Direct observation of excitonic instability in Ta2NiSe5

Abstract

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.

Imaging the coherent propagation of collective modes in the excitonic insulator Ta 2 NiSe 5 at room temperature

“…The reduced dimensionality of TMCs results in weak screening of the Coulomb interaction and consequently large exciton binding energies, which could lead to condensation at noncryogenic temperatures. Among this family of materials, particular attention has been devoted to Ta 2 NiSe 5 following hints of the potential existence of an EI phase below 328 K (8)(9)(10)(12)(13)(14)(15)(16)(17)(18)(19)(20)(21)(22). These works have provided insights into the properties in Ta 2 NiSe 5 , for instance, by using equilibrium (8,12) and time-resolved (9,17,18,21,23) angle-resolved photoemission spectroscopy (ARPES), by analyzing the effect of physical and chemical pressure (14,22), by doping (16), and by detecting anomalies in charge transport (14), optical signatures (24,25), and phonon properties (10,15,19,20,22,26) as a function of temperature.…”

Lattice fluctuation induced pseudogap in quasi-one-dimensional Ta2NiSe5

“…While we qualitatively explain the observed spectral behaviours using a single phonon mode, further experiments have indicated multiple phonons involved in this transition: the diffuse scattering and inelastic x-ray scattering results point to extensive transverse acoustic phonon softening 38 ; Raman scattering also suggest strong involvement of optical phonons [20][21][22] . Distinct from the acoustic-phonon-driven structural transition in 3D perovskite PrAlO3 45 , the strong fluctuations in Ta2NiSe5 seem to suggest additional contributions from the lower system dimension and optical phonons.…”

“…In the pursuit of a potential room-temperature Bose-Einstein condensate, recent studies revealed evidence for exciton formation, strong lattice instability and electron-phonon coupling in quasi-1D ternary chalcogenide Ta2NiSe5 [19][20][21][22] . Upon warming, a q = 0 monoclinic-to-orthorhombic structural transition happens at Ts~329K, above which a semi-metallic electronic structure is supposedly restored and protected by the mirror symmetry of Ta chains about the Ni chain 23 .…”

“…Crossing the transition, the Ta atoms slightly shear along the chain direction, resulting in an increase of the β angle from 90° to 90.53° (as exaggerated in Fig 1(c)) 19 . The second order nature of the transition is indicated by the gradual separation of |Q( 1 38 and anharmonic optical modes [20][21][22] .…”

“…Therefore, other phonon modes are decoupled from electrons and can be ignored. The phonon frequency is chosen to be 2THz [20][21][22] . To generate the simulations in Fig.…”