52 research outputs found
Imaging ellipsometry of graphene
Imaging ellipsometry studies of graphene on SiO2/Si and crystalline GaAs are
presented. We demonstrate that imaging ellipsometry is a powerful tool to
detect and characterize graphene on any flat substrate. Variable angle
spectroscopic ellipsometry is used to explore the dispersion of the optical
constants of graphene in the visible range with high lateral resolution. In
this way the influence of the substrate on graphene's optical properties can be
investigatedComment: 3 pages, 3 figure
Tunneling Anisotropic Spin Polarization in lateral (Ga,Mn)As/GaAs spin Esaki diode devices
We report here on anisotropy of spin polarization obtained in lateral
all-semiconductor all-electrical spin injection devices, employing
(Ga,Mn)As/GaAs Esaki diode structures as spin aligning
contacts, resulting from the dependence of the efficiency of spin tunneling on
the orientation of spins with respect to different crystallographic directions.
We observed an in-plane anisotropy of in case of spins oriented either
along or directions and anisotropy between
in-plane and perpendicular-to-plane orientation of spins.Comment: 9 pages, 3 figure
Magnetic anisotropy of epitaxial (Ga,Mn)As on (113)A GaAs
The temperature dependence of magnetic anisotropy in (113)A (Ga,Mn)As layers
grown by molecular beam epitaxy is studied by means of superconducting quantum
interference device (SQUID) magnetometry as well as by ferromagnetic resonance
(FMR) and magnetooptical effects. Experimental results are described
considering cubic and two kinds of uniaxial magnetic anisotropy. The magnitude
of cubic and uniaxial anisotropy constants is found to be proportional to the
fourth and second power of saturation magnetization, respectively. Similarly to
the case of (001) samples, the spin reorientation transition from uniaxial
anisotropy with the easy along the [-1, 1, 0] direction at high temperatures to
the biaxial anisotropy at low temperatures is observed around 25 K. The
determined values of the anisotropy constants have been confirmed by FMR
studies. As evidenced by investigations of the polar magnetooptical Kerr
effect, the particular combination of magnetic anisotropies allows the
out-of-plane component of magnetization to be reversed by an in-plane magnetic
field. Theoretical calculations within the p-d Zener model explain the
magnitude of the out-of-plane uniaxial anisotropy constant caused by epitaxial
strain, but do not explain satisfactorily the cubic anisotropy constant. At the
same time the findings point to the presence of an additional uniaxial
anisotropy of unknown origin. Similarly to the case of (001) films, this
additional anisotropy can be explained by assuming the existence of a shear
strain. However, in contrast to the (001) samples, this additional strain has
an out-of-the-(001)-plane character.Comment: 13 pages, 9 figure
Tuning the Fr\"ohlich exciton-phonon scattering in monolayer MoS2
A direct band gap, remarkable light-matter coupling as well as strong
spin-orbit and Coulomb interaction establish two-dimensional (2D) crystals of
transition metal dichalcogenides (TMDs) as an emerging material class for
fundamental studies as well as novel technological concepts. Valley selective
optical excitation allows for optoelectronic applications based on the momentum
of excitons. In addition to lattice imperfections and disorder, scattering by
phonons is a significant mechanism for valley depolarization and decoherence in
TMDs at elevated temperatures preventing high-temperature valley polarization
required for realistic applications. Thus, a detailed knowledge about strength
and nature of the interaction of excitons with phonons is vital. We directly
access exciton-phonon coupling in charge tunable single layer MoS2 devices by
polarization resolved Raman spectroscopy. We observe a strong defect mediated
coupling between the long-range oscillating electric field induced by the
longitudinal optical (LO) phonon in the dipolar medium and the exciton. We find
that this so-called Fr\"ohlich exciton LO-phonon interaction is suppressed by
doping. This suppression correlates with a distinct increase of the degree of
valley polarization of up to 20 % even at elevated temperatures of 220 K. Our
result demonstrates a promising strategy to increase the degree of valley
polarization towards room temperature valleytronic applications.Comment: 18 pages, 3 figures, S
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