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

We present low-temperature volume thermal expansion, β, and specific heat, C, measurements on high-quality single crystals of CeNi₂Ge₂ and YbRh₂(Si0.95Ge0.05)₂ which are located very near to quantum critical points. For both systems, β shows a more singular temperature dependence than C, and thus the Gr\"uneisen ratio Γ∝β/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(Si0.95Ge0.05)₂ cannot be explained by the itinerant SDW theory but is qualitatively consistent with a locally quantum critical picture.