Critical charge fluctuations and emergent coherence in a strongly correlated excitonic insulator

Abstract

Excitonic insulator is a coherent electronic phase that results from the formation of a macroscopic population of bound particle-hole pairs—excitons. With only a few candidate materials known, the collective excitonic behavior is challenging to observe, being obscured by crystalline lattice effects. Here we use polarization-resolved Raman spectroscopy to reveal the quadrupolar excitonic mode in the candidate zero-gap semiconductor Ta2NiSe5 disentangling it from the lattice phonons. The excitonic mode pronouncedly softens close to the phase transition, showing its electronic character, while its coupling to noncritical lattice modes is shown to enhance the transition temperature. On cooling, we observe the gradual emergence of coherent superpositions of band states at the correlated insulator gap edge, with strong departures from mean-field theory predictions. Our results demonstrate the realization of a strongly correlated excitonic state in an equilibrium bulk material.

Common microscopic origin of the phase transitions in Ta2NiS5 and the excitonic insulator candidate Ta2NiSe5

“…Further analysis of the phonon symmetries allows us to identify all three B 2g and eight A g phonon modes of the orthorhombic and all A g phonon modes of the monoclinic geometry. Their comparison with the phononic peak positions from recent experimental Raman measurements that are sensitive to B 2g and A g symmetry channel 33,56,58,59 shows a good agreement of all monoclinic A g modes, except for one, and all orthorhombic A g modes, even if our results are at T = 0. Only the first two B 2g modes, which are labelled 2 and 5 in the orthorhombic phase, and mode 5 in the monoclinic phase show a substantial energy difference between the calculated and the experimental results (see Figs.…”

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.…”

Fresnel-Floquet theory of light-induced terahertz reflectivity amplification in Ta2NiSe5

“…A number of state-of-the-art experiments have been performed to shed light on the origin of the phase transition at 326 K as well as what role the excitonic and lattice instabilities play in this phase transition process. However, the results show contradictory results [10,[13][14][15][16][17][18][22][23][24][25][26][27][28][29][30][31][32][33][34][35][36][37] and the question whether Ta 2 NiSe 5 hosts an excitonic insulator phase remains an open question. Using DFT frozen phonon calculations, we find that the electron-phonon coupling for the 4.7 THz phonon effectively vanishes in the high temperature orthorhombic phase.…”