The spontaneous symmetry breaking in Ta$_2$NiSe$_5$ is structural in nature

Abstract

FIG. 2. Light-induced modification of the electronic structure. a,b Snapshots of the trARPES spectra along the kx momentum direction and for kz = 0 Å−1 . The data have been measured at 11 K with a probe photon energy of 10.75 eV and an absorbed pump fluence of 0.4 mJ/cm 2 . (a) Snapshot before photoexcitation (t < 0). At the Γ point of the Brillouin zone (kx = 0 Å−1 ), the flat anti-bonding VB is located around -0.16 eV, whereas the bonding VB appears around -0.65 eV. (b) Snapshot measured at the maximum of the pump-probe response (t = 0.3 ps). Upon photoexcitation, the VB is depleted in intensity and broadens significantly. Spectral weight is transfered above EF and accumulates close to Γ. c-f, Evolution of the photoexcited state (at t = 0.3 ps) along kx at representative kz momenta, as indicated in the labels. Note that the color scale is different from that of panels (a,b). The spectral weight above EF assumes a W-like shape consistent with the dispersion of the CB. The VB and CB never crosses each other and thus the gap size remains finite in the whole kx-kz momentum space around Γ. The white lines denote the energy-momentum dispersion calculated at the GW level (Fig. 1b). A rigid shift of -84 meV has been applied to the VB to account for the underestimated gap resulting from the GW method. The calculated dispersions have an excellent match with the experimental findings.

Failed excitonic quantum phase transition in Ta$_2$Ni(Se$_{1-x}$S$_x$)$_5$

“…EI results from a proliferation of excitons driven by Coulomb attraction between electrons and holes in a semiconductor or a semimetal [25][26][27][28][29]. Ta 2 NiSe 5 exhibits a phase transition at T c = 328 K; while the pronounced changes in band structure [18], transport [19] and optical [30] properties are consistent with the ones expected for an EI, they allow an alternative interpretation in terms of a purely structural phase transition [31][32][33]. Indeed, EI state in Ta 2 NiSe 5 is expected to break mirror symmetries of the lattice due to the distinct symmetries of the electron and hole states forming the exciton [34], similar to a structural transition [31].…”

Unveiling the underlying interactions in Ta2NiSe5 from photo-induced lifetime change

“…First, the sensitivity of the gap renormalization dynamics to the position in the BEC-BCS crossover may explain the apparent contradiction between theory and experiment in Fig. 3(k), as well as various TR-ARPES experiments which showed a wide range of responses, including gap reductions [39,40,57], gap enhancements [13] and rigid gap shifts [12]. Second, the contradiction between equilibrium ARPES results, suggesting a BCS nature of the ground state, and the transient gap enhancement observed in TRARPES remains, an open question.…”

“…In the semimetallic case, the phase transition is described as a binding of weakly interacting electron-hole pairs [5,7] in analogy to the binding of electron-electron pairs described in the Bardeen-Cooper-Schrieffer (BCS) theory of superconductivity. However, in solid state systems, an EI is typically strongly coupled to lattice degrees of freedom and the interplay between electronic and electron-phonon interactions is a recurrent question in the field [8][9][10][11][12][13][14].…”

“…Early equilibrium ARPES results on TNS were interpreted within a purely electronic picture [24,28,34], but subsequent additional experimental probes, including Raman [8,10,11,35] and optical spectroscopy [25,[36][37][38], have suggested a strong coupling between the EI and lattice distortions raising the question of the quantitative contribution of the electronic and lattice instabilities to the opening of the gap in TNS. To resolve this question, TR-ARPES experiments have focused on the ultrafast response of the electronic gap, reporting either a transient modulation of the gap amplitude (interpreted within an electronic picture [13,39,40]) or a rigid gap response (interpreted within a largely lattice-driven scenario [12]).…”

“…Theoretical developments have followed a similar trajectory, where initial studies have focused on a purely excitonic description that successfully reproduced the equilibrium ARPES spectrum of TNS [28,33] and optical absorption spectra [41]. However, subsequent studies have revealed the importance of the electron-lattice coupling [12,42,43], and the interplay between electronelectron and electron-phonon contributions is currently debated with nonlinear optical responses [25,38,44], collective dynamics [26,45], and transient protocols for the order enhancement [46][47][48][49] all discussed as highlighting either the electron-electron or electron-lattice contributions to the excitonic insulator state.…”