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.

Complex correlated states emerging from many-body interactions between quasiparticles (electrons, excitons and phonons) are at the core of condensed matter physics and material science. In low-dimensional materials, quantum confinement affects the electronic, and subsequently, optical properties for these correlated states. Here, by combining photoluminescence, optical reflection measurements and ab initio theoretical calculations, we demonstrate an unconventional excitonic state and its bound phonon sideband in layered silicon diphosphide (SiP2), where the bound electron–hole pair is composed of electrons confined within one-dimensional phosphorus–phosphorus chains and holes extended in two-dimensional SiP2 layers. The excitonic state and emergent phonon sideband show linear dichroism and large energy redshifts with increasing temperature. Our ab initio many-body calculations confirm that the observed phonon sideband results from the correlated interaction between excitons and optical phonons. With these results, we propose layered SiP2 as a platform for the study of excitonic physics and many-particle effects.

The excitonic insulator is an electronically driven phase of matter that emerges upon the spontaneous formation and Bose condensation of excitons. Detecting this exotic order in candidate materials is a subject of paramount importance, as the size of the excitonic gap in the band structure establishes the potential of this collective state for superfluid energy transport. However, the identification of this phase in real solids is hindered by the coexistence of a structural order parameter with the same symmetry as the excitonic order. Only a few materials are currently believed to host a dominant excitonic phase, Ta 2 NiSe 5 being the most promising. Here, we test this scenario by using an ultrashort laser pulse to quench the broken-symmetry phase of this transition metal chalcogenide. Tracking the dynamics of the material’s electronic and crystal structure after light excitation reveals spectroscopic fingerprints that are compatible only with a primary order parameter of phononic nature. We rationalize our findings through state-of-the-art calculations, confirming that the structural order accounts for most of the gap opening. Our results suggest that the spontaneous symmetry breaking in Ta 2 NiSe 5 is mostly of structural character, hampering the possibility to realize quasi-dissipationless energy transport.