The Bloch theorem mathematically proves that in a periodic crystal, electrons can acquire a negative mass. The present work aims to provide a physical understanding for why this is so. We successively analyze the consequences of the 3-fold orbital valence state coupling to (i) a non-degenerate orbital level in the conduction band, (ii) a 3-fold orbital level in the conduction band, and (iii) spin states through spin-orbit interaction. We show that it is not at all trivial for valence electrons to acquire a negative mass for whatever their momentum with respect to the crystal axes: it is necessary to not only have a coupling to a degenerate orbital conduction level, but also a symmetry breaking of the 3-fold valence subspace by the spin quantization axis, as induced by spin-orbit interaction. Due to the relativistic origin of this interaction, the existence of negative valence masses thus constitutes an unexpected signature of quantum relativity.
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