Ultrafast melting and recovery of collective order in the excitonic insulator Ta2NiSe5

Abstract

The layered chalcogenide Ta2NiSe5 has been proposed to host an excitonic condensate in its ground state, a phase that could offer a unique platform to study and manipulate many-body states at room temperature. However, identifying the dominant microscopic contribution to the observed spontaneous symmetry breaking remains challenging, perpetuating the debate over the ground state properties. Here, using broadband ultrafast spectroscopy we investigate the out-of-equilibrium dynamics of Ta2NiSe5 and demonstrate that the transient reflectivity in the near-infrared range is connected to the system’s low-energy physics. We track the status of the ordered phase using this optical signature, establishing that high-fluence photoexcitations can suppress this order. From the sub-50 fs quenching timescale and the behaviour of the photoinduced coherent phonon modes, we conclude that electronic correlations provide a decisive contribution to the excitonic order formation. Our results pave the way towards the ultrafast control of an exciton condensate at room temperature.

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

“…In addition, for this type of excitation, we would expect to measure higher propagation velocities (>5 × 10 6 m/s) because of the light-like nature of these modes (see section S8). Last, the disappearance of the propagation above T c is unlikely to be explained by this scenario, as the observed phonon modes characterize both the low-and high-temperature phase of this material, which several recent works suggest being an insulator even above T c (25,37). We next consider the possibility of the propagation of QPs emanating from the excitation spot with velocity v k = (∂ϵ k /∂k) of the order of 10 5 m/s, which can excite phonons in their wake.…”

“…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.…”

Algorithm advances and applications of time‐dependent first‐principles simulations for ultrafast dynamics

“…The majority of experiments have reported the ultrafast melting and recovery of structures following photoexcitation 63–65,101 . However, the rt‐TDDFT methods used by most researchers do not satisfy the detailed balance of electron state transitions, and it cannot describe the hot carrier cooling process 30,31,98 .…”

“…Because of this, it cannot be used to describe hot‐carrier cooling 35–37 . This is problematic in dealing with many laser‐induced phenomena like phase transition 63–65 . The early stage of such phase transitions can be described without hot‐carrier cooling, but in order to yield the recovery of the system from the initial excitation, typically in the 200–1000 fs range, the hot‐carrier cooling becomes important 63–65 .…”

Third-harmonic generation in excitonic insulators

“…Recently, several transition-metal compounds, including TiSe 2 [7,[32][33][34], Ta 2 NiSe 5 [6,[35][36][37][38][39][40], and WTe 2 [8,41,42], are considered as candidates for the EIs. In particular, the origin of the ordered state in Ta 2 NiSe 5 are actively debated by the Raman and nonequilibrium pump-probe spectroscopies [43][44][45][46][47][48][49][50][51][52][53][54][55][56][57].…”