21 research outputs found
Long wavelength local density of states oscillations near graphene step edges
Using scanning tunneling microscopy and spectroscopy, we have studied the
local density of states (LDOS) of graphene over step edges in boron nitride.
Long wavelength oscillations in the LDOS are observed with maxima parallel to
the step edge. Their wavelength and amplitude are controlled by the energy of
the quasiparticles allowing a direct probe of the graphene dispersion relation.
We also observe a faster decay of the LDOS oscillations away from the step edge
than in conventional metals. This is due to the chiral nature of the Dirac
fermions in graphene.Comment: 5 pages, 4 figures, to appear in Phys. Rev. Let
Quantum Hall Effect, Screening and Layer-Polarized Insulating States in Twisted Bilayer Graphene
We investigate electronic transport in dual-gated twisted bilayer graphene.
Despite the sub-nanometer proximity between the layers, we identify independent
contributions to the magnetoresistance from the graphene Landau level spectrum
of each layer. We demonstrate that the filling factor of each layer can be
independently controlled via the dual gates, which we use to induce Landau
level crossings between the layers. By analyzing the gate dependence of the
Landau level crossings, we characterize the finite inter-layer screening and
extract the capacitance between the atomically-spaced layers. At zero filling
factor, we observe magnetic and displacement field dependent insulating states,
which indicate the presence of counter-propagating edge states with inter-layer
coupling.Comment: 4 pages, 3 figure
Tunneling in graphene-topological insulator hybrid devices
Hybrid graphene-topological insulator (TI) devices were fabricated using a
mechanical transfer method and studied via electronic transport. Devices
consisting of bilayer graphene (BLG) under the TI BiSe exhibit
differential conductance characteristics which appear to be dominated by
tunneling, roughly reproducing the BiSe density of states. Similar
results were obtained for BLG on top of BiSe, with 10-fold greater
conductance consistent with a larger contact area due to better surface
conformity. The devices further show evidence of inelastic phonon-assisted
tunneling processes involving both BiSe and graphene phonons. These
processes favor phonons which compensate for momentum mismatch between the TI
and graphene points. Finally, the utility of these tunnel
junctions is demonstrated on a density-tunable BLG device, where the
charge-neutrality point is traced along the energy-density trajectory. This
trajectory is used as a measure of the ground-state density of states
Electrically-driven amplification of terahertz acoustic waves in graphene
In graphene devices, the electronic drift velocity can easily exceed the
speed of sound in the material at moderate current biases. Under this
condition, the electronic system can efficiently amplify acoustic phonons,
leading to the exponential growth of sound waves in the direction of the
carrier flow. Here, we demonstrate that such phonon amplification can
significantly modify the electrical properties of graphene devices. We observe
a super-linear growth of the resistivity in the direction of the carrier flow
when the drift velocity exceeds the speed of sound, causing up to a 7 times
increase over 8 micrometers. The resistance growth is observable for carrier
densities away from the Dirac point and is enhanced at cryogenic temperatures.
These observations are explained by a theoretical model for the
electrical-amplification of acoustic phonons, which reach frequencies up to 2.2
terahertz with the nanoscale wavelength set by gate-tunable ~kF transitions
across the Fermi surface. These findings offer a route to high-frequency
on-chip sound generation and detection, which can be used to modulate and probe
electronic physics in van der Waals heterostructures in the terahertz frequency
range
Controllable Strain-driven Topological Phase Transition and Dominant Surface State Transport in High-Quality HfTe5 Samples
Controlling materials to create and tune topological phases of matter could
potentially be used to explore new phases of topological quantum matter and to
create novel devices where the carriers are topologically protected. It has
been demonstrated that a trivial insulator can be converted into a topological
state by modulating the spin-orbit interaction or the crystal lattice. However,
there are limited methods to controllably and efficiently tune the crystal
lattice and at the same time perform electronic measurements at cryogenic
temperatures. Here, we use large controllable strain to demonstrate the
topological phase transition from a weak topological insulator phase to a
strong topological insulator phase in high-quality HfTe5 samples. After
applying high strain to HfTe5 and converting it into a strong topological
insulator, we found that the sample's resistivity increased by more than two
orders of magnitude (24,000%) and that the electronic transport is dominated by
the topological surface states at cryogenic temperatures. Our findings show
that HfTe5 is an ideal material for engineering topological properties, and it
could be generalized to study topological phase transitions in van der Waals
materials and heterostructures. These results can pave the way to create novel
devices with applications ranging from spintronics to fault-tolerant
topologically protected quantum computers
Emergence of Superlattice Dirac Points in Graphene on Hexagonal Boron Nitride
The Schr\"odinger equation dictates that the propagation of nearly free
electrons through a weak periodic potential results in the opening of band gaps
near points of the reciprocal lattice known as Brillouin zone boundaries.
