-cr, where beta(cr) and C-cr denote the thermal expansion and specific heat after subtraction of non-critical background contributions, diverges. The related critical exponent epsilon in Gamma(cr)similar to T-epsilon 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 Grimeisen ratio. For the two former systems the critical exponent epsilon = 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 epsilon < 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. (c) 2007 Published by Elsevier Ltd.