Probing the exciton condensate phase in 1<i>T</i>-TiSe<sub>2</sub>with photoemission

Monney, Claude; Schwier, Eike F.; Garnier, M. G.; Mariotti, N.; Didiot, Clément; Cercellier, H.; Marcus, J.; Berger, H.; Titov, A. N.; Beck, H. P.; Aebi, Philipp

doi:10.1088/1367-2630/12/12/125019

Cited by 61 publications

(59 citation statements)

References 36 publications

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“…As a candidate for EI, quasi-one-dimensional 1T -TiSe 2 has been studied both theoretically and experimentally. 4,5 TaNi 2 Se 5 has also been studied by angle-resolved photoemission spectroscopy (ARPES) measurements. It was reported that the valence-band top is extremely flat, and the material can be a new candidate for an EI of bound pairs between Ni 3d-Se 4p holes and Ta 5d electrons.…”

Section: Introductionmentioning

confidence: 99%

BCS-BEC crossover in the extended Falicov-Kimball model: Variational cluster approach

2011

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We study the spontaneous symmetry breaking of the excitonic insulator state induced by the Coulomb interaction U in the two-dimensional extended Falicov-Kimball model. Using the variational cluster approximation (VCA) and Hartree-Fock approximation (HFA), we evaluate the order parameter, single-particle excitation gap, momentum distribution functions, coherence length of excitons, and single-particle and anomalous excitation spectra, as a function of U at zero temperature. We find that in the weak-to-intermediate coupling regime, the Fermi surface plays an essential role and calculated results can be understood in close correspondence with the BCS theory, whereas in the strong-coupling regime, the Fermi surface plays no role and results are consistent with the picture of BEC. Moreover, we find that HFA works well both in the weak-and strong-coupling regime, and that the difference between the results of VCA and HFA mostly appears in the intermediate-coupling regime. The reason for this is discussed from a viewpoint of the self-energy. We thereby clarify the excitonic insulator state that typifies either a BCS condensate of electron-hole pairs (weak-coupling regime) or a Bose-Einstein condensate of preformed excitons (strong-coupling regime).

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Section: Introductionmentioning

confidence: 99%

BCS-BEC crossover in the extended Falicov-Kimball model: Variational cluster approach

2011

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“…An EI state in semiconducting Ta 2 NiSe 5 was recently probed by photoemission 9 . On the other hand, in the layered transition-metal dichalcogenide 1T -TiSe 2 , which is a SM, a BCS-like electron-hole pairing was considered as the driving force for the periodic lattice distortion 10 . Here evidence suggests electron-hole 'Cooper-pair' fluctuations above the SM-EI transition temperature.…”

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confidence: 99%

Electron-hole pair condensation at the semimetal-semiconductor transition: A BCS-BEC crossover scenario

