303 research outputs found
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
Fabrication of comb-drive actuators for straining nanostructured suspended graphene
We report on the fabrication and characterization of an optimized comb-drive
actuator design for strain-dependent transport measurements on suspended
graphene. We fabricate devices from highly p-doped silicon using deep reactive
ion etching with a chromium mask. Crucially, we implement a gold layer to
reduce the device resistance from k to
at room temperature in order to allow for
strain-dependent transport measurements. The graphene is integrated by
mechanically transferring it directly onto the actuator using a
polymethylmethacrylate membrane. Importantly, the integrated graphene can be
nanostructured afterwards to optimize device functionality. The minimum feature
size of the structured suspended graphene is 30 nm, which allows for
interesting device concepts such as mechanically-tunable nanoconstrictions.
Finally, we characterize the fabricated devices by measuring the Raman spectrum
as well as the a mechanical resonance frequency of an integrated graphene sheet
for different strain values.Comment: 10 pages, 9 figure
Tunable mechanical coupling between driven microelectromechanical resonators
We present a microelectromechanical system, in which a silicon beam is
attached to a comb-drive actuator, that is used to tune the tension in the
silicon beam, and thus its resonance frequency. By measuring the resonance
frequencies of the system, we show that the comb-drive actuator and the silicon
beam behave as two strongly coupled resonators. Interestingly, the effective
coupling rate (~ 1.5 MHz) is tunable with the comb-drive actuator (+10%) as
well as with a side-gate (-10%) placed close to the silicon beam. In contrast,
the effective spring constant of the system is insensitive to either of them
and changes only by 0.5%. Finally, we show that the comb-drive actuator
can be used to switch between different coupling rates with a frequency of at
least 10 kHz.Comment: 5 pages, 4 figures, 1 tabl
Cavity-enhanced single photon emission from a single impurity-bound exciton
Impurity-bound excitons in ZnSe quantum wells are bright single photon
emitters--a crucial element in photonics-based quantum technology. But to
achieve the efficiencies required for practical applications, these emitters
must be integrated into optical cavities that enhance their radiative
properties and far-field emission pattern. In this work, we demonstrate
cavity-enhanced emission from a single impurity-bound exciton in a ZnSe quantum
well. We utilize a bullseye cavity structure optimized to feature a small mode
volume and a nearly Gaussian far-field transverse mode that can efficiently
couple to an optical fiber. The fabricated device displays emission that is
more than an order of magnitude brighter than bulk impurity-bound exciton
emitters in the ZnSe quantum well, as-well-as clear anti-bunching, which
verifies the single photon emission from the source. Time-resolved
photoluminescence spectroscopy reveals a Purcell-enhanced radiative decay
process with a Purcell factor of 1.43. This work paves the way towards high
efficiency spin-photon interfaces using an impurity-doped II-VI semiconductor
coupled to nanophotonics
Interfacial mixing in heteroepitaxial growth
We investigate the growth of a film of some element B on a substrate made of
another substrance A in a model of molecular beam epitaxy. A vertical exchange
mechanism allows the A-atoms to stay on the growing surface with a certain
probability. Using kinetic Monte Carlo simulations as well as scaling
arguments, the incorporation of the A's into the growing B-layer is
investigated. Moreover we develop a rate equation theory for this process. In
the limit of perfect layer-by-layer growth, the density of A-atoms decays in
the B-film like the inverse squared distance from the interface. The power law
is cut off exponentially at a characteristic thickness of the interdiffusion
zone that depends on the rate of exchange of a B-adatom with an A-atom in the
surface and on the system size. Kinetic roughening changes the exponents. Then
the thickness of the interdiffusion zone is determined by the diffusion length.Comment: 11 pages, 11 figure
Electrical resistance of individual defects at a topological insulator surface
Three-dimensional topological insulators host surface states with linear
dispersion, which manifest as a Dirac cone. Nanoscale transport measurements
