2 research outputs found
Measured potential profile in a quantum anomalous Hall system suggests bulk-dominated current flow
Ideally, quantum anomalous Hall systems should display zero longitudinal
resistance. Yet in experimental quantum anomalous Hall systems elevated
temperature can make the longitudinal resistance finite, indicating dissipative
flow of electrons. Here, we show that the measured potentials at multiple
locations within a device at elevated temperature are well-described by
solution of Laplace's equation, assuming spatially-uniform conductivity,
suggesting non-equilibrium current flows through the two-dimensional bulk.
Extrapolation suggests that at even lower temperatures current may still flow
primarily through the bulk rather than, as had been assumed, through edge
modes. An argument for bulk current flow previously applied to quantum Hall
systems supports this picture.Comment: 6 pages, 4 figures, plus supplemental material
Unusual magnetotransport in twisted bilayer graphene from strain-induced open Fermi surfaces
Anisotropic hopping in a toy Hofstadter model was recently invoked to explain
a rich and surprising Landau spectrum measured in twisted bilayer graphene away
from the magic angle. Suspecting that such anisotropy could arise from
unintended uniaxial strain, we extend the Bistritzer-MacDonald model to include
uniaxial heterostrain. We find that such strain strongly influences band
structure, shifting the three otherwise-degenerate van Hove points to different
energies. Coupled to a Boltzmann magnetotransport calculation, this reproduces
previously-unexplained non-saturating magnetoresistance over broad ranges
of density near filling , and predicts subtler features that had not
been noticed in the experimental data. In contrast to these distinctive
signatures in longitudinal resistivity, the Hall coefficient is barely
influenced by strain, to the extent that it still shows a single sign change on
each side of the charge neutrality point -- surprisingly, this sign change no
longer occurs at a van Hove point. The theory also predicts a marked rotation
of the electrical transport principal axes as a function of filling even for
fixed strain and for rigid bands. More careful examination of
interaction-induced nematic order versus strain effects in twisted bilayer
graphene could thus be in order.Comment: 11 pages main text (4 figures) + 8 pages supplementary material (11
figures