1,579 research outputs found
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
Double Diffusion Encoding Prevents Degeneracy in Parameter Estimation of Biophysical Models in Diffusion MRI
Purpose: Biophysical tissue models are increasingly used in the
interpretation of diffusion MRI (dMRI) data, with the potential to provide
specific biomarkers of brain microstructural changes. However, the general
Standard Model has recently shown that model parameter estimation from dMRI
data is ill-posed unless very strong magnetic gradients are used. We analyse
this issue for the Neurite Orientation Dispersion and Density Imaging with
Diffusivity Assessment (NODDIDA) model and demonstrate that its extension from
Single Diffusion Encoding (SDE) to Double Diffusion Encoding (DDE) solves the
ill-posedness and increases the accuracy of the parameter estimation. Methods:
We analyse theoretically the cumulant expansion up to fourth order in b of SDE
and DDE signals. Additionally, we perform in silico experiments to compare SDE
and DDE capabilities under similar noise conditions. Results: We prove
analytically that DDE provides invariant information non-accessible from SDE,
which makes the NODDIDA parameter estimation injective. The in silico
experiments show that DDE reduces the bias and mean square error of the
estimation along the whole feasible region of 5D model parameter space.
Conclusions: DDE adds additional information for estimating the model
parameters, unexplored by SDE, which is enough to solve the degeneracy in the
NODDIDA model parameter estimation.Comment: 22 pages, 7 figure
A Semiconductor Nanowire-Based Superconducting Qubit
We introduce a hybrid qubit based on a semiconductor nanowire with an
epitaxially grown superconductor layer. Josephson energy of the transmon-like
device ("gatemon") is controlled by an electrostatic gate that depletes
carriers in a semiconducting weak link region. Strong coupling to an on-chip
microwave cavity and coherent qubit control via gate voltage pulses is
demonstrated, yielding reasonably long relaxation times (0.8 {\mu}s) and
dephasing times (1 {\mu}s), exceeding gate operation times by two orders of
magnitude, in these first-generation devices. Because qubit control relies on
voltages rather than fluxes, dissipation in resistive control lines is reduced,
screening reduces crosstalk, and the absence of flux control allows operation
in a magnetic field, relevant for topological quantum information
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
Parity lifetime of bound states in a proximitized semiconductor nanowire
Quasiparticle excitations can compromise the performance of superconducting
devices, causing high frequency dissipation, decoherence in Josephson qubits,
and braiding errors in proposed Majorana-based topological quantum computers.
Quasiparticle dynamics have been studied in detail in metallic superconductors
but remain relatively unexplored in semiconductor-superconductor structures,
which are now being intensely pursued in the context of topological
superconductivity. To this end, we introduce a new physical system comprised of
a gate-confined semiconductor nanowire with an epitaxially grown superconductor
layer, yielding an isolated, proximitized nanowire segment. We identify
Andreev-like bound states in the semiconductor via bias spectroscopy, determine
the characteristic temperatures and magnetic fields for quasiparticle
excitations, and extract a parity lifetime (poisoning time) of the bound state
in the semiconductor exceeding 10 ms.Comment: text and supplementary information combine
Expresión incompleta de incompatibilidad trimórfica en Oxalis compacta Gill. ex Hook. et Arn. subsp. compacta en los Andes de Chile central
The expression of trimorphic incompatibility was investigated in a high altitude population of Oxalis compacta subsp.compacta distributed in the upper alpine belt (3100-3470 m) of the central Chile Andes. Stigma-anther reciprocity (2populations), morph representation (7 populations), unassisted selfing capacity and open-pollination fruit set (6 populations)were determined. O. compacta ssp. compacta es highly dependent on external pollinators. Although it has typical tristylousflowers, considerable seed set is possible following illegitimate intermorph and intramorph cross pollination. Openpollination fruit set varied from 9-83% and on average was lower in the higher-elevation populations. The possibilityof seed set following two kinds of illegitimate cross pollination is seen as a means of enhancing reproductive output byenabling more effective use of the limited pollinator resource in the high alpine environment, where fruit set shows highinter-annual variability and there is some evidence for pollination limitation in Oxalis compacta subsp. compacta.Se investigó la expresión de incompatibilidad trimórfica en una población de Oxalis compacta subsp. compacta distribuidaen la zona alpina superior de los Andes de Chile central. Se determinó la reciprocidad de los