518 research outputs found
Spin Blockade in Capacitively Coupled Quantum Dots
We present transport measurements on a lateral double dot produced by
combining local anodic oxidation and electron beam lithography. We investigate
the tunability of our device and demonstrate, that we can switch between
capacitive and tunnel coupling. In the regime of capacitive coupling we observe
the phenomenon of spin blockade in a magnetic field and analyze the influence
of capacitive interdot coupling on this effect.Comment: 4 pages, 3 figure
Tuning the onset voltage of resonant tunneling through InAs quantum dots by growth parameters
We investigated the size dependence of the ground state energy in
self-assembled InAs quantum dots embedded in resonant tunneling diodes.
Individual current steps observed in the current-voltage characteristics are
attributed to resonant single-electron tunneling via the ground state of
individual InAs quantum dots. The onset voltage of the first step observed is
shown to decrease systematically from 200 mV to 0 with increasing InAs
coverage. We relate this to a coverage-dependent size of InAs dots grown on
AlAs. The results are confirmed by atomic force micrographs and
photoluminescence experiments on reference samples.Comment: 3 pages, 3 figure
Kondo effect in a few-electron quantum ring
A small quantum ring with less than 10 electrons was studied by transport
spectroscopy. For strong coupling to the leads a Kondo effect is observed and
used to characterize the spin structure of the system in a wide range of
magnetic fields. At small magnetic fields Aharonov-Bohm oscillations influenced
by Coulomb interaction appear. They exhibit phase jumps by at the
Coulomb-blockade resonances. Inside Coulomb-blockade valleys the Aharonov-Bohm
oscillations can also be studied due to the finite conductance caused by the
Kondo effect. Astonishingly, the maxima of the oscillations show linear shifts
with magnetic field and gate voltage.Comment: 4 pages, 4 figure
Combined atomic force microscope and electron-beam lithography used for the fabrication of variable-coupling quantum dots
We have combined direct nanofabrication by local anodic oxidation with
conventional electron-beam lithography to produce a parallel double quantum dot
based on a GaAs/AlGaAs heterostructure. The combination of both nanolithography
methods allows to fabricate robust in-plane gates and Cr/Au top gate electrodes
on the same device for optimal controllability. This is illustrated by the
tunability of the interdot coupling in our device. We describe our fabrication
and alignment scheme in detail and demonstrate the tunability in
low-temperature transport measurements.Comment: 4 pages, 3 figure
Fabrication of quantum point contacts by engraving GaAs/AlGaAs-heterostructures with a diamond tip
We use the all-diamond tip of an atomic force microscope for the direct
engraving of high quality quantum point contacts in
GaAs/AlGaAs-heterostructures. The processing time is shortened by two orders of
magnitude compared to standard silicon tips. Together with a reduction of the
line width to below 90 nm the depletion length of insulating lines is reduced
by a factor of two with the diamond probes. The such fabricated defect free
ballistic constrictions show well resolved conductance plateaus and the 0.7
anomaly in electronic transport measurements.Comment: 3 pages, 3 figure
Aharonov-Bohm oscillations of a tunable quantum ring
With an atomic force microscope a ring geometry with self-aligned in-plane
gates was directly written into a GaAs/AlGaAs-heterostructure. Transport
measurements in the open regime show only one transmitting mode and
Aharonov-Bohm oscillations with more than 50% modulation are observed in the
conductance. The tuning via in-plane gates allows to study the Aharonov-Bohm
effect in the whole range from the open ring to the Coulomb-blockade regime.Comment: 3 pages, 3 figure
Interacting electrons on a quantum ring: exact and variational approach
We study a system of interacting electrons on a one-dimensional quantum ring
using exact diagonalization and the variational quantum Monte Carlo method. We
examine the accuracy of the Slater-Jastrow -type many-body wave function and
compare energies and pair distribution functions obtained from the two
approaches. Our results show that this wave function captures most correlation
effects. We then study the smooth transition to a regime where the electrons
localize in the rotating frame, which for the ultrathin quantum ring system
happens at quite high electron density.Comment: 19 pages, 10 figures. Accepted for publication in the New Journal of
Physic
Coulomb interaction effects on the electronic structure of radial polarized excitons in nanorings
The electronic structure of radially polarized excitons in structured
nanorings is analyzed, with emphasis in the ground-state properties and their
dependence under applied magnetic fields perpendicular to the ring plane. The
electron-hole Coulomb attraction has been treated rigorously, through numerical
diagonalization of the full exciton Hamiltonian in the non-interacting
electron-hole pairs basis. Depending on the relative weight of the kinetic
energy and Coulomb contributions, the ground-state of polarized excitons has
"extended" or "localized" features. In the first case, corresponding to small
rings dominated by the kinetic energy, the ground-state shows Aharonov-Bohm
(AB) oscillations due to the individual orbits of the building particles of the
exciton. In the localized regime, corresponding to large rings dominated by the
Coulomb interaction, the only remaining AB oscillations are due to the magnetic
flux trapped between the electron and hole orbits. This dependence of the
exciton, a neutral excitation, on the flux difference confirms this feature as
a signature of Coulomb dominated polarized excitons. Analytical approximations
are provided in both regimens, which accurate reproduce the numerical results.Comment: 9 pages, including 6 figure
Single-cell microfluidics facilitates the rapid quantification of antibiotic accumulation in Gram-negative bacteria (article)
This is the final version. Available on open access from the Royal Society of Chemistry via the DOI in this recordData availability:
All the data is available in the main text or in the supplementary
materials.The code associated with this article is located in ORE at: http://hdl.handle.net/10871/121661The double-membrane cell envelope of Gram-negative bacteria is a formidable barrier to intracellular antibiotic accumulation. A quantitative understanding of antibiotic transport in these cells is crucial for drug development, but this has proved elusive due to a dearth of suitable investigative techniques. Here we combine microfluidics and time-lapse auto-fluorescence microscopy to rapidly quantify antibiotic accumulation in hundreds of individual Escherichia coli cells. By serially manipulating the microfluidic environment, we demonstrated that stationary phase Escherichia coli, traditionally more refractory to antibiotics than growing cells, display reduced accumulation of the antibiotic ofloxacin compared to actively growing cells. Our novel microfluidic method facilitates the quantitative comparison of the role of the microenvironment versus various membrane transport pathways in cellular drug accumulation. Unlike traditional techniques, our assay is rapid, studying accumulation as the cells are dosed with the drug. This platform provides a powerful new tool for studying antibiotic accumulation in bacteria, which will be critical for the rational development of the next generation of antibiotics.European CommissionBiotechnology and Biological Sciences Research Council (BBSRC)Engineering and Physical Sciences Research Council (EPSRC)University of Exeter School of BiosciencesEuropean Union Horizon 2020Medical Research Council (MRC)Royal SocietyWellcome TrustGW4 Initiator awar
- …