Constraints on the merging of the transition lines at the tricritical   point in a wing-structure phase diagram
  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. 
Valentin Taufour, Udhara S. Kaluarachchi, Vladimir G. Kogan
cond-mat.str-el cond-mat.other