427 research outputs found
Measurements of a Quantum Dot with an Impedance-Matching On-Chip LC Resonator at GHz Frequencies
We report the realization of a bonded-bridge on-chip superconducting coil and
its use in impedance-matching a highly ohmic quantum dot (QD) to a
measurement setup. The coil, modeled as a lumped-element resonator, is
more compact and has a wider bandwidth than resonators based on coplanar
transmission lines (e.g. impedance transformers and stub tuners) at
potentially better signal-to-noise ratios. In particular for measurements of
radiation emitted by the device, such as shot noise, the 50 larger
bandwidth reduces the time to acquire the spectral density. The resonance
frequency, close to 3.25 GHz, is three times higher than that of the one
previously reported wire-bonded coil. As a proof of principle, we fabricated an
circuit that achieves impedance-matching to a load
and validate it with a load defined by a carbon nanotube QD of which we measure
the shot noise in the Coulomb blockade regime.Comment: 7 pages, 6 figure
Vortex soliton tori with multiple nested phase singularities in dissipative media
We show the existence of stable two- and three-dimensional vortex solitons
carrying multiple, spatially separated, single-charge topological dislocations
nested around a vortex-ring core. Such new nonlinear states are supported by
elliptical gain landscapes in focusing nonlinear media with two-photon
absorption. The separation between the phase dislocations is dictated mostly by
the geometry of gain landscape and it only slightly changes upon variation of
the gain or absorption strength.Comment: 17 pages, 5 figures, to appear in Physical Review
Electrophysiological and Morphological Properties of Embryonic Neocortical Grafts Developing in Different Regions of the Host Rat Brain
Parallel morphological and electrophysiological studies of embryonic neocortical tissue (primordia of anterior parietalâpresumptive sensorimotorâcortex) grafted into different regions of the host adult brain (sensori-motor cortex, caudate-putamen, septum or thalamus) were carried out to investigate to what extent the properties of transplanted embryonic neocortexâan advanced organizational form of neuronal tissueâare affected by homotopic or heterotopic surroundings
Quasi-linear analysis of the extraordinary electron wave destabilized by runaway electrons
Runaway electrons with strongly anisotropic distributions present in
post-disruption tokamak plasmas can destabilize the extraordinary electron
(EXEL) wave. The present work investigates the dynamics of the quasi-linear
evolution of the EXEL instability for a range of different plasma parameters
using a model runaway distribution function valid for highly relativistic
runaway electron beams produced primarily by the avalanche process. Simulations
show a rapid pitch-angle scattering of the runaway electrons in the high energy
tail on the time scale. Due to the wave-particle
interaction, a modification to the synchrotron radiation spectrum emitted by
the runaway electron population is foreseen, exposing a possible experimental
detection method for such an interaction
Impact of copper and iron binding properties on the anticancer activity of 8-hydroxyquinoline derived Mannich bases.
