Orthorhombic-to-Monoclinic Phase Transition of Ta2NiSe5 Induced by the   Bose-Einstein Condensation of Excitons
  Using the band structure calculation and mean-field analysis of the derived three-chain Hubbard model with phonon degrees of freedom, we discuss the origin of the orthorhombic-to-monoclinic phase transition of the layered chalcogenide Ta$_2$NiSe$_5$. We show that the Bose-Einstein condensation of excitonic electron-hole pairs cooperatively induces the instability of the phonon mode at momentum $q\rightarrow 0$ in the quasi-one-dimensional Ta-NiSe-Ta chain, resulting in the structural phase transition of the system. The calculated single-particle spectra reproduce the deformation of the band structure observed in the angle-resolved photoemission spectroscopy experiment. 
T. Kaneko, T. Toriyama, T. Konishi, and Y. Ohta
cond-mat.str-el cond-mat.mtrl-sci