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BEGIN:VEVENT
DTSTAMP:20200229T071521Z
LAST-MODIFIED:20190524T145437Z
DTSTART:20190618T140000Z
DTEND:20190618T150000Z
UID:event2139@bu.edu
URL:http://physics.bu.edu/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 Greenâ€™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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