We present low-temperature volume thermal expansion, β, and specific heat, C, measurements on high-quality single crystals of CeNi 2 Ge 2 and YbRh 2 (Si 0.95 Ge 0.05 ) 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 Grüneisen ratio Γ ∝ β/C diverges as T → 0. For CeNi 2 Ge 2 , our results are in accordance with the spin-density wave (SDW) scenario for three-dimensional critical spinfluctuations. By contrast, the observed singularity in YbRh 2 (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.

We present low-temperature heat and charge transport as well as caloric properties of a ThAsSe single crystal. An extra -AT(1/2) term in the electrical resistivity, independent of magnetic fields as high as 14 T, provides evidence for an unusual scattering of conduction electrons. Additionally, both the thermal conductivity and the specific heat show a glass-type temperature dependence which signifies the presence of tunneling states. These observations apparently point to an experimental realization of a two-channel Kondo effect derived from structural two-level systems.

We have observed an unexpected enhancement of the lower critical field H(c1)(T) and the critical current I(c)(T) deep in the superconducting state below T approximately 0.6 K (T/T(c) approximately 0.3) in the filled skutterudite heavy fermion superconductor PrOs(4)Sb(12). From a comparison of the behavior of H(c1)(T) with that of the heavy fermion superconductors U(1-x)Th(x)Be(13) and UPt(3), we speculate that the enhancement of H(c1)(T) and I(c)(T) in PrOs(4)Sb(12) reflects a transition into another superconducting phase that occurs below T/T(c) approximately 0.3. An examination of the literature reveals unexplained anomalies in other physical properties of PrOs(4)Sb(12) near T/T(c) approximately 0.3 that correlate with the features we have observed in H(c1)(T) and I(c)(T).

No abstract