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CeCoIn5 exhibits the highest superconducting transition temperature (Tc = 2.3 K) among the Ce‐based heavy‐fermion compounds. Power‐law dependencies in specific heat (C), thermal conductivity (κ), and nuclear spin‐lattice relaxation rate (1/T1) for T ≪ Tc point to an unconventional superconducting state. Within the superconducting state specific‐heat shows an anomaly in the vicinity of the upper critical field Hc2 = 11.6 T for magnetic field parallel to the c‐axis. This phase‐transition is discussed as possible realization of a Fulde‐Ferrell‐Larkin‐Ovchinnikov (FFLO) phase. On the other hand, CeCoIn5 is located close to an antiferromagnetic (AFM) quantum critical point (QCP) at ambient pressure and underlying magnetic fluctuations may be responsible for the specific‐heat feature. We present first results of a specific‐heat study under pressure and in high magnetic fields to clarify the origin of this phase.

The newly‐discovered clathrates Ba6Ge25 and its relatives consist of a rigidG ermanium framework, in which Barium or other metallic atoms are trapped. These ‘guest’ atoms can ‘rattle’ freely in the oversized cages at high temperature. In Ba6Ge25, some of Ba atoms aref rozen into the split positions randomly below 200 K. This structural phase transition results in a two‐step resistivity jump from metallic behavior to semiconductor‐like behavior characterized by a mean free path of 3A. Surprisingly, a BCS‐like superconducting transition occursa t Tc ≈ 0.24 K in such a ‘bad metal’. By applying hydrostatic pressure, the structural phase transition is suppressed but Tc increases drastically. Tc reaches a maximum value of 3.85K at a critical pressure pc ≈ 2.8 GPa, where the structural distortion is completely suppressed and the system exhibits metallic behavior over the whole temperature range. Higher pressures lead to a slight decrease of Tc. On replacing 1/3 of the Ba atoms with Na, no structural phase transi...