Negative Isotopic Shift of the LO Phonon in 3C-SiC: A Universal Mechanism of Mass Effect Dominance in Cubic Crystals

A new physical phenomenon—the dominance of the isotopic mass effect over the strain contribution in cubic crystals, which is manifested in a negative shift of a phonon mode—has been discovered. The universality of the relation ω ~ μ –1/2 for symmetric lattices has been experimentally proven on the example of 3C-SiC. The contrast with a positive shift in anisotropic 4H/6H-SiC proves the key role of crystal symmetry in the competitive separation of the mass effect and strain contributions.