Violation of the Entropic Area Law for Fermions

We investigate the scaling of the entanglement entropy in an infinite translational invariant fermionic system of any spatial dimension. The states under consideration are ground states and excitations of tight-binding Hamiltonians with arbitrary interactions. We show that the entropy of a finite region typically scales with the area of the surface times a logarithmic correction. Thus, in contrast with analogous bosonic systems, the entropic area law is violated for fermions. The relation between the entanglement entropy and the structure of the Fermi surface is discussed, and it is proven that the presented scaling law holds whenever the Fermi surface is finite. This is, in particular, true for all ground states of Hamiltonians with finite range interactions.

Figure 1

Consider the Fermi surface and shift it by a vector $q$ in $k$ space. $\Xi (q)$ is then given by the (dark gray) area of the Fermi sea, which is no longer covered by its translation. The scaling of the entanglement entropy depends only on the behavior of $\Xi$ in the vicinity of $q=0$.