443 research outputs found
Characteristics of Arctic low-tropospheric humidity inversions based on radio soundings
Humidity inversions have a high potential importance in the Arctic climate
system, especially for cloud formation and maintenance, in wide spatial and
temporal scales. Here we investigate the climatology and characteristics of
humidity inversions in the Arctic, including their spatial and temporal
variability, sensitivity to the methodology applied and differences from the
Antarctic humidity inversions. The study is based on data of the Integrated
Global Radiosonde Archive (IGRA) from 36 Arctic stations between the years
2000 and 2009. The results indicate that humidity inversions are present on
multiple levels nearly all the time in the Arctic atmosphere. Almost half
(48%) of the humidity inversions were found at least partly within the
same vertical layer with temperature inversions, whereas the existence of
the other half may, at least partly, be linked to uneven vertical
distribution of horizontal moisture transport. A high atmospheric surface
pressure was found to increase the humidity inversion occurrence, whereas
relationships between humidity inversion properties and cloud cover were
generally relatively weak, although for some inversion properties they were
systematic. For example, humidity inversions occurred slightly more often
and were deeper under clear sky than in overcast conditions for almost
all stations. The statistics of Arctic humidity inversion properties,
especially inversion strength, depth and base height, proved to be very
sensitive to the instruments and methodology applied. For example, the
median strength of the strongest inversion in a profile was twice as large
as the median of all Arctic inversions. The most striking difference between
the Arctic and Antarctic humidity inversions was the much larger range of
the seasonal cycle of inversion properties in the Arctic. Our results offer
a baseline for validation of weather prediction and climate models and also
encourage further studies on humidity inversions due to the vital, but so
far poorly understood, role of humidity inversions in Arctic cloud
processes
Coupling of shells in a carbon nanotube quantum dot
We systematically study the coupling of longitudinal modes (shells) in a
carbon nanotube quantum dot. Inelastic cotunneling spectroscopy is used to
probe the excitation spectrum in parallel, perpendicular and rotating magnetic
fields. The data is compared to a theoretical model including coupling between
shells, induced by atomically sharp disorder in the nanotube. The calculated
excitation spectra show good correspondence with experimental data.Comment: 8 pages, 4 figure
Effective g-factor in Majorana Wires
We use the effective g-factor of subgap states, g*, in hybrid InAs nanowires
with an epitaxial Al shell to investigate how the superconducting density of
states is distributed between the semiconductor core and the metallic shell. We
find a step-like reduction of g* and improved hard gap with reduced carrier
density in the nanowire, controlled by gate voltage. These observations are
relevant for Majorana devices, which require tunable carrier density and g*
exceeding the g-factor of the proximitizing superconductor. Additionally, we
observe the closing and reopening of a gap in the subgap spectrum coincident
with the appearance of a zero-bias conductance peak
Nonequilibrium Cotunneling through a Three-Level Quantum Dot
We calculate the nonlinear cotunneling conductance through a quantum dot with
3 electrons occupying the three highest lying energy levels. Starting from a
3-orbital Anderson model, we apply a generalized Schrieffer-Wolff
transformation to derive an effective Kondo model for the system. Within this
model we calculate the nonequilibrium occupation numbers and the corresponding
cotunneling current to leading order in the exchange couplings. We identify the
inelastic cotunneling thresholds and their splittings with applied magnetic
field, and make a qualitative comparison to recent experimental data on carbon
nanotube and InAs quantum-wire quantum dots. Further predictions of the model
like cascade resonances and a magnetic-field dependence of the orbital level
splitting are not yet observed but within reach of recent experimental work on
carbon nanotube and InAs nanowire quantum dots.Comment: 12 pages, 13 figure
Nonlocality of Majorana modes in hybrid nanowires
