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Entanglement of electrons and the lattice in a Luttinger system

The coupling between electronic and lattice degrees of freedom lies at the core of many important properties of solids. Nevertheless, surprisingly little is know about the entanglement between these degrees of freedom. We here calculate the entanglement entropy at zero temperature as well as the mutual information and the entanglement negativity at finite temperatures between the electrons and the lattice of a one-dimensional chain. The electrons are described within Luttinger-liquid theory. Our results show that the entanglement entropy diverges when one approaches the limit of stability, the so-called Wentzel-Bardeen singularity. We have found that the mutual information and the entanglement negativity decrease with the temperature. The mutual information reaches a finite value in the infinite-temperature limit, which is the consequence of the infinite linear electron spectrum of Luttinger theory. The entanglement negativity becomes exactly zero above a certain temperature, i.e., the lattice and the electrons become non-entangled above this temperature. If the electron-electron interaction is unscreened or weakly screened, this characteristic temperature diverges with the system size. However, if the interaction is strongly screened the characteristic temperature is finite and independent of the system size.