1,182 research outputs found
Spin and charge transport in graphene-based spin transport devices with Co/MgO spin injection and spin detection electrodes
In this review we discuss spin and charge transport properties in
graphene-based single-layer and few-layer spin-valve devices. We give an
overview of challenges and recent advances in the field of device fabrication
and discuss two of our fabrication methods in more detail which result in
distinctly different device performances. In the first class of devices, Co/MgO
electrodes are directly deposited onto graphene which results in rough
MgO-to-Co interfaces and favor the formation of conducting pinholes throughout
the MgO layer. We show that the contact resistance area product (RA) is a
benchmark for spin transport properties as it scales with the measured spin
lifetime in these devices indicating that contact-induced spin dephasing is the
bottleneck for spin transport even in devices with large RA values. In a
second class of devices, Co/MgO electrodes are first patterned onto a silicon
substrate. Subsequently, a graphene-hBN heterostructure is directly transferred
onto these prepatterned electrodes which provides improved interface
properties. This is seen by a strong enhancement of both charge and spin
transport properties yielding charge carrier mobilities exceeding 20000
cm/(Vs) and spin lifetimes up to 3.7 ns at room temperature. We discuss
several shortcomings in the determination of both quantities which complicates
the analysis of both extrinsic and intrinsic spin scattering mechanisms.
Furthermore, we show that contacts can be the origin of a second charge
neutrality point in gate dependent resistance measurements which is influenced
by the quantum capacitance of the underlying graphene layer.Comment: 19 pages, 8 figure
Stability of strained heteroepitaxial systems in (1+1) dimensions
We present a simple analytical model for the determination of the stable
phases of strained heteroepitaxial systems in (1+1) dimensions. In order for
this model to be consistent with a subsequent dynamic treatment, all
expressions are adjusted to an atomistic Lennard-Jones system. Good agreement
is obtained when the total energy is assumed to consist of two contributions:
the surface energy and the elastic energy. As a result, we determine the stable
phases as a function of the main ``control parameters'' (binding energies,
coverage and lattice mismatch). We find that there exists no set of parameters
leading to an array of islands as a stable configuration. We however show that
a slight modification of the model can lead to the formation of stable arrays
of islands.Comment: 11 pages, 14 figures, submitted to Physical Review
Dynamics of homogeneous nucleation
The classical nucleation theory for homogeneous nucleation is formulated as a
theory for a density fluctuation in a supersaturated gas at a given
temperature. But Molecular Dynamics simulations reveal that it is small cold
clusters which initiates the nucleation. The temperature in the nucleating
clusters fluctuate, but the mean temperature remains below the temperature in
the supersaturated gas until they reach the critical nucleation size. The
critical nuclei have, however, a temperature equal to the supersaturated gas.
The kinetics of homogeneous nucleation is not only caused by a grow or shrink
by accretion or evaporation of monomers only, but by an exponentially declining
change in cluster size per time step equal to the cluster distribution in the
supersaturated gas.Comment: 5 figure
Contact-induced charge contributions to non-local spin transport measurements in Co/MgO/graphene devices
Recently, it has been shown that oxide barriers in graphene-based non-local
spin-valve structures can be the bottleneck for spin transport. The barriers
may cause spin dephasing during or right after electrical spin injection which
limit spin transport parameters such as the spin lifetime of the whole device.
An important task is to evaluate the quality of the oxide barriers of both spin
injection and detection contacts in a fabricated device. To address this issue,
we discuss the influence of spatially inhomogeneous oxide barriers and
especially conducting pinholes within the barrier on the background signal in
non-local measurements of graphene/MgO/Co spin-valve devices. By both
simulations and reference measurements on devices with non-ferromagnetic
electrodes, we demonstrate that the background signal can be caused by
inhomogeneous current flow through the oxide barriers. As a main result, we
demonstrate the existence of charge accumulation next to the actual spin
accumulation signal in non-local voltage measurements, which can be explained
by a redistribution of charge carriers by a perpendicular magnetic field
similar to the classical Hall effect. Furthermore, we present systematic
studies on the phase of the low frequency non-local ac voltage signal which is
measured in non-local spin measurements when applying ac lock-in techniques.
