Nature of Symmetry Breaking at the Excitonic Insulator Transition: 
<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msub><mml:mrow><mml:mi>Ta</mml:mi></mml:mrow><mml:mrow><mml:mn>2</mml:mn></mml:mrow></mml:msub><mml:msub><mml:mrow><mml:mi>NiSe</mml:mi></mml:mrow><mml:mrow><mml:mn>5</mml:mn></mml:mrow></mml:msub></mml:mrow></mml:math>

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

doi:10.1103/physrevlett.124.197601

Cited by 114 publications

(124 citation statements)

References 44 publications

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“…Finally, let us comment on the effects of electronic terms that explicitly break the symmetry, which have been pointed out recently [47,48]. We numerically confirmed that these terms make the phase mode massive as well, and the dependence of the mass depends sensitively on the form of such terms.…”

Section: B Effects Of Electron-phonon Couplingsupporting

confidence: 68%

“…The breaking of the U(1) symmetry leads to a massless Goldstone mode, which can result in interesting coherent phenomena such as supertransport. However, the crucial issue regarding the EI phase in real materials is that the system may weakly break the U(1) symmetry, either by coupling to phonons [36,45,46] or by a direct hybridization term [47,48]. In particular, such electron-phonon (elph) coupling can cooperate with the Coulomb interaction to stabilize the ordered phase.…”

Section: Introductionmentioning

confidence: 99%

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Collective modes in excitonic insulators: Effects of electron-phonon coupling and signatures in the optical response

et al. 2020

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We consider a two-band spinless model describing an excitonic insulator (EI) on the two-dimensional square lattice with anisotropic hopping parameters and electron-phonon (el-ph) coupling, inspired by the EI candidate Ta 2 NiSe 5. We systematically study the nature of the collective excitations in the ordered phase which originates from the interband Coulomb interaction and the el-ph coupling. When the ordered phase is stabilized only by the Coulomb interaction (pure EI phase), its collective response exhibits a massless phase mode in addition to the amplitude mode. We show that in the BEC regime, the signal of the amplitude mode becomes less prominent and that the anisotropy in the phase-mode velocities is reduced. Through coupling to the lattice, the phase mode acquires a mass and the signal of the amplitude mode becomes less prominent. Importantly, the character of the softening mode at the boundary between the normal semiconductor phase and the ordered phase depends on the parameters. In particular, we point out that even for el-ph coupling smaller than the Coulomb interaction, the mode that softens to zero at the boundary can have a phonon character. We also discuss how the collective modes can be observed in the optical conductivity. Furthermore, we study the effects of nonlocal interactions on the collective modes and show the possibility of realizing a coexistence of an in-gap mode and an above-gap mode split off from the single-amplitude mode in the system with local interactions only.

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Section: B Effects Of Electron-phonon Couplingsupporting

confidence: 68%

Section: Introductionmentioning

confidence: 99%

Collective modes in excitonic insulators: Effects of electron-phonon coupling and signatures in the optical response

et al. 2020

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“…There have been various DFT studies of Ta 2 NiSe 5 since 2012, and mainly two difficulties were encountered for this complicated material. First, the description of its normal phase band structure above T c is delicate because the electronic structure of Ta 2 NiSe 5 is close to being either a semimetal or a semiconductor, depending on the fine details of the DFT calculation, especially the choice of an appropriate functional for describing this material [11,12,17,18,33]. Second, the evaluation of the band gap in the low-temperature semiconducting phase is a difficult task because it is a combination of a hybridization band gap due to the low-symmetry monoclinic phase and a correlation gap due to the excitonic insulator phase.…”

Section: Resultsmentioning

confidence: 99%

“…In a configuration interaction picture, Ta 2 NiSe 5 has been shown to be a negative charge transfer material, and its Ni site has a mainly d 9 L ground state with some d 10 L 2 contribution [2]. In parallel, studies based on density functional theory (DFT) calculations disagree on its capability to capture the normal state of Ta 2 NiSe 5 , notably due to the difficulty of finding an appropriate functional for describing this material [11,12,17,18]. In addition, time-resolved studies based on pump-probe techniques support the existence of an excitonic insulator ground state at low temperature [14,[19][20][21][22].…”

Section: Introductionmentioning

confidence: 99%

Mapping the unoccupied state dispersions in Ta2NiSe5 with resonant inelastic x-ray scattering

et al. 2020

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The transition metal chalcogenide Ta 2 NiSe 5 undergoes a second-order phase transition at T c = 328 K involving a small lattice distortion. Below T c , a band gap at the center of its Brillouin zone increases up to about 0.35 eV. In this work, we study the electronic structure of Ta 2 NiSe 5 in its low-temperature semiconducting phase, using resonant inelastic x-ray scattering (RIXS) at the Ni L 3 edge. In addition to a weak fluorescence response, we observe a collection of intense Raman-like peaks that we attribute to electron-hole excitations. Using density functional theory calculations of its electronic band structure, we identify the main Raman-like peaks as interband transitions between valence and conduction bands. By performing angle-dependent RIXS measurements, we uncover the dispersion of these electron-hole excitations that allows us to extract the low-energy boundary of the electron-hole continuum. From the dispersion of the valence band measured by angle-resolved photoemission spectroscopy, we derive the effective mass of the lowest unoccupied conduction band.

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“…In this work, we consider ultrafast many-particle correlations in an excitonic-insulator system acting as a prototypical ordered-phase material [1,7,10,65]. Out-of-equilibrium dynamics in such systems with a symmetry-broken ground state has been shown to be extremely sensitive to all the intricacies in the electronic and lattice structure [7,10,29,53,66].…”

Section: Introductionmentioning

confidence: 99%

Comparing the generalized Kadanoff-Baym ansatz with the full Kadanoff-Baym equations for an excitonic insulator out of equilibrium

Tuovinen¹,

Golež²,

Eckstein³

et al. 2020

Phys. Rev. B

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We investigate out-of-equilibrium dynamics in an excitonic insulator (EI) with a finite-momentum pairing perturbed by a laser-pulse excitation and a sudden coupling to fermionic baths. The transient dynamics of the excitonic order parameter is resolved using the full nonequilibrium Green's function approach and the generalized Kadanoff-Baym ansatz (GKBA) within the second-order Born approximation. The comparison between the two approaches after a laser-pulse excitation shows a good agreement in the weak and the intermediate photodoping regime. In contrast, the laser-pulse dynamics resolved by the GKBA does not show a complete melting of the excitonic order after a strong excitation. Instead we observe persistent oscillations of the excitonic order parameter with a predominant frequency given by the renormalized equilibrium band gap. This anomalous behavior can be overcome within the GKBA formalism by coupling to an external bath, which leads to a transition of the EI system toward the normal state. We analyze the long-time evolution of the system and distinguish decay timescales related to dephasing and thermalization.

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Nature of Symmetry Breaking at the Excitonic Insulator Transition: Ta2NiSe5

Cited by 114 publications

References 44 publications

Collective modes in excitonic insulators: Effects of electron-phonon coupling and signatures in the optical response

Collective modes in excitonic insulators: Effects of electron-phonon coupling and signatures in the optical response

Mapping the unoccupied state dispersions in Ta2NiSe5 with resonant inelastic x-ray scattering

Comparing the generalized Kadanoff-Baym ansatz with the full Kadanoff-Baym equations for an excitonic insulator out of equilibrium

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