Flux quantization for a superconducting ring in the shape of a M\"obius band

We give two derivations of magnetic flux quantization in a superconducting ring in the shape of a M\"obius band, one using direct study of the Schr\"odinger equation and the other using the holonomy of flat U(1)-gauge bundles. Both methods show that the magnetic flux must be quantized in integral or half-integral multiples of $\Phi_0=hc/(2e)$. Half-integral quantization shows up in "nodal states" whose wavefunction vanishes along the center of the ring, for which there is now some experimental evidence.