provide direct access to the transport properties of the Dirac cone in real
space and allow the detailed investigation of charge carrier scattering. Here,
we use scanning tunnelling potentiometry to analyse the resistance of different
kinds of defects at the surface of a (Bi0.53Sb0.47)2Te3 topological insulator
thin film. The largest localized voltage drop we find to be located at domain
boundaries in the topological insulator film, with a resistivity about four
times higher than that of a step edge. Furthermore, we resolve resistivity
dipoles located around nanoscale voids in the sample surface. The influence of
such defects on the resistance of the topological surface state is analysed by
means of a resistor network model. The effect resulting from the voids is found
to be small compared to the other defects
(Si)GeSn nanostructures for light emitters
Energy-efficient integrated circuits for on-chip or chip-to-chip data transfer via photons could be tackled by monolithically grown group IV photonic devices. The major goal here is the realization of fully integrated group IV room temperature electrically driven lasers. An approach beyond the already demonstrated optically-pumped lasers would be the introduction of GeSn/(Si)Ge(Sn) heterostructures and exploitation of quantum mechanical effects by reducing the dimensionality, which affects the density of states. In this contribution we present epitaxial growth, processing and characterization of GeSn/(Si)Ge(Sn) heterostructures, ranging from GeSn/Ge multi quantum wells (MQWs) to GeSn quantum dots (QDs) embedded in a Ge matrix. Light emitting diodes (LEDs) were fabricated based on the MQW structure and structurally analyzed via TEM, XRD and RBS. Moreover, EL measurements were performed to investigate quantum confinement effects in the wells. The GeSn QDs were formed via Sn diffusion /segregation upon thermal annealing of GeSn single quantum wells (SQW) embedded in Ge layers. The evaluation of the experimental results is supported by band structure calculations of GeSn/(Si)Ge(Sn) heterostructures to investigate their applicability for photonic devices
Integrated impedance bridge for absolute capacitance measurements at cryogenic temperatures and finite magnetic fields
We developed an impedance bridge that operates at cryogenic temperatures
(down to 60 mK) and in perpendicular magnetic fields up to at least 12 T. This
is achieved by mounting a GaAs HEMT amplifier perpendicular to a printed
circuit board containing the device under test and thereby parallel to the
magnetic field. The measured amplitude and phase of the output signal allows
for the separation of the total impedance into an absolute capacitance and a
resistance. Through a detailed noise characterization, we find that the best
resolution is obtained when operating the HEMT amplifier at the highest gain.
We obtained a resolution in the absolute capacitance of
6.4~aF at 77 K on a comb-drive actuator, while maintaining
a small excitation amplitude of 15~. We show the magnetic field
functionality of our impedance bridge by measuring the quantum Hall plateaus of
a top-gated hBN/graphene/hBN heterostructure at 60~mK with a probe signal of
12.8~.Comment: 7 pages, 5 figure
Sweet's syndrome in a patient with Crohn's disease: a case report
<p>Abstract</p> <p>Background</p> <p>Sweet's syndrome, also known as acute febrile neutrophilic dermatosis, has been associated with malignancy, autoimmune disease and collagen vascular disease. The association of Crohn's disease and Sweet's syndrome is rare. We report a case of Sweet's syndrome in a patient with Crohn's disease.</p> <p>Case presentation</p> <p>A 63-year-old man with a history of Crohn's disease presented with one-week duration of abdominal pain, diarrhea and hematochezia. He also noticed eruption of painful skin rashes all over his body at the same time. Colonoscopy and esophagogastroduodenoscopy (EGD) showed inflammation involving different parts of the gastrointestinal tract consistent with Crohn's disease. Punch biopsy of the skin lesion was consistent with Sweet's syndrome, which has a rare association with Crohn's disease.</p> <p>Conclusion</p> <p>Crohn's disease should be excluded in patients presenting with Sweet's syndrome and diarrhea. Alternatively, Sweet's syndrome should be considered as a diagnosis when a patient with Crohn's disease develops skin lesions.</p
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