Probing Charge Density Wave Dynamics in TiSe₂ Using Ultrafast Electron Diffraction

Paul Xhori

doi:10.17918/00000891

The origin of the charge density wave (CDW) state in TiSe₂ has been under intense scrutiny since its discovery in 1976. Theoretical and experimental evidence of the origin of the CDW has been inconclusive, with findings in favor of either correlated electron effects such as the excitonic insulator mechanism, or conventional electron-phonon coupling mechanism, or the Jahn-Teller effect. The recent discovery of superconductivity upon application of pressure or doping with copper has renewed the interest in the material, as has the discovery of the chiral nature of the CDW, neither of which have an accepted theoretical explanation. We have used an ultrafast electron diffraction setup, that has been developed over the course of this thesis, to observe the ultrafast relaxation dynamics in thin (~50 nm) single crystals of TiSe₂. The dynamics are initiated by a 100 fs laser pump pulses that almost instantaneously excite the electronic subsystem and create a non-equilibrium state with highly excited electrons in the presence of cold ionic lattice. As the energetic electrons interact with ionic lattice of the crystal, the ultrafast electron diffraction method is used to monitor the electron-phonon relaxation dynamics. Both incoherent processes as well as the coherent breathing modes initiated along the van der Waals bonded layers are studied with time resolution of ~100 fs over several hundreds of picoseconds. The complex lattice dynamics are initiated by the photoinduced strain that originates from combination of thermoelastic effects due to lattice heating, displacive forces related to the sudden modification of electronic screening in the crystal, and non-thermal phase transitions initiated in the material. The above processes contribute to the complex relaxation dynamics observed through the changes in the instantaneous electron diffraction pattern. The dependence of the photoinduced strain on the pump laser fluence is monitored on picosecond timescale at low temperatures (TiSe₂ in CDW phase) and at room temperature (TiSe₂ in normal phase). The experimentally observed chirality of the CDW can be explained by the existence of a metastable chiral phase that can be accessed within a certain range of electronic temperatures. Our observed strain dynamics correlates with recent density functional theory (DFT) calculations. The DFT results reproduce the experimental findings without the need to invoke correlated electron effects such as the excitonic insulator condensate. Our results suggest that the chiral nature of the CDW as well as the observed non-thermal melting of the CDW under ultrafast laser excitation, could be explained solely through the modification of the electronic susceptibility at elevated temperatures. The findings in this work will stimulate further study of the chiral nature of charge ordering with possible future application of these phenomena in electronic devices and systems.

Probing Charge Density Wave Dynamics in TiSe₂ Using Ultrafast Electron Diffraction

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Probing Charge Density Wave Dynamics in TiSe₂ Using Ultrafast Electron Diffraction

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