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DTSTAMP:20261020T041255Z
LAST-MODIFIED:20190524T145437Z
DTSTART:20190618T140000Z
DTEND:20190618T150000Z
UID:event2139@bu.edu
URL:http://physics.bu.edu/internal/events/show/2139
SUMMARY:INTERACTION OF PHONONS AND DIRAC FERMIONS ON THE (001) SURFACE OF T
	HE TOPOLOGICAL CRYSTALLINE INSULATOR LEAD TIN SELENIDE
DESCRIPTION:Featuring Samuel Kalish\n\nPart of the PhD Final Oral Exams.\n\
	nExamining Committee:  Michael El-Batanouny\, Claudio Chamon\, Karl Ludwig\
	, John Butler\, Wanzheng Hu\n\nAbstract: \n\nThe following dissertation pre
	sents a study of the interaction between Dirac fermions and surface phonons
	 at the (001) surface of the topological crystalline insulator Pb0.7Sn0.3Se
	.  Nontrivial topological crystalline insulator phases are protected by an 
	element or elements of their crystal symmetries and not by time-reversal sy
	mmetry as is the case of topological insulators.  Both material types suppo
	rt robust metallic surface states while maintaining their bulk insulating c
	haracter.  Pb1-xSnxSe exhibits a temperature-dependent topological transiti
	on and thus allows the application of inelastic helium atom scattering spec
	troscopy and time-of-flight techniques to measure the (001) surface phonon 
	dispersion in both the topological and trivial phases.  Surface lattice dyn
	amics calculations are performed to interpret and characterize measured dis
	persion curves.  Comparing the results of the two phases reveals a remarkab
	le change in an optical phonon branch.  A nearly-flat 9.0 meV shear-vertica
	l surface resonance branch in the trivial phase is replaced in the topologi
	cal phase with a new shear-vertical resonance that exhibits dramatic moment
	um-dependent softening down to a minimum of 5.8 meV at a momentum that coin
	cides with 2kF of the emergent Dirac fermions.  This softening clearly indi
	cates strong screening of these surface phonon modes by the Dirac fermions 
	and suggests further investigation of the underlying electron-phonon coupli
	ng.\n\nA quantitative calculation of the coupling between the electronic st
	ates and surface phonons is performed with the aid of a microscopic model b
	ased on linear Coulomb coupling of ionic displacements to the Dirac fermion
	 density\, which is incorporated in the phonon Matsubara Green’s function
	.  The real part of the self-energy is fit to the experimental dispersion d
	ata and the imaginary part is obtained through a Kramers-Kronig transformat
	ion.  Finally\, this is used to calculate the corresponding momentum-depend
	ent mode-specific electron-phonon coupling parameter λν(q).  The 
	coupling magnitude is found to reach values as high as 0.47\, which is stro
	nger than that in typical metals\, but weaker than the values reported for 
	the surfaces of the topological insulators Bi2Te3 and Bi2Se3.
LOCATION:SCI 352\, 590 Commonwealth Avenue\, 02215
STATUS:CONFIRMED
CLASS:PUBLIC
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