30 research outputs found
Kondo physics in tunable semiconductor nanowire quantum dots
We have observed the Kondo effect in strongly coupled semiconducting nanowire
quantum dots. The devices are made from indium arsenide nanowires, grown by
molecular beam epitaxy, and contacted by titanium leads. The device
transparency can be tuned by changing the potential on a gate electrode, and
for increasing transparencies the effects dominating the transport changes from
Coulomb Blockade to Universal Conductance Fluctuations with Kondo physics
appearing in the intermediate region.Comment: 4 pages, 4 figure
Single nanowire solar cells beyond the Shockley-Queisser limit
Light management is of great importance to photovoltaic cells, as it
determines the fraction of incident light entering the device. An optimal
pn-junction combined with an optimal light absorption can lead to a solar cell
efficiency above the Shockley-Queisser limit. Here, we show how this is
possible by studying photocurrent generation for a single core-shell p-i-n
junction GaAs nanowire solar cell grown on a silicon substrate. At one sun
illumination a short circuit current of 180 mA/cm^2 is obtained, which is more
than one order of magnitude higher than what would be predicted from
Lambert-Beer law. The enhanced light absorption is shown to be due to a light
concentrating property of the standing nanowire as shown by photocurrent maps
of the device. The results imply new limits for the maximum efficiency
obtainable with III-V based nanowire solar cells under one sun illumination.Comment: 19 pages, 3 figure
Shadow epitaxy for in-situ growth of generic semiconductor/superconductor devices
Uniform, defect-free crystal interfaces and surfaces are crucial ingredients
for realizing high-performance nanoscale devices. A pertinent example is that
advances in gate-tunable and topological superconductivity using
semiconductor/superconductor electronic devices are currently built on the hard
proximity-induced superconducting gap obtained from epitaxial indium
arsenide/aluminium heterostructures. Fabrication of devices requires selective
etch processes; these exist only for InAs/Al hybrids, precluding the use of
other, potentially superior material combinations. We present a crystal growth
platform -- based on three-dimensional structuring of growth substrates --
which enables synthesis of semiconductor nanowire hybrids with in-situ
patterned superconductor shells. This platform eliminates the need for etching,
thereby enabling full freedom in choice of hybrid constituents. We realise and
characterise all the most frequently used architectures in superconducting
hybrid devices, finding increased yield and electrostatic stability compared to
etched devices, along with evidence of ballistic superconductivity. In addition
to aluminium, we present hybrid devices based on tantalum, niobium and
vanadium.
This is the submitted version of the manuscript. The accepted, peer reviewed
version is available from Advanced Materials:
http://doi.org/10.1002/adma.201908411
Previous title: Shadow lithography for in-situ growth of generic
semiconductor/superconductor device
Advances in the theory of III-V Nanowire Growth Dynamics
Nanowire (NW) crystal growth via the vapour_liquid_solid mechanism is a
complex dynamic process involving interactions between many atoms of various
thermodynamic states. With increasing speed over the last few decades many
works have reported on various aspects of the growth mechanisms, both
experimentally and theoretically. We will here propose a general continuum
formalism for growth kinetics based on thermodynamic parameters and transition
state kinetics. We use the formalism together with key elements of recent
research to present a more overall treatment of III_V NW growth, which can
serve as a basis to model and understand the dynamical mechanisms in terms of
the basic control parameters, temperature and pressures/beam fluxes.
Self-catalysed GaAs NW growth on Si substrates by molecular beam epitaxy is
used as a model system.Comment: 63 pages, 25 figures and 4 tables. Some details are explained more
carefully in this version aswell as a new figure is added illustrating
various facets of a WZ crysta
Kondo-enhanced Andreev tunneling in InAs nanowire quantum dots
We report measurements of the nonlinear conductance of InAs nanowire quantum
dots coupled to superconducting leads. We observe a clear alternation between
odd and even occupation of the dot, with sub-gap-peaks at
markedly stronger(weaker) than the quasiparticle tunneling peaks at
for odd(even) occupation. We attribute the enhanced
-peak to an interplay between Kondo-correlations and Andreev tunneling
in dots with an odd number of spins, and substantiate this interpretation by a
poor man's scaling analysis
Clinical Description of a Completed Outbreak of SARS in Vietnam, February–May, 2003
We investigated the clinical manifestations and course of all probable severe acute respiratory syndrome (SARS) patients in the Vietnam outbreak. Probable SARS cases were defined by using the revised World Health Organization criteria. We systematically reviewed medical records and undertook descriptive statistical analyses. All 62 patients were hospitalized. On admission, the most prominent symptoms were malaise (82.3%) and fever (79.0%). Cough, chest pain, and shortness of breath were present in approximately one quarter of the patients; 79.0% had lymphopenia; 40.3% had thrombocytopenia; 19.4% had leukopenia; and 75.8% showed changes on chest radiograph. Fever developed on the first day of illness onset, and both respiratory symptoms and radiographic changes occurred on day 4. On average, maximal radiographic changes were observed on day 10, and fevers subsided by day 13. Symptoms on admission were nonspecific, although fever, malaise, and lymphopenia were common. The complications of SARS included invasive intubation and ventilation (11.3%) and death (9.7%)
Formation of GaAs nanwires grown without catalyst seed crystals on <sup>111</sup>Si:Proceedings of 17th International Congress, #M1.22, Rio-Brazil
Influence of the oxide layer for growth of self-assisted InAs nanowires on Si(111)
The growth of self-assisted InAs nanowires (NWs) by molecular beam epitaxy (MBE) on Si(111) is studied for different growth parameters and substrate preparations. The thickness of the oxide layer present on the Si(111) surface is observed to play a dominant role. Systematic use of different pre-treatment methods provides information on the influence of the oxide on the NW morphology and growth rates, which can be used for optimizing the growth conditions. We show that it is possible to obtain 100% growth of vertical NWs and no parasitic bulk structures between the NWs by optimizing the oxide thickness. For a growth temperature of 460°C and a V/III ratio of 320 an optimum oxide thickness of 9 ± 3 Å is found