Ab-initio theory of the symmetry-breaking transitions in Ta2NiS5 and Ta2NiSe5

Ta2NiSe5 is one of the most promising materials for hosting an excitonic insulator ground state. While several experimental observations have been interpreted in this way, the precise nature of the symmetry breaking occurring in Ta2NiSe5, the electronic order parameter, and a realistic microscopic description of the transition mechanism are, however, missing and the role of structural and electronic instabilities on crystal symmetry breaking has yet to be disentangled. Meanwhile, the structural phase transition in its complementary material Ta2NiS5 does not show any experimental hints of an excitonic insulating phase.

I will present a microscopic investigation of the electronic and phononic effects involved in the structural phase transition in Ta2NiSe5 and Ta2NiS5 using extensive first-principles calculations. I will discuss the discrete lattice symmetries which are broken at the transition of Ta2NiSe5, which we obtained from a symmetry analysis based on first-principles calculations. From this, we can identify a purely electronic order parameter that breaks these discrete crystal symmetries and that can contribute to the experimentally observed lattice distortion from an orthorombic to a monoclinic phase. Additionally, I will show that in both materials phonon instabilities also lead to crystal symmetry breakings, which in turn lead to changes in the electronic bandstructure in line with experimental observation. A total energy landscape analysis shows no tendency towards a purely electronic instability, and we find that a sizeable lattice distortion is needed to open a bandgap. Based on this, we can conclude that a purely electronic excitonic instability is not needed to explain the phase transition in both Ta2NiSe5 and Ta2NiS5.