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

TL;DR

This study investigates the divergence of the Grueneisen ratio at quantum critical points in heavy fermion metals CeNi2Ge2 and YbRh2(Si,Ge)2, revealing different underlying quantum critical behaviors.

Contribution

It provides experimental evidence of divergent Grueneisen ratios near quantum critical points and distinguishes between SDW and local quantum criticality in different materials.

Findings

Grueneisen ratio diverges as T approaches 0 in both materials.

CeNi2Ge2 behavior aligns with SDW scenario for 3D spin fluctuations.

YbRh2(Si,Ge)2 shows singularity inconsistent with SDW, supporting local quantum criticality.

Abstract

We present low-temperature volume thermal expansion, $\beta$, 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, $\beta$ shows a more singular temperature dependence than $C$, and thus the Grueneisen ratio ${\Gamma \propto \beta/C}$ 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.