et al. 2012

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We act on the suggestion that an excitonic insulator state might separate-at very low temperatures-a semimetal from a semiconductor and ask for the nature of these transitions. Based on the analysis of electron-hole pairing in the extended Falicov-Kimball model, we show that tuning the Coulomb attraction between both species, a continuous crossover between a BCS-like transition of Cooper-type pairs and a Bose-Einstein condensation of preformed tightly-bound excitons might be achieved in a solid-state system. The precursor of this crossover in the normal state might cause the transport anomalies observed in several strongly correlated mixed-valence compounds.PACS numbers: 71.30.+h, 71.35.Lk The challenging suggestion of electron-hole pair condensation in thermal equilibrium into the excitonic insulator (EI) phase at the semimetal (SM) to semiconductor (SC) transition 1 , where the SM-EI transition may be described in analogy with BCS theory of superconductivity and the SC-EI transition is discussed in terms of a BoseEinstein condensation (BEC) of preformed excitons 2-4 , is of topical interest. This is due to the growing amount of experimental data on materials which are candidates for the realization of the EI, where different situations with respect to the SM/SC-EI transition are given. For example, in the rare-earth chalcogenide TmSe 0.45 Te 0.55 , that is, an intermediate-valent SC, the pressure-induced resistivity anomaly at low temperatures was ascribed to exciton formation and a subsequent SC-EI transition 5-8 . An EI state in semiconducting Ta 2 NiSe 5 was recently probed by photoemission 9 . On the other hand, in the layered transition-metal dichalcogenide 1T -TiSe 2 , which is a SM, a BCS-like electron-hole pairing was considered as the driving force for the periodic lattice distortion 10 . Here evidence suggests electron-hole 'Cooper-pair' fluctuations above the SM-EI transition temperature. A BCSlike electron-hole pair condensation was also studied for graphene bilayers 11 . In this system a BCS-BEC crossover might be realized by a magnetic field that creates a gap and magneto-excitons which may condense. From a theoretical point of view, one of the main issues in this field is the better understanding and a detailed description of the normal phase above the SM/SC-EI transition, especially of the electron-hole pair fluctuations and of the BCS-BEC crossover scenario 12 that characterizes the EI instability and has not been observed in a solid so far.In this Rapid Communication we address this topic and the mechanisms behind in terms of a minimal two-band model, the so-called extended Falicov-Kimball model (EFKM) 3,13,14 which covers direct c-and f -band hopping and admits the pairing of c electrons with f holes via a strongly screened Coulomb interaction. Thereby we focus on the normal phase that surrounds the EI and look for precursor effects in the electron-hole pair susceptibility. In particular we analyze the nature of the electronhole bound states and determine their number and spectral weight....

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“…For these reasons the EC remains an elusive phase of matter [1,2,4]. Possible SM candidates suggested to undergo a transition to the excitonic insulating phase with an EC are transition-metal dichalcogenide TiSe 2 [5,6] and HgTe quantum well with a thickness of 20 nm [7][8][9]. However, there is no conclusive evidence for an EC in such systems.…”

Section: Transport Properties Of An Electron-hole Bilayer In Contact mentioning

confidence: 99%

Transport Properties of an Electron-Hole Bilayer in Contact with a Superconductor Hybrid Junction

2017

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We investigate the transport properties of a junction consisting of an electron-hole bilayer in contact with normal and superconducting leads. The electron-hole bilayer is considered as a semimetal with two electronic bands. We assume that in the region between the contacts the system hosts an exciton condensate described by a BCS-like model with a gap Γ in the quasiparticle density of states. We first discuss how the subgap electronic transport through the junction is mainly governed by the interplay between two kinds of reflection processes at the interfaces: the standard Andreev reflection at the interface between the superconductor and the exciton condensate, and a coherent crossed reflection at the semimetal-excitoncondensate interface that converts electrons from one layer into the other. We show that the differential conductance of the junction shows a minimum at voltages of the order of Γ=e. Such a minimum can be seen as a direct hallmark of the existence of the gapped excitonic state.

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Probing the exciton condensate phase in 1T-TiSe₂with photoemission

Cited by 61 publications

References 36 publications

BCS-BEC crossover in the extended Falicov-Kimball model: Variational cluster approach

BCS-BEC crossover in the extended Falicov-Kimball model: Variational cluster approach

Electron-hole pair condensation at the semimetal-semiconductor transition: A BCS-BEC crossover scenario

Transport Properties of an Electron-Hole Bilayer in Contact with a Superconductor Hybrid Junction

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Probing the exciton condensate phase in 1T-TiSe2with photoemission

Cited by 61 publications

References 36 publications

BCS-BEC crossover in the extended Falicov-Kimball model: Variational cluster approach

BCS-BEC crossover in the extended Falicov-Kimball model: Variational cluster approach

Electron-hole pair condensation at the semimetal-semiconductor transition: A BCS-BEC crossover scenario

Transport Properties of an Electron-Hole Bilayer in Contact with a Superconductor Hybrid Junction

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Probing the exciton condensate phase in 1T-TiSe₂with photoemission