7 research outputs found
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
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 single-aligned moiré 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-moiré superlattice. For both cases we evaluate how well the thermopower agrees with Mott's equation. Finally, we show the same moiré 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
Exploring room temperature spin transport under band gap opening in bilayer graphene
Abstract We study the room-temperature electrical control of charge and spin transport in high-quality bilayer graphene, fully encapsulated with hBN and contacted via 1D spin injectors. We show that spin transport in this device architecture is measurable at room temperature and its spin transport parameters can be modulated by opening of a band gap via a perpendicular displacement field. The modulation of the spin current is dominated by the control of the spin relaxation time with displacement field, demonstrating the basic operation of a spin-based field-effect transistor
Author Correction: Exploring room temperature spin transport under band gap opening in bilayer graphene
Tuneable spin injection in high-quality graphene with one-dimensional contacts
Spintronics involves the development of low-dimensional electronic systems
with potential use in quantum-based computation. In graphene, there has been
significant progress in improving spin transport characteristics by
encapsulation and reducing impurities, but the influence of standard
two-dimensional (2D) tunnel contacts, via pinholes and doping of the graphene
channel, remains difficult to eliminate. Here, we report the observation of
spin injection and tuneable spin signal in fully-encapsulated graphene, enabled
by van der Waals heterostructures with one-dimensional (1D) contacts. This
architecture prevents significant doping from the contacts, enabling
high-quality graphene channels, currently with mobilities up to 130,000
cmVs and spin diffusion lengths approaching 20 m. The
nanoscale-wide 1D contacts allow spin injection both at room and at low
temperature, with the latter exhibiting efficiency comparable with 2D tunnel
contacts. At low temperature, the spin signals can be enhanced by as much as an
order of magnitude by electrostatic gating, adding new functionality.Comment: Manuscript and Supporting Informatio