Berlin 2018 – wissenschaftliches Programm
O 101: Focus Session: Structural Dynamics in Nanoscale Materials, Probed by Ultrafast Electron Pulses II
O 101.2: Hauptvortrag
Donnerstag, 15. März 2018, 15:30–16:00, HE 101
Ultrafast Electronic Band Gap Control and Self-Protection from a Photoinduced Phase Transition in an Excitonic Insulator — •Julia Stähler — Dept. of Physical Chemistry, Fritz Haber Institute Berlin, Faradayweg 4-6, 14195 Berlin
Ta2NiSe5 is proposed to support an excitonic insulator phase below TC = 328 K combined with a structural change. The former occurs in small gap semiconductors with strong electron-hole interaction where excitons form spontaneously and condense into a new insulating ground state. We study the ultrafast electron and lattice dynamics of Ta2NiSe5 by means of time- and angle-resolved photoemission spectroscopy (trARPES) and time-resolved coherent optical phonon spectroscopy. We find that the low temperature structural phase persists even for high excitation densities and the photoinduced structural phase transition is hindered by absorption saturation of excitation pulses at a fluence of FC = 0.2 mJ cm2. We also show that the electronic band gap can be optically controlled by tuning the excitation density. Below FC, the band gap shrinks transiently due to photoenhanced screening of the Coulomb interaction. However, above FC, the band gap transiently widens at the Gamma point 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 density, persisting until interband carrier relaxation occurs. These results demonstrate the possibility to manipulate exciton condensates with light and gain ultrafast band gap control.