Divergence of the Grueneisen Ratio at Quantum Critical Points in Heavy Fermion Metals

Abstract

We present low-temperature volume thermal expansion, ${\hat{I}}^2$, and specific heat, $C$, measurements on high-quality single crystals of CeNi2Ge2 and YbRh2(Si${}_{0.95}$Ge${}_{0.05}$)${}_2$ which are located very near to quantum critical points. For both systems, ${\hat{I}}^2$ shows a more singular temperature dependence than $C$, and thus the Grueneisen ratio $\hat{I}“\propto {\hat{I}}^2/C$ diverges as T --> 0. For CeNi2Ge2, our results are in accordance with the spin-density wave (SDW) scenario for three-dimensional critical spin-fluctuations. By contrast, the observed singularity in YbRh2$(Si$_{0.95}$Ge$_{0.05}$)$_2$ cannot be explained by the itinerant SDW theory but is qualitatively consistent with a locally quantum critical picture.

11 pages, 4 figures