66 research outputs found
Detecting Topological Currents in Graphene Superlattices
Topological materials may exhibit Hall-like currents flowing transversely to
the applied electric field even in the absence of a magnetic field. In graphene
superlattices, which have broken inversion symmetry, topological currents
originating from graphene's two valleys are predicted to flow in opposite
directions and combine to produce long-range charge neutral flow. We observe
this effect as a nonlocal voltage at zero magnetic field in a narrow energy
range near Dirac points at distances as large as several microns away from the
nominal current path. Locally, topological currents are comparable in strength
to the applied current, indicating large valley-Hall angles. The long-range
character of topological currents and their transistor-like control by gate
voltage can be exploited for information processing based on the valley degrees
of freedom.Comment: 19 pgs, 9 fg
High-temperature quantum oscillations caused by recurring Bloch states in graphene superlattices
Cyclotron motion of charge carriers in metals and semiconductors leads to Landau quantization and magneto-oscillatory behavior in their properties. Cryogenic temperatures are usually required to observe these oscillations. We show that graphene superlattices support a different type of quantum oscillations that do not rely on Landau quantization. The oscillations are extremely robust and persist well above room temperature in magnetic fields of only a few T. We attribute this phenomenon to repetitive changes in the electronic structure of superlattices such that charge carriers experience effectively no magnetic field at simple fractions of the flux quantum per superlattice unit cell. Our work points at unexplored physics in Hofstadter butterfly systems at high temperatures
Edge currents shunt the insulating bulk in gapped graphene
An energy gap can be opened in the spectrum of graphene reaching values as large as 0.2 eV in the case of bilayers. However, such gaps rarely lead to the highly insulating state expected at low temperatures. This long-standing puzzle is usually explained by charge inhomogeneity. Here we revisit the issue by investigating proximity-induced superconductivity in gapped graphene and comparing normal-state measurements in the Hall bar and Corbino geometries. We find that the supercurrent at the charge neutrality point in gapped graphene propagates along narrow channels near the edges. This observation is corroborated by using the edgeless Corbino geometry in which case resistivity at the neutrality point increases exponentially with increasing the gap, as expected for an ordinary semiconductor. In contrast, resistivity in the Hall bar geometry saturates to values of about a few resistance quanta. We attribute the metallic-like edge conductance to a nontrivial topology of gapped Dirac spectra
Nanoscale mapping and spectroscopy of non-radiative hyperbolic modes in hexagonal boron nitride nanostructures
The inherent crystal anisotropy of hexagonal boron nitride (hBN) sustains
naturally hyperbolic phonon polaritons, i.e. polaritons that can propagate with
very large wavevectors within the material volume, thereby enabling optical
confinement to exceedingly small dimensions. Indeed, previous research has
shown that nanometer-scale truncated nanocone hBN cavities, with deep
subwavelength dimensions, support three-dimensionally confined optical modes in
the mid-infrared. Due to optical selection rules, only a few of many such modes
predicted theoretically have been observed experimentally via far-field
reflection and scattering-type scanning near-field optical microscopy. The
Photothermal induced resonance (PTIR) technique probes optical and vibrational
resonances overcoming weak far-field emission by leveraging an atomic force
microscope (AFM) probe to transduce local sample expansion due to light
absorption. Here we show that PTIR enables the direct observation of previously
unobserved, dark hyperbolic modes of hBN nanostructures. Leveraging these
optical modes could yield a new degree of control over the electromagnetic
near-field concentration, polarization and angular momentum in nanophotonic
applications.Comment: 14 pages with references, 4 figure
Unintentional high density p-type modulation doping of a GaAs/AlAs core-multi-shell nanowire
Achieving significant doping in GaAs/AlAs core/shell nanowires (NWs) is of
considerable technological importance but remains a challenge due to the
amphoteric behavior of the dopant atoms. Here we show that placing a narrow
GaAs quantum well in the AlAs shell effectively getters residual carbon
acceptors leading to an \emph{unintentional} p-type doping. Magneto-optical
studies of such a GaAs/AlAs core multi-shell NW reveal quantum confined
emission. Theoretical calculations of NW electronic structure confirm quantum
confinement of carriers at the core/shell interface due to the presence of
ionized carbon acceptors in the 1~nm GaAs layer in the shell.
Micro-photoluminescence in high magnetic field shows a clear signature of
avoided crossings of the Landau level emission line with the Landau
level TO phonon replica. The coupling is caused by the resonant hole-phonon
interaction, which points to a large 2D hole density in the structure.Comment: just published in Nano Letters
(http://pubs.acs.org/doi/full/10.1021/nl500818k
Hierarchy of Hofstadter states and replica quantum Hall ferromagnetism in graphene superlattices
Self-similarity and fractals have fascinated researchers across various
disciplines. In graphene placed on boron nitride and subjected to a magnetic
field, self-similarity appears in the form of numerous replicas of the original
Dirac spectrum, and their quantization gives rise to a fractal pattern of
Landau levels, referred to as the Hofstadter butterfly. Here we employ
capacitance spectroscopy to probe directly the density of states (DoS) and
energy gaps in this spectrum. Without a magnetic field, replica spectra are
seen as pronounced DoS minima surrounded by van Hove singularities. The
Hofstadter butterfly shows up as recurring Landau fan diagrams in high fields.
Electron-electron interactions add another twist to the self-similar behaviour.
We observe suppression of quantum Hall ferromagnetism, a reverse Stoner
transition at commensurable fluxes and additional ferromagnetism within replica
spectra. The strength and variety of the interaction effects indicate a large
playground to study many-body physics in fractal Dirac systems.Comment: Nature Phys. (2014
Large tunable valley splitting in edge-free graphene quantum dots on boron nitride
Coherent manipulation of binary degrees of freedom is at the heart of modern
quantum technologies. Graphene offers two binary degrees: the electron spin and
the valley. Efficient spin control has been demonstrated in many solid state
systems, while exploitation of the valley has only recently been started, yet
without control on the single electron level. Here, we show that van-der Waals
stacking of graphene onto hexagonal boron nitride offers a natural platform for
valley control. We use a graphene quantum dot induced by the tip of a scanning
tunneling microscope and demonstrate valley splitting that is tunable from -5
to +10 meV (including valley inversion) by sub-10-nm displacements of the
quantum dot position. This boosts the range of controlled valley splitting by
about one order of magnitude. The tunable inversion of spin and valley states
should enable coherent superposition of these degrees of freedom as a first
step towards graphene-based qubits
Resonant tunnelling between the chiral Landau states of twisted graphene lattices
A class of multilayered functional materials has recently emerged in which the component atomic layers are held together by weak van der Waals forces that preserve the structural integrity and physical properties of each layer. An exemplar of such a structure is a transistor device in which relativistic Dirac Fermions can resonantly tunnel through a boron nitride barrier, a few atomic layers thick, sandwiched between two graphene electrodes. An applied magnetic field quantises graphene's gapless conduction and valence band states into discrete Landau levels, allowing us to resolve individual inter-Landau level transitions and thereby demonstrate that the energy, momentum and chiral properties of the electrons are conserved in the tunnelling process. We also demonstrate that the change in the semiclassical cyclotron trajectories, following an inter-layer tunnelling event, is analogous to the case of intra-layer Klein tunnelling
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