Spectroscopic signatures of crystal momentum fractionalization

We consider gapped Z(2) spin liquids, where spinon quasiparticles may carry fractional quantum numbers of space-group symmetry. In particular, spinons can carry fractional crystal momentum. We show that such quantum number fractionalization has dramatic, spectroscopically accessible consequences, namely, enhanced periodicity of the two-spinon density of states in the Brillouin zone, which can be detected via inelastic neutron scattering. This effect is a sharp signature of certain topologically ordered spin liquids and other symmetry-enriched topological phases. Considering square lattice space-group and time-reversal symmetry, we show that exactly four distinct types of spectral periodicity are possible.