41 research outputs found
Transport Gap in Suspended Bilayer Graphene at Zero Magnetic Field
We report a change of three orders of magnitudes in the resistance of a
suspended bilayer graphene flake which varies from a few ks in the high
carrier density regime to several Ms around the charge neutrality point
(CNP). The corresponding transport gap is 8 meV at 0.3 K. The sequence of
appearing quantum Hall plateaus at filling factor followed by
suggests that the observed gap is caused by the symmetry breaking of the lowest
Landau level. Investigation of the gap in a tilted magnetic field indicates
that the resistance at the CNP shows a weak linear decrease for increasing
total magnetic field. Those observations are in agreement with a spontaneous
valley splitting at zero magnetic field followed by splitting of the spins
originating from different valleys with increasing magnetic field. Both, the
transport gap and field response point toward spin polarized layer
antiferromagnetic state as a ground state in the bilayer graphene sample. The
observed non-trivial dependence of the gap value on the normal component of
suggests possible exchange mechanisms in the system.Comment: 8 pages, 5 figure
Imaging ultra thin layers with helium ion microscopy: Utilizing the channeling contrast mechanism
Background: Helium ion microscopy is a new high-performance alternative to classical scanning electron microscopy. It provides superior resolution and high surface sensitivity by using secondary electrons.\ud
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Results: We report on a new contrast mechanism that extends the high surface sensitivity that is usually achieved in secondary electron images, to backscattered helium images. We demonstrate how thin organic and inorganic layers as well as self-assembled monolayers can be visualized on heavier element substrates by changes in the backscatter yield. Thin layers of light elements on heavy substrates should have a negligible direct influence on backscatter yields. However, using simple geometric calculations of the opaque crystal fraction, the contrast that is observed in the images can be interpreted in terms of changes in the channeling probability.\ud
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Conclusion: The suppression of ion channeling into crystalline matter by adsorbed thin films provides a new contrast mechanism for HIM. This dechanneling contrast is particularly well suited for the visualization of ultrathin layers of light elements on heavier substrates. Our results also highlight the importance of proper vacuum conditions for channeling-based experimental methods\u
Linear scaling between momentum and spin scattering in graphene
Spin transport in graphene carries the potential of a long spin diffusion
length at room temperature. However, extrinsic relaxation processes limit the
current experimental values to 1-2 um. We present Hanle spin precession
measurements in gated lateral spin valve devices in the low to high (up to
10^13 cm^-2) carrier density range of graphene. A linear scaling between the
spin diffusion length and the diffusion coefficient is observed. We measure
nearly identical spin- and charge diffusion coefficients indicating that
electron-electron interactions are relatively weak and transport is limited by
impurity potential scattering. When extrapolated to the maximum carrier
mobilities of 2x10^5 cm^2/Vs, our results predict that a considerable increase
in the spin diffusion length should be possible
Spin transport in high quality suspended graphene devices
We measure spin transport in high mobility suspended graphene (\mu ~ 10^5
cm^2/Vs), obtaining a (spin) diffusion coefficient of 0.1 m^2/s and giving a
lower bound on the spin relaxation time (\tau_s ~ 150 ps) and spin relaxation
length (\lambda_s=4.7 \mu m) for intrinsic graphene. We develop a theoretical
model considering the different graphene regions of our devices that explains
our experimental data.Comment: 22 pages, 6 figures; Nano Letters, Article ASAP (2012)
(http://pubs.acs.org/doi/abs/10.1021/nl301050a
Electronic spin transport in graphene field effect transistors
Spin transport experiments in graphene, a single layer of carbon atoms,
indicate spin relaxation times that are significantly shorter than the
theoretical predictions. We investigate experimentally whether these short spin
relaxation times are due to extrinsic factors, such as spin relaxation caused
by low impedance contacts, enhanced spin flip processes at the device edges or
the presence of an aluminium oxide layer on top of graphene in some samples.
