100 research outputs found
Time-resolved lateral spin-caloric transport of optically generated spin packets in n-GaAs
We report on lateral spin-caloric transport (LSCT) of electron spin packets
which are optically generated by ps laser pulses in the non-magnetic
semiconductor n-GaAs at K. LSCT is driven by a local temperature
gradient induced by an additional cw heating laser. The spatio-temporal
evolution of the spin packets is probed using time-resolved Faraday rotation.
We demonstrate that the local temperature-gradient induced spin diffusion is
solely driven by a non-equilibrium hot spin distribution, i.e. without
involvement of phonon drag effects. Additional electric field-driven spin drift
experiments are used to verify directly the validity of the non-classical
Einstein relation for moderately doped semiconductors at low temperatures for
near band-gap excitation.Comment: 12 pages, 8 figure
Dry-transferred CVD graphene for inverted spin valve devices
Integrating high-mobility graphene grown by chemical vapor deposition (CVD)
into spin transport devices is one of the key tasks in graphene spintronics. We
use a van der Waals pickup technique to transfer CVD graphene by hexagonal
boron nitride (hBN) from the copper growth substrate onto predefined Co/MgO
electrodes to build inverted spin valve devices. Two approaches are presented:
(i) a process where the CVD-graphene/hBN stack is first patterned into a bar
and then transferred by a second larger hBN crystal onto spin valve electrodes
and (ii) a direct transfer of a CVD-graphene/hBN stack. We report record high
spin lifetimes in CVD graphene of up to 1.75 ns at room temperature. Overall,
the performances of our devices are comparable to devices fabricated from
exfoliated graphene also revealing nanosecond spin lifetimes. We expect that
our dry transfer methods pave the way towards more advanced device geometries
not only for spintronic applications but also for CVD-graphene-based
nanoelectronic devices in general where patterning of the CVD graphene is
required prior to the assembly of final van der Waals heterostructures.Comment: 5 pages, 3 figure
High mobility dry-transferred CVD bilayer graphene
We report on the fabrication and characterization of high-quality chemical
vapor-deposited (CVD) bilayer graphene (BLG). In particular, we demonstrate
that CVD-grown BLG can mechanically be detached from the copper foil by an
hexagonal boron nitride (hBN) crystal after oxidation of the copper-to-BLG
interface. Confocal Raman spectroscopy reveals an AB-stacking order of the BLG
crystals and a high structural quality. From transport measurements on fully
encapsulated hBN/BLG/hBN Hall bar devices we extract charge carrier mobilities
up to 180,000 cm/(Vs) at 2 K and up to 40,000 cm/(Vs) at 300 K,
outperforming state-of-the-art CVD bilayer graphene devices. Moreover, we show
an on-off ration of more than 10,000 and a band gap opening with values of up
to 15 meV for a displacement field of 0.2 V/nm in such CVD grown BLG.Comment: 5 pages, 4 figure
Nanosecond spin lifetimes in bottom-up fabricated bilayer graphene spin-valves with atomic layer deposited AlO spin injection and detection barriers
We present spin transport studies on bi- and trilayer graphene non-local
spin-valves which have been fabricated by a bottom-up fabrication method. By
this technique, spin injection electrodes are first deposited onto
Si/SiO substrates with subsequent mechanical transfer of a
graphene/hBN heterostructure. We showed previously that this technique allows
for nanosecond spin lifetimes at room temperature combined with carrier
mobilities which exceed 20,000 cm/(Vs). Despite strongly enhanced spin and
charge transport properties, the MgO injection barriers in these devices
exhibit conducting pinholes which still limit the measured spin lifetimes. We
demonstrate that these pinholes can be partially diminished by an oxygen
treatment of a trilayer graphene device which is seen by a strong increase of
the contact resistance area products of the Co/MgO electrodes. At the same
time, the spin lifetime increases from 1 ns to 2 ns. We believe that the
pinholes partially result from the directional growth in molecular beam
epitaxy. For a second set of devices, we therefore used atomic layer deposition
of AlO which offers the possibility to isotropically deposit more
homogeneous barriers. While the contacts of the as-fabricated bilayer graphene
devices are non-conductive, we can partially break the oxide barriers by
voltage pulses. Thereafter, the devices also exhibit nanosecond spin lifetimes.Comment: 6 pages, 4 figure
From diffusive to ballistic transport in etched graphene constrictions and nanoribbons
Graphene nanoribbons and constrictions are envisaged as fundamental
components of future carbon-based nanoelectronic and spintronic devices. At
nanoscale, electronic effects in these devices depend heavily on the dimensions
of the active channel and the nature of edges. Hence, controlling both these
parameters is crucial to understand the physics in such systems. This review is
about the recent progress in the fabrication of graphene nanoribbons and
constrictions in terms of low temperature quantum transport. In particular,
recent advancements using encapsulated graphene allowing for quantized
conductance and future experiments towards exploring spin effects in these
