Zero-gap semiconductor to excitonic insulator transition in Ta2NiSe5

Lu, Yangfan; Kōno, Hiroshi; Larkin, T. I.; Rost, Andreas W.; Takayama, T.; Boris, A. V.; Keimer, B.; Takagi, H.

doi:10.1038/ncomms14408

Cited by 256 publications

(325 citation statements)

References 22 publications

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“…(In contrast, the amplitude oscillations in the BCS regime decay as 1=t 0.5 [33], which is also consistent with the corresponding predictions for SC [41][42][43][44]. Therefore, the existence of prominent amplitude oscillations with the frequency of the gap and a decay ∼1=t 0.5 may be used to distinguish the BCS from the BEC nature of the system [21]. )…”

supporting

confidence: 73%

“…An EI state is formed by the macroscopic condensation of electron-hole pairs [13,14], and its theoretical description is analogous to the BCS or Bose-Einstein condensate (BEC) theory for SC. Although the idea of exciton condensation was proposed already in the 1960s [13][14][15], the interest in this topic has been renewed recently by the study of some candidate materials such as 1T-TiSe 2 and Ta 2 NiSe 5 (TNS) [16][17][18][19][20][21]. Their analogy to SC makes the EI an interesting system for nonequilibrium studies.…”

mentioning

confidence: 99%

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Photoinduced Enhancement of Excitonic Order

et al. 2017

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We study the dynamics of excitonic insulators coupled to phonons using the time-dependent mean-field theory. Without phonon couplings, the linear response is given by the damped amplitude oscillations of the order parameter with a frequency equal to the minimum band gap. A phonon coupling to the interband transfer integral induces two types of long-lived collective oscillations of the amplitude, one originating from the phonon dynamics and the other from the phase mode, which becomes massive. We show that, even for small phonon coupling, a photoinduced enhancement of the exciton condensation and the gap can be realized. Using the Anderson pseudospin picture, we argue that the origin of the enhancement is a cooperative effect of the massive phase mode and the Hartree shift induced by the photoexcitation. We also discuss how the enhancement of the order and the collective modes can be observed with time-resolved photoemission spectroscopy. [7][8][9][10][11][12]. An EI state is formed by the macroscopic condensation of electron-hole pairs [13,14], and its theoretical description is analogous to the BCS or Bose-Einstein condensate (BEC) theory for SC. Although the idea of exciton condensation was proposed already in the 1960s [13][14][15], the interest in this topic has been renewed recently by the study of some candidate materials such as 1T-TiSe 2 and Ta 2 NiSe 5 (TNS) [16][17][18][19][20][21]. Their analogy to SC makes the EI an interesting system for nonequilibrium studies. In particular, for TNS, time-and angle-resolved photoemission spectroscopy (trARPES) experiments showed that the direct band gap can be either decreased or increased depending on the pump fluence [11], which was interpreted in terms of a photoinduced enhancement or suppression of excitonic order. A more recent report of the amplitude mode [12] provides further confirmation of an EI state in TNS.So far, the theoretical works on EIs have mainly focused on the equilibrium properties such as the BEC-BCS crossover [22,23], the coupling of the EI to phonons [24][25][26], linear susceptibilities [27][28][29], and the effect of strong interactions and new emergent phases [30][31][32]. In contrast, the nonequilibrium investigation of EIs has just begun [10]. In this work, using TNS as a model system, we clarify two basic effects of the electron-phonon (e-ph) coupling on the dynamics of EIs: (i) the effect on collective modes and (ii) the impact on the excitonic order after

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confidence: 99%

Photoinduced Enhancement of Excitonic Order

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“…An angle-resolve photoemission (ARPES) study on Ta 2 NiSe 5 showed that the top of the valence band at the Γ-point flattened at temperatures below its structural transition temperature ( = 325 K), and this flat band was interpreted as an excitonic insulating ground state of condensed electron-hole pairs of Ta 5 d -electrons and Ni 3 d - and Se 4 p -holes 9 . A recent study also shows that Ta 2 NiSe 5 is a zero-gap semiconductor, with a transitions to an EI occurring near 326 K (referred as the onset temperature ()) 15 . There was another very recent ellipsometry spectroscopic study on Ta 2 NiSe 5 16 ; the authors claimed that exciton-phonon complexes in Ta 2 NiS 5 and Ta 2 NiSe 5 are confirmed and their observation agrees with the hypothesis of an excitonic insulator ground state.…”

