Seminars Archive
Ultrafast electronic and structural dynamics in an excitonic insulator
Department of Physical Chemistry, Fritz Haber Institute, Berlin, Germany
Abstract
Ultrafast control of matter phases is of both fundamental and technological interest. Here, we study the ultrafast electronic and structural dynamics of Ta2NiSe5 by means of time- and angle-resolved photoelectron spectroscopy (trARPES) [1] and transient mid-IR reflectivity measurements [2]. Ta2NiSe5 was proposed to undergo a semiconductor-to-excitonic-insulator (EI) phase transition below TC ≈ 328 K, combined with a structural change from an orthorhombic to a monoclinic symmetry [3]. Such an EI phase is expected to occur in small-band-gap semiconductors with strong electron-hole interaction as excitons can form spontaneously and condense into a ground state at a sufficiently low temperature [4].
Below TC, trARPES shows a strong fluence-dependent depopulation of the valence band at Γ, until the optical absorption saturates at a critical pump fluence Fsat = 0.2 mJ cm−2. This is reflected in a saturation of the mid-IR optical response at Fsat. Particularly, two coherent optical phonons at 2.1 and 4.0 THz, markers of the EI/monoclinic phase, persist above Fsat, indicating that the photoinduced structural phase transition is hindered by optical absorption saturation.
Upon tuning of the pump fluence, the electronic band gap can be either reduced or increased on an ultrafast timescale. Below Fsat, trARPES of both the occupied and unoccupied electronic band structure reveals a band gap shrinking due to photocarrier-induced screening of the Coulomb interaction. These dynamics are analogous to the behavior of a photoexcited semiconductor (photoinduced band gap renormalization). Conversely, above Fsat, trARPES of the valence band top at Γ indicates a band gap widening that persists up to approximately 1 ps. We find that this nontrivial out-of-equilibrium widening originates from the interplay of transient screening of the Coulomb interaction and photoenhancement of the EI condensate density, supported by theoretical calculations. Thus, our results prove it is possible to optically manipulate an exciton condensate and gain ultrafast control of its electronic band gap.
[1] Mor et al., Phys. Rev. Lett. 119, 086401 (2017)
[2] Mor et al., Phys. Rev. B 97, 115154 (2018)
[3] Wakisaka et al., Phys Rev. Lett. 103, 026402 (2009)
[4] Halperin and Rice, Rev. Mod. Phys. 40, 4, 755 (1968)
