254 research outputs found
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
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
Scanning-gate microscopy of semiconductor nanostructures: an overview
This paper presents an overview of scanning-gate microscopy applied to the
imaging of electron transport through buried semiconductor nanostructures.
After a brief description of the technique and of its possible artifacts, we
give a summary of some of its most instructive achievements found in the
literature and we present an updated review of our own research. It focuses on
the imaging of GaInAs-based quantum rings both in the low magnetic field
Aharonov-Bohm regime and in the high-field quantum Hall regime. In all of the
given examples, we emphasize how a local-probe approach is able to shed new, or
complementary, light on transport phenomena which are usually studied by means
of macroscopic conductance measurements.Comment: Invited talk by SH at 39th "Jaszowiec" International School and
Conference on the Physics of Semiconductors, Krynica-Zdroj, Poland, June 201
Imaging Electron Wave Functions Inside Open Quantum Rings
Combining Scanning Gate Microscopy (SGM) experiments and simulations, we
demonstrate low temperature imaging of electron probability density
in embedded mesoscopic quantum rings (QRs). The tip-induced
conductance modulations share the same temperature dependence as the
Aharonov-Bohm effect, indicating that they originate from electron wavefunction
interferences. Simulations of both and SGM conductance maps
reproduce the main experimental observations and link fringes in SGM images to
.Comment: new titl
Transport inefficiency in branched-out mesoscopic networks: An analog of the Braess paradox
We present evidence for a counter-intuitive behavior of semiconductor
mesoscopic networks that is the analog of the Braess paradox encountered in
classical networks. A numerical simulation of quantum transport in a two-branch
mesoscopic network reveals that adding a third branch can paradoxically induce
transport inefficiency that manifests itself in a sizable conductance drop of
the network. A scanning-probe experiment using a biased tip to modulate the
transmission of one branch in the network reveals the occurrence of this
paradox by mapping the conductance variation as a function of the tip voltage
and position.Comment: 2nd version with minor stylistic corrections. To appear in Phys. Rev.
Lett.: Editorially approved for publication 6 January 201
Structural determination of bilayer graphene on SiC(0001) using synchrotron radiation photoelectron diffraction
In recent years there has been growing interest in the electronic properties of 'few layer' graphene films. Twisted layers, different stacking and register with the substrate result in remarkable unconventional couplings. These distinctive electronic behaviours have been attributed to structural differences, even if only a few structural determinations are available. Here we report the results of a structural study of bilayer graphene on the Si-terminated SiC(0001) surface, investigated using synchrotron radiation-based photoelectron diffraction and complemented by angle-resolved photoemission mapping of the electronic valence bands. Photoelectron diffraction angular distributions of the graphene C 1s component have been measured at different kinetic energies and compared with the results of multiple scattering simulations for model structures. The results confirm that bilayer graphene on SiC(0001) has a layer spacing of 3.48 Å and an AB (Bernal) stacking, with a distance between the C buffer layer and the first graphene layer of 3.24 Å. Our work generalises the use of a versatile and precise diffraction method capable to shed light on the structure of low-dimensional materials
Long dephasing time and high temperature ballistic transport in an InGaAs open quantum dot
We report on measurements of the magnetoconductance of an open circular
InGaAs quantum dot between 1.3K and 204K. We observe two types of
magnetoconductance fluctuations: universal conductance fluctuations (UCFs), and
'focusing' fluctuations related to ballistic trajectories between openings. The
electron phase coherence time extracted from UCFs amplitude is larger than in
GaAs/AlGaAs quantum dots and follows a similar temperature dependence (between
T^-1 and T^-2). Below 150K, the characteristic length associated with
'focusing' fluctuations shows a slightly different temperature dependence from
that of the conductivity.Comment: 6 pages, 4 figures, proceedings of ICSNN2002, to appear in Physica
Planning the electron traffic in semiconductor networks: A mesoscopic analog of the Braess paradox encountered in road networks
By combining quantum simulations of electron transport and scanning-gate
microscopy, we have shown that the current transmitted through a semiconductor
two-path rectangular network in the ballistic and coherent regimes of transport
can be paradoxically degraded by adding a third path to the network. This is
analogous to the Braess paradox occurring in classical networks. Simulations
reported here enlighten the role played by congestion in the network.Comment: 31st Int. Conf. Phys. Semiconductors, Zurich, July-August 201
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