High Pressure Effects
  Experiments under hydrostatic and uniaxial pressure serve not only as a guide in the synthesis of materials with superior superconducting properties but also allow a quantitative test of theoretical models. In this chapter the pressure dependence of the superconducting properties of elemental, binary, and multi-atom superconductors are explored, with an emphasis on those exhibiting relatively high values of the transition temperature Tc. In contrast to the vast majority of superconductors, where Tc decreases under pressure, in the cuprate oxides Tc normally increases. Uniaxial pressure studies give evidence that this increase arises mainly from the reduction in the area of the CuO2 planes (Tc approximately proportional to inverse square area), rather than in the separation between the planes, thus supporting theoretical models which attribute the superconductivity primarily to intraplanar pairing interactions. More detailed information would be provided by future experiments in which the hydrostatic and uniaxial pressure dependences of several basic parameters, such as Tc, the superconducting gap, the pseudo-gap, the carrier concentration, and the exchange interaction are determined for a given material over the full range of doping. 
James S. Schilling