Electronic excitation and atomic forces in optically excited group V semimetals

O'Mahony, Shane M.

Electronic excitation and atomic forces in optically excited group V semimetals

dc.availability.bitstream	openaccess
dc.check.chapterOfThesis	NA	en
dc.contributor.advisor	Fahy, Stephen B.	en
dc.contributor.advisor	Murphy-Armando, Felipe	en
dc.contributor.author	O'Mahony, Shane M.
dc.contributor.funder	Irish Research Council	en
dc.contributor.funder	Science Foundation Ireland	en
dc.date.accessioned	2020-09-25T11:13:10Z
dc.date.available	2020-09-25T11:13:10Z
dc.date.issued	2020-09-02
dc.date.submitted	2020-09-02
dc.description.abstract	The main focus of this work is to investigate how electronic excitation changes the forces between atoms in materials, causing large amplitude collective atomic motion, referred to as coherent phonon motion. The recent development of ultrafast optical spectroscopy has greatly advanced our understanding of these phenomena. However, many aspects of ultrafast phenomena are still poorly understood. For instance, experiments on a variety of different materials have shown that symmetry-breaking coherent modes have a much smaller amplitude than fully symmetric coherent modes. Recent experimental work in the group V semimetals have shown that the symmetry-breaking $E_g$ mode is driven by a force which decays in $\sim 10$ fs, a timescale much shorter than the period of the mode, explaining its small amplitude. However, the exact mechanism for this decay has remained unclear. We show that the $E_g$ force in optically excited group V semimetals decays due to coupling of the excited electrons to thermal vibrations. We propose a method for increasing the impact of photoexcitation on symmetry-breaking modes by a suitable choice of photon energy, which would apply to a variety of materials. We are also interested in the dynamics of the coherent phonons themselves. Recently, it has been shown that optical excitation can drive collective structural transformations to phases that do not exist on the equilibrium phase diagram of materials as diverse as transition metal oxides, transition metal chalcogenides, and the group V semimetals. Experimentally reaching many of these states requires a high degree of photoexcitation, with a significant fraction ($1$-$5$\%) of the valence electrons excited to the conduction bands. We use first-principles calculations to simulate coherent phonon dynamics in Sb, and show that at high enough excited carrier concentration, Sb undergoes a reversal of its Peierls distortion. Our results are in good agreement with pump-probe experiments carried out by our experimental collaborators, providing good evidence that such a reversal can be induced by optical excitation.	en
dc.description.status	Not peer reviewed	en
dc.description.version	Accepted Version	en
dc.format.mimetype	application/pdf	en
dc.identifier.citation	O'Mahony, S. M. 2020. Electronic excitation and atomic forces in optically excited group V semimetals. PhD Thesis, University College Cork.	en
dc.identifier.endpage	150	en
dc.identifier.uri	https://hdl.handle.net/10468/10587
dc.language.iso	en	en
dc.publisher	University College Cork	en
dc.relation.project	Irish Research Council (Grant number GOIPG/2015/2784)	en
dc.rights	© 2020, Shane M. O'Mahony.	en
dc.rights.uri	https://creativecommons.org/licenses/by-nc-nd/4.0/	en
dc.subject	Ultrafast excitation	en
dc.subject	Electronic structure	en
dc.subject	Phonons	en
dc.subject	Relaxation	en
dc.subject	Symmetry	en
dc.subject	Group V semimetals	en
dc.title	Electronic excitation and atomic forces in optically excited group V semimetals	en
dc.type	Doctoral thesis	en
dc.type.qualificationlevel	Doctoral	en
dc.type.qualificationname	PhD - Doctor of Philosophy	en