The anticancer activity of 8-hydroxyquinolines relies on complex formation with redox active copper and iron ions. Here we employ UV-visible spectrophotometry and EPR spectroscopy to compare proton dissociation and complex formation processes of the reference compound 8-hydroxyquinoline (Q-1) and three related Mannich bases to reveal possible correlations with biological activity. The studied derivatives harbor a CH2-N moiety at position 7 linked to morpholine (Q-2), piperidine (Q-3), and chlorine and fluorobenzylamino (Q-4) substituents. Solid phase structures of Q-3, Q-4·HCl·H2O, [(Cu(HQ-2)2)2]·(CH3OH)2·Cl4·(H2O)2, [Cu(Q-3)2]·Cl2 and [Cu(HQ-4)2(CH3OH)]·ZnCl4·CH3OH were characterized by single-crystal X-ray diffraction analysis. In addition, the redox properties of the copper and iron complexes were studied by cyclic voltammetry, and the direct reaction with physiologically relevant reductants (glutathione and ascorbic acid) was monitored. In vitro cytotoxicity studies conducted with the human uterine sarcoma MES-SA/Dx5 cell line reveal the significant cytotoxicity of Q-2, Q-3, and Q-4 in the sub- to low micromolar range (IC50 values 0.2-3.3 ΌM). Correlation analysis of the anticancer activity and the metal binding properties of the compound series indicates that, at physiological pH, weaker copper(ii) and iron(iii) binding results in elevated toxicity (e.g.Q4: pCu = 13.0, pFe = 6.8, IC50 = 0.2 ΌM vs.Q1: pCu = 15.1, pFe = 13.0 IC50 = 2.5 ΌM). Although the studied 8-hydroxyquinolines preferentially bind copper(ii) over iron(iii), the cyclic voltammetry data revealed that the more cytotoxic ligands preferentially stabilize the lower oxidation state of the metal ions. A linear relationship between the pKa (OH) and IC50 values of the studied 8-hydroxyquinolines was found. In summary, we identify Q-4 as a potent and selective anticancer candidate with significant toxicity in drug resistant cells
Local electrical tuning of the nonlocal signals in a Cooper pair splitter
A Cooper pair splitter consists of a central superconducting contact, S, from
which electrons are injected into two parallel, spatially separated quantum
dots (QDs). This geometry and electron interactions can lead to correlated
electrical currents due to the spatial separation of spin-singlet Cooper pairs
from S. We present experiments on such a device with a series of bottom gates,
which allows for spatially resolved tuning of the tunnel couplings between the
QDs and the electrical contacts and between the QDs. Our main findings are
gate-induced transitions between positive conductance correlation in the QDs
due to Cooper pair splitting and negative correlations due to QD dynamics.
Using a semi-classical rate equation model we show that the experimental
findings are consistent with in-situ electrical tuning of the local and
nonlocal quantum transport processes. In particular, we illustrate how the
competition between Cooper pair splitting and local processes can be optimized
in such hybrid nanostructures.Comment: 9 pages, 6 figures, 2 table
Impaired cumulus mucification and female sterility in tumor necrosis factor-induced protein-6 deficient mice
Blocking-state influence on shot noise and conductance in quantum dots
Quantum dots (QDs) investigated through electron transport measurements often
exhibit varying, state-dependent tunnel couplings to the leads. Under specific
conditions, weakly coupled states can result in a strong suppression of the
electrical current and they are correspondingly called blocking states. Using
the combination of conductance and shot noise measurements, we investigate
blocking states in carbon nanotube (CNT) QDs. We report negative differential
conductance and super-Poissonian noise. The enhanced noise is the signature of
electron bunching, which originates from random switches between the strongly
and weakly conducting states of the QD. Negative differential conductance
appears here when the blocking state is an excited state. In this case, at the
threshold voltage where the blocking state becomes populated, the current is
reduced. Using a master equation approach, we provide numerical simulations
reproducing both the conductance and the shot noise pattern observed in our
measurements.Comment: 10 pages, 7 figure
Magnetic field tuning and quantum interference in a Cooper pair splitter
Cooper pair splitting (CPS) is a process in which the electrons of naturally
occurring spin-singlet pairs in a superconductor are spatially separated using
two quantum dots. Here we investigate the evolution of the conductance
correlations in an InAs CPS device in the presence of an external magnetic
field. In our experiments the gate dependence of the signal that depends on
both quantum dots continuously evolves from a slightly asymmetric Lorentzian to
a strongly asymmetric Fano-type resonance with increasing field. These
experiments can be understood in a simple three - site model, which shows that
the nonlocal CPS leads to symmetric line shapes, while the local transport
processes can exhibit an asymmetric shape due to quantum interference. These
findings demonstrate that the electrons from a Cooper pair splitter can
propagate coherently after their emission from the superconductor and how a
magnetic field can be used to optimize the performance of a CPS device. In
addition, the model calculations suggest that the estimate of the CPS
efficiency in the experiments is a lower bound for the actual efficiency.Comment: 5 pages + 4 pages supplementary informatio
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