Structural dynamics of 1T-TiSe2 using femtosecond electron diffraction

Suleiman, Aminat Oyiza (2014-12)

Thesis (MSc)--Stellenbosch University, 2014.

Thesis

ENGLISH ABSTRACT: Trilayered transition metal dichalcogenides such as our sample (1T-TiSe2) have been studied for many years as systems with strong electron-electron and electron-phonon correlations. The main attraction to this family of compound is its potential to exhibit ground state phenomena known as charge density waves whose detailed physical origin has been controversially determined. In this study, we have used an ultrafast femtosecond laser based on a pump-probe technique, namely ultrafast electron diffraction, to investigate these exotic features associated with the crystal. A pump laser pulse photo-excites the crystal from its ground state and the probe pulse (ultrashort electron pulse) takes the snapshot of the evolution of the lattice generating an electron diffraction pattern of the crystal. Hence the dynamical structural behaviour can be observed in time with a subpicosecond temporal resolution. As a hexagonal close-packed structure, its signature is expected to be seen in the diffraction pattern in both a steady-state and electron time-resolved femtosecond electron diffraction. In addition, simulations of electron diffractions pattern for room and low temperature structural data via a software called Simulation and Analysis of Electron diffraction (SAED) have been carried out. Clear signatures of charge density waves were seen at low temperature.

AFRIKAANSE OPSOMMING: Drie-laag oorgangsmetaal dikhalkogeniedes soos ons voorbeeld (1T-TiSe2), word reeds vir baie jare bestudeer as sisteme met sterk elektron-elektron en elektron-fonon korrelasies. Die hoof aantrekkingskrag van hierdie sisteme is die verskynsel van ladingdigtheidsgolwe in die grondtoestand. Die fisiese oorsprong van hierdie ladingdigtheidsgolwe was bepaal te midde van verskeie teenstrydighede. In hierdie studie, maak ons gebruik van die ultravinnige femtosekonde laser gebaseerde aktiveer-interogeer tegniek, genaamd ultravinnige elektron diffraksie (UED) om unicke eienskappe wat met die kristal geassosieer is te bestudeer. In UED wek ’n ultravinnige laserpuls (aktivering) die kristal op vanaf die grondtoestand waartydens n ultravinnige elektronpuls (interogering) ’n foto neem van die evolusie van die elektron diffraksiepatroon wat deur die kristalrooster gegenereer word. Hierdie wisselwerking van die interogerings elektronpuls en die sisteem kan gevolglik teen verskeie vasgetelde tye toegelaat word. Dus kan die dinamiese strukturele gedrag waargeneem word met ’n tydresolusie in die orde van die elektronpuls (sub-pikosekondes). Siende dat die kristal ’n diggepakte-heksagonale struktuur vorm, behoort die kenmerkende diffraksiepatroon daarvan waarneembaar te wees in beide die bestendige diffraksie en femtosekonde elektron diffraksie tegnieke. In hierdie konteks was duidelike tekens van ladingdigtheidsgolwe waargeneem. Benewens was daar ook simulasies uitgevoer om die elektron diffraksiepatrone asook die strukturele data by kamer en lae temperature vas te pen. Die sagteware wat hiervoor gebruik word is genaamd Simulasie en Ontleding van Elektronendiffraksie (SAED) - Simulation and Analysis of Electron Diffraction (SAED).

Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/95990
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