236 research outputs found
Efficient dynamical nuclear polarization in quantum dots: Temperature dependence
We investigate in micro-photoluminescence experiments the dynamical nuclear
polarization in individual InGaAs quantum dots. Experiments carried out in an
applied magnetic field of 2T show that the nuclear polarization achieved
through the optical pumping of electron spins is increasing with the sample
temperature between 2K and 55K, reaching a maximum of about 50%. Analysing the
dependence of the Overhauser shift on the spin polarization of the optically
injected electron as a function of temperature enables us to identify the main
reasons for this increase.Comment: 5 pages, 3 figure
Electrochemical polymerisation of phenol in aqueous solution on a Ta/PbO2 anode
This paper deals with the treatment of aqueous phenol solutions using an electrochemical technique. Phenol can be partly eliminated from aqueous solution by electrochemically initiated polymerisation. Galvanostatic electrolyses of phenol solutions at concentration up to 0.1 mol dmâ3 were carried out on a Ta/PbO2 anode. The polymers formed are insoluble in acidic medium but soluble in alkaline. These polymers were filtered and then dissolved in aqueous solution of sodium hydroxide (1 mol dmâ3). The polymers formed were quantified by total organic carbon (TOC) measurement. It was found that the conversion of phenol into polymers increases as a function of initial concentration, anodic current density, temperature, and solution pH. The percentage of phenol polymerised can reach 15%
Impact of heavy hole-light hole coupling on optical selection rules in GaAs quantum dots
We report strong heavy hole-light mixing in GaAs quantum dots grown by
droplet epitaxy. Using the neutral and charged exciton emission as a monitor we
observe the direct consequence of quantum dot symmetry reduction in this strain
free system. By fitting the polar diagram of the emission with simple
analytical expressions obtained from kp theory we are able to extract
the mixing that arises from the heavy-light hole coupling due to the
geometrical asymmetry of the quantum dot.Comment: 4 pages, 2 figure
Bunching visibility for correlated photons from single GaAs quantum dots
We study photon bunching phenomena associated with biexciton-exciton cascade
in single GaAs self-assembled quantum dots. Experiments carried out with a
pulsed excitation source show that significant bunching is only detectable at
very low excitation, where the typical intensity of photon streams is less than
the half of their saturation value. Our findings are qualitatively understood
with a model which accounts for Poissonian statistics in the number of
excitons, predicting the height of a bunching peak being determined by the
inverse of probability of finding more than one exciton.Comment: 6 pages, 6 figs to appear in Phys. Rev.
Synthesis and Biological Evaluation of New Chromenes and Chromeno[2,3-d] pyrimidines
A simple and efficient approach has been developed to synthesise novel and functionalised 5H-chromeno[2,3-d] pyrimidines derivatives (4aâh). This approach entails treating 2-amino-3-cyano-4H-chromenes (3aâh) with formamidine acetate under microwave irradiations and solvent-free conditions. All structures of new compounds obtained in this study were characterised by IR, MS, 1H and 13C NMR analysis. Additionally, the synthesised compounds were investigated for their antibacterial and antioxidant potential. Compounds 3b, 3c, 3e, 4c and 4e showed significant activities
A comparison of electrochemical degradation of phenol on boron doped diamond and lead dioxide anodes
This work compares two electrode materials used to mineralize phenol contained in waste waters. Two disks covered with either boron doped diamond (BDD) or PbO2 were used as anodes in a one compartment flow cell under the same hydrodynamic conditions. Efficiencies of galvanostatic electrolyses are compared on the basis of measurements of Total Organic Carbon (TOC) and Chemical Oxygen Demand (COD). Galvanostatic electrolyses were monitored by analysis of phenol and of its oxidation derivatives to evaluate the operating time needed for complete elimination of toxic aromatics. The experimental current efficiency is close to the theoretical value for the BDD electrode. Other parameters being equal, phenol species disappeared at the same rate using the two electrode materials but the BDD anode showed better efficiency to eliminate TOC and COD. Moreover, during the electrolysis less intermediates are formed with BDD compared to PbO2 whatever the current density. A comparison of energy consumption is given based on the criterion of 99% removal of aromatic compounds
Nonlinear absorption reversing between an electroactive ligand and its metal complexes
We present the nonlinear absorption investigation of an electroactive ligand and two ruthenium and iron metal complexes under 532 nm, 30 ps laser excitation, by the open aperture Z-scan technique. Significant nonlinear optical parameters have in all cases been measured, while the nonlinear attribute has been found to change from saturable to reverse saturable absorption between the initial ligand and its complexes
Demagnetization of Quantum Dot Nuclear Spins: Breakdown of the Nuclear Spin Temperature Approach
The physics of interacting nuclear spins arranged in a crystalline lattice is
typically described using a thermodynamic framework: a variety of experimental
studies in bulk solid-state systems have proven the concept of a spin
temperature to be not only correct but also vital for the understanding of
experimental observations. Using demagnetization experiments we demonstrate
that the mesoscopic nuclear spin ensemble of a quantum dot (QD) can in general
not be described by a spin temperature. We associate the observed deviations
from a thermal spin state with the presence of strong quadrupolar interactions
within the QD that cause significant anharmonicity in the spectrum of the
nuclear spins. Strain-induced, inhomogeneous quadrupolar shifts also lead to a
complete suppression of angular momentum exchange between the nuclear spin
ensemble and its environment, resulting in nuclear spin relaxation times
exceeding an hour. Remarkably, the position dependent axes of quadrupolar
interactions render magnetic field sweeps inherently non-adiabatic, thereby
causing an irreversible loss of nuclear spin polarization.Comment: 15 pages, 3 figure
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