24 research outputs found
Quantum Transport and Field Induced Insulating States in Bilayer Graphene pnp Junctions
We perform transport measurements in high quality bilayer graphene pnp
junctions with suspended top gates. At a magnetic field B=0, we demonstrate
band gap opening by an applied perpendicular electric field, with an On/Off
ratio up to 20,000 at 260mK. Within the band gap, the conductance decreases
exponentially by 3 orders of magnitude with increasing electric field, and can
be accounted for by variable range hopping with a gate-tunable density of
states, effective mass, and localization length. At large B, we observe quantum
Hall conductance with fractional values, which arise from equilibration of edge
states between differentially-doped regions, and the presence of an insulating
state at filling factor {\nu}=0. Our work underscores the importance of bilayer
graphene for both fundamental interest and technological applications.Comment: 4 figures, to appear in Nano Lett. Minor typos correcte
Suspension and Measurement of Graphene and Bi2Se3 Atomic Membranes
Coupling high quality, suspended atomic membranes to specialized electrodes
enables investigation of many novel phenomena, such as spin or Cooper pair
transport in these two dimensional systems. However, many electrode materials
are not stable in acids that are used to dissolve underlying substrates. Here
we present a versatile and powerful multi-level lithographical technique to
suspend atomic membranes, which can be applied to the vast majority of
substrate, membrane and electrode materials. Using this technique, we
fabricated suspended graphene devices with Al electrodes and mobility of 5500
cm^2/Vs. We also demonstrate, for the first time, fabrication and measurement
of a free-standing thin Bi2Se3 membrane, which has low contact resistance to
electrodes and a mobility of >~500 cm^2/Vs
Visualizing the Effect of an Electrostatic Gate with Angle-Resolved Photoemission Spectroscopy
Electrostatic gating is pervasive in materials science, yet its effects on
the electronic band structure of materials has never been revealed directly by
angle-resolved photoemission spectroscopy (ARPES), the technique of choice to
non-invasively probe the electronic band structure of a material. By means of a
state-of-the-art ARPES setup with sub-micron spatial resolution, we have
investigated a heterostructure composed of Bernal-stacked bilayer graphene
(BLG) on hexagonal boron nitride and deposited on a graphite flake. By voltage
biasing the latter, the electric field effect is directly visualized on the
valence band as well as on the carbon 1s core level of BLG. The band gap
opening of BLG submitted to a transverse electric field is discussed and the
importance of intralayer screening is put forward. Our results pave the way for
new studies that will use momentum-resolved electronic structure information to
gain insight on the physics of materials submitted to the electric field
effect
Control of Giant Topological Magnetic Moment and Valley Splitting in Trilayer Graphene
Bloch states of electrons in honeycomb two-dimensional crystals with
multi-valley band structure and broken inversion symmetry have orbital magnetic
moments of a topological nature. In crystals with two degenerate valleys, a
perpendicular magnetic field lifts the valley degeneracy via a Zeeman effect
due to these magnetic moments, leading to magnetoelectric effects which can be
leveraged for creating valleytronic devices. In this work, we demonstrate that
trilayer graphene with Bernal stacking, (ABA TLG) hosts topological magnetic
moments with a large and widely tunable valley g-factor, reaching a value 1050
at the extreme of the studied parametric range. The reported experiment
consists in sublattice-resolved scanning tunneling spectroscopy under
perpendicular electric and magnetic fields that control the TLG bands. The
tunneling spectra agree very well with the results of theoretical modeling that
includes the full details of the TLG tight-binding model and accounts for a
quantum-dot-like potential profile formed electrostatically under the scanning
tunneling microscope tip.Comment: Manuscript and Supporting Information update
Magnetoconductance Oscillations in High-Mobility Suspended Bilayer and Trilayer Graphene
We report pronounced magnetoconductance oscillations observed on suspended
bilayer and trilayer graphene devices with mobilities up to 270,000 cm2/Vs. For
bilayer devices, we observe conductance minima at all integer filling factors
nu between 0 and -8, as well as a small plateau at {\nu}=1/3. For trilayer
devices, we observe features at nu=-1, -2, -3 and -4, and at {\nu}~0.5 that
persist to 4.5K at B=8T. All of these features persist for all accessible
values of Vg and B, and could suggest the onset of symmetry breaking of the
first few Landau (LL) levels and fractional quantum Hall states.Comment: to appear in Phys. Rev. Let