Common microscopic origin of the phase transitions in Ta2NiS5 and the excitonic insulator candidate Ta2NiSe5

Lukas Windgätter,Hannes Hübener,Antoine Georges,Malte Rösner,Simone Latini,Andrew J Millis,Giacomo Mazza,Angel Rubio

doi:10.1038/s41524-021-00675-6

Abstract

The structural phase transition in Ta2NiSe5 has been envisioned as driven by the formation of an excitonic insulating phase. However, the role of structural and electronic instabilities on crystal symmetry breaking has yet to be disentangled. Meanwhile, the phase transition in its complementary material Ta2NiS5 does not show any experimental hints of an excitonic insulating phase. We 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. In both materials the crystal symmetries are broken by phonon instabilities, which in turn lead to changes in the electronic bandstructure also observed in the experiment. 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. We conclude that an excitonic instability is not needed to explain the phase transition in both Ta2NiSe5 and Ta2NiS5.

Highlights

The excitonic insulator phase has been theoretically proposed in the 1960s1–6 and is predicted to appear in semiconductors with excitonic binding energies larger than the bandgap of their conventional groundstate
We investigate bulk Ta2NiSe5 (TNSe), which is considered to be a very promising candidate to host an excitonic condensate[21], with several experimental evidence suggesting an excitonic insulating (EI) groundstate: angle-resolved photo-electron spectroscopy (ARPES) measurements have shown a characteristic band flattening near the Γpoint upon cooling below the critical temperature[22,23,24,25], a domeshaped bandgap–temperature phase diagram, similar to the theoretically predicted one, has been found[26], and the opening of a gap has been measured in scanning tunnel spectroscopy (STS)[27], optics[26] and ARPES24,28 experiments below the critical temperature[27]
While we find that the fine electronic properties are sensitive to the details of exchange and correlation, the underlying mechanism of bandgap opening due to the structural phase transition is independent of the functional choice for both TNSe and TNS

Summary

INTRODUCTION

The excitonic insulator phase has been theoretically proposed in the 1960s1–6 and is predicted to appear in semiconductors (or semimetals) with excitonic binding energies larger than the bandgap (or bandoverlap) of their conventional groundstate. For TNSe, we can establish the orthorhombic groundstate to be metallic and observe a sizeable gap opening as well as a band flattening with the phase transition This shows that for this material the structural distortion is essential for the bandgap opening observed in optics and STS measurements and cannot be explained considering only the electronic degrees of freedom. Performing an equivalent DFT analysis for TNS shows that both the orthorhombic and monoclinic phases are gapped systems with parabolic electronic dispersions at Γ This explains why a metal to semiconductor transition and band flattening, which have been characterized as a signature of an excitonic insulator in TNSe, has not been observed for TNS34. As the relaxed structure predicted by the vdWoptB88 functional has the best agreement with the experimentally measured values for both the monoclinic and orthorhombic phase, we have chosen it for all following structural calculations

The effect of electronic heating

DISCUSSION

METHODS

CODE AVAILABILITY