Constraints on the merging of the transition lines at the tricritical point in a wing-structure phase diagram

Here, we consider the phase diagram of a ferromagnetic system driven to a quantum phase transition with a tuning parameter p. Before being suppressed, the transition becomes of the first order at a tricritical point, from which wings emerge under application of the magnetic field H in the T − p − H phase diagram. We show that the edge of the wings merge with tangent slopes at the tricritical point.