4,678 research outputs found
Spin-Orbit Torques in Transition Metal Dichalcogenide/Ferromagnet Heterostructures
In recent years, there has been a growing interest in spin-orbit torques
(SOTs) for manipulating the magnetization in nonvolatile magnetic memory
devices. SOTs rely on the spin-orbit coupling of a nonmagnetic material coupled
to a ferromagnetic layer to convert an applied charge current into a torque on
the magnetization of the ferromagnet (FM). Transition metal dichalcogenides
(TMDs) are promising candidates for generating these torques with both high
charge-to-spin conversion ratios, and symmetries and directions which are
efficient for magnetization manipulation. Moreover, TMDs offer a wide range of
attractive properties, such as large spin-orbit coupling, high crystalline
quality and diverse crystalline symmetries. Although numerous studies were
published on SOTs using TMD/FM heterostructures, we lack clear understanding of
the observed SOT symmetries, directions, and strengths. In order to shine some
light on the differences and similarities among the works in literature, in
this mini-review we compare the results for various TMD/FM devices,
highlighting the experimental techniques used to fabricate the devices and to
quantify the SOTs, discussing their potential effect on the interface quality
and resulting SOTs. This enables us to both identify the impact of particular
fabrication steps on the observed SOT symmetries and directions, and give
suggestions for their underlying microscopic mechanisms. Furthermore, we
highlight recent progress of the theoretical work on SOTs using TMD
heterostructures and propose future research directions.Comment: 14 pages, 1 figure, 1 tabl
Enhancing magneto-optic effects in two-dimensional magnets by thin-film interference
The magneto-optic Kerr effect is a powerful tool for measuring magnetism in
thin films at microscopic scales, as was recently demonstrated by the major
role it played in the discovery of two-dimensional (2D) ferromagnetism in
monolayer CrI and CrGeTe. These 2D magnets are often stacked
with other 2D materials in van der Waals heterostructures on a SiO/Si
substrate, giving rise to thin-film interference. This can strongly affect
magneto-optical measurements, but is often not taken into account in
experiments. Here, we show that thin-film interference can be used to engineer
the magneto-optical signals of 2D magnetic materials and optimize them for a
given experiment or setup. Using the transfer matrix method, we analyze the
magneto-optical signals from realistic systems composed of van der Waals
heterostructures on SiO/Si substrates, using CrI as a prototypical 2D
magnet, and hexagonal boron nitride (hBN) to encapsulate this air-sensitive
layer. We observe a strong modulation of the Kerr rotation and ellipticity,
reaching several tens to hundreds of milliradians, as a function of the
illumination wavelength, and the thickness of the SiO and layers composing
the van der Waals heterostructure. Similar results are obtained in
heterostructures composed by other 2D magnets, such as CrCl, CrBr and
CrGeTe. Designing samples for the optimal trade-off between
magnitude of the magneto-optical signals and intensity of the reflected light
should result in a higher sensitivity and shorter measurement times. Therefore,
we expect that careful sample engineering, taking into account thin-film
interference effects, will further the knowledge of magnetization in
low-dimensional structures.Comment: manuscript: 12 pages, 4 figures. supplementary material: 22 pages, 20
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
24 \textmu m length spin relaxation length in boron nitride encapsulated bilayer graphene
We have performed spin and charge transport measurements in dual gated high
mobility bilayer graphene encapsulated in hexagonal boron nitride. Our results
show spin relaxation lengths up to 13~\textmu m at room temperature
with relaxation times of 2.5~ns. At 4~K, the diffusion coefficient
rises up to 0.52~m/s, a value 5 times higher than the best achieved for
graphene spin valves up to date. As a consequence, rises up to
24~\textmu m with as high as 2.9~ns. We characterized 3 different
samples and observed that the spin relaxation times increase with the device
length. We explain our results using a model that accounts for the spin
relaxation induced by the non-encapsulated outer regions.Comment: 5 pages and 4 figure
Unconventional spin Hall effects in nonmagnetic solids
Direct and inverse spin Hall effects lie at the heart of novel applications
that utilize spins of electrons as information carriers, allowing generation of
spin currents and detecting them via the electric voltage. In the standard
arrangement, applied electric field induces transverse spin current with
perpendicular spin polarization. Although conventional spin Hall effects are
