A pre-requisite for the emergence of Berry curvature is either a broken
inversion symmetry or a broken time-reversal symmetry. Thus two-dimensional
materials such as transition metal dichalcogenides and gated bilayer graphene
are widely studied for valleytronics as they exhibit broken inversion symmetry.
For most of the studies related to graphene and other two-dimensional
materials, these materials are encapsulated with hexagonal boron nitride (hBN),
a wide band gap material which has comparable lattice parameter to that of
graphene.
Encapsulation with hBN layer protects the graphene and other
two-dimensional materials from unwanted adsorption of stray molecules while
keeping their properties intact. hBN also acts as a smooth twodimensional
substrate unlike SiO2 which is highly non-uniform, increasing the mobility of
carriers in graphene.
However, most of the valleytronics studies on bilayer graphene with hBN
encapsulation has not taken into account the effect of hBN layer in breaking
the layer symmetry of bilayer graphene and inducing Berry curvature.
This is why Japan Advanced Institute of Science and Technology (JAIST) postdoc Afsal Kareekunnan, senior lecturer Manoharan Muruganathan and Professor Hiroshi Mizuta decided it was vital to take into account the effect of hBN as a substrate and as an encapsulation layer on the valleytronics properties of bilayer graphene.
By using
first-principles calculations, they have found that for hBN/bilayer graphene
commensurate heterostructures, the configuration, as well as the orientation of
the hBN layer, has an immense effect on the polarity as well as the magnitude
of the Berry curvature (Physical Review B, "Manipulating Berry curvature
in hBN/bilayer graphene commensurate heterostructures").
For
non-encapsulated hBN/bilayer graphene heterostructure, where hBN is present
only at the bottom, the layer symmetry is broken due to the difference in the
potential experienced by the two layers of the bilayer graphene. This layer
asymmetry induces a non-zero Berry curvature.
However,
encapsulation of the bilayer graphene with hBN (where the top and bottom hBN
are out of phase with each other) nullifies the effect of hBN and drives the
system towards symmetry, reducing the magnitude of the Berry curvature.
A small
Berry curvature which is still present is the feature of pristine bilayer
graphene where the spontaneous charge transfer from the valleys to one of the
layers results in a slight asymmetry between the layers as reported by the
group earlier.
Nonetheless,
encapsulating bilayer graphene with the top and bottom hBN in phase with each
other enhances the effect of hBN, leading to an increase in the asymmetry
between the layers and a large Berry curvature. This is due to the asymmetric
potential experienced by the two layers of bilayer graphene from the top and
bottom hBN.
The group
has also found that the magnitude and the polarity of the Berry curvature can
be tuned in all the above-mentioned cases with the application of an
out-of-plane electric field.
"We
believe that, from both theoretical and experimental perspective, such precise
analysis of the effect of the use of hBN both as a substrate and as an
encapsulation layer for graphene-based devices gives deep insight into the
system which has great potential to be an ideal valleytronic material,"
Professor Mizuta said.
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