Lateral spin valve devices using a field effect transistor geometry allowed for
the investigation of the spin relaxation as a function of the charge density,
going continuously from metallic hole to electron conduction (charge densities
of cm) via the Dirac charge neutrality point (). The results are quantitatively described by a one dimensional spin
diffusion model where the spin relaxation via the contacts is taken into
account. Spin valve experiments for various injector/detector separations and
spin precession experiments reveal that the longitudinal (T) and the
transversal (T) relaxation times are similar. The anisotropy of the spin
relaxation times and , when the spins are injected
parallel or perpendicular to the graphene plane, indicates that the effective
spin orbit fields do not lie exclusively in the two dimensional graphene plane.
Furthermore, the proportionality between the spin relaxation time and the
momentum relaxation time indicates that the spin relaxation mechanism is of the
Elliott-Yafet type. For carrier mobilities of 2-5 cm2^/Vs and
for graphene flakes of 0.1-2 m in width, we found spin relaxation times of
the order of 50-200 ps, times which appear not to be determined by the
extrinsic factors mentioned above.Comment: 11 pages, 13 figure
Field induced quantum-Hall ferromagnetism in suspended bilayer graphene
We have measured the magneto-resistance of freely suspended high-mobility
bilayer graphene. For magnetic fields T we observe the opening of a field
induced gap at the charge neutrality point characterized by a diverging
resistance. For higher fields the eight-fold degenerated lowest Landau level
lifts completely. Both the sequence of this symmetry breaking and the strong
transition of the gap-size point to a ferromagnetic nature of the insulating
phase developing at the charge neutrality point.Comment: 7 pages, 5 figure
Large yield production of high mobility freely suspended graphene electronic devices on a PMGI based organic polymer
The recent observation of fractional quantum Hall effect in high mobility
suspended graphene devices introduced a new direction in graphene physics, the
field of electron-electron interaction dynamics. However, the technique used
currently for the fabrication of such high mobility devices has several
drawbacks. The most important is that the contact materials available for
electronic devices are limited to only a few metals (Au, Pd, Pt, Cr and Nb)
since only those are not attacked by the reactive acid (BHF) etching
fabrication step. Here we show a new technique which leads to mechanically
stable suspended high mobility graphene devices which is compatible with almost
any type of contact material. The graphene devices prepared on a
polydimethylglutarimide based organic resist show mobilities as high as 600.000
cm^2/Vs at an electron carrier density n = 5.0 10^9 cm^-2 at 77K. This
technique paves the way towards complex suspended graphene based spintronic,
superconducting and other types of devices.Comment: 14 pages, 4 figure
Coexistence of electron and hole transport in graphene
When sweeping the carrier concentration in monolayer graphene through the
charge neutrality point, the experimentally measured Hall resistivity shows a
smooth zero crossing. Using a two- component model of coexisting electrons and
holes around the charge neutrality point, we unambiguously show that both types
of carriers are simultaneously present. For high magnetic fields up to 30 T the
electron and hole concentrations at the charge neutrality point increase with
the degeneracy of the zero-energy Landau level which implies a quantum Hall
metal state at \nu=0 made up by both electrons and holes.Comment: 5 pages, 6 figure
Anisotropic spin relaxation in graphene
Spin relaxation in graphene is investigated in electrical graphene spin valve
devices in the non-local geometry. Ferromagnetic electrodes with in-plane
magnetizations inject spins parallel to the graphene layer. They are subject to
Hanle spin precession under a magnetic field applied perpendicular to the
graphene layer. Fields above 1.5 T force the magnetization direction of the
ferromagnetic contacts to align to the field, allowing injection of spins
perpendicular to the graphene plane. A comparison of the spin signals at B = 0
and B = 2 T shows a 20 % decrease in spin relaxation time for spins
perpendicular to the graphene layer compared to spins parallel to the layer. We
analyze the results in terms of the different strengths of the spin orbit
effective fields in the in-plane and out-of-plane directions.Comment: 5 pages, 4 figure