Ultrafast electronic band gap control and inhibition of the photoinduced structural phase transition in an excitonic insulator - LACUS Seminar

Ultrafast control of matter phases is of both fundamental and technological interest. Here, we study the ultrafast dynamics of Ta 2 NiSe 5 by means of time- and angle-resolved photoelectron spectroscopy (trARPES) [1] and transient reflectivity measurements [2]. Ta 2 NiSe 5 is proposed to support an excitonic insulator (EI) phase below T C ≈ 328 K, combined with a structural change from orthorombic to monoclinic symmetry. Such an EI phase is expected to occur in small gap semiconductors with strong electron-hole interaction as excitons can form spontaneously and condense into a ground state. Below T C, trARPES around  shows a strong fluence-dependent valence band depopulation, until absorption saturates at a critical fluence F sat = 0.2 mJ cm −2 . This is reflected in a saturation of the mid-IR optical response at F sat . A coherent phonon at 4 THz, marker of the EI/monoclinic phase, persists above T C , indicating that the photoinduced structural transition is hindered by pump absorption saturation. trARPES shows that below F sat the band gap shrinks transiently due to photoenhanced screening of the Coulomb interaction, while above F sat it widens and recovers to its equilibrium value after ~1.5 ps. Hartree-Fock calculations reveal that the band gap widening is due to photoenhancement of the exciton condensate, persisting until the system undergoes interband relaxation. Our results prove it is possible to manipulate exciton condensates optically and gain ultrafast control of semiconductor band gaps.

[1] Mor et al., Phys. Rev. Lett. 119, 086401 (2017).