Fractional Quantum Numbers, Complex Orbifolds and Noncommutative Geometry

This paper studies the conductance on the universal homology covering space $Z$ of 2D orbifolds in a strong magnetic field, thereby removing the integrality constraint on the magnetic field in earlier works in the literature. We consider a natural Landau Hamiltonian on $Z$ and show that its low-lying spectrum consists of a finite number of isolated points. We calculate the von Neumann degree of the associated holomorphic spectral orbibundles when the magnetic field $B$ is large, and obtain fractional quantum numbers as the conductance.