Exciton condensation due to electron-phonon interaction

Abstract

We show that the coupling to vibrational degrees of freedom can drive a semimetal excitonicinsulator quantum phase transition in an one-dimensional two-band f -c electron system at zero temperature. The insulating state typifies an excitonic condensate accompanied by a finite lattice distortion. Using the projector-based renormalization method we analyze the ground-state and spectral properties of the interacting electron-phonon model at half-filling. In particular we calculate the momentum dependence of the excitonic order parameter function and determine the finite critical interaction strength for the metal-insulator transition to appear. The electron spectral function reveals the strong hybridization of f -and c-electron states and the opening of a singleparticle excitation gap. The phonon spectral function indicates that the phonon mode involved in the transition softens (hardens) in the adiabatic (non-adiabatic and extreme anti-adiabatic) phonon frequency regime.

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.…”

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

“…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.…”

Stripe and Short Range Order in the Charge Density Wave of 1T−CuxTiSe2

“…Two possible scenarios are being considered, one based on a purely electronic process characterized by an excitonic instability [8], while the other one involves a Jahn-Teller (JT) distortion [10]. More refined theories also propose a mixture of these two possible contributions, in the so called indirect JT transition [11][12][13].…”

“…Two possible scenarios are being considered, one based on a purely electronic process characterized by an excitonic instability [8], while the other one involves a Jahn-Teller (JT) distortion [10]. More refined theories also propose a mixture of these two possible contributions, in the so called indirect JT transition [11][12][13].1T-TiSe 2 becomes superconducting when intercalating more than x ¼ 0.04 copper into the vdW gap, with a maximum critical temperature T c ¼ 4.1 K near x ¼ 0.08 [14]. Transport measurements [14,15] indicate the CDW is suppressed upon increasing the Cu content which would suggest a competition with superconductivity.…”