82 research outputs found
Multitip scanning gate microscopy for ballistic transport studies in systems with two-dimensional electron gas
We consider conductance mapping of systems based on the two-dimensional
electron gas with scanning gate microscopy using two and more tips of the
atomic force microscope. The paper contains results of numerical simulations
for a model tip potential with a proposal of a few procedures for extraction
and manipulation the ballistic transport properties. In particular, we
demonstrate that the multi-tip techniques can be used for readout of the Fermi
wavelength, detection of potential defects, filtering specific transverse
modes, tuning the system into resonant conditions under which a stable map of a
local density of states can be extracted from conductance maps using a third
tip.Comment: 9 pages, 12 figure
Identifying and abating copper foil impurities to optimize graphene growth
Copper foil impurities are hampering scalable production of high-quality
graphene by chemical vapor deposition (CVD). Here, we conduct a thorough study
on the origin of these unavoidable contaminations at the surface of copper
after the CVD process. We identify two distinct origins for the impurities. The
first type is intrinsic impurities, originating from the manufacturing process
of the copper foils, already present at the surface before any high-temperature
treatment, or buried into the bulk of copper foils. The buried impurities
diffuse towards the copper surface during high-temperature treatment and
precipitate. The second source is external: silica contamination arising from
the quartz tube that also precipitate on copper. The problem of the extrinsic
silica contamination is readily solved upon using an adequate confinement the
copper foil samples. The intrinsic impurities are much more difficult to remove
since they appear spread in the whole foil. Nevertheless, electropolishing
proves particularly efficient in drastically reducing the issue.Comment: 26 pages, 12 figure
2D Rutherford-Like Scattering in Ballistic Nanodevices
Ballistic injection in a nanodevice is a complex process where electrons can
either be transmitted or reflected, thereby introducing deviations from the
otherwise quantized conductance. In this context, quantum rings (QRs) appear as
model geometries: in a semiclassical view, most electrons bounce against the
central QR antidot, which strongly reduces injection efficiency. Thanks to an
analogy with Rutherford scattering, we show that a local partial depletion of
the QR close to the edge of the antidot can counter-intuitively ease ballistic
electron injection. On the contrary, local charge accumulation can focus the
semi-classical trajectories on the hard-wall potential and strongly enhance
reflection back to the lead. Scanning gate experiments on a ballistic QR, and
simulations of the conductance of the same device are consistent, and agree to
show that the effect is directly proportional to the ratio between the strength
of the perturbation and the Fermi energy. Our observation surprisingly fits the
simple Rutherford formalism in two-dimensions in the classical limit
Imaging and controlling electron transport inside a quantum ring
Traditionally, the understanding of quantum transport, coherent and
ballistic1, relies on the measurement of macroscopic properties such as the
conductance. While powerful when coupled to statistical theories, this approach
cannot provide a detailed image of "how electrons behave down there". Ideally,
understanding transport at the nanoscale would require tracking each electron
inside the nano-device. Significant progress towards this goal was obtained by
combining Scanning Probe Microscopy (SPM) with transport measurements2-7. Some
studies even showed signatures of quantum transport in the surrounding of
nanostructures4-6. Here, SPM is used to probe electron propagation inside an
open quantum ring exhibiting the archetype of electron wave interference
phenomena: the Aharonov-Bohm effect8. Conductance maps recorded while scanning
the biased tip of a cryogenic atomic force microscope above the quantum ring
show that the propagation of electrons, both coherent and ballistic, can be
investigated in situ, and even be controlled by tuning the tip potential.Comment: 11 text pages + 3 figure
Spin-Orbit Coupling, Antilocalization, and Parallel Magnetic Fields in Quantum Dots
We investigate antilocalization due to spin-orbit coupling in ballistic GaAs
quantum dots. Antilocalization that is prominent in large dots is suppressed in
small dots, as anticipated theoretically. Parallel magnetic fields suppress
both antilocalization and also, at larger fields, weak localization, consistent
with random matrix theory results once orbital coupling of the parallel field
is included. In situ control of spin-orbit coupling in dots is demonstrated as
a gate-controlled crossover from weak localization to antilocalization.Comment: related papers at http://marcuslab.harvard.ed
Scanning Gate Spectroscopy of transport across a Quantum Hall Nano-Island
We explore transport across an ultra-small Quantum Hall Island (QHI) formed
by closed quan- tum Hall edge states and connected to propagating edge channels
through tunnel barriers. Scanning gate microscopy and scanning gate
spectroscopy are used to first localize and then study a single QHI near a
quantum point contact. The presence of Coulomb diamonds in the spectroscopy
con- firms that Coulomb blockade governs transport across the QHI. Varying the
microscope tip bias as well as current bias across the device, we uncover the
QHI discrete energy spectrum arising from electronic confinement and we extract
estimates of the gradient of the confining potential and of the edge state
velocity.Comment: 13 pages, 3 figure
Imaging Coulomb Islands in a Quantum Hall Interferometer
In the Quantum Hall regime, near integer filling factors, electrons should
only be transmitted through spatially-separated edge states. However, in
mesoscopic systems, electronic transmission turns out to be more complex,
giving rise to a large spectrum of magnetoresistance oscillations. To explain
these observations, recent models put forward that, as edge states come close
to each other, electrons can hop between counterpropagating edge channels, or
tunnel through Coulomb islands. Here, we use scanning gate microscopy to
demonstrate the presence of quantum Hall Coulomb islands, and reveal the
spatial structure of transport inside a quantum Hall interferometer. Electron
islands locations are found by modulating the tunneling between edge states and
confined electron orbits. Tuning the magnetic field, we unveil a continuous
evolution of active electron islands. This allows to decrypt the complexity of
high magnetic field magnetoresistance oscillations, and opens the way to
further local scale manipulations of quantum Hall localized states
Formation of quantum dots in the potential fluctuations of InGaAs heterostructures probed by scanning gate microscopy
The disordered potential landscape in an InGaAs/InAlAs two-dimensional
electron gas patterned into narrow wires is investigated by means of scanning
gate microscopy. It is found that scanning a negatively charged tip above
particular sites of the wires produces conductance oscillations that are
periodic in the tip voltage. These oscillations take the shape of concentric
circles whose number and diameter increase for more negative tip voltages until
full depletion occurs in the probed region. These observations cannot be
explained by charging events in material traps, but are consistent with Coulomb
blockade in quantum dots forming when the potential fluctuations are raised
locally at the Fermi level by the gating action of the tip. This interpretation
is supported by simple electrostatic simulations in the case of a disorder
potential induced by ionized dopants. This work represents a local
investigation of the mechanisms responsible for the disorder-induced
metal-to-insulator transition observed in macroscopic two-dimensional electron
systems at low enough density
Classical analogy for the deflection of flux avalanches by a metallic layer
Sudden avalanches of magnetic flux bursting into a superconducting sample
undergo deflections of their trajectories when encountering a conductive layer
deposited on top of the superconductor. Remarkably, in some cases flux is
totally excluded from the area covered by the conductive layer. We present a
simple classical model that accounts for this behaviour and considers a
magnetic monopole approaching a semi-infinite conductive plane. This model
suggests that magnetic braking is an important mechanism responsible for
avalanche deflection.Comment: 14 pages, 5 figure
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