229 research outputs found
Comment on "Dynamic range of nanotube- and nanowire-based electromechanical systems"
We investigate the role of quantum effects (e.g. zero-point energy
fluctuations) in the physics of nanotube- and nanowire-based electromechanical
sensors as discussed in a recent article [Postma et al., Appl. Phys. Lett. 86,
223105 (2005)]. Employing the quantum fluctuation-dissipation theorem we find
that these effects pose additional limits on the dynamic range of
nanomechanical resonators.Comment: 1 page, 1 figure, Appl. Phys. Lett. (in print
Local transport measurements on epitaxial graphene
Growth of large-scale graphene is still accompanied by imperfections. By
means of a four-tip STM/SEM the local structure of graphene grown on SiC(0001)
was correlated with scanning electron microscope images and spatially resolved
transport measurements. The systematic variation of probe spacings and
substrate temperature has clearly revealed two-dimensional transport regimes of
Anderson localization as well as of diffusive transport. The detailed analysis
of the temperature dependent data demonstrates that the local on-top nano-sized
contacts do not induce significant strain to the epitaxial graphene films.Comment: 3 figure
Readout of carbon nanotube vibrations based on spin-phonon coupling
We propose a scheme for spin-based detection of the bending motion in
suspended carbon-nanotubes, using the curvature-induced spin-orbit interaction.
We show that the resulting effective spin-phonon coupling can be used to
down-convert the high-frequency vibration-modulated spin-orbit field to
spin-flip processes at a much lower frequency. This vibration-induced
spin-resonance can be controlled with an axial magnetic field. We propose a
Pauli spin blockade readout scheme and predict that the leakage current shows
pronounced peaks as a function of the external magnetic field. Whereas the
resonant peaks allow for frequency readout, the slightly off-resonant current
is sensitive to the vibration amplitude.Comment: 3 pages(+), 3 figure
Low B Field Magneto-Phonon Resonances in Single-Layer and Bilayer Graphene
Many-body effects resulting from strong electron-electron and electron-phonon
interactions play a significant role in graphene physics. We report on their
manifestation in low B field magneto-phonon resonances in high quality
exfoliated single-layer and bilayer graphene encapsulated in hexagonal boron
nitride. These resonances allow us to extract characteristic effective Fermi
velocities, as high as m/s, for the observed "dressed"
Landau level transitions, as well as the broadening of the resonances, which
increases with Landau level index
Asymmetric Franck-Condon factors in suspended carbon nanotube quantum dots
Electronic states and vibrons in carbon nanotube quantum dots have in general
different location and size. As a consequence, the conventional
Anderson-Holstein model, coupling vibrons to the dot total charge only, may no
longer be appropriated in general. Here we explicitly address the role of the
spatial fluctuations of the electronic density, yielding space-dependent
Franck-Condon factors. We discuss the consequent marked effects on transport
which are compatible with recent measurements. This picture can be relevant for
tunneling experiments in generic nano-electromechanical systems.Comment: 4+ pages, 3 figures (2 color, 1 BW
Dry-transferred CVD graphene for inverted spin valve devices
Integrating high-mobility graphene grown by chemical vapor deposition (CVD)
into spin transport devices is one of the key tasks in graphene spintronics. We
use a van der Waals pickup technique to transfer CVD graphene by hexagonal
boron nitride (hBN) from the copper growth substrate onto predefined Co/MgO
electrodes to build inverted spin valve devices. Two approaches are presented:
(i) a process where the CVD-graphene/hBN stack is first patterned into a bar
and then transferred by a second larger hBN crystal onto spin valve electrodes
and (ii) a direct transfer of a CVD-graphene/hBN stack. We report record high
spin lifetimes in CVD graphene of up to 1.75 ns at room temperature. Overall,
the performances of our devices are comparable to devices fabricated from
exfoliated graphene also revealing nanosecond spin lifetimes. We expect that
our dry transfer methods pave the way towards more advanced device geometries
not only for spintronic applications but also for CVD-graphene-based
nanoelectronic devices in general where patterning of the CVD graphene is
