1. A summary about AC-Calorimetric measurements at High Pressure and Low Temperatures appeared in Adv. Phys. Solid State, 43, 889 (2003)
2. The use of different high pressure technqiues in the investigation of various systems is desrcibed in Strongly correlated electron systems under high pressure: structural, magnetic, and superconducting phase transitions by H. Wilhelm, Habiliation Thesis, University Geneva, Mai 1999

1.) Heavy Fermions:

The application of external pressure on metals with strong correlations is an established technique to tune their ground state properties. The electronic interactions in heavy-Fermion (HF) compounds can be influenced in such a way that high pressure favors the Kondo interaction in Ce-compounds and the RKKY interaction in Yb-systems. Thus, pressure suppresses (favors) long range magnetic order and enhances (weakens) the screening of the localized 4f-electrons. If both interactions are of similar strength in the vicinity of a critical pressure often a deviation from the Fermi-liquid (FL) behavior is observed and some Ce-compounds even attain a superconducting ground state. More information about this topic can be found in the article by D. Jaccard et al., Physica B 259-261, 1 (1999).
 
 
 
 

2.) 1D-organic compounds

The existence of a common border between the superconducting (SC) ground state and the insulating phase of spin density wave (SDW) nature, was recognized as a remarkable property of the phase diagram of the Bechgaard salt (TMTSF)₂PF₆. It belongs to a broad family of isostructural compounds (TM)₂X, where the flat organic molecule TM is either tetramethyltetraselenafulvalene (TMTSF) or tetramethyltetrathiafulvalene (TMTTF). Here X denotes a monovalent anion such as PF₆, AsF₆, ClO₄ or B. In the crystal, these molecules form stacks separated by chains of anions X. The overlap between the electron clouds of neighboring TM molecules along the stacking direction (parallel the a-axis) is about 10 (500) times larger than that between the stacks in the transverse b-(c-)direction. Provided that the longitudinal overlap is large compared to the on--site Coulomb repulsion, these organic materials become conducting with a pronounced one dimensional (1-D) character. The 1-D character of the Fermi surface of (TMTSF)₂PF₆, the presence of a spin-Peierls (SP) transition instead of the usual Peierls instability as well as the existence of enhanced antiferromagnetic (AF) fluctuations at low temperature, evidenced by NMR relaxation experiments, raised several questions about the mechanism responsible for superconductivity in organic conductors. Since 1-D physics is a relevant concept in these low dimensional systems, SDW and electron-electron pairing can develop simultaneously at low temperature in the interacting electron gas. A cross-over from SDW to SC correlations could possibly be achieved through a small variation of the coupling constants either by applying pressure or changing X. Furthermore, the nuclear spin-lattice relaxation rate data of (TMTSF)₂PF₆ suggest that SDW correlations prevail at low temperature even under pressure when superconductivity is stabilized.

The experimental and theoretical results are reported in:
D. Jaccard et al., J. Phys.: Cond. Matter 13, L89 (2001); From spin-Peierls to superconductivity:(TMTTF)₂PF₆ under high pressure
H. Wilhelm et al., Eur. Phys. J. B 21, 175 (2001); The case of universality of the phase diagram of the Fabre and Bechgaard salts

H. Wilhelm et al. in Frontiers of High Pressure Research II: Application of High Pressure to Low Dimensional Novel Electronic Materials, ed. H.D. Hochheimer et al. NATO Sciences Series II, Vol 48, Kluwer Academic Press, Dordrecht, p 423 (2001); Pressure-induced superconductivity in the spin-Peierls compound (TMTTF)₂PF₆

3.) High-Tc compounds and related cuprates