We present a model for viable gravitational reheating involving a scalar field directly coupled to the Ricci curvature scalar .
Crucial to the model is a period of kination after inflation , which causes the Ricci scalar to change sign thus inducing a tachyonic effective mass m ^ { 2 } \propto - H ^ { 2 } for the scalar field .
The resulting tachyonic growth of the scalar field provides the energy for reheating , allowing for temperatures high enough for thermal leptogenesis .
Additionally , the required period of kination necessarily leads to a blue-tilted primordial gravitational wave spectrum with the potential to be detected by future experiments .
We find that for reheating temperatures T _ { RH } \lesssim 1 GeV , the possibility exists for the Higgs field to play the role of the scalar field .