Quantum fluctuations of order parameters in structural phase transitions and the pressure dependence of transition temperatures

Low-temperature saturation of the temperature dependence of order parameters Q ( T ) leads to highly nonlinear phase boundaries T c ( x ) in ( T c , x ) space where x is a secondary field variable. A typical experimental parameter x is the external pressure p or an electric field E which couple with the square of the order parameter ( H coupling = xQ 2 ). The general characteristics of T c ( x ) are derived in mean field theory for 4 and p models. It is shown that in several cases the function T c ( x ) can be found by direct inversion of the function Q ( T ) with the parameter mapping Q p -2 x .