No eigenstate of the critical transverse-field Ising chain satisfies the area law (2303.09525v2)

Abstract: We argue that, in a basis common to all one-site shift invariant conserved charges, there is no eigenstate of a noninteracting local spin-1/2 chain Hamiltonian that satisfies the area law if the ground state has half-integer central charge. That is to say, in those models all (quasi)local one-site shift invariant conserved operators are gapless. From the standpoint of bipartite entanglement properties, we show indeed that there are three distinct one-site shift invariant noninteracting models, two of which are equivalent to the XX model (for one of them the transformation breaks one-site shift invariance) and the other to the critical Ising model. The former class has two locally distinct one-site shift invariant excited states satisfying the area law; the latter two classes have none.