DeMell, Jennifer E.Naumov, IvanStephen, Gregory M.Blumenschein, Nicholas A.Leo Sun, Y.-J.Fedorko, AdrianRobinson, Jeremy T.Campbell, Paul M.Taylor, Patrick J.Heiman, DonDev, PratibhaHanbicki, Aubrey T.Friedman, Adam L.Mechanical stacking of two dissimilar materials often has surprising consequences for heterostructure behavior. In particular, a 2D electron gas (2DEG) is formed in the heterostructure of the topological crystalline insulator Pb0.24Sn0.76Te and graphene due to contact of a polar with a nonpolar surface and the resulting changes in electronic structure needed to avoid polar catastrophe. The spintronic properties of this heterostructure with non-local spin valve devices are studied. This study observes spin-momentum locking at lower temperatures that transitions to regular spin channel transport only at ≈40 K. Hanle spin precession measurements show a spin relaxation time as high as 2.18 ns. Density functional theory calculations confirm that the spin-momentum locking is due to a giant Rashba effect in the material and that the phase transition is a Lifshitz transition. The theoretically predicted Lifshitz transition is further evident in the phase transition-like behavior in the Landé g-factor and spin relaxation time.en-USArticle