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DTSTAMP:20261001T191933Z
LAST-MODIFIED:20160208T211508Z
DTSTART:20160218T190000Z
DTEND:20160218T200000Z
UID:event1551@bu.edu
URL:http://physics.bu.edu/internal/events/show/1551
SUMMARY:Interaction between Phonons and Dirac Fermions on the [001] Surface
	 of the Topological Crystalline Insulator Pb1-xSnxSe
DESCRIPTION:Featuring Samuel Kalish\n\nPart of the Preliminary Oral Exam.\n
	\nExamining Committee: Michael El-Batanouny\, Claudio Chamon\, Karl Ludwig\
	, and John Butler\n\nAbstract:\n\nTopological crystalline insulators (TCIs)
	 belong to a class of topological insulators whose surface Dirac fermion me
	tallic states are protected by crystalline symmetry\, as opposed to time-re
	versal symmetry.  These surface states exhibit chirality and spin helicity.
	  The material  Pb1-xSnxSe\, with rocksalt crystal structure\, becomes a TC
	I above a critical Sn concentration of xc=18%.  In the topological phase\, 
	Pb1-xSnxSe has a bulk band inversion around the gap at the L-point. It exhi
	bits a rich assortment of novel phenomena including a temperature-dependent
	 transition from the trivial to the topological phase\, double Dirac cones 
	that give rise to a prominent van Hove singularity\, as well as a Lifshitz 
	Fermi surface topology transition. Revealing the surface dynamical signatur
	es associated with such phenomena will provide a solid basis for explaining
	 the fundamental physics of these systems. For example\, dynamical changes 
	manifest in the Lifshitz transition will bring to light how modifications o
	f the Fermi surface topology affect a physical system’s dynamic response.
	 \nThe Dirac fermion states are robust against backscattering by impurities
	\, phonons\, and defects\, making TCIs promising for applications in spintr
	onics and quantum computing.  While material improvements may minimize impu
	rities and defects\, phonons are always present\, which makes understanding
	 the physics of the electron-phonon coupling in these materials especially 
	important.\nI am using the technique of inelastic scattering of helium beam
	s from surfaces to investigate changes in surface dynamics as the system un
	dergoes the transition from the trivial to the topological phase. Special a
	ttention is given to the appearance of topological signatures in the measur
	ed surface phonon dispersions along high-symmetry directions of the [001] s
	urface Brillouin zone.  These signatures will be analyzed using temperature
	-dependent Matsubara Green’s functions to precisely determine phonon-mode
	-specific electron-phonon couplings λν(q).  I am also using elastic He sc
	attering to track the evolution of the surface charge density topographies 
	through the transition.\n\n![Kalish](/resources/event-image/1551/ac692df_sm
	all)
LOCATION:SCI 352\, 590 Commonwealth Avenue\, 02215
STATUS:CONFIRMED
CLASS:PUBLIC
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