  Quantum Monte Carlo simulations reveal an exotic metallic phase with a
single-particle gap but gapless spin and charge excitations and a nonsaturating
resistivity in a two-dimensional SU(2) Falicov-Kimball model. An exact duality
between this model and an unconstrained slave-spin theory leads to a
classification of the phase as a fractionalized or orthogonal metal whose
low-energy excitations have different quantum numbers than the original
electrons. Whereas the fractionalized metal corresponds to the regime of
disordered slave spins, the regime of ordered slave spins is a Fermi liquid. At
a critical temperature, an Ising phase transition to a spontaneously generated
constrained slave-spin theory of the Hubbard model is observed.