This phase has so far widely been neglected in the analysis of non-local spin
transport. We demonstrate that this phase is another hallmark of the
homogeneity of the MgO spin injection and detection barriers. We link backgate
dependent changes of the phase to the interplay between the capacitance of the
oxide barrier to the quantum capacitance of graphene.Comment: 19 pages, 7 figure
Inter-valley dark trion states with spin lifetimes of 150 ns in WSe
We demonstrate long trion spin lifetimes in a WSe monolayer of up to 150
ns at 5 K. Applying a transverse magnetic field in time-resolved Kerr-rotation
measurements reveals a complex composition of the spin signal of up to four
distinct components. The Kerr rotation signal can be well described by a model
which includes inhomogeneous spin dephasing and by setting the trion spin
lifetimes to the measured excitonic recombination times extracted from
time-resolved reflectivity measurements. We observe a continuous shift of the
Kerr resonance with the probe energy, which can be explained by an
adsorbate-induced, inhomogeneous potential landscape of the WSe flake. A
further indication of extrinsic effects on the spin dynamics is given by a
change of both the trion spin lifetime and the distribution of g-factors over
time. Finally, we detect a Kerr rotation signal from the trion's higher-energy
triplet state when the lower-energy singlet state is optically pumped by
circularly polarized light. We explain this by the formation of dark trion
states, which are also responsible for the observed long trion spin lifetimes.Comment: 23 pages, 13 figure
Free energy of cluster formation and a new scaling relation for the nucleation rate
Recent very large molecular dynamics simulations of homogeneous nucleation
with Lennard-Jones atoms [Diemand et al. J. Chem. Phys. {\bf
139}, 074309 (2013)] allow us to accurately determine the formation free energy
of clusters over a wide range of cluster sizes. This is now possible because
such large simulations allow for very precise measurements of the cluster size
distribution in the steady state nucleation regime. The peaks of the free
energy curves give critical cluster sizes, which agree well with independent
estimates based on the nucleation theorem. Using these results, we derive an
analytical formula and a new scaling relation for nucleation rates: is scaled by , where the supersaturation ratio is ,
is the dimensionless surface energy, and is a dimensionless
nucleation rate. This relation can be derived using the free energy of cluster
formation at equilibrium which corresponds to the surface energy required to
form the vapor-liquid interface. At low temperatures (below the triple point),
we find that the surface energy divided by that of the classical nucleation
theory does not depend on temperature, which leads to the scaling relation and
implies a constant, positive Tolman length equal to half of the mean
inter-particle separation in the liquid phase.Comment: 7 figure
Nucleolar localization of influenza A NS1: striking differences between mammalian and avian cells
In mammalian cells, nucleolar localization of influenza A NS1 requires the presence of a C-terminal nucleolar localization signal. This nucleolar localization signal is present only in certain strains of influenza A viruses. Therefore, only certain NS1 accumulate in the nucleolus of mammalian cells. In contrast, we show that all NS1 tested in this study accumulated in the nucleolus of avian cells even in the absence of the above described C-terminal nucleolar localization signal. Thus, nucleolar localization of NS1 in avian cells appears to rely on a different nucleolar localization signal that is more conserved among influenza virus strains
Role of MgO barriers for spin and charge transport in Co/MgO/graphene non-local spin-valve devices
We investigate spin and charge transport in both single and bilayer graphene
non-local spin-valve devices. Similar to previous studies on bilayer graphene,
we observe an inverse dependence of the spin lifetime on the carrier mobility
in our single layer devices. This general trend is only observed in devices
with large contact resistances. Furthermore, we observe a second Dirac peak in
devices with long spin lifetimes. This results from charge transport underneath
the contacts. In contrast, all devices with low ohmic contact resistances only
exhibit a single Dirac peak. Additionally, the spin lifetime is significantly
reduced indicating that an additional spin dephasing occurs underneath the
electrodes.Comment: 5 pages, 3 figure
Evaluation of a Mutually Coupled Diversity Receiver
A quick, reliable, and simple evaluation of mutual coupling effects is essential for the optimization of antenna arrays for small mobile communications devices. In recent papers we have proposed novel figures of merit that quantify the impact on diversity reception in terms of scattering matrix of the array and have confirmed the validity of these formulas by practical diversity measurements. The present paper provides an extended analysis of the measurement data and contrasts the benefits of this method of array characterization with existing approaches
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