Extremal positive semidefinite matrices whose sparsity pattern is given by graphs without K5 minors
L.T. Solus, C. Uhler, R. Yoshida, Linear Algebra and Its Applications 509 (2016) 247–275.

Facets of the r-stable (n, k)-hypersimplex
T. Hibi, L.T. Solus, Annals of Combinatorics 20 (2016) 815–829.