Erratum: Orthorhombic-to-monoclinic phase transition of Ta2NiSe5induced by the Bose-Einstein condensation of excitons [Phys. Rev. B87, 035121 (2013)]

Competition between excitonic charge and spin density waves: Influence of electron-phonon and Hund's rule couplings

“…In these systems, the valence and conduction bands are formed by orbitals located on different atoms. For example, in 1T -TiSe 2 , the 4p orbitals of Se ions account for the valence bands and the 3d orbitals of Ti ions account for the conduction bands [7][8][9][10][11][12][13][14] , and in Ta 2 NiSe 5 , the 3d orbitals of Ni ions form the valence bands and the 5d orbitals of Ta ions form the conduction bands [21][22][23][24] . Hund's rule coupling, acting between electrons on different orbitals of a single ion and favoring the spin-triplet excitons, is therefore negligible.…”

“…Including thereby the Hund's rule coupling is known to stabilize the spin-triplet excitonic phase in the otherwise degenerate spin-singlet and spintriplet excitonic phases [18][19][20]28,42,43 . On the other hand, we have shown in our previous work 43 that taking into account electronic interactions only, a spin-singlet excitonic phase cannot be stabilized, which may however be realized in 1T -TiSe 2 and Ta 2 NiSe 5 , where the importance of electron-phonon coupling was recently pointed out [10][11][12][13][14]23 . Although the spin-singlet excitonic state has been investigated in the spinless multiband model with electron-phonon coupling 12,44,45 , not much is known about the role of the electron-phonon coupling played in the excitonic density wave states in the spinful multiband Hubbard model.…”

“…The condensation of spin-triplet excitons was also predicted to occur in the proximity of the spin-state transition, 18 of which Pr 0.5 Ca 0.5 CoO 3 is an example. 19,20 Likewise, the structural phase transition of the layered chalcogenide Ta 2 NiSe 5 has been attributed to a spin-singlet EI [21][22][23][24] . The spin-density-wave (SDW) state of iron-pnictide superconductors has sometimes been argued to be of the excitonic origin as well [25][26][27][28] .…”

Fate of the excitonic insulator in the presence of phonons

“…First attempts to include a coupling to the lattice degrees of freedom have been made quite recently, motivated by several experiments indicating that the lattice is involved at the phase transition to the anticipated EI phase. [17][18][19][20][21] For example, in the TmSe 0. 45 Te 0.55 compound a drop of the specific heat and an increase of the lattice constant have been interpreted as a strong coupling between excitons and phonons.…”

Excitonic Insulator State of the Extended Falicov–Kimball Model in the Cluster Dynamical Impurity Approximation

“…[7][8][9] In recent years, the possible realization of spinsinglet excitonic condensation has been suggested for transition-metal chalcogenides such as 1T -TiSe 2 and Ta 2 NiSe 5 . [10][11][12][13][14][15][16][17][18][19][20][21][22][23] The spin-triplet excitonic condensation has also been suggested to occur in the high-spin/lowspin crossover region of some cobalt oxide materials with the cubic perovskite structure. [24][25][26][27][28][29] Because these materials are among transition-metal chalcogenides and oxides, where the effects of electron correlations are strong, one must reconsider the excitonic phases from the standpoint of strongly correlated electron systems.…”