Optical spectroscopy of transition metal dichalcogenide semiconductors and semimetals
Optical spectroscopy of transition metal dichalcogenide semiconductors and semimetals
Jun Yan, Department of Physics, UMass Amherst
Date and time:
Thu, Oct 27, 2016 - 11:30am
Refreshments at 11:15am
Location:
LGRT 1033
Category:
Condensed Matter Seminar
Speaker link:
http://blogs.umass.edu/yan1/
Abstract:
Molybdenum and tungsten based transition metal dichalcogenides (TMDCs)
with polymorphs in the hexagonal (H) and distorted octahedral (T’)
phases are a class of layered materials with fascinating electronic
properties. H-TMDCs host massive Dirac fermions and have become a
material system of choice for realizing valleytronics; bulk and
monolayer T’-TMDCs are predicted to be Weyl semimetals and topological
insulators respectively. In this seminar I will discuss our recent
studies using optical spectroscopy as a powerful non-destructive tool to
probe the excitonic, vibrational and symmetry properties of H- and T’-
TMDCs. Using helicity-resolved measurements, we show that in
H-(Mo,W)(S,Se)2 atomic layers, due to conservation of pseudo-angular
momentum, the in-plane zone-center optical phonons switch the circular
polarization of incident photons completely; in drastic contrast to
valley-polarized luminescence that maintains the circular polarization
of incident photons. The strong Coulomb interaction in reduced
dimensions makes the Rydberg series of the tightly bound excitons
well-separated in energy, and offers an intriguing platform for
manipulating valley polarization and coherence. In T’-MoTe2, we found
the complex-looking phonon structures can be classified into zigzag
modes and mirror modes respectively. This is confirmed by our
measurements in bulk and monolayer samples. We further discovered that
the low energy shear modes in bulk T’-MoTe2 provide a sensitive tool for
probing the breaking of inversion symmetry in the crystal, which is key
to the existence of Weyl fermions in non-magnetic systems.
Department of Physics