Gapless fermions and gauge fields in dielectrics

To study the nonmagnetic dielectric state and Mott transitions we consider an example of a two-dimensional modified Hubbard model with a large number of colors. Low-energy excitations in this phase are fermionic excitations and Bose excitations described by gauge fields of the U(1) group. The transition into the metal state has little effect on the fermionic spectrum, but it results in the local U(1) symmetry being broken and fermions becoming able to transfer charge excitations. Apart from the half-filling, scalar Bose excitations also appear. Due to the presence of additional gauge fields the physical conductivity is determined by the lowest conductivity of the Fermi or Bose subsystems.