Pressure-induced quantum phase transitions

A quantum critical point is approached by applying pressure in a number of ferromagnetic and antiferromagnetic metals. The observed dependence of Tc on pressure necessarily means that the magnetic energy is coupled to the lattice. A first-order phase transition occurs if this coupling exceeds a critical value: this is inevitable if $\frac{\partial T_c}{\partial p}$ diverges as Tc approaches zero. It is argued that this is the cause of the first-order transition that is observed in many systems. Landau theory is used to obtain the phase diagram and also to predict the regions where metastable phases occur that agree well with experiments done on MnSi and other materials. The theory can be used to obtain very approximate values for the temperature and pressure at the tricritical point in terms of measured quantities. The values of the tricritical temperature for various materials obtained from Landau theory are too low but it is shown that the predicted values will rise if the effects of fluctuations are included.