1,044 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
A hybrid metal/semiconductor electron pump for quantum metrology
Electron pumps capable of delivering a current higher than 100pA with
sufficient accuracy are likely to become the direct mise en pratique of the
possible new quantum definition of the ampere. Furthermore, they are essential
for closing the quantum metrological triangle experiment which tests for
possible corrections to the quantum relations linking e and h, the electron
charge and the Planck constant, to voltage, resistance and current. We present
here single-island hybrid metal/semiconductor transistor pumps which combine
the simplicity and efficiency of Coulomb blockade in metals with the
unsurpassed performances of silicon switches. Robust and simple pumping at
650MHz and 0.5K is demonstrated. The pumped current obtained over a voltage
bias range of 1.4mV corresponds to a relative deviation of 5e-4 from the
calculated value, well within the 1.5e-3 uncertainty of the measurement setup.
Multi-charge pumping can be performed. The simple design fully integrated in an
industrial CMOS process makes it an ideal candidate for national measurement
institutes to realize and share a future quantum ampere
Optical properties of an ensemble of G-centers in silicon
We addressed the carrier dynamics in so-called G-centers in silicon
(consisting of substitutional-interstitial carbon pairs interacting with
interstitial silicons) obtained via ion implantation into a
silicon-on-insulator wafer. For this point defect in silicon emitting in the
telecommunication wavelength range, we unravel the recombination dynamics by
time-resolved photoluminescence spectroscopy. More specifically, we performed
detailed photoluminescence experiments as a function of excitation energy,
incident power, irradiation fluence and temperature in order to study the
impact of radiative and non-radiative recombination channels on the spectrum,
yield and lifetime of G-centers. The sharp line emitting at 969 meV (1280
nm) and the broad asymmetric sideband developing at lower energy share the same
recombination dynamics as shown by time-resolved experiments performed
selectively on each spectral component. This feature accounts for the common
origin of the two emission bands which are unambiguously attributed to the
zero-phonon line and to the corresponding phonon sideband. In the framework of
the Huang-Rhys theory with non-perturbative calculations, we reach an
estimation of 1.60.1 \angstrom for the spatial extension of the
electronic wave function in the G-center. The radiative recombination time
measured at low temperature lies in the 6 ns-range. The estimation of both
radiative and non-radiative recombination rates as a function of temperature
further demonstrate a constant radiative lifetime. Finally, although G-centers
are shallow levels in silicon, we find a value of the Debye-Waller factor
comparable to deep levels in wide-bandgap materials. Our results point out the
potential of G-centers as a solid-state light source to be integrated into
opto-electronic devices within a common silicon platform
Applying Machine Learning to Catalogue Matching in Astrophysics
We present the results of applying automated machine learning techniques to
the problem of matching different object catalogues in astrophysics. In this
study we take two partially matched catalogues where one of the two catalogues
has a large positional uncertainty. The two catalogues we used here were taken
from the HI Parkes All Sky Survey (HIPASS), and SuperCOSMOS optical survey.
Previous work had matched 44% (1887 objects) of HIPASS to the SuperCOSMOS
catalogue.
A supervised learning algorithm was then applied to construct a model of the
matched portion of our catalogue. Validation of the model shows that we
achieved a good classification performance (99.12% correct).
Applying this model, to the unmatched portion of the catalogue found 1209 new
matches. This increases the catalogue size from 1887 matched objects to 3096.
The combination of these procedures yields a catalogue that is 72% matched.Comment: 8 Pages, 5 Figure
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