Section: Introductionmentioning

confidence: 97%

Temperature-dependent excitonic superfluid plasma frequency evolution in an excitonic insulator, Ta2NiSe5

Seo¹,

Eom²,

Kim³

et al. 2018

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An interesting van der Waals material, Ta2NiSe5 has been known one of strong excitonic insulator candidates since it has very small or zero bandgap and can have a strong exciton binding energy because of its quasi-one-dimensional crystal structure. Here we investigate a single crystal Ta2NiSe5 using optical spectroscopy. Ta2NiSe5 has quasi-one-dimensional chains along the a-axis. We have obtained anisotropic optical properties of a single crystal Ta2NiSe5 along the a- and c-axes. The measured a- and c-axis optical conductivities exhibit large anisotropic electronic and phononic properties. With regard to the a-axis optical conductivity, a sharp peak near 3050 cm−1 at 9 K, with a well-defined optical gap ( 1800 cm−1) and a strong temperature-dependence, is observed. With an increase in temperature, this peak broadens and the optical energy gap closes around ∼325 K (). The spectral weight redistribution with respect to the frequency and temperature indicates that the normalized optical energy gap () is . The temperature-dependent superfluid plasma frequency of the excitonic condensation in Ta2NiSe5 has been determined from measured optical data. Our study may pave new avenues in the future research on excitonic insulators.

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“…One of the highlights is a set of experiments on semimetals, including TiSe 2 and Ta 2 NiSe 5 , which undergo phase transitions into insulating ground states upon cooling [30][31][32] . Experimental evidence suggests that this transition is induced by the formation of excitons due to Coulomb attraction between electrons and holes in small pockets near the Fermi level.…”

Section: Excitonic Insulatorsmentioning

confidence: 99%

The physics of quantum materials

2017

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The physical description of all materials is rooted in quantum mechanics, which describes how atoms bond and electrons interact at a fundamental level. Although these quantum e ects can in many cases be approximated by a classical description at the macroscopic level, in recent years there has been growing interest in material systems where quantum e ects remain manifest over a wider range of energy and length scales. Such quantum materials include superconductors, graphene, topological insulators, Weyl semimetals, quantum spin liquids, and spin ices. Many of them derive their properties from reduced dimensionality, in particular from confinement of electrons to two-dimensional sheets. Moreover, they tend to be materials in which electrons cannot be considered as independent particles but interact strongly and give rise to collective excitations known as quasiparticles. In all cases, however, quantum-mechanical e ects fundamentally alter properties of the material. This Review surveys the electronic properties of quantum materials through the prism of the electron wavefunction, and examines how its entanglement and topology give rise to a rich variety of quantum states and phases; these are less classically describable than conventional ordered states also driven by quantum mechanics, such as ferromagnetism.T he way we think about manifestations of quantum physics in materials has recently undergone a profound change of perspective. Although materials scientists and engineers have long exploited quantum effects in a range of electronic deviceswell-known examples are the quantized electronic energy levels and optical selection rules at the heart of optoelectronics, and the tunnel effect that underlies the upcoming generation of harddisk drives 1,2 -the past decade has seen a dramatic increase in our understanding of how subtle quantum effects control the macroscopic behaviour of a whole range of different materials.Two strange and beautiful aspects of quantum mechanics have come to the fore. One is the topological nature of quantum wavefunctions. A familiar example is the existence of quantized vortices in superconductors. These vortices exist because of the requirement that the superconducting condensate have a well-defined phase, and gauge invariance fixes how this phase couples to magnetic flux. The phase can wind only by an integer multiple of 2π around a vortex, and this integer winding number is a simple example of a topological invariant: a quantity that remains fixed under smooth changes of a system. Similar topological quantities turn out to govern many other kinds of materials, not just superconductors, and these support phenomena ranging from dissipationless transport to novel quasiparticle excitations.Another deep feature of quantum mechanics is the non-local entanglement of some quantum states that is spectacularly highlighted in teleportation experiments with two photons separated over macroscopic distances 3 . Even the wavefunction of two spins in a singlet is entangled, in that the wavefunction of...

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Zero-gap semiconductor to excitonic insulator transition in Ta2NiSe5

Cited by 256 publications

References 22 publications

Photoinduced Enhancement of Excitonic Order

Photoinduced Enhancement of Excitonic Order

Temperature-dependent excitonic superfluid plasma frequency evolution in an excitonic insulator, Ta2NiSe5

The physics of quantum materials

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