Another type of these materials may be characterized by a Z2 index, and they exhibit spin-momentum locking, and possess edge states protected by time-reversal symmetry.
One type of these materials uniquely experience two sign changes in their Hall coefficient as electron doping proceeds from n equals zero to n equals two. That example of these materials are described by Hubbard theory. Another type of these materials may be characterized by a Z2 index, and they exhibit spin-momentum locking, and possess edge states protected by time-reversal symmetry. Another of these materials deviates from conventional (*) band theory, and was first proposed with a nickel(II) oxide model by Nevill Mott. In that type of these materials, electrons hop between unit cells, unlike in their "charge-transfer" and "topological" versions. These materials typically possess large band gaps and high dielectric constants. For 10 points, name these materials in which electric currents do not flow. 
One type of these materials uniquely experience two sign changes in their Hall coefficient as electron doping proceeds from n equals zero to n equals two.
electrical insulators [accept topological insulators, or Mott insulators; prompt on dielectrics] 