24 research outputs found
Snake Trajectories in Ultraclean Graphene p-n Junctions
Snake states are trajectories of charge carriers curving back and forth along
an interface. There are two types of snake states, formed by either inverting
the magnetic field direction or the charge carrier type at an interface.
Whereas the former has been demonstrated in GaAs-AlGaAs heterostructures, the
latter has become conceivable only with the advance of ballistic graphene where
a gapless p-n interface governed by Klein tunneling can be formed. Such snake
states were hidden in previous experiments due to limited sample quality. Here
we report on magneto-conductance oscillations due to snake states in a
ballistic suspended graphene p-n-junction which occur already at a very small
magnetic field of 20mT. The visibility of 30% is enabled by Klein collimation.
Our finding is firmly supported by quantum transport simulations. We
demonstrate the high tunability of the device and operate it in different
magnetic field regimesComment: Accepted for publication in Nature Communication
Near-surface InAs 2DEG on a GaAs substrate: characterization and superconducting proximity effect
We have studied a near-surface two-dimensional electron gas based on an InAs
quantum well on a GaAs substrate. In devices without a dielectric layer we
estimated large electron mobilities on the order of cm/Vs. We have
observed quantized conductance in a quantum point contact, and determined the
g-factor. Using samples with an epitaxial Al layer, we defined multiple
Josephson junctions and found the critical current to be gate tunable. Based on
multiple Andreev reflections the semiconductor-superconductor interface is
transparent, with an induced gap of 125 eV. Our results demonstrate the
viability of this platform for hybrid topological superconductor devices
Exfoliation of single layer BiTeI flakes
Spin orbit interaction can be strongly boosted when a heavy element is
embedded into an inversion asymmetric crystal field. A simple structure to
realize this concept in a 2D crystal contains three atomic layers, a middle one
built up from heavy elements generating strong atomic spin-orbit interaction
and two neighboring atomic layers with different electron negativity. BiTeI is
a promising candidate for such a 2D crystal, since it contains heavy Bi layer
between Te and I layers. Recently the bulk form of BiTeI attracted considerable
attention due to its giant Rashba interaction, however, 2D form of this crystal
was not yet created. In this work we report the first exfoliation of single
layer BiTeI using a recently developed exfoliation technique on stripped gold.
Our combined scanning probe studies and first principles calculations show that
SL BiTeI flakes with sizes of 100 m were achieved which are stable at
ambient conditions. The giant Rashba splitting and spin-momentum locking of
this new member of 2D crystals open the way towards novel spintronic
applications and synthetic topological heterostructures.Comment: 20 pages, 5 figure
Signatures of single quantum dots in graphene nanoribbons within the quantum Hall regime
We report on the observation of periodic conductance
oscillations near quantum Hall plateaus in suspended graphene
nanoribbons. They are attributed to single quantum dots that
are formed in the narrowest part of the ribbon, in the
valleys and hills of a disorder potential. In a wide flake
with two gates, a double-dot system's signature has been
observed. Electrostatic confinement is enabled in single-
layer graphene due to the gaps that are formed between the
Landau levels, suggesting a way to create gate-defined
quantum dots that can be accessed with quantum Hall edge
states
Revealing the band structure of ZrTe using Multicarrier Transport
The layered material ZrTe appears to exhibit several exotic behaviors
which resulted in significant interest recently, although the exact properties
are still highly debated. Among these we find a Dirac/Weyl semimetallic
behavior, nontrivial spin textures revealed by low temperature transport, and a
potential weak or strong topological phase. The anomalous behavior of
resistivity has been recently elucidated as originating from band shifting in
the electronic structure. Our work examines magnetotransport behavior in
ZrTe samples in the context of multicarrier transport. The results, in
conjunction with ab-initio band structure calculations, indicate that many of
the transport features of ZrTe across the majority of the temperature range
can be adequately explained by the semiclassical multicarrier transport model
originating from a complex Fermi surface.Comment: Main Text: 10 pages, 5 figures; Supporting Information: 10 pages, 7
figure
Boosting proximity spin orbit coupling in graphene/WSe heterostructures via hydrostatic pressure
Van der Waals heterostructures composed of multiple few layer crystals allow
the engineering of novel materials with predefined properties. As an example,
coupling graphene weakly to materials with large spin orbit coupling (SOC)
allows to engineer a sizeable SOC in graphene via proximity effects. The
strength of the proximity effect depends on the overlap of the atomic orbitals,
therefore, changing the interlayer distance via hydrostatic pressure can be
utilized to enhance the interlayer coupling between the layers. In this work,
we report measurements on a graphene/WSe heterostructure exposed to
increasing hydrostatic pressure. A clear transition from weak localization to
weak anti-localization is visible as the pressure increases, demonstrating the
increase of induced SOC in graphene
Exfoliation of single layer BiTeI flakes
Spin orbit interaction is strongly enhanced in structures where a heavy element is embedded in an inversion asymmetric crystal field. A simple way for realizing such a setup is to take a single atomic layer of a heavy element and encapsulate it between two atomic layers of different elemental composition. BiTeI is a promising candidate for such a 2D crystal. In its bulk form BiTeI consists of loosely coupled three atom thick layers where a layer of high atomic number Bi are sandwiched between Te and I sheets. Despite considerable recent attention to bulk BiTeI due to its giant Rashba spin splitting, the isolation of a single layer remained elusive. In this work we report the first successful isolation and characterization of a single layer of BiTeI using a novel exfoliation technique on stripped gold. Our scanning probe studies and first principles calculations show that the fabricated 100 mu m sized BiTeI flakes are stable at ambient conditions. Giant Rashba splitting and spin-momentum locking of this new 2D crystal opens the way towards novel spintronic applications and synthetic topological heterostructures
Gate-controlled conductance enhancement from quantum Hall channels along graphene p-n junctions
The formation of quantum Hall channels inside the bulk of graphene is studied using various contact and gate geometries. p-n junctions are created along the longitudinal direction of samples, and enhanced conductance is observed in the case of bipolar doping due to new conducting channels forming in the bulk, whose position, propagating direction and, in one geometry, coupling to electrodes are determined by the gate-controlled filling factor across the device. This effect could be exploited to probe the behavior and interaction of quantum Hall channels protected against uncontrolled scattering at the edges
Thermopower in hBN/graphene/hBN superlattices
Thermoelectric effects are highly sensitive to the asymmetry in the density
of states around the Fermi energy and can be exploited as probes of the
electronic structure. We experimentally study thermopower in high-quality
monolayer graphene, within heterostructures consisting of complete hBN
encapsulation and 1D edge contacts, where the graphene and hBN lattices are
aligned. When graphene is aligned to one of the hBN layers, we demonstrate the
presence of additional sign reversals in the thermopower as a function of
carrier density, directly evidencing the presence of the moir\'e superlattice.
We show that the temperature dependence of the thermopower enables the
assessment of the role of built-in strain variation and van Hove singularities
and hints at the presence of Umklapp electron-electron scattering processes. As
the thermopower peaks around the neutrality point, this allows to probe the
energy spectrum degeneracy. Further, when graphene is double-aligned with the
top and bottom hBN crystals, the thermopower exhibits features evidencing
multiple cloned Dirac points caused by the differential super-moir\'e lattice.
For both cases we evaluate how well the thermopower agrees with Mott's
equation. Finally, we show the same superlattice device can exhibit a
temperature-driven thermopower reversal from positive to negative and vice
versa, by controlling the carrier density. The study of thermopower provides an
alternative approach to study the electronic structure of 2D superlattices,
whilst offering opportunities to engineer the thermoelectric response on these
heterostructures.Comment: 9 pages, 3 figure