摘要
The structural phase transition in Ta-NiSes 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 Ta_(2)NiS_(5)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 Ta_(2)NiSe_(5)and Ta-Niss using extensive first-principles calculations.In both materials the crystal symmetries are broken by phonon instabilities,which in tum 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 Ta_(2)NiSe_(5)and Ta_(2)NiS_(5).
基金
We are grateful to E.Baldini,I.Mazin and Yann Gallais for enlightening discussions throughout the course of this work.We would like to thank M.Ye,G.Blumberg,K.Kim,BJ.Kim,MJ.Kim and S.Kaiser for sharing the experimental data of their Raman measurements and valuable discussions.This work is supported by the European Research Council(ERC-2015-AdG-694097)
Grupos Consolidados(IT1249-19)
the Flatiron Institute,a division of the Simons Foundation.We acknowledge funding by the Deutsche Forschungsgemeinschaft(DFG)under Germany's Excellence Strategy-Cluster of Excellence Advanced Imaging of Matter(AIM)EXC 2056-390715994
by the Deutsche Forschungsgemeinschaft(DFG,German Research Foundation)-SFB-925-project 170620586
Support by the MaxPlanck Institute-New York City Center for Non-Equilibrium Quantum Phenomena is acknowledged
S.L.acknowledges support from the Alexander von Humboldt foundation
G.M.acknowledges support of the Swiss National Science Foundation FNS/SNF through an Ambizione grant.
