Activity Report 2010 - CNRS

SCANNING GATE
NANOELECTRONICS
In the framework of Vincent Bayot’s Chair
of Excellence, two major results were
obtained by scanning gate microscopy
(SGM): a theoretical understanding of
SGM images in the coherent regime of
transport both in the presence of defects
and weak magnetic field; and the
discovery of Coulomb islands in a
quantum Hall interferometer.
In the quantum Hall (QH) regime, near
integer Landau level filling factors,
electrons should be perfectly transmitted
through spatially separated edge states
(Fig. 2).
SGM uses the electrically polarized tip of
a low-temperature AFM to scan above a
semiconductor device while the conductance
changes, due to the tip perturbation,
are simultaneously mapped in real
space. Previously, we have applied the
SGM technique to InGaAs-based quantum
rings (QRs) at low temperature and
under the effect of an external magnetic
field (Hackens et al., Nature Phys 2006,
Martins et al., PRL 2007). When the AFM
tip scans over the QR surface, fringes are
observed in the tip-induced conductance
changes which are essentially radial with
the QR. To understand the physics behind
SGM experiments, we have focused on
the correspondence between the local
density of states (LDOS) and SGM
conductance images by including an
external magnetic field in the
simulations. We also generate a realistic
potential profile to account for disorder,
and include many conduction channels
contributing to the transport. We find
that, in contrast with the current density
distribution, the LDOS can often be
determined by recursive semi-classical
trajectories with energies close to the
Fermi energy. As a result, the
correspondence between LDOS and SGM
images is clearly established (Fig. 1).
Fig. 2: An artist’s view of edge states in the
quantum ring confining potential. The tip (in
green) induces a local perturbation of the
potential that can be scanned over the
quantum Hall interferometer.
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 (Rosenow et
al., PRL 2007) put forward the theory
that, as edge states come close to each
other, electrons can hop between
counterpropagating edge channels, or
tunnel through Coulomb islands, giving
rise to a new kind of Coulomb blockade
effect. We have used SGM to
demonstrate the presence of QH Coulomb
islands, and reveal the spatial structure
of transport inside a QH interferometer.
The locations of electron islands are
found by modulating the tunneling
between edge states and confined
electron orbits, i.e. the SGM polarized tip
is used to modulate Coulomb blockade,
resulting in concentric fringes
surrounding the active island (Fig. 3).
HIGHLIGHT : NANO-CHARACTERIZATION AND METROLOGY
Fig. 1: Comparison between the LDOS (A) and
conductance variation images (B) when
negatively charged defects are included in the
system.
Moreover, by varying the Fermi
level and magnetic field strength, we find
that the LDOS and conductance images
are periodical with the external field and
that they bear the same periodicity as
the Aharonov-Bohm effect. This finding
strengthens our view that SGM in the
weak tip-potential limit is the analogue of
STM for imaging the electronic LDOS in
buried open mesoscopic systems.
Fig. 3: SGM image in the QH regime. The
center of the resulting concentric fringes mark
the position of the active quantum Coulomb
island.
Tuning the magnetic field, we
were able to unveil a continuous
evolution of active quantum Coulomb
islands. This allows to decrypt the
complexity of high-magnetic field
magnetoresistance oscillations, and
opens the way to further local-scale
manipulations of QH localized states.
CONTACTS
vincent.bayot@uclouvain.be
serge.huant@grenoble.cnrs.fr
herve.courtois@grenoble.cnrs.fr
FURTHER READING
M. G. Pala et al., Nanotechnology, 20,
264021 (2009)
B. Hackens et al., Nature Communications
1:39 (2010).
12