BEGIN:VCALENDAR
VERSION:2.0
PRODID:-//RLASKEY//CALENDEROUS//EN
CALSCALE:GREGORIAN
METHOD:PUBLISH
BEGIN:VEVENT
DTSTAMP:20261021T115516Z
LAST-MODIFIED:20150317T183811Z
DTSTART:20150408T170000Z
DTEND:20150408T180000Z
UID:event1427@bu.edu
URL:http://physics.bu.edu/internal/events/show/1427
SUMMARY:Measuring\, Interpreting and Translating Electron Quasiparticle-pho
	non Interactions on the Surfaces of the Topological Insulators Bismuth Sele
	nide and Bismuth Telluride
DESCRIPTION:Featuring Colin Howard\n\nPart of the PhD Final Oral Exams.\n\n
	Examining Committee: Michael El-Batanouny\, Claudio Chamon\, Karl Ludwig\, 
	Robert Carey\, William Skocpol\n\nABSTRACT\n\nThe following dissertation pr
	esents a comprehensive study of the interaction between Dirac fermion quasi
	particles (DFQs) and surface phonons on the surfaces of the topological ins
	ulators Bi$_2$Se$_3$ and Bi$_2$Te$_3$. Inelastic helium atom surface scatte
	ring (HASS) spectroscopy and time of flight (TOF) techniques were used to m
	easure the surface phonon dispersion of these materials along the two high-
	symmetry directions of the surface Brillouin zone (SBZ). Two anomalies comm
	on to both materials are exhibited in the experimental data. First\, there 
	is an absence of Rayleigh acoustic waves on the surface of these materials\
	, pointing to weak coupling between the surface charge density and the surf
	ace acoustic phonon modes and potential applications for soundproofing tech
	nologies. Secondly\, both materials exhibit an out-of-plane polarized optic
	al phonon mode beginning at the SBZ center and dispersing to lower energy w
	ith increasing wave vector along both high-symmetry directions of the SBZ. 
	 This trend terminates in a V-shaped minimum at a wave vector corresponding
	 to 2$k_F$ for each material\, after which the dispersion resumes its upwar
	d trend. This phenomenon constitutes a strong Kohn anomaly and can be attri
	buted to the interaction between the surface phonons and DFQs.\n\nTo quanti
	fy the coupling between the optical phonons experiencing strong renormaliza
	tion and the DFQs at the surface\, a phenomenological model was constructed
	 based within the random phase approximation. Fitting the theoretical model
	 to the experimental data allowed for the extraction of the matrix elements
	 of the coupling Hamiltonian and the modifications to the surface phonon pr
	opagator encoded in the phonon self energy. This allowed\, for the first ti
	me\, calculation of phonon mode-specific quasiparticle-phonon coupling $\la
	mbda_\nu({\bf q})$ from experimental data. Additionally\, an averaged coupl
	ing parameter was determined for both materials yielding $\bar\lambda^{\tex
	t{Te}} \approx 2$ and $\bar\lambda^{\text{Se}} \approx 0.7$. These values a
	re significantly higher than those of typical metals\, underscoring the str
	ong coupling between optical surface phonons and DFQs in topological insula
	tors.\n\nIn an effort to connect experimental results obtained from phonon 
	and photoemission spectroscopies\, a computational process for taking coupl
	ing information from the phonon perspective and translating it to the DFQ p
	erspective was derived. The procedure involves using information obtained f
	rom HASS measurements (namely the coupling matrix elements and optical phon
	on dispersion) as input to a Matsubara Green function formalism\, from whic
	h one can obtain the real and imaginary parts of the DFQ self energy. With 
	these at hand it is possible to calculate the DFQ spectral function and den
	sity of states\, allowing for comparison with photoemission and scanning tu
	nneling spectroscopies. The results set the necessary energy resolution and
	 extraction methodology for calculating $\bar\lambda$ from the DFQ perspect
	ive. Additionally\, determining $\bar\lambda$ from the calculated spectral 
	functions yields results identical to those obtained from HASS\, proving th
	e self-consistency of the approach. \n\n\n![Howard](/resources/event-image/
	1427/5e5e53d_original)
LOCATION:SCI 352\, 590 Commonwealth Avenue\, 02215
STATUS:CONFIRMED
CLASS:PUBLIC
END:VEVENT
END:VCALENDAR