527 research outputs found
Intermediate states in Andreev bound state fusion
Hybridization is a very fundamental quantum mechanical phenomenon, with the
text book example of binding two hydrogen atoms in a hydrogen molecule. In
semiconductor physics, a quantum dot (QD) can be considered as an artificial
atom, with two coupled QDs forming a molecular state, and two electrons on a
single QD the equivalent of a helium atom. Here we report tunnel spectroscopy
experiments illustrating the hybridization of another type of discrete quantum
states, namely of superconducting subgap states that form in segments of a
semiconducting nanowire in contact with superconducting reservoirs. We show and
explain a collection of intermediate states found in the process of merging
individual bound states, hybridizing with a central QD and eventually
coherently linking the reservoirs. These results may serve as a guide in future
Majorana fusion experiments and explain a large variety of recent bound state
experiments
Analyzing capacitance-voltage measurements of vertical wrapped-gated nanowires
The capacitance of arrays of vertical wrapped-gate InAs nanowires are
analyzed. With the help of a Poisson-Schr"odinger solver, information about the
doping density can be obtained directly. Further features in the measured
capacitance-voltage characteristics can be attributed to the presence of
surface states as well as the coexistence of electrons and holes in the wire.
For both scenarios, quantitative estimates are provided. It is furthermore
shown that the difference between the actual capacitance and the geometrical
limit is quite large, and depends strongly on the nanowire material.Comment: 15 pages, 6 Figures included, to appear in Nanotechnolog
Magnetic field independent sub-gap states in hybrid Rashba nanowires
Sub-gap states in semiconducting-superconducting nanowire hybrid devices are
controversially discussed as potential topologically non-trivial quantum
states. One source of ambiguity is the lack of an energetically and spatially
well defined tunnel spectrometer. Here, we use quantum dots directly integrated
into the nanowire during the growth process to perform tunnel spectroscopy of
discrete sub-gap states in a long nanowire segment. In addition to sub-gap
states with a standard magnetic field dependence, we find topologically trivial
sub-gap states that are independent of the external magnetic field, i.e. that
are pinned to a constant energy as a function of field. We explain this effect
qualitatively and quantitatively by taking into account the strong spin-orbit
interaction in the nanowire, which can lead to a decoupling of Andreev bound
states from the field due to a spatial spin texture of the confined
eigenstates
Coherent Single Charge Transport in Molecular-Scale Silicon Nanowire Transistors
We report low-temperature electrical transport studies of molecule-scale
silicon nanowires. Individual nanowires exhibit well-defined Coulomb blockade
oscillations characteristic of charge addition to a single nanostructure with
length scales up to at least 400 nm. Further studies demonstrate coherent
charge transport through discrete single particle quantum levels extending the
whole device, and show that the ground state spin configuration follows the
Lieb-Mattis theorem. In addition, depletion of the nanowires suggests that
phase coherent single-dot characteristics are accessible in a regime where
correlations are strong.Comment: 4 pages and 4 figure
GaAs:Mn nanowires grown by molecular beam epitaxy of (Ga,Mn)As at MnAs segregation conditions
GaAs:Mn nanowires were obtained on GaAs(001) and GaAs(111)B substrates by
molecular beam epitaxial growth of (Ga,Mn)As at conditions leading to MnAs
phase separation. Their density is proportional to the density of catalyzing
MnAs nanoislands, which can be controlled by the Mn flux and/or the substrate
temperature. Being rooted in the ferromagnetic semiconductor (Ga,Mn)As, the
nanowires combine one-dimensional properties with the magnetic properties of
(Ga,Mn)As and provide natural, self assembled structures for nanospintronics.Comment: 13 pages, 6 figure
Measuring Temperature Gradients over Nanometer Length Scales
When a quantum dot is subjected to a thermal gradient, the temperature of
electrons entering the dot can be determined from the dot's thermocurrent if
the conductance spectrum and background temperature are known. We demonstrate
this technique by measuring the temperature difference across a 15 nm quantum
dot embedded in a nanowire. This technique can be used when the dot's energy
states are separated by many kT and will enable future quantitative
investigations of electron-phonon interaction, nonlinear thermoelectric
effects, and the effciency of thermoelectric energy conversion in quantum dots.Comment: 6 pages, 5 figure
Correlating the nanostructure and electronic properties of InAs nanowires
The electronic properties and nanostructure of InAs nanowires are correlated
by creating multiple field effect transistors (FETs) on nanowires grown to have
low and high defect density segments. 4.2 K carrier mobilities are ~4X larger
in the nominally defect-free segments of the wire. We also find that dark field
optical intensity is correlated with the mobility, suggesting a simple route
