The emphasis of this article lies on the properties of heavy fermion systems at the lowest temperatures obtained so far. Methods for measuring specific heat capacity and magnetic susceptibility in the milliKelvin regime are described for both low magnetic fields (<20 mT ) and high fields (<8 T ). Experimental results on UPt 3, UBe 13, and CeCu 6, are presented, and remaining problems are discussed. UPt 3 is widely regarded as the heavy Fermion system which exhibits unconventional superconductivity as demonstrated by the existence of multiple superconducting phases. Whether power laws for instance for the specific heat capacity in the superconducting state extend to T ≈ 0 instead of an exponential behavior as for BCS superconductors is a long-standing question. We have measured the specific heat capacity of several single crystals of UPt 3 in magnetic fields varying from 0 up to 7 T down to a final temperature of 10 mK . Instead of an extended power law a maximum of c(T) occurs around 20 mK, and this maximum persists in magnetic fields above B c2. It is obviously due to a new phase transition which is present both in the normal and in the superconducting state of UPt 3, slightly modified in the latter. Entropy balance above T c is fulfilled if the low-temperature peaks are included. DC-magnetization measurements on two single crystals of UPt 3 in a SQUID system yield a temperature dependence of the penetration depth ~T2 between 150 and 20 mK, considerably extending the temperature range of earlier experiments. Measurements of the anisotropic part of the magnetic susceptibility in a torque-meter indicate an additional phase line from a temperature-dependent anisotropic susceptibility to a T-independent state which is closely connected to the Bc2-line over a wide field range. No indication for a re-entrance of superconductivity is found down to 20 mK. For UBe 13 (in the superconducting state) no specific heat anomaly above 24 mK is found but a deviation from the T2.7 power law valid at higher temperatures. On a single crystal of CeCu 6 dc-magnetization measurements in various magnetic fields in a SQUID system show a plateau of the magnetic susceptibility between 400 and 50 mK, followed by an increase towards lower temperatures. An attempt to fit the low-temperature magnetization curves in several fields between 0.01 mT and 1.6 mT (minus the background from the plateau) with a Brillouin function revealed significant deviations. In 2.7 mT, the highest field applied in this experiment, however, the magnetization can be perfectly fitted assuming a tiny concentration of Gd impurities (1.5 ppm). After subtraction of the contribution due to the Gd 3+ ions from the magnetization curves in each of the lower fields a drop is revealed below 3–5 mK which gets successively quenched by the magnetic field and which has disappeared in 2.7 mT. The specific heat capacity of the same single crystal of CeCu 6 in zero magnetic field shows an increase of c/T from 1.55 J/moleK 2 at higher temperatures up to 2.8 J/moleK 2 at 11 mK. We interpret both results as due to an antiferromagnetic phase transition at 3–5 mK with the peak just not reached in the specific heat experiments.
Multiple superconducting phases in a nearly ferromagnetic system
