283 research outputs found
Question-answering, relevance feedback and summarisation : TREC-9 interactive track report
In this paper we report on the effectiveness of query-biased summaries for a question-answering task. Our summarisation system presents searchers with short summaries of documents, composed of a series of highly matching sentences extracted from the documents. These summaries are also used as evidence for a query expansion algorithm to test the use of summaries as evidence for interactive and automatic query expansion
Separating spin and charge transport in single wall carbon nanotubes
We demonstrate spin injection and detection in single wall carbon nanotubes
using a 4-terminal, non-local geometry. This measurement geometry completely
separates the charge and spin circuits. Hence all spurious magnetoresistance
effects are eliminated and the measured signal is due to spin accumulation
only. Combining our results with a theoretical model, we deduce a spin
polarization at the contacts of approximately 25 %. We show that the
magnetoresistance changes measured in the conventional two-terminal geometry
are dominated by effects not related to spin accumulation.Comment: Number of pages: 11 Number of figures:
The Magneto-coulomb effect in spin valve devices
We discuss the influence of the magneto-coulomb effect (MCE) on the
magnetoconductance of spin valve devices. We show that MCE can induce
magnetoconductances of several per cents or more, dependent on the strength of
the coulomb blockade. Furthermore, the MCE-induced magnetoconductance changes
sign as a function of gate voltage. We emphasize the importance of separating
conductance changes induced by MCE from those due to spin accumulation in spin
valve devices.Comment: This paper includes 3 figure
Controlling the efficiency of spin injection into graphene by carrier drift
Electrical spin injection from ferromagnetic metals into graphene is hindered
by the impedance mismatch between the two materials. This problem can be
reduced by the introduction of a thin tunnel barrier at the interface. We
present room temperature non-local spin valve measurements in
cobalt/aluminum-oxide/graphene structures with an injection efficiency as high
as 25%, where electrical contact is achieved through relatively transparent
pinholes in the oxide. This value is further enhanced to 43% by applying a DC
current bias on the injector electrodes, that causes carrier drift away from
the contact. A reverse bias reduces the AC spin valve signal to zero or
negative values. We introduce a model that quantitatively predicts the behavior
of the spin accumulation in the graphene under such circumstances, showing a
good agreement with our measurements.Comment: 4 pages, 3 color figure
Electronic spin drift in graphene field effect transistors
We studied the drift of electron spins under an applied DC electric field in
single layer graphene spin valves in a field effect transport geometry at room
temperature. In the metallic conduction regime (
m), for DC fields of about 70 kV/m applied between the spin
injector and spin detector, the spin valve signals are increased/decreased,
depending on the direction of the DC field and the carrier type, by as much as
50%. Sign reversal of the drift effect is observed when switching from
hole to electron conduction. In the vicinity of the Dirac neutrality point the
drift effect is strongly suppressed. The experiments are in quantitative
agreement with a drift-diffusion model of spin transport.Comment: 4 figure
Fast pick up technique for high quality heterostructures of bilayer graphene and hexagonal boron nitride
We present a fast method to fabricate high quality heterostructure devices by
picking up crystals of arbitrary sizes. Bilayer graphene is encapsulated with
hexagonal boron nitride to demonstrate this approach, showing good electronic
quality with mobilities ranging from 17 000 cm^2/V/s at room temperature to 49
000 cm^2/V/s at 4.2 K, and entering the quantum Hall regime below 0.5 T. This
method provides a strong and useful tool for the fabrication of future high
quality layered crystal devices.Comment: 5 pages, 3 figure
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
Controlling spin relaxation in hexagonal BN-encapsulated graphene with a transverse electric field
We experimentally study the electronic spin transport in hBN encapsulated
single layer graphene nonlocal spin valves. The use of top and bottom gates
allows us to control the carrier density and the electric field independently.
The spin relaxation times in our devices range up to 2 ns with spin relaxation
lengths exceeding 12 m even at room temperature. We obtain that the ratio
of the spin relaxation time for spins pointing out-of-plane to spins in-plane
is 0.75 for zero applied perpendicular
electric field. By tuning the electric field this anisotropy changes to
0.65 at 0.7 V/nm, in agreement with an electric field tunable in-plane
Rashba spin-orbit coupling
A transfer technique for high mobility graphene devices on commercially available hexagonal boron nitride
We present electronic transport measurements of single- and bilayer graphene
on commercially available hexagonal boron nitride. We extract mobilities as
high as 125 000 cm^2/V/s at room temperature and 275 000 cm^2/V/s at 4.2 K. The
excellent quality is supported by the early development of the nu = 1 quantum
Hall plateau at a magnetic field of 5 T and temperature of 4.2 K. We also
present a new and accurate transfer technique of graphene to hexagonal boron
nitride crystals. This technique is simple, fast and yields atomically flat
graphene on boron nitride which is almost completely free of bubbles or
wrinkles. The potential of commercially available boron nitride combined with
our transfer technique makes high mobility graphene devices more accessible.Comment: 3 pages, 3 figure
- …