49 research outputs found
Imaging Localized States in Graphene Nanostructures
Probing techniques with spatial resolution have the potential to lead to a
better understanding of the microscopic physical processes and to novel routes
for manipulating nanostructures. We present scanning-gate images of a graphene
quantum dot which is coupled to source and drain via two constrictions. We
image and locate conductance resonances of the quantum dot in the
Coulomb-blockade regime as well as resonances of localized states in the
constrictions in real space.Comment: 18 pages, 7 figure
Imaging the lateral shift of a quantum-point contact using scanning-gate microscopy
We perform scanning-gate microscopy on a quantum-point contact. It is defined
in a high-mobility two-dimensional electron gas of an AlGaAs/GaAs
heterostructure, giving rise to a weak disorder potential. The lever arm of the
scanning tip is significantly smaller than that of the split gates defining the
conducting channel of the quantum-point contact. We are able to observe that
the conducting channel is shifted in real space when asymmetric gate voltages
are applied. The observed shifts are consistent with transport data and
numerical estimations.Comment: 5 pages, 3 figure
Observation of excited states in a graphene quantum dot
We demonstrate that excited states in single-layer graphene quantum dots can
be detected via direct transport experiments. Coulomb diamond measurements show
distinct features of sequential tunneling through an excited state. Moreover,
the onset of inelastic cotunneling in the diamond region could be detected. For
low magnetic fields, the positions of the single-particle energy levels
fluctuate on the scale of a flux quantum penetrating the dot area. For higher
magnetic fields, the transition to the formation of Landau levels is observed.
Estimates based on the linear energy-momentum relation of graphene give carrier
numbers of the order of 10 for our device.Comment: 3 pages, 3 figure
Observation of excited states in a graphene quantum dot
We demonstrate that excited states in single-layer graphene quantum dots can
be detected via direct transport experiments. Coulomb diamond measurements show
distinct features of sequential tunneling through an excited state. Moreover,
the onset of inelastic cotunneling in the diamond region could be detected. For
low magnetic fields, the positions of the single-particle energy levels
fluctuate on the scale of a flux quantum penetrating the dot area. For higher
magnetic fields, the transition to the formation of Landau levels is observed.
Estimates based on the linear energy-momentum relation of graphene give carrier
numbers of the order of 10 for our device.Comment: 3 pages, 3 figure
Transport through a strongly coupled graphene quantum dot in perpendicular magnetic field
We present transport measurements on a strongly coupled graphene quantum dot
in a perpendicular magnetic field. The device consists of an etched
single-layer graphene flake with two narrow constrictions separating a 140 nm
diameter island from source and drain graphene contacts. Lateral graphene gates
are used to electrostatically tune the device. Measurements of Coulomb
resonances, including constriction resonances and Coulomb diamonds prove the
functionality of the graphene quantum dot with a charging energy of around 4.5
meV. We show the evolution of Coulomb resonances as a function of perpendicular
magnetic field, which provides indications of the formation of the graphene
specific 0th Landau level. Finally, we demonstrate that the complex pattern
superimposing the quantum dot energy spectra is due to the formation of
additional localized states with increasing magnetic field.Comment: 6 pages, 4 figure
Coherent Electron-Phonon Coupling in Tailored Quantum Systems
The coupling between a two-level system and its environment leads to
decoherence. Within the context of coherent manipulation of electronic or
quasiparticle states in nanostructures, it is crucial to understand the sources
of decoherence. Here, we study the effect of electron-phonon coupling in a
graphene and an InAs nanowire double quantum dot. Our measurements reveal
oscillations of the double quantum dot current periodic in energy detuning
between the two levels. These periodic peaks are more pronounced in the
nanowire than in graphene, and disappear when the temperature is increased. We
attribute the oscillations to an interference effect between two alternative
inelastic decay paths involving acoustic phonons present in these materials.
This interpretation predicts the oscillations to wash out when temperature is
increased, as observed experimentally.Comment: 11 pages, 4 figure
Topological insulator quantum dot with tunable barriers
Thin (6-7 quintuple layer) topological insulator Bi2Se3 quantum dot devices
are demonstrated using ultrathin (2~4 quintuple layer) Bi2Se3 regions to
realize semiconducting barriers which may be tuned from Ohmic to tunneling
conduction via gate voltage. Transport spectroscopy shows Coulomb blockade with
large charging energy >5 meV, with additional features implying excited states
Dirac electrons in graphene-based quantum wires and quantum dots
In this paper we analyse the electronic properties of Dirac electrons in
finite-size ribbons and in circular and hexagonal quantum dots made of
graphene.Comment: Contribution for J. Phys.: Cond. Mat. special issue on graphene
physic
Properties of Graphene: A Theoretical Perspective
In this review, we provide an in-depth description of the physics of
monolayer and bilayer graphene from a theorist's perspective. We discuss the
physical properties of graphene in an external magnetic field, reflecting the
chiral nature of the quasiparticles near the Dirac point with a Landau level at
zero energy. We address the unique integer quantum Hall effects, the role of
electron correlations, and the recent observation of the fractional quantum
Hall effect in the monolayer graphene. The quantum Hall effect in bilayer
graphene is fundamentally different from that of a monolayer, reflecting the
unique band structure of this system. The theory of transport in the absence of
an external magnetic field is discussed in detail, along with the role of
disorder studied in various theoretical models. We highlight the differences
and similarities between monolayer and bilayer graphene, and focus on
thermodynamic properties such as the compressibility, the plasmon spectra, the
weak localization correction, quantum Hall effect, and optical properties.
Confinement of electrons in graphene is nontrivial due to Klein tunneling. We
review various theoretical and experimental studies of quantum confined
structures made from graphene. The band structure of graphene nanoribbons and
the role of the sublattice symmetry, edge geometry and the size of the
nanoribbon on the electronic and magnetic properties are very active areas of
research, and a detailed review of these topics is presented. Also, the effects
of substrate interactions, adsorbed atoms, lattice defects and doping on the
band structure of finite-sized graphene systems are discussed. We also include
a brief description of graphane -- gapped material obtained from graphene by
attaching hydrogen atoms to each carbon atom in the lattice.Comment: 189 pages. submitted in Advances in Physic