  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.
