11,644 research outputs found
Low-density silicon allotropes for photovoltaic applications
Silicon materials play a key role in many technologically relevant fields,
ranging from the electronic to the photovoltaic industry. A systematic search
for silicon allotropes was performed by employing a modified ab initio minima
hopping crystal structure prediction method. The algorithm was optimized to
specifically investigate the hitherto barely explored low-density regime of the
silicon phase diagram by imitating the guest-host concept of clathrate
compounds. In total 44 metastable phases are presented, of which 11 exhibit
direct or quasi-direct band-gaps in the range of 1.0-1.8 eV, close to
the optimal Shockley-Queisser limit of 1.4 eV, with a stronger overlap
of the absorption spectra with the solar spectrum compared to conventional
diamond silicon. Due to the structural resemblance to known clathrate compounds
it is expected that the predicted phases can be synthesized
Seeking Quantum Speedup Through Spin Glasses: The Good, the Bad, and the Ugly
There has been considerable progress in the design and construction of
quantum annealing devices. However, a conclusive detection of quantum speedup
over traditional silicon-based machines remains elusive, despite multiple
careful studies. In this work we outline strategies to design hard tunable
benchmark instances based on insights from the study of spin glasses - the
archetypal random benchmark problem for novel algorithms and optimization
devices. We propose to complement head-to-head scaling studies that compare
quantum annealing machines to state-of-the-art classical codes with an approach
that compares the performance of different algorithms and/or computing
architectures on different classes of computationally hard tunable spin-glass
instances. The advantage of such an approach lies in having to only compare the
performance hit felt by a given algorithm and/or architecture when the instance
complexity is increased. Furthermore, we propose a methodology that might not
directly translate into the detection of quantum speedup, but might elucidate
whether quantum annealing has a "`quantum advantage" over corresponding
classical algorithms like simulated annealing. Our results on a 496 qubit
D-Wave Two quantum annealing device are compared to recently-used
state-of-the-art thermal simulated annealing codes.Comment: 14 pages, 8 figures, 3 tables, way too many reference
A fabrication guide for planar silicon quantum dot heterostructures
We describe important considerations to create top-down fabricated planar
quantum dots in silicon, often not discussed in detail in literature. The
subtle interplay between intrinsic material properties, interfaces and
fabrication processes plays a crucial role in the formation of
electrostatically defined quantum dots. Processes such as oxidation, physical
vapor deposition and atomic-layer deposition must be tailored in order to
prevent unwanted side effects such as defects, disorder and dewetting. In two
directly related manuscripts written in parallel we use techniques described in
this work to create depletion-mode quantum dots in intrinsic silicon, and
low-disorder silicon quantum dots defined with palladium gates. While we
discuss three different planar gate structures, the general principles also
apply to 0D and 1D systems, such as self-assembled islands and nanowires.Comment: Accepted for publication in Nanotechnology. 31 pages, 12 figure
Directional supercontinuum generation: the role of the soliton
In this paper we numerically study supercontinuum generation by pumping a
silicon nitride waveguide, with two zero-dispersion wavelengths, with
femtosecond pulses. The waveguide dispersion is designed so that the pump pulse
is in the normal-dispersion regime. We show that because of self-phase
modulation, the initial pulse broadens into the anomalous-dispersion regime,
which is sandwiched between the two normal-dispersion regimes, and here a
soliton is formed. The interaction of the soliton and the broadened pulse in
the normal-dispersion regime causes additional spectral broadening through
formation of dispersive waves by non-degenerate four-wave mixing and
cross-phase modulation. This broadening occurs mainly towards the second
normal-dispersion regime. We show that pumping in either normal-dispersion
regime allows broadening towards the other normal-dispersion regime. This
ability to steer the continuum extension towards the direction of the other
normal-dispersion regime beyond the sandwiched anomalous-dispersion regime
underlies the directional supercontinuum notation. We numerically confirm the
approach in a standard silica microstructured fiber geometry with two
zero-dispersion wavelengths
4D visualization of embryonic, structural crystallization by single-pulse microscopy
In many physical and biological systems the transition from an amorphous to ordered native structure involves complex energy landscapes, and understanding such transformations requires not only their thermodynamics but also the structural dynamics during the process. Here, we extend our 4D visualization method with electron imaging to include the study of irreversible processes with a single pulse in the same ultrafast electron microscope (UEM) as used before in the single-electron mode for the study of reversible processes. With this augmentation, we report on the transformation of amorphous to crystalline structure with silicon as an example. A single heating pulse was used to initiate crystallization from the amorphous phase while a single packet of electrons imaged selectively in space the transformation as the structure continuously changes with time. From the evolution of crystallinity in real time and the changes in morphology, for nanosecond and femtosecond pulse heating, we describe two types of processes, one that occurs at early time and involves a nondiffusive motion and another that takes place on a longer time scale. Similar mechanisms of two distinct time scales may perhaps be important in biomolecular folding
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The Impact of Radiation Damage on Electron Multiplying CCD Technology for the WFIRST Coronagraph
This thesis follows an investigation into the effects of radiation damage on the e2v CCD201-20; the detector baselined for use in the WFIRST coronagraph imaging and spectroscopy camera systems (hereafter, WFIRST CGI). The CCD201 is an EM-CCD, a variant of traditional CCD technology that is well suited for operation in light starved conditions. Despite successful implementation on many ground-based instruments, the technology has yet to be used within a space environment and therefore has low technological maturity compared to the standard CCD counterpart. Improvement of the technological maturity rested upon in-depth investigations into the effect of radiation damage on the CCD201, which in turn could be used to estimate the End Of Life (EOL) performance of the instrument and de-risk the utilisation of EM-CCDs for the mission. An in-depth radiation campaign was completed whereby multiple CCD201s were irradiated to multiple fluence levels at both room temperature and the nominal operating temperature of the mission (165 K). Performance was measured prior to and following each irradiation, including measurements of low-signal Charge Transfer Inefficiency (CTI), dark current and Clock Induced Charge (CIC). Significant performance differences were noted between the room temperature and cryogenic irradiation case, indicating that cryogenic irradiations are instrumental to accurate EOL performance estimates. CTI was identified as the key limitation to CGI science performance, and so attention then turned to amelioration strategies focused on improving performance in the presence of radiation damage, including trap pumping and narrow-channel modelling. The results presented in this thesis have helped lead to the adoption of the CCD201-20 for the WFIRST mission, have provided key insight into the differences between room temperature and cryogenic irradiations, have advanced the “trap pumping” technique for use on EM-CCDs and presented the properties on the dominant traps that impact CTI for radiation damaged CCDs. The findings are not only useful for the WFIRST CGI, but for any future space mission that will utilise EM-CCD technology in an environment where radiation has the potential to degrade science performance
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