However, in the case of massless Dirac fermions, it has been predicted that the
chirality of the charge carriers prevents the opening of a band gap and instead
new Dirac points appear in the electronic structure of the material. Graphene
on hexagonal boron nitride (hBN) exhibits a rotation dependent Moir\'e pattern.
In this letter, we show experimentally and theoretically that this Moir\'e
pattern acts as a weak periodic potential and thereby leads to the emergence of
a new set of Dirac points at an energy determined by its wavelength. The new
massless Dirac fermions generated at these superlattice Dirac points are
characterized by a significantly reduced Fermi velocity. The local density of
states near these Dirac cones exhibits hexagonal modulations indicating an
anisotropic Fermi velocity.Comment: 16 pages, 6 figure
STM Spectroscopy of ultra-flat graphene on hexagonal boron nitride
Graphene has demonstrated great promise for future electronics technology as
well as fundamental physics applications because of its linear energy-momentum
dispersion relations which cross at the Dirac point. However, accessing the
physics of the low density region at the Dirac point has been difficult because
of the presence of disorder which leaves the graphene with local microscopic
electron and hole puddles, resulting in a finite density of carriers even at
the charge neutrality point. Efforts have been made to reduce the disorder by
suspending graphene, leading to fabrication challenges and delicate devices
which make local spectroscopic measurements difficult. Recently, it has been
shown that placing graphene on hexagonal boron nitride (hBN) yields improved
device performance. In this letter, we use scanning tunneling microscopy to
show that graphene conforms to hBN, as evidenced by the presence of Moire
patterns in the topographic images. However, contrary to recent predictions,
this conformation does not lead to a sizable band gap due to the misalignment
of the lattices. Moreover, local spectroscopy measurements demonstrate that the
electron-hole charge fluctuations are reduced by two orders of magnitude as
compared to those on silicon oxide. This leads to charge fluctuations which are
as small as in suspended graphene, opening up Dirac point physics to more
diverse experiments than are possible on freestanding devices.Comment: Nature Materials advance online publication 13/02/201
Exceptional electronic transport and quantum oscillations in thin bismuth crystals grown inside van der Waals materials
Confining materials to two-dimensional forms changes the behavior of
electrons and enables new devices. However, most materials are challenging to
produce as uniform thin crystals. Here, we present a new synthesis approach
where crystals are grown in a nanoscale mold defined by atomically-flat van der
Waals (vdW) materials. By heating and compressing bismuth in a vdW mold made of
hexagonal boron nitride (hBN), we grow ultraflat bismuth crystals less than 10
nanometers thick. Due to quantum confinement, the bismuth bulk states are
gapped, isolating intrinsic Rashba surface states for transport studies. The
vdW-molded bismuth shows exceptional electronic transport, enabling the
observation of Shubnikov-de Haas quantum oscillations originating from the
(111) surface state Landau levels, which have eluded previous studies. By
measuring the gate-dependent magnetoresistance, we observe multi-carrier
quantum oscillations and Landau level splitting, with features originating from
both the top and bottom surfaces. Our vdW-mold growth technique establishes a
platform for electronic studies and control of bismuth's Rashba surface states
and topological boundary modes. Beyond bismuth, the vdW-molding approach
provides a low-cost way to synthesize ultrathin crystals and directly integrate
them into a vdW heterostructure
Correlated insulator behaviour at half-filling in magic-angle graphene superlattices
Van der Waals (vdW) heterostructures are an emergent class of metamaterials comprised of vertically stacked two-dimensional (2D) building blocks, which provide us with a vast tool set to engineer their properties on top of the already rich tunability of 2D materials. 1 One of the knobs, the twist angle between different layers, plays a crucial role in the ultimate electronic properties of a vdW heterostructure and does not have a direct analog in other systems such as MBE-grown semiconductor heterostructures. For small twist angles, the moiré pattern produced by the lattice misorientation creates a long-range modulation. So far, the study of the effect of twist angles in vdW heterostructures has been mostly concentrated in graphene/hex a gonal boron nitride (h-BN) twisted structures, which exhibit relatively weak interlayer interaction due to the presence of a large bandgap in h-BN. 2-5 Here we show that when two graphene sheets are twisted by an angle close to the theoretically predicted ‘magic angle’, the resulting flat band structure near charge neutrality gives rise to a strongly-correlated electronic system . 6 These flat bands exhibit half-filling insulating phases at zero magnetic field, which we show to be a Mott-like insulator arising from electrons localized in the moiré superlattice. These unique properties of magic-angle twisted bilayer graphene (TwBLG) open up a new playground for exotic many-body quantum phases in a 2D platform made of pure carbon and without mag netic field. The easy accessibility of the flat bands, the electrical tunability, and the bandwidth tunability though twist angle may pave the way towards more exotic correlated systems, such as unconventional superconductors or quantum spin liquids