43 research outputs found
Imaging resonant dissipation from individual atomic defects in graphene
Conversion of electric current into heat involves microscopic processes that
operate on nanometer length-scales and release minute amounts of power. While
central to our understanding of the electrical properties of materials,
individual mediators of energy dissipation have so far eluded direct
observation. Using scanning nano-thermometry with sub-micro K sensitivity we
visualize and control phonon emission from individual atomic defects in
graphene. The inferred electron-phonon 'cooling power spectrum' exhibits sharp
peaks when the Fermi level comes into resonance with electronic quasi-bound
states at such defects, a hitherto uncharted process. Rare in the bulk but
abundant at graphene's edges, switchable atomic-scale phonon emitters define
the dominant dissipation mechanism. Our work offers new insights for addressing
key materials challenges in modern electronics and engineering dissipation at
the nanoscale
Nanoscale imaging of equilibrium quantum Hall edge currents and of the magnetic monopole response in graphene
The recently predicted topological magnetoelectric effect and the response to
an electric charge that mimics an induced mirror magnetic monopole are
fundamental attributes of topological states of matter with broken time
reversal symmetry. Using a SQUID-on-tip, acting simultaneously as a tunable
scanning electric charge and as ultrasensitive nanoscale magnetometer, we
induce and directly image the microscopic currents generating the magnetic
monopole response in a graphene quantum Hall electron system. We find a rich
and complex nonlinear behavior governed by coexistence of topological and
nontopological equilibrium currents that is not captured by the monopole
models. Furthermore, by utilizing a tuning fork that induces nanoscale
vibrations of the SQUID-on-tip, we directly image the equilibrium currents of
individual quantum Hall edge states for the first time. We reveal that the edge
states that are commonly assumed to carry only a chiral downstream current, in
fact carry a pair of counterpropagating currents, in which the topological
downstream current in the incompressible region is always counterbalanced by
heretofore unobserved nontopological upstream current flowing in the adjacent
compressible region. The intricate patterns of the counterpropagating
equilibrium-state orbital currents provide new insights into the microscopic
origins of the topological and nontopological charge and energy flow in quantum
Hall systems
Visualizing thickness-dependent magnetic textures in few-layer
Magnetic ordering in two-dimensional (2D) materials has recently emerged as a
promising platform for data storage, computing, and sensing. To advance these
developments, it is vital to gain a detailed understanding of how the magnetic
order evolves on the nanometer-scale as a function of the number of atomic
layers and applied magnetic field. Here, we image few-layer
using a combined scanning superconducting
quantum interference device and atomic force microscopy probe. Maps of the
material's stray magnetic field as a function of applied magnetic field reveal
its magnetization per layer as well as the thickness-dependent magnetic
texture. Using a micromagnetic model, we correlate measured stray-field
patterns with the underlying magnetization configurations, including labyrinth
domains and skyrmionic bubbles. Comparison between real-space images and
simulations demonstrates that the layer dependence of the material's magnetic
texture is a result of the thickness-dependent balance between crystalline and
shape anisotropy. These findings represent an important step towards 2D
spintronic devices with engineered spin configurations and controlled
dependence on external magnetic fields.Comment: 15 pages, 4 figures, and supplementary informatio