devices are presented. The influence of charge carrier inhomogeneity and the
important length scales which play a crucial role for transport in high quality
samples are also discussed.Comment: 32 pages, 6 figures. Will appear in Annalen der Physi
Charge carrier density-dependent Raman spectra of graphene encapsulated in hexagonal boron nitride
We present low-temperature Raman measurements on gate tunable graphene
encapsulated in hexagonal boron nitride, which allows to study in detail the
Raman G and 2D mode frequencies and line widths as function of the charge
carrier density. We observe a clear softening of the Raman G mode (of up to 2.5
cm) at low carrier density due to the phonon anomaly and a residual
G~mode line width of 3.5 cm at high doping. From analyzing the
G mode dependence on doping and laser power we extract an
electron-phonon-coupling constant of 4.4 10 (for the
G mode phonon). The ultra-flat nature of encapsulated graphene results in a
minimum Raman 2D peak line width of 14.5 cm and allows to observe the
intrinsic electron-electron scattering induced broadening of the 2D peak of up
to 18 cm for an electron density of 510 cm (laser
excitation energy of 2.33 eV). Our findings not only provide insights into
electron-phonon coupling and the role of electron-electron scattering for the
broadening of the 2D peak, but also crucially shows the limitations when it
comes to the use of Raman spectroscopy (i.e. the use of the frequencies and the
line widths of the G and 2D modes) to benchmark graphene in terms of charge
carrier density, strain and strain inhomogenities. This is particularly
relevant when utilizing spatially-resolved 2D Raman line width maps to assess
substrate-induced nanometer-scale strain variations.Comment: 10 pages, 5 figure
Identifying suitable substrates for high-quality graphene-based heterostructures
We report on a scanning confocal Raman spectroscopy study investigating the
strain-uniformity and the overall strain and doping of high-quality chemical
vapour deposited (CVD) graphene-based heterostuctures on a large number of
different substrate materials, including hexagonal boron nitride (hBN),
transition metal dichalcogenides, silicon, different oxides and nitrides, as
well as polymers. By applying a hBN-assisted, contamination free, dry transfer
process for CVD graphene, high-quality heterostructures with low doping
densities and low strain variations are assembled. The Raman spectra of these
pristine heterostructures are sensitive to substrate-induced doping and strain
variations and are thus used to probe the suitability of the substrate material
for potential high-quality graphene devices. We find that the flatness of the
substrate material is a key figure for gaining, or preserving high-quality
graphene.Comment: 6 pages, 5 figure
Quantum transport through MoS constrictions defined by photodoping
We present a device scheme to explore mesoscopic transport through molybdenum
disulfide (MoS) constrictions using photodoping. The devices are based on
van-der-Waals heterostructures where few-layer MoS flakes are partially
encapsulated by hexagonal boron nitride (hBN) and covered by a few-layer
graphene flake to fabricate electrical contacts. Since the as-fabricated
devices are insulating at low temperatures, we use photo-induced remote doping
in the hBN substrate to create free charge carriers in the MoS layer. On
top of the device, we place additional metal structures, which define the shape
of the constriction and act as shadow masks during photodoping of the
underlying MoS/hBN heterostructure. Low temperature two- and four-terminal
transport measurements show evidence of quantum confinement effects.Comment: 9 pages, 6 figure
Low B Field Magneto-Phonon Resonances in Single-Layer and Bilayer Graphene
Many-body effects resulting from strong electron-electron and electron-phonon
interactions play a significant role in graphene physics. We report on their
manifestation in low B field magneto-phonon resonances in high quality
exfoliated single-layer and bilayer graphene encapsulated in hexagonal boron
nitride. These resonances allow us to extract characteristic effective Fermi
velocities, as high as m/s, for the observed "dressed"
Landau level transitions, as well as the broadening of the resonances, which
increases with Landau level index
Spin lifetimes exceeding 12 nanoseconds in graphene non-local spin valve devices
We show spin lifetimes of 12.6 ns and spin diffusion lengths as long as 30.5
\mu m in single layer graphene non-local spin transport devices at room
temperature. This is accomplished by the fabrication of Co/MgO-electrodes on a
Si/SiO substrate and the subsequent dry transfer of a graphene-hBN-stack on
top of this electrode structure where a large hBN flake is needed in order to
diminish the ingress of solvents along the hBN-to-substrate interface.
Interestingly, long spin lifetimes are observed despite the fact that both
conductive scanning force microscopy and contact resistance measurements reveal
the existence of conducting pinholes throughout the MgO spin
injection/detection barriers. The observed enhancement of the spin lifetime in
single layer graphene by a factor of 6 compared to previous devices exceeds
current models of contact-induced spin relaxation which paves the way towards
probing intrinsic spin properties of graphene.Comment: 8 pages, 5 figure
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