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O: Fachverband Oberflächenphysik

O 75: Ultrafast Electron and Spin Dynamics

O 75.1: Vortrag

Mittwoch, 22. März 2017, 15:00–15:15, TRE Phy

Ultrafast electronic band gap control in an excitonic insulator — •Selene Mor¹, Marc Herzog², Denis Golez³, Philipp Werner³, Martin Eckstein⁴, Claude Monney⁵, and Julia Stähler¹ — ¹Fritz-Haber-Institut, Berlin, Germany — ²Potsdam Univ., Potsdam, Germany — ³Fribourg Univ., Fribourg, Switzerland — ⁴MPG for Structural Dynamics, Hamburg, Germany — ⁵Zurich Univ., Zurich, Switzerland

Ultrafast control of material properties is of both fundamental and technological interest. Here, we investigated the sub-picosecond dynamics of the electronic structure of Ta₂NiSe₅ by means of time- and angle-resolved photoelectron spectroscopy (trARPES). The system was proposed to support an excitonic insulator (EI) phase below T_c≈ 328 K. 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 with a larger gap. trARPES below T_c reveals a strong excitation-density-dependent valence band depopulation, until absorption saturates at a critical fluence F_C = 0.2 mJ cm⁻². Below F_C the band gap shrinks transiently due to photoenhanced screening of Coulomb interaction, while it widens above F_C for and relaxes after ≈ 1.5 ps. Hartree-Fock calculations reveal that the band gap widening can be explained by photoenhancement of the density of the exciton condensate, an effect that persists until the system undergoes interband relaxation. These results prove the possibility to manipulate condensates of excitons with light and to gain ultrafast control of the size of semiconductor band gaps.