5 research outputs found
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
Imaging de Haas-van Alphen quantum oscillations and milli-Tesla pseudomagnetic fields
A unique attribute of atomically thin quantum materials is the in-situ
tunability of their electronic band structure by externally controllable
parameters like electrostatic doping, electric field, strain, electron
interactions, and displacement or twisting of atomic layers. This unparalleled
control of the electronic bands has led to the discovery of a plethora of
exotic emergent phenomena. But despite its key role, there is currently no
versatile method for mapping the local band structure in advanced 2D materials
devices in which the active layer is commonly embedded in various insulating
layers and metallic gates. Utilizing a scanning superconducting quantum
interference device, we image the de Haas-van Alphen quantum oscillations in a
model system, the Bernal-stacked trilayer graphene with dual gates, which
displays multiple highly-tunable bands. By resolving thermodynamic quantum
oscillations spanning over 100 Landau levels in low magnetic fields, we
reconstruct the band structure and its controllable evolution with the
displacement field with unprecedented precision and spatial resolution of 150
nm. Moreover, by developing Landau level interferometry, we reveal
shear-strain-induced pseudomagnetic fields and map their spatial dependence. In
contrast to artificially-induced large strain, which leads to pseudomagnetic
fields of hundreds of Tesla, we detect naturally occurring pseudomagnetic
fields as low as 1 mT corresponding to graphene twisting by just 1 millidegree
over one {\mu}m distance, two orders of magnitude lower than the typical angle
disorder in high-quality twisted bilayer graphene devices. This ability to
resolve the local band structure and strain on the nanoscale opens the door to
the characterization and utilization of tunable band engineering in practical
van der Waals devices.Comment: Nature (2023
Scanning SQUID-on-tip microscope in a top-loading cryogen-free dilution refrigerator
The scanning superconducting quantum interference device (SQUID) fabricated
on the tip of a sharp quartz pipette (SQUID-on-tip) has emerged as a versatile
tool for nanoscale imaging of magnetic, thermal, and transport properties of
microscopic devices of quantum materials. We present the design and performance
of a scanning SQUID-on-tip microscope in a top-loading probe of a cryogen-free
dilution refrigerator. The microscope is enclosed in a custom-made vacuum-tight
cell mounted at the bottom of the probe and is suspended by springs to suppress
vibrations caused by the pulse tube cryocooler. Two capillaries allow in-situ
control of helium exchange gas pressure in the cell that is required for
thermal imaging. A nanoscale heater is used to create local temperature
gradients in the sample, which enables quantitative characterization of the
relative vibrations between the tip and the sample. The spectrum of the
vibrations shows distinct resonant peaks with maximal power density of about 27
nm/Hz in the in-plane direction. The performance of the SQUID-on-tip
microscope is demonstrated by magnetic imaging of the MnBiTe magnetic
topological insulator, magnetization and current distribution imaging in a
SrRuO ferromagnetic oxide thin film, and by thermal imaging of dissipation
in graphene.Comment: Submitted to Review of Scientific Instrument
Proteins evolve on the edge of supramolecular self-assembly
The self-association of proteins into symmetric complexes is ubiquitous in all kingdoms of life1,2,3,4,5,6. Symmetric complexes possess unique geometric and functional properties, but their internal symmetry can pose a risk. In sickle-cell disease, the symmetry of haemoglobin exacerbates the effect of a mutation, triggering assembly into harmful fibrils7. Here we examine the universality of this mechanism and its relation to protein structure geometry. We introduced point mutations solely designed to increase surface hydrophobicity among 12 distinct symmetric complexes from Escherichia coli. Notably, all responded by forming supramolecular assemblies in vitro, as well as in vivo upon heterologous expression in Saccharomyces cerevisiae. Remarkably, in four cases, micrometre-long fibrils formed in vivo in response to a single point mutation. Biophysical measurements and electron microscopy revealed that mutants self-assembled in their folded states and so were not amyloid-like. Structural examination of 73 mutants identified supramolecular assembly hot spots predictable by geometry. A subsequent structural analysis of 7,471 symmetric complexes showed that geometric hot spots were buffered chemically by hydrophilic residues, suggesting a mechanism preventing mis-assembly of these regions. Thus, point mutations can frequently trigger folded proteins to self-assemble into higher-order structures. This potential is counterbalanced by negative selection and can be exploited to design nanomaterials in living cells.</p