required prior to the assembly of final van der Waals heterostructures.Comment: 5 pages, 3 figure
High mobility dry-transferred CVD bilayer graphene
We report on the fabrication and characterization of high-quality chemical
vapor-deposited (CVD) bilayer graphene (BLG). In particular, we demonstrate
that CVD-grown BLG can mechanically be detached from the copper foil by an
hexagonal boron nitride (hBN) crystal after oxidation of the copper-to-BLG
interface. Confocal Raman spectroscopy reveals an AB-stacking order of the BLG
crystals and a high structural quality. From transport measurements on fully
encapsulated hBN/BLG/hBN Hall bar devices we extract charge carrier mobilities
up to 180,000 cm/(Vs) at 2 K and up to 40,000 cm/(Vs) at 300 K,
outperforming state-of-the-art CVD bilayer graphene devices. Moreover, we show
an on-off ration of more than 10,000 and a band gap opening with values of up
to 15 meV for a displacement field of 0.2 V/nm in such CVD grown BLG.Comment: 5 pages, 4 figure
A world of smartphone experiments with the app phyphox
SMARTPHONES AS MEASUREMENT DEVICES
The concept of the app phyphox is based on the simple idea that smartphones and tablets come with a plethora of sensors, which can be used for data acquisition in science education. Phyphox was developed at the RWTH Aachen University for this purpose and presents itself as an open source tool with many options to customise data sources, data analysis and data presentation, while not overwhelming students with these options while they use their own devices to discover the world.
Experimentation with device sensors
There are many situations in which these readily available measurement devices can enhance science education. These range from classical educational experiments that can be reproduced with household items (radial acceleration in a salad spinner), over casually discovering the world around us (determine the speed of an elevator with the pressure sensor) to projects on technical applications (build a Pitot tube based on this pressure sensor).
DIY-Sensors with Arduino and MicroPython
Beyond these typical experiments, phyphox can be used in modern microcontroller-based projects. Smartphone sensors can easily be combined with cheap external sensors using an Arduino or MicroPython library for phyphox. This allows us to combine the visualisation capabilities of the phone with the wide choice of sensors of DIY electronics and is accessible even to programming beginners.
Collaborative experiments for large audiences
While these examples are suitable on the scale of typical school classes, the connectivity of smartphones allows us to scale experimental data acquisition to large audiences. Automated data collection and analysis allow for entire lecture halls to participate in live experiments during a lecture and even worldwide experiments to determine Earth’s axial tilt have been demonstrated.
FURTHER READING
Sebastian Staacks, Simon Hütz, Heidrun Heinke, Christoph Stampfer. (2018). Advanced tools for smartphone-based experiments: phyphox. Physics Education, 53(4), 045009. https://doi.org/10.1088/1361-6552/aac05e
Sebastian Staacks, Dominik Dorsel, Simon Hütz, Frank Stallmach, Tobias Splith, Heidrun Heinke, Christoph Stampfer. (2022). Collaborative smartphone experiments for large audiences with phyphox. European Journal of Physics, 43(5), 055702. https://doi.org/10.1088/1361-6404/ac783
Diffractive paths for weak localization in quantum billiards
We study the weak localization effect in quantum transport through a clean
ballistic cavity with regular classical dynamics. We address the question which
paths account for the suppression of conductance through a system where
disorder and chaos are absent. By exploiting both quantum and semiclassical
methods, we unambiguously identify paths that are diffractively backscattered
into the cavity (when approaching the lead mouths from the cavity interior) to
play a key role. Diffractive scattering couples transmitted and reflected paths
and is thus essential to reproduce the weak-localization peak in reflection and
the corresponding anti-peak in transmission. A comparison of semiclassical
calculations featuring these diffractive paths yields good agreement with full
quantum calculations and experimental data. Our theory provides system-specific
predictions for the quantum regime of few open lead modes and can be expected
to be relevant also for mixed as well as chaotic systems.Comment: 5 pages, 3 figures, final version with extended discussion and added
reference
- …