1,248 research outputs found
Electron transport nonlocality in monolayer graphene modified with hydrogen silsesquioxane polymerization
A number of practical and fundamental applications of graphene requires modification of some of its properties. In this paper we study the effect of polymerization of a hydrogen silsesquioxane film on top of monolayer graphene with the intent to increase the strength of the spin-orbit interaction. The measured nonlocal resistances R-NL were found to be up to 700 Omega, significantly exceeding the expected contribution from conventional Ohmic currents. The RNL dependence on the channel length resembles exponential decay with a characteristic length of lambda similar or equal to 500 nm that is close to the spin-relaxation length in graphene reported elsewhere. The sensitivity of the measured effect to the electron-beam exposure was shown to decrease with an increased level of the surface contamination. However, no modulation of the effect is observed when an in-plane magnetic field is applied. This implies that a spin Hall model fails to explain the observed phenomenon and an alternative interpretation is required.</p
Spin injection and spin accumulation in permalloy-copper mesoscopic spin valves
We study the electrical injection and detection of spin currents in a lateral
spin valve device, using permalloy (Py) as ferromagnetic injecting and
detecting electrodes and copper (Cu) as non-magnetic metal. Our multi-terminal
geometry allows us to experimentally distinguish different magneto resistance
signals, being 1) the spin valve effect, 2) the anomalous magneto resistance
(AMR) effect and 3) Hall effects. We find that the AMR contribution of the Py
contacts can be much bigger than the amplitude of the spin valve effect, making
it impossible to observe the spin valve effect in a 'conventional' measurement
geometry. However, these 'contact' magneto resistance signals can be used to
monitor the magnetization reversal process, making it possible to determine the
magnetic switching fields of the Py contacts of the spin valve device. In a
'non local' spin valve measurement we are able to completely isolate the spin
valve signal and observe clear spin accumulation signals at T=4.2 K as well as
at room temperature. We obtain spin diffusion lengths in copper of 1 micrometer
and 350 nm at T=4.2 K and room temperature respectively.Comment: 8 pages (incl. figures), 7 figures, RevTex, conferenc
The controllable pi - SQUID
We have fabricated and studied a new kind of DC SQUID in which the magnitude
and sign of the critical current of the individual Josephson junctions can be
controlled by additional voltage probes connected to the junctions. We show
that the amplitude of the voltage oscillations of the SQUID as a function of
the applied magnetic field can be tuned and that the phase of the oscillations
can be switched between 0 and in the temperature range of 0.1 - 4.2 K
using a suitable control voltage. This is equivalent to the external
application of (n+1/2) flux quantum.Comment: 3 Figures, submitted to Applied Physics Letter
Nonlocal Spin Transport as a Probe of Viscous Magnon Fluids
Magnons in ferromagnets behave as a viscous fluid over a length scale, the
momentum-relaxation length, below which momentum-conserving scattering
processes dominate. We show theoretically that in this hydrodynamic regime
viscous effects lead to a sign change in the magnon chemical potential, which
can be detected as a sign change in the nonlocal resistance measured in spin
transport experiments. This sign change is observable when the
injector-detector distance becomes comparable to the momentum-relaxation
length. Taking into account momentum- and spin-relaxation processes, we
consider the quasiconservation laws for momentum and spin in a magnon fluid.
The resulting equations are solved for nonlocal spin transport devices in which
spin is injected and detected via metallic leads. Because of the finite
viscosity we also find a backflow of magnons close to the injector lead. Our
work shows that nonlocal magnon spin transport devices are an attractive
platform to develop and study magnon-fluid dynamics
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
Proximity and Josephson effects in superconductor - two dimensional electron gas planar junctions
The DC Josephson effect is theoretically studied in a planar junction in
which a two dimensional electron gas (2DEG) infinite in lateral directions is
in contact with two superconducting electrodes placed on top of the 2DEG. An
energy gap in the excitation spectrum is created in the 2DEG due to the
proximity effect. It is shown that under certain conditions, the region of the
2DEG underneath the superconductors is analogous to a superconducting region
with an order parameter \/, where
\/ depends on the interface transmittance
and the Fermi velocity mismatch between the superconductors and the 2DEG.Comment: 9 pages REVTeX, 5 figures available on reques
Dynamic Data Selection for Neural Machine Translation
Intelligent selection of training data has proven a successful technique to
simultaneously increase training efficiency and translation performance for
phrase-based machine translation (PBMT). With the recent increase in popularity
of neural machine translation (NMT), we explore in this paper to what extent
and how NMT can also benefit from data selection. While state-of-the-art data
selection (Axelrod et al., 2011) consistently performs well for PBMT, we show
that gains are substantially lower for NMT. Next, we introduce dynamic data
selection for NMT, a method in which we vary the selected subset of training
data between different training epochs. Our experiments show that the best
results are achieved when applying a technique we call gradual fine-tuning,
with improvements up to +2.6 BLEU over the original data selection approach and
up to +3.1 BLEU over a general baseline.Comment: Accepted at EMNLP201
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