15 research outputs found
The Nature of Interlayer Binding and Stacking of - Hybridized Carbon Layers: A Quantum Monte Carlo Study
-graphyne is a two-dimensional sheet of - hybridized carbon
atoms in a honeycomb lattice. While the geometrical structure is similar to
that of graphene, the hybridized triple bonds give rise to electronic structure
that is different from that of graphene. Similar to graphene, -graphyne
can be stacked in bilayers with two stable configurations, but the different
stackings have very different electronic structures: one is predicted to have
gapless parabolic bands and the other a tunable band gap which is attractive
for applications. In order to realize applications, it is crucial to understand
which stacking is more stable. This is difficult to model, as the stability is
a result of weak interlayer van der Waals interactions which are not well
captured by density functional theory (DFT). We have used quantum Monte Carlo
simulations that accurately include van der Waals interactions to calculate the
interlayer binding energy of bilayer graphyne and to determine its most stable
stacking mode. Our results show that interlayer bindings of - and
-bonded carbon networks are significantly underestimated in a Kohn-Sham
DFT approach, even with an exchange-correlation potential corrected to include,
in some approximation, van der Waals interactions. Finally, our quantum Monte
Carlo calculations reveal that the interlayer binding energy difference between
the two stacking modes is only 0.9(4) meV/atom. From this we conclude that the
two stable stacking modes of bilayer -graphyne are almost degenerate
with each other, and both will occur with about the same probability at room
temperature unless there is a synthesis path that prefers one stacking over the
other.Comment: 25 pages, 6 figure
Layer-dependent optically-induced spin polarization in InSe
Optical control of spin in semiconductors has been pioneered using
nanostructures of III-V and II-VI semiconductors, but the emergence of
two-dimensional van der Waals materials offers an alternative low-dimensional
platform for spintronic phenomena. Indium selenide (InSe), a group-III
monochalcogenide van der Waals material, has shown promise for opto-electronics
due to its high electron mobility, tunable direct bandgap, and quantum
transport. There are predictions of spin-dependent optical selection rules
suggesting potential for all-optical excitation and control of spin in a
two-dimensional layered material. Despite these predictions, layer-dependent
optical spin phenomena in InSe have yet to be explored. Here, we present
measurements of layer-dependent optical spin dynamics in few-layer and bulk
InSe. Polarized photoluminescence reveals layer-dependent optical orientation
of spin, thereby demonstrating the optical selection rules in few-layer InSe.
Spin dynamics are also studied in many-layer InSe using time-resolved Kerr
rotation spectroscopy. By applying out-of-plane and in-plane static magnetic
fields for polarized emission measurements and Kerr measurements, respectively,
the -factor for InSe was extracted. Further investigations are done by
calculating precession values using a model,
which is supported by \textit{ab-initio} density functional theory. Comparison
of predicted precession rates with experimental measurements highlights the
importance of excitonic effects in InSe for understanding spin dynamics.
Optical orientation of spin is an important prerequisite for opto-spintronic
phenomena and devices, and these first demonstrations of layer-dependent
optical excitation of spins in InSe lay the foundation for combining
layer-dependent spin properties with advantageous electronic properties found
in this material.Comment: 11 pages, 6 figures, supplemental materia