Spatial separation of Majorana zero modes distinguishes trivial from topological midgap states and is key to topological protection in quantum computing applications. Although signatures of Majorana zero modes in tunneling spectroscopy have been reported in numerous studies, a quantitative measure of the degree of separation, or nonlocality, of the emergent zero modes has not been reported. Here, we present results of an experimental study of nonlocality of emergent zero modes in superconductor-semiconductor hybrid nanowire devices. The approach takes advantage of recent theory showing that nonlocality can be measured from splitting due to hybridization of the zero mode in resonance with a quantum dot state at one end of the nanowire. From these splittings as well as anticrossing of the dot states, measured for even and odd occupied quantum dot states, we extract both the degree of nonlocality of the emergent zero mode, as well as the spin canting angles of the nonlocal zero mode. Depending on the device measured, we obtain either a moderate degree of nonlocality, suggesting a partially separated Andreev subgap state, or a highly nonlocal state consistent with a well-developed Majorana modeThis research was supported by Microsoft, the Danish National Research Foundation, the European Commission, and the Spanish Ministry of Economy and Competitiveness through Grants No. FIS2015-65706-P, No. FIS2015-64654-P, and No. FIS2016-80434-P (AEI/FEDER, EU), the Ramón y Cajal programme Grant No. RYC-2011-09345, and the María de Maeztu Programme for Units of Excellence in R&D (Grant No. MDM-2014-0377). C.M.M. acknowledges support from the Villum Foundation. M.-T.D. acknowledges support from State Key Laboratory of High Performance Computing, Chin
The impact of radiosounding observations on numerical weather prediction analyses in the Arctic
The radiosounding network in the Arctic, despite being sparse, is a crucial part of the atmospheric observing system for weather prediction and reanalysis. The spatial coverage of the network was evaluated using a numerical weather prediction model, comparing radiosonde observations from Arctic land stations and expeditions in the central Arctic Ocean with operational analyses and background fields (12h forecasts) from ECMWF for January 2016 – September 2018. The results show that the impact of radiosonde observations on analyses has large geographical variation. In data‐sparse areas, such as the central Arctic Ocean, high‐quality radiosonde observations substantially improve the analyses, while satellite observations are not able to compensate for the large spatial gap in the radiosounding network. In areas where the network is reasonably dense, the quality of background field is more related to how radiosonde observations are utilized in the assimilation and to the quality of those observations
Majorana bound states in a coupled quantum-dot hybrid-nanowire system
Hybrid nanowires combining semiconductor and superconductor materials appear
well suited for the creation, detection, and control of Majorana bound states
(MBSs). We demonstrate the emergence of MBSs from coalescing Andreev bound
states (ABSs) in a hybrid InAs nanowire with epitaxial Al, using a quantum dot
at the end of the nanowire as a spectrometer. Electrostatic gating tuned the
nanowire density to a regime of one or a few ABSs. In an applied axial magnetic
field, a topological phase emerges in which ABSs move to zero energy and remain
there, forming MBSs. We observed hybridization of the MBS with the end-dot
bound state, which is in agreement with a numerical model. The ABS/MBS spectra
provide parameters that are useful for understanding topological
superconductivity in this system.Comment: Article and Supplementary Materia
Voltage-Controlled Superconducting Quantum Bus
We demonstrate the ability of an epitaxial semiconductor-superconductor
nanowire to serve as a field-effect switch to tune a superconducting cavity.
Two superconducting gatemon qubits are coupled to the cavity, which acts as a
quantum bus. Using a gate voltage to control the superconducting switch yields
up to a factor of 8 change in qubit-qubit coupling between the on and off
states without detrimental effect on qubit coherence. High-bandwidth operation
of the coupling switch on nanosecond timescales degrades qubit coherence
Superconductivity-enhanced bias spectroscopy in carbon nanotube quantum dots
We study low-temperature transport through carbon nanotube quantum dots in
the Coulomb blockade regime coupled to niobium-based superconducting leads. We
observe pronounced conductance peaks at finite source-drain bias, which we
ascribe to elastic and inelastic cotunneling processes enhanced by the
coherence peaks in the density of states of the superconducting leads. The
inelastic cotunneling lines display a marked dependence on the applied gate
voltage which we relate to different tunneling-renormalizations of the two
subbands in the nanotube. Finally, we discuss the origin of an especially
pronounced sub-gap structure observed in every fourth Coulomb diamond
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