139 research outputs found
Bound states and magnetic field-induced valley splitting in gate-tunable graphene quantum dots
The magnetic field dependence of energy levels in gapped single- and bilayer
graphene quantum dots (QDs) defined by electrostatic gates is studied
analytically in terms of the Dirac equation. Due to the absence of sharp edges
in these types of QDs, the valley degree of freedom is a good quantum number.
We show that its degeneracy is efficiently and controllably broken by a
magnetic field applied perpendicular to the graphene plane. This opens up a
feasible route to create well-defined and well controlled spin- and
valley-qubits in graphene QDs. We also point out the similarities and
differences in the spectrum between single- and bilayer graphene quantum dots.
Striking in the case of bilayer graphene is the anomalous bulk Landau level
(LL) that crosses the gap which results in crossings of QD states with this
bulk LL at large magnetic fields in stark contrast to the single-layer case
where this LL is absent. The tunability of the gap in the bilayer case allows
us to observe different regimes of level spacings directly related to the
formation of a pronounced ``Mexican hat'' in the bulk bandstructure. We discuss
the applicability of such QDs to control and measure the valley isospin and
their potential use for hosting and controlling spin qubits.Comment: 12 pages, 10 figure
Electrical observation of a tunable band gap in bilayer graphene nanoribbons at room temperature
We investigate the transport properties of double-gated bilayer graphene
nanoribbons at room temperature. The devices were fabricated using conventional
CMOS-compatible processes. By analyzing the dependence of the resistance at the
charge neutrality point as a function of the electric field applied
perpendicular to the graphene surface, we show that a band gap in the density
of states opens, reaching an effective value of ~sim50 meV. This demonstrates
the potential of bilayer graphene as FET channel material in a conventional
CMOS environment.Comment: 3 pages, 3 figure
Quantum Hall effect in narrow graphene ribbons
The edge states in the integer quantum Hall effect are known to be
significantly affected by electrostatic interactions leading to the formation
of compressible and incompressible strips at the boundaries of Hall bars. We
show here, in a combined experimental and theoretical analysis, that this does
not hold for the quantum Hall effect in narrow graphene ribbons. In our
graphene Hall bar, which is only 60 nm wide, we observe the quantum Hall effect
up to Landau level index k=2 and show within a zero free-parameter model that
the spatial extent of the compressible and incompressible strips is of a
similar magnitude as the magnetic length. We conclude that in narrow graphene
ribbons the single-particle picture is a more appropriate description of the
quantum Hall effect and that electrostatic effects are of minor importance.Comment: RevTex, 5 pages, 4 figures (matches published version
Robustness of the optical-conductivity sum rule in Bilayer Graphene
We calculate the optical sum associated with the in-plane conductivity of a
graphene bilayer. A bilayer asymmetry gap generated in a field-effect device
can split apart valence and conduction bands, which otherwise would meet at two
K points in the Brillouin zone. In this way one can go from a compensated
semimetal to a semiconductor with a tunable gap. However, the sum rule turns
out to be 'protected' against the opening of this semiconducting gap, in
contrast to the large variations observed in other systems where the gap is
induced by strong correlation effects.Comment: 6 pages, 3 figures. Final versio
Tuning impurity states in bilayer graphene
We study the impurity states in bilayer graphene in the unitary limit using
Green's function method. Unlike in single layer graphene, the presence of
impurities at two non-equivalent sites in bilayer graphene produce different
impurity states which is understood as the change in the band structure due to
interlayer hopping of electrons. The impurity states can also be tuned by
changing the band structure of bilayer grahene through external electric field
bias.Comment: 7 pages, 9 figures, sumbitted to PR
Coulomb-driven broken-symmetry states in doubly gated suspended bilayer graphene
The non-interacting energy spectrum of graphene and its bilayer counterpart
consists of multiple degeneracies owing to the inherent spin, valley and layer
symmetries. Interactions among charge carriers are expected to spontaneously
break these symmetries, leading to gapped ordered states. In the quantum Hall
regime these states are predicted to be ferromagnetic in nature whereby the
system becomes spin polarized, layer polarized or both. In bilayer graphene,
due to its parabolic dispersion, interaction-induced symmetry breaking is
already expected at zero magnetic field. In this work, the underlying order of
the various broken-symmetry states is investigated in bilayer graphene that is
suspended between top and bottom gate electrodes. By controllably breaking the
spin and sublattice symmetries we are able to deduce the order parameter of the
various quantum Hall ferromagnetic states. At small carrier densities, we
identify for the first time three distinct broken symmetry states, one of which
is consistent with either spontaneously broken time-reversal symmetry or
spontaneously broken rotational symmetry
Gate-defined graphene double quantum dot and excited state spectroscopy
A double quantum dot is formed in a graphene nanoribbon device using three
top gates. These gates independently change the number of electrons on each dot
and tune the inter-dot coupling. Transport through excited states is observed
in the weakly coupled double dot regime. We extract from the measurements all
relevant capacitances of the double dot system, as well as the quantized level
spacing
Strong Suppression of Electrical Noise in Bilayer Graphene Nano Devices
Low-frequency 1/f noise is ubiquitous, and dominates the signal-to-noise
performance in nanodevices. Here we investigate the noise characteristics of
single-layer and bilayer graphene nano-devices, and uncover an unexpected 1/f
noise behavior for bilayer devices. Graphene is a single layer of graphite,
where carbon atoms form a 2D honeycomb lattice. Despite the similar
composition, bilayer graphene (two graphene monolayers stacked in the natural
graphite order) is a distinct 2D system with a different band structure and
electrical properties. In graphene monolayers, the 1/f noise is found to follow
Hooge's empirical relation with a noise parameter comparable to that of bulk
semiconductors. However, this 1/f noise is strongly suppressed in bilayer
graphene devices, and exhibits an unusual dependence on the carrier density,
different from most other materials. The unexpected noise behavior in graphene
bilayers is associated with its unique band structure that varies with the
charge distribution among the two layers, resulting in an effective screening
of potential fluctuations due to external impurity charges. The findings here
point to exciting opportunities for graphene bilayers in low-noise
applications
Atomic Hole Doping of Graphene
Graphene is an excellent candidate for the next generation of electronic
materials due to the strict two-dimensionality of its electronic structure as
well as the extremely high carrier mobility. A prerequisite for the development
of graphene based electronics is the reliable control of the type and density
of the charge carriers by external (gate) and internal (doping) means. While
gating has been successfully demonstrated for graphene flakes and epitaxial
graphene on silicon carbide, the development of reliable chemical doping
methods turns out to be a real challenge. In particular hole doping is an
unsolved issue. So far it has only been achieved with reactive molecular
adsorbates, which are largely incompatible with any device technology. Here we
show by angle-resolved photoemission spectroscopy that atomic doping of an
epitaxial graphene layer on a silicon carbide substrate with bismuth, antimony
or gold presents effective means of p-type doping. Not only is the atomic
doping the method of choice for the internal control of the carrier density. In
combination with the intrinsic n-type character of epitaxial graphene on SiC,
the charge carriers can be tuned from electrons to holes, without affecting the
conical band structure
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