estigmas y anteras (dospoblaciones), representación de morfos florales (7 poblaciones), capacidad para autofecundación no asistida y polinizaciónabierta (6 poblaciones). O. compacta subsp. compacta es altamente dependiente de polinizadores externos. No obstantela presencia de flores tristilicas, las dos categorías ilegítimas de polinización dieron semillas. La producción de frutosmediante polinización abierta fluctuó entre 9-83% y en promedio fue menor en las poblaciones de mayor elevación. Seplantea que la capacidad de formar semillas mediante polinización ilegítima permitirá el uso más eficaz del recurso limitadode polinización en la zona andina superior, donde los niveles de fructificación varían entre años y existe evidencia preliminarde limitación de polen en Oxalis compacta subsp. compacta
Classification of BATSE, Swift, and Fermi Gamma-Ray Bursts from Prompt Emission Alone
Although it is generally assumed that there are two dominant classes of
gamma-ray bursts (GRB) with different typical durations, it has been difficult
to unambiguously classify GRBs as short or long from summary properties such as
duration, spectral hardness, and spectral lag. Recent work used t-distributed
stochastic neighborhood embedding (t-SNE), a machine learning algorithm for
dimensionality reduction, to classify all Swift gamma-ray bursts as short or
long. Here, the method is expanded, using two algorithms, t-SNE and UMAP, to
produce embeddings that are used to provide a classification for the 1911 BATSE
bursts, 1321 Swift bursts, and 2294 Fermi bursts for which both spectra and
metadata are available. Although the embeddings appear to produce a clear
separation of each catalog into short and long bursts, a resampling-based
approach is used to show that a small fraction of bursts cannot be robustly
classified. Further, 3 of the 304 bursts observed by both Swift and Fermi have
robust but conflicting classifications. A likely interpretation is that in
addition to the two predominant classes of GRBs, there are additional, uncommon
types of bursts which may require multi-wavelength observations in order to
separate from more typical short and long GRBs.Comment: ApJ, in pres
Transport signatures of quasiparticle poisoning in a Majorana island
We investigate effects of quasiparticle poisoning in a Majorana island with
strong tunnel coupling to normal-metal leads. In addition to the main Coulomb
blockade diamonds, "shadow" diamonds appear, shifted by 1e in gate voltage,
consistent with transport through an excited (poisoned) state of the island.
Comparison to a simple model yields an estimate of parity lifetime for the
strongly coupled island (~ 1 {\mu}s) and sets a bound for a weakly coupled
island (> 10 {\mu}s). Fluctuations in the gate-voltage spacing of Coulomb peaks
at high field, reflecting Majorana hybridization, are enhanced by the reduced
lever arm at strong coupling. In energy units, fluctuations are consistent with
previous measurements.Comment: includes supplementary materia
Quantization of Hall Resistance at the Metallic Interface between an Oxide Insulator and SrTiO
The two-dimensional metal forming at the interface between an oxide insulator
and SrTiO3 provides new opportunities for oxide electronics. However, the
quantum Hall effect, one of the most fascinating effects of electrons confined
in two dimensions, remains underexplored at these complex oxide
heterointerfaces. Here, we report the experimental observation of quantized
Hall resistance in a SrTiO3 heterointerface based on the modulation-doped
amorphous-LaAlO/SrTiO heterostructure, which exhibits both high
electron mobility exceeding 10000 cm/Vs and low carrier density on the
order of ~10 cm. Along with unambiguous Shubnikov-de Haas
oscillations, the spacing of the quantized Hall resistance suggests that the
interface is comprised of a single quantum well with ten parallel conducting
two-dimensional subbands. This provides new insight into the electronic
structure of conducting oxide interfaces and represents an important step
towards designing and understanding advanced oxide devices
Mesoscopic conductance fluctuations in InAs nanowire-based SNS junctions
We report a systematic experimental study of mesoscopic conductance
fluctuations in superconductor/normal/superconductor (SNS) devices
Nb/InAs-nanowire/Nb. These fluctuations far exceed their value in the normal
state and strongly depend on temperature even in the low-temperature regime.
This dependence is attributed to high sensitivity of perfectly conducting
channels to dephasing and the SNS fluctuations thus provide a sensitive probe
of dephasing in a regime where normal transport fails to detect it. Further,
the conductance fluctuations are strongly non-linear in bias voltage and reveal
sub-gap structure. The experimental findings are qualitatively explained in
terms of multiple Andreev reflections in chaotic quantum dots with imperfect
contacts.Comment: Manuscript and supplemen
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