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HL: Fachverband Halbleiterphysik

HL 43: Ultra-fast phenomena

HL 43.2: Vortrag

Mittwoch, 18. März 2015, 09:45–10:00, EW 202

Ultrafast coherent phonon dynamics during the phase transition of the quasi one-dimensional Ta₂NiSe₅ — •Selene Mor¹, Marc Herzog¹, Claude Monney^{1, 2}, Martin Wolf¹, and Julia Stähler¹ — ¹Fritz-Haber-Institut der MPG, Dep. of Phys. Chem., Berlin, Germany — ²Institut für Physik, University of Zürich, Switzerland

Ta₂NiSe₅ is a layered compound in which atomic chains are aligned in layers, forming a quasi one-dimensional crystal structure. At 328 K, the system shows a structural change accompanied by an electronic phase transition (PT) from a low temperature (LT) (potentially excitonic) insulator to a semiconductor. Our aim is to unveil the microscopic mechanisms underlying this PT by investigating the non-equilibrium dynamics after optical excitation. The system is excited with a femtosecond laser pulse at 800 nm and the mid-infrared (MIR) transient optical response is monitored by ultrafast optical spectroscopy. We observe a fast rise of transient reflectivity, which decays exponentially within 2-3 ps. This incoherent response is superimposed by several coherent phonon oscillations. In particular, we observe a 4 THz phonon that is specific of the LT phase, which is confirmed by temperature dependent Raman spectroscopy. The time evolution of this LT phase phonon reveals a finite lifetime of few ps. Furthermore, a non linear behavior is observed for both the coherent and incoherent response as a function of excitation fluence. The fluence and sample temperature dependence suggest that the PT can be driven by photoexcitation on an ultrafast timescale.