Regensburg 2016 – scientific programme

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O: Fachverband Oberflächenphysik

O 86: Ultrafast Surface Dynamics I

O 86.10: Talk

Thursday, March 10, 2016, 17:15–17:30, S052

Transient band gap enhancement of the photoexcited excitonic insulator phase in quasi-1D Ta₂NiSe₅ — •Selene Mor¹, Marc Herzog¹, Claude Monney², Martin Wolf¹, and Julia Staehler¹ — ¹Fritz-Haber-Institut der MPG, Dept. of Phys. Chem., Berlin, Germany — ²University of Zurich, Physics Dept., Switzerland

Strong electron-hole interaction in small gap semiconductors can lead to spontaneous formation of excitons resulting in an excitonic insulator (EI) phase. Ta₂NiSe₅ (TNS) has been proposed as candidate for such a phase transition (PT) in combination with a structural change at T_c≈ 328 K. In order to unveil how the monoclinic/EI phase stabilizes in TNS, we monitor the non-equilibrium dynamics after photoexcitation using time-resolved optical and photoemission spectroscopy. Time-resolved photoemission in the EI phase shows a strong excitation-density-dependent valence band depletion, until absorption saturation is reached at a critical fluence F_sat. This is also reflected in a saturation threshold of the overall transient optical response. A coherent phonon at 4 THz, which is specific of the monoclinic phase, persists above F_sat, indicative of a hindered photoinduced PT. Time-resolved photoemission below F_sat reveals a band gap shrinking due to photoenhanced screening of Coulomb interaction. However, above F_sat this process competes with a delayed band gap widening that we attribute to increased excitonic correlations. After ∼ 1.5 ps, excess energy is transferred to the lattice and the band gap shrinking is driven quasi-thermally. These complex dynamics support the key role of electron-hole correlations as origin of the EI phase in TNS.