commonly used in spin-orbit torques or spin Hall magnetoresistance experiments,
the possibilities to configure electronic devices according to specific needs
are quite limited. Here, we investigate unconventional spin Hall effects that
have the same origin as conventional ones, but manifest only in low-symmetry
crystals where spin polarization, spin current and charge current are not
enforced to be orthogonal. Based on the symmetry analysis for all 230 space
groups, we have identified crystal structures that could exhibit unusual
configurations of charge-to-spin conversion. The most relevant geometries have
been explored in more detail; in particular, we have analyzed the collinear
components yielding transverse charge and spin current with spin polarization
parallel to one of them, as well as the longitudinal ones, where charge and
spin currents are parallel. In addition, we have demonstrated that
unconventional spin Hall effect can be induced by controllable breaking the
crystal symmetries by an external electric field, which opens a perspective for
external tuning of spin injection and detection by electric fields. The results
have been confirmed by density functional theory calculations performed for
various materials relevant for spintronics. We are convinced that our findings
will stimulate further computational and experimental studies of unconventional
spin Hall effects
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
Group theory analysis of electrons and phonons in N-layer graphene systems
In this work we study the symmetry properties of electrons and phonons in
graphene systems as function of the number of layers. We derive the selection
rules for the electron-radiation and for the electron-phonon interactions at
all points in the Brillouin zone. By considering these selection rules, we
address the double resonance Raman scattering process. The monolayer and
bilayer graphene in the presence of an applied electric field are also
discussed.Comment: 8 pages, 6 figure
Charge dynamics in the 2D/3D semiconductor heterostructure WSe/GaAs
Understanding the relaxation and recombination processes of excited states in two-dimensional (2D)/three-dimensional (3D) semiconductor heterojunctions is essential for developing efficient optical and (opto)electronic devices which integrate new 2D materials with more conventional 3D ones. In this work, we unveil the carrier dynamics and charge transfer in a monolayer of WSe on a GaAs substrate. We use time-resolved differential reflectivity to study the charge relaxation processes involved in the junction and how they change when compared to an electrically decoupled heterostructure, WSe/hBN/GaAs. We observe that the monolayer in direct contact with the GaAs substrate presents longer optically-excited carrier lifetimes (3.5 ns) when compared with the hBN-isolated region (1 ns), consistent with a strong reduction of radiative decay and a fast charge transfer of a single polarity. Through low-temperature measurements, we find evidence of a type-II band alignment for this heterostructure with an exciton dissociation that accumulates electrons in the GaAs and holes in the WSe. The type-II band alignment and fast photo-excited carrier dissociation shown here indicate that WSe/GaAs is a promising junction for new photovoltaic and other optoelectronic devices, making use of the best properties of new (2D) and conventional (3D) semiconductors
Magnetic field control of light-induced spin accumulation in monolayer MoSe
Semiconductor transition metal dichalcogenides (TMDs) have equivalent
dynamics for their two spin/valley species. This arises from their
energy-degenerated spin states, connected via time-reversal symmetry. When an
out-of-plane magnetic field is applied, time-reversal symmetry is broken and
the energies of the spin-polarized bands shift, resulting in different bandgaps
and dynamics in the K and K valleys. Here, we use time-resolved Kerr
rotation to study the magnetic field dependence of the spin dynamics in
monolayer MoSe. We show that the magnetic field can control the
light-induced spin accumulation of the two valley states, with a small effect
on the recombination lifetimes. We unveil that the magnetic field-dependent
spin accumulation is in agreement with hole spin dynamics at the longer
timescales, indicating that the electron spins have faster relaxation rates. We
propose a rate equation model that suggests that lifting the energy-degeneracy
of the valleys induces an ultrafast spin-flip toward the stabilization of the
valley with the higher valence band energy. Our results provide an experimental
insight into the ultrafast charge and spin dynamics in TMDs and a way to
control it, which will be useful for the development of new spintronic and
valleytronic applications.Comment: 6 pages, 4 figure
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