for selecting wires with a low defect density. At low temperatures, FETs
fabricated on high defect density segments of InAs nanowires showed transport
properties consistent with single electron charging, even on devices with low
resistance ohmic contacts. The charging energies obtained suggest quantum dot
formation at defects in the wires. These results reinforce the importance of
controlling the defect density in order to produce high quality electrical and
optical devices using InAs nanowires.Comment: Related papers at http://pettagroup.princeton.ed
Fatal poisoning in drug addicts in the Nordic countries in 2017
This study is the seventh report on fatal poisonings among drug addicts in the Nordic countries. In this report, we analyse data from the five Nordic countries: Denmark, Finland, Iceland, Norway and Sweden. Data on gender, number of deaths, places of deaths, age, main intoxicants and substances detected in blood were recorded to obtain national and comparable Nordic data, and to allow comparison with earlier studies conducted in 1984, 1991, 1997, 2002, 2007 and 2012. The death rate (number of deaths per 100,000 inhabitants) was highest in Iceland (6.58) followed closely by Sweden (6.46) and then lowest in Denmark (4.29). The death rate increased in Finland (5.84), Iceland and Sweden and decreased in Denmark compared to earlier studies. The death rate in Norway, which has decreased since 2002, has stabilised around 5.7 as of 2017. Women accounted for 7-23% of the fatal poisonings. The percentage was lowest in Iceland and highest in Finland and Norway. The age range was 14-70 years. The median age (41 years) was highest in Denmark and Norway. The other countries had a median age between 33 and 35 years. Opioids were the main cause of death. Methadone remained the main intoxicant in Denmark, while heroin/morphine was still the main intoxicant in Norway, as was buprenorphine in Finland. However, the picture has changed in Sweden compared to 2012, where heroin/morphine caused most deaths in 2017. Sweden also experienced the highest number of deaths from fentanyl analogues (67 deaths) and buprenorphine (61 deaths). Deaths from fentanyl analogues also occurred in Denmark, Finland and Norway, but to a smaller extent. Over the years, the proportion of opioid deaths has decreased in all countries except Sweden, which has experienced an increase. This decline has been replaced by deaths from CNS stimulants like cocaine, amphetamine and methylenedioxymethamphetamine (MDMA). Cocaine deaths have occurred in all countries but most frequently in Denmark. MDMA deaths have increased in all countries but mostly in Finland. Poly-drug use was widespread, as seen in the earlier studies. The median number of detected drugs per case varied from 4-6. Heroin/morphine, methadone, buprenorphine, cocaine, amphetamine, methamphetamine, MDMA, tetrahydrocannabinol (THC) and benzodiazepines were frequently detected. Pregabalin and gabapentin were detected in all countries, especially pregabalin, which was detected in 42% of the Finnish cases. New psychoactive substances (NPS) occurred in all countries except Iceland. (C) 2020 Elsevier B.V. All rights reserved.Peer reviewe
Observation of degenerate one-dimensional sub-bands in cylindrical InAs nanowires
One-dimensional (1D) sub-bands in cylindrical InAs nanowires (NWs) are
electrically mapped as a function of NW diameter in the range of 15-35 nm. At
low temperatures, stepwise current increases with the gate voltage are clearly
observed and attributed to the electron transport through individual 1D
sub-bands. The two-fold degeneracy in certain sub-band energies predicted by
simulation due to structural symmetry is experimentally observed for the first
time. The experimentally obtained sub-band energies match the simulated
results, shedding light on both the energies of the sub-bands as well as the
number of sub-bands populated per given gate voltage and diameter. This work
serves to provide better insight into the electrical transport behavior of 1D
semiconductors.Comment: 20 pages, 5 figures, supporting information include
Unintentional high density p-type modulation doping of a GaAs/AlAs core-multi-shell nanowire
Achieving significant doping in GaAs/AlAs core/shell nanowires (NWs) is of
considerable technological importance but remains a challenge due to the
amphoteric behavior of the dopant atoms. Here we show that placing a narrow
GaAs quantum well in the AlAs shell effectively getters residual carbon
acceptors leading to an \emph{unintentional} p-type doping. Magneto-optical
studies of such a GaAs/AlAs core multi-shell NW reveal quantum confined
emission. Theoretical calculations of NW electronic structure confirm quantum
confinement of carriers at the core/shell interface due to the presence of
ionized carbon acceptors in the 1~nm GaAs layer in the shell.
Micro-photoluminescence in high magnetic field shows a clear signature of
avoided crossings of the Landau level emission line with the Landau
level TO phonon replica. The coupling is caused by the resonant hole-phonon
interaction, which points to a large 2D hole density in the structure.Comment: just published in Nano Letters
(http://pubs.acs.org/doi/full/10.1021/nl500818k
- …