133 research outputs found
All-electrical time-resolved spin generation and spin manipulation in n-InGaAs
We demonstrate all-electrical spin generation and subsequent manipulation by
two successive electric field pulses in an n-InGaAs heterostructure in a
time-resolved experiment at zero external magnetic field. The first electric
field pulse along the crystal axis creates a current induced spin
polarization (CISP) which is oriented in the plane of the sample. The
subsequent electric field pulse along [110] generates a perpendicular magnetic
field pulse leading to a coherent precession of this spin polarization with
2-dimensional electrical control over the final spin orientation. Spin
precession is probed by time-resolved Faraday rotation. We determine the
build-up time of CISP during the first field pulse and extract the spin
dephasing time and internal magnetic field strength during the spin
manipulation pulse.Comment: 5 pages, 4 figure
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
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
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
Anisotropic Electron Spin Lifetime in (In,Ga)As/GaAs (110) Quantum Wells
Anisotropic electron spin lifetimes in strained undoped (In,Ga)As/GaAs (110)
quantum wells of different width and height are investigated by time-resolved
Faraday rotation and time-resolved transmission and are compared to the
(001)-orientation. From the suppression of spin precession, the ratio of
in-plane to out-of-plane spin lifetimes is calculated. Whereas the ratio
increases with In concentration in agreement with theory, a surprisingly high
anisotropy of 480 is observed for the broadest quantum well, when expressed in
terms of spin relaxation times.Comment: 4 pages, 4 figures, revise
Limitations to Carrier Mobility and Phase-Coherent Transport in Bilayer Graphene
We present transport measurements on high-mobility bilayer graphene fully
encapsulated in hexagonal boron nitride. We show two terminal quantum Hall
effect measurements which exhibit full symmetry broken Landau levels at low
magnetic fields. From weak localization measurements, we extract gate-tunable
phase coherence times as well as the inter- and intra-valley
scattering times and . While is in qualitative
agreement with an electron-electron interaction mediated dephasing mechanism,
electron spin-flip scattering processes are limiting at low
temperatures. The analysis of and points to local strain
fluctuation as the most probable mechanism for limiting the mobility in
high-quality bilayer graphene
Phase-coherent transport in catalyst-free vapor phase deposited BiSe crystals
Free-standing BiSe single crystal flakes of variable thickness are
grown using a catalyst-free vapor-solid synthesis and are subsequently
transferred onto a clean Si/SiO substrate where the flakes are
contacted in Hall bar geometry. Low temperature magneto-resistance measurements
are presented which show a linear magneto-resistance for high magnetic fields
and weak anti-localization (WAL) at low fields. Despite an overall strong
charge carrier tunability for thinner devices, we find that electron transport
is dominated by bulk contributions for all devices. Phase coherence lengths
\l_\phi as extracted from WAL measurements increase linearly with increasing
electron density exceeding m at 1.7 K. While \l_\phi is in
qualitative agreement with electron electron interaction-induced dephasing, we
find that spin flip scattering processes limit \l_\phi at low temperatures.Comment: 8 pages, 5 figure
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