Berlin 2018 – scientific programme

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O: Fachverband Oberflächenphysik

O 101: Focus Session: Structural Dynamics in Nanoscale Materials, Probed by Ultrafast Electron Pulses II

O 101.2: Invited Talk

Thursday, March 15, 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 T_C = 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 F_C = 0.2 mJ cm². We also show that the electronic band gap can be optically controlled by tuning the excitation density. Below F_C, the band gap shrinks transiently due to photoenhanced screening of the Coulomb interaction. However, above F_C, 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.