| dc.description.abstract | Strain, both naturally occurring and deliberately engineered, can have a considerable
effect on the structural, electronic, and transport properties of 2D and layered materials.
Uniaxial or biaxial heterostrain (i.e. different strain applied to different layers) modifies
the stacking arrangement of bilayer graphene (BLG) forming Moiré superlattices. This
subsequently influences the electronic structure and the transport properties of the bilayer.
We performed Density Functional Theory (DFT) calculations to investigate the interplay
between heterostrain and the resulting stacking in BLG. We found that above a critical
strain of 1%, it is energetically favorable for the free layer to be unstrained, indicating a
transition between uniform AB stacking and nonuniform mixed stacking. This suggests
that even small levels of strain can provide a platform to reversibly engineer stacking or
der and Moiré features in BLGs, providing a viable alternative to twistronics to tune the
stacking order of the system, and consequently its properties.
The domain walls between the AB and BAstacked gapped BLG have garnered in
tense interest, as they host topologically protected, valleypolarised transport channels.
The introduction of a twist angle θ between the bilayers and the associated formation of
a Moiré pattern has been the dominant method used to study these topological channels,
but heterostrain can also give rise to similar stacking domains and interfaces. We theoreti
cally investigated the electronic structure of a uniaxially heterostrained BLG. We discussed
the formation and evolution of interface localized channels in the onedimensional Moiré
pattern that emerges due to the different stacking registries between the two layers. We
found that a uniform heterostrain is not sufficient to create onedimensional topological
channels in biased BLG. Instead, using a simple model to account for the inplane atomic
reconstruction driven by the changing stacking registry, we showed that the resulting expanded Bernalstacked domains and sharper interfaces are required for robust topological
interfaces to emerge. These states are highly localized in the AAor SPstacked interface
regions and exhibit differences in their layer and sublattice distribution depending on the
interface stacking. We conclude that heterostrain can be used as a mechanism to tune the
presence and distribution of topological channels in gapped BLG systems, complementary
to the field of twistronics.
We then investigated the effect of the Poisson contraction, which can occur with the
application of uniaxial heterostrain for heterostrained BLG. In this initial investigation,
we do not include the effect of atomic relaxation on the electronic and transport properties
of the interface channels. The inclusion of Poisson contraction ν with the application of
heterostrain leads to the formation of 2D Moiré superlattices, similar to the ones created
in twisted BLG. For low energies, the states in the AAstacked regions are highly local
ized, whereas the SPstacked regions host states that form a network throughout the lattice
(topological channels). Moreover, the SPstacked interface channels occur over a range of
energies (dispersive channels), but they do not have the expected pattern. We explained
this, due to the fact that different strains are applied along x and y directions. The over
all transmission has very low values due to the existence of the AB/BAstacked gapped
domains in heterostrained BLG with the application of large interlayer bias, compared to
no bias. Consequently, heterostrained untwisted BLG is an alternative way to tune the
electronic and transport properties of BLG, compared to twisted BLG, overcoming limitations relevant to achieving precise twist angle θ. In future work, we will extend this study
for lower and larger values of heterostrain to investigate how the localization of states is
affected. Also, we could study how transmission is modified by changing the value of
heterostrain for this system, as similar studies have been done for twisted BLG. | en |
