Non-Coulomb strong electron-hole binding in 
<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:msub><mml:mi>Ta</mml:mi><mml:mn>2</mml:mn></mml:msub><mml:msub><mml:mi>NiSe</mml:mi><mml:mn>5</mml:mn></mml:msub></mml:mrow></mml:math>
 revealed by time- and angle-resolved photoemission spectroscopy

Tang, Tian-wei; Wang, Hongyuan; Duan, Shaofeng; Yang, Yuanyuan; Huang, Chaozhi; Guo, Yanfeng; Qian, Dong; Zhang, Wentao

doi:10.1103/physrevb.101.235148

Cited by 56 publications

(55 citation statements)

References 41 publications

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“…In the monoclinic phase, the valence band and the conduction band belong to the same irreducible representation (IR) along the Γ- Z direction, where they overlap with each other, thus allowing a band gap to open by hybridization between them 35 , 37 . Thus, as pointed out earlier , 14 – 17 , 20 , 23 , 24 , 26 , 33 , 37 , 38 , the distinction between an excitonic insulator and a band insulator in the presence of a lattice distortion may ultimately be only quantitative. However, if the transition is primarily of electronic origin and the structural distortion is only a secondary effect, the latter may, in principle, be engineered away by a clever materials design.…”

Section: Introductionmentioning

confidence: 89%

“…For a similar reason, it is difficult to differentiate the insulating phase of Ta 2 NiSe 5 from a trivial band insulator 18 – 38 . At its semimetal-to-insulator transition, Ta 2 NiSe 5 undergoes a simultaneous structural phase transition 39 , 40 from the high-temperature orthorhombic ( C m c m ) phase to the low-temperature monoclinic ( C 2/ c ) phase (Fig.…”

Section: Introductionmentioning

confidence: 99%

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Direct observation of excitonic instability in Ta2NiSe5

Kim

et al. 2021

Nat Commun

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Coulomb attraction between electrons and holes in a narrow-gap semiconductor or a semimetal is predicted to lead to an elusive phase of matter dubbed excitonic insulator. However, direct observation of such electronic instability remains extremely rare. Here, we report the observation of incipient divergence in the static excitonic susceptibility of the candidate material Ta2NiSe5 using Raman spectroscopy. Critical fluctuations of the excitonic order parameter give rise to quasi-elastic scattering of B2g symmetry, whose intensity grows inversely with temperature toward the Weiss temperature of TW ≈ 237 K, which is arrested by a structural phase transition driven by an acoustic phonon of the same symmetry at TC = 325 K. Concurrently, a B2g optical phonon becomes heavily damped to the extent that its trace is almost invisible around TC, which manifests a strong electron-phonon coupling that has obscured the identification of the low-temperature phase as an excitonic insulator for more than a decade. Our results unambiguously reveal the electronic origin of the phase transition.

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Section: Introductionmentioning

confidence: 89%

Section: Introductionmentioning

confidence: 99%

Direct observation of excitonic instability in Ta2NiSe5

Kim

et al. 2021

Nat Commun

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“…ARPES experiments revealed a flattening of the valence band when cooling below the transition temperature [11,12], which has been taken as indication of an EI state within a model simulation [13]. Motivated by this, various experiments have recently performed for this material, using NMR [14], inelastic xray scattering [15], scanning tunneling microscopy or spectroscopy [16,17], and time-and angle-resolved photoemission spectroscopy (t-ARPES) [18][19][20]. Besides there is the isostructual compound Ta 2 NiS 5 , which opens up the possibility to perform comparative measurements and analysis, not only of the photoemission [21] but also of the magnetic susceptibility [10] and the optical conductivity [22,23].…”

Section: Introductionmentioning

confidence: 99%

Finite-temperature photoemission in the extended Falicov-Kimball model: a case study for Ta$_2$NiSe$_5$

Lange

Fehske

2021

SciPost Phys.

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Utilizing the unbiased time-dependent density-matrix renormalization group technique, we examine the photoemission spectra in the extended Falicov-Kimball model at zero and finite temperatures, particularly with regard to the excitonic insulator state most likely observed in the quasi-one-dimensional material Ta_22NiSe_55. Working with infinite boundary conditions, we are able to simulate all dynamical correlation functions directly in the thermodynamic limit. For model parameters best suited for Ta_22NiSe_55 the photoemission spectra show a weak but clearly visible two-peak structure, around the Fermi momenta k\simeq\pm k_{F}k≃±kF, which suggests that Ta_22NiSe_55 develops an excitonic insulator of BCS-like type. At higher temperatures, the leakage of the conduction-electron band beyond the Fermi energy becomes distinct, which provides a possible explanation for the bare non-interacting band structure seen in time- and angle-resolved photoemission spectroscopy experiments.

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“…Impulsive laser excitation below T c has been shown to coherently excite at least five distinct Raman-active phonons with frequencies near 1, 2, 3, 3.7, and 4 THz. The 1, 2, and 4 THz modes are sensitive to the EI transition at T c [18,[28][29][30] and thus constitute the OPCPs, while the 3 and 3.7 THz modes are reportedly not coupled to the EI order parameter and thus serve as a control.…”

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confidence: 99%

Signatures of Ultrafast Reversal of Excitonic Order in Ta2NiSe5

et al. 2020

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In the presence of electron-phonon coupling, an excitonic insulator harbors two degenerate ground states described by an Ising-type order parameter. Starting from a microscopic Hamiltonian, we derive the equations of motion for the Ising order parameter in the phonon coupled excitonic insulator Ta 2 NiSe 5 and show that it can be controllably reversed on ultrashort timescales using appropriate laser pulse sequences. Using a combination of theory and time-resolved optical reflectivity measurements, we report evidence of such order parameter reversal in Ta 2 NiSe 5 based on the anomalous behavior of its coherently excited orderparameter-coupled phonons. Our Letter expands the field of ultrafast order parameter control beyond spin and charge ordered materials.

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Non-Coulomb strong electron-hole binding in Ta2NiSe5 revealed by time- and angle-resolved photoemission spectroscopy

Cited by 56 publications

References 41 publications

Direct observation of excitonic instability in Ta2NiSe5

Direct observation of excitonic instability in Ta2NiSe5

Finite-temperature photoemission in the extended Falicov-Kimball model: a case study for Ta$_2$NiSe$_5$

Signatures of Ultrafast Reversal of Excitonic Order in Ta2NiSe5

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