Merons and Thomas precession

Abstract

In a recent study of Thomas precession of electron spin in the context of the Dirac equation, Shankar and Mathur identify the non-Abelian Berry potential (arising from Kramers degeneracy) with the meron. We point out that there is a global mathematical subtlety which prevents such an identification. We go on to clarify the physical context in which merons do arise as Berry potentials.