329 research outputs found
Distinguishing Majorana bound states from localized Andreev bound states by interferometry
Experimental evidence for Majorana bound states (MBSs) is so far mainly based
on the robustness of a zero-bias conductance peak. However, similar features
can also arise due to Andreev bound states (ABSs) localized at the end of an
island. We show that these two scenarios can be distinguished by an
interferometry experiment based on embedding a Coulomb-blockaded island into an
Aharonov-Bohm ring. For two ABSs, when the ground state is nearly degenerate,
cotunneling can change the state of the island and interference is suppressed.
By contrast, for two MBSs the ground state is nondegenerate and cotunneling has
to preserve the island state, which leads to -periodic conductance
oscillations with magnetic flux. Such interference setups can be realized with
semiconducting nanowires or two-dimensional electron gases with
proximity-induced superconductivity and may also be a useful spectroscopic tool
for parity-flip mechanisms
Coupling and braiding Majorana bound states in networks defined in proximitized two-dimensional electron gases
Two-dimensional electron gases with strong spin-orbit coupling covered by a
superconducting layer offer a flexible and potentially scalable platform for
Majorana networks. We predict Majorana bound states (MBSs) to appear for
experimentally achievable parameters and realistic gate potentials in two
designs: either underneath a narrow stripe of a superconducting layer
(S-stripes) or where a narrow stripe has been removed from a uniform layer
(N-stripes). The coupling of the MBSs can be tuned for both types in a wide
range (10 eV) using gates placed adjacent to the stripes. For
both types, we numerically compute the local density of states for two parallel
Majorana-stripe ends as well as Majorana trijunctions formed in a tuning-fork
geometry. The MBS coupling between parallel Majorana stripes can be suppressed
below 1 neV for potential barriers in the meV range for separations of about
200 nm. We further show that the MBS couplings in a trijunction can be
gate-controlled in a range similar to the intra-stripe coupling while
maintaining a sizable gap to the excited states (tens of eV). Altogether,
this suggests that braiding can carried out on a time scale of 10-100 ns
Spin quadrupoletronics: moving spin anisotropy around
We show that spin anisotropy can be transferred to an isotropic system by
transport of spin quadrupole moment. We derive the quadrupole moment current
and continuity equation and study a high-spin valve structure consisting of two
ferromagnets coupled to a quantum dot probing an impurity spin. The quadrupole
back-action on their coupled spin results in spin torques and anisotropic spin
relaxation which do not follow from standard spin current considerations. We
demonstrate the detection of the impurity spin by charge transport and its
manipulation by electric fields.Comment: v2 updated arXiv reference [6
Transport of spin-anisotropy without spin currents
We revisit the transport of spin-degrees of freedom across an electrically
and thermally biased tunnel junction between two ferromagnets with
non-collinear magnetizations. Besides the well-known charge and spin currents
we show that a non-zero spin-quadrupole current flows between the ferromagnets.
This tensor-valued current describes the non-equilibrium transport of
spin-anisotropy relating to both local and non-local multi-particle spin
correlations of the circuit. This quadratic spin-anisotropy, quantified in
terms of the spin-quadrupole moment, is fundamentally a two-electron quantity.
In spin-valves with an embedded quantum dot such currents have been shown to
result in a quadrupole accumulation that affects the measurable quantum dot
spin and charge dynamics. The spin-valve model studied here allows fundamental
questions about spin-quadrupole storage and transport to be worked out in
detail, while ignoring the detection by a quantum dot. This physical
understanding of this particular device is of importance for more complex
devices where spin-quadrupole transport can be detected. We demonstrate that,
as far as storage and transport are concerned, the spin anisotropy is only
partly determined by the spin polarization. In fact, for a thermally biased
spin-valve the charge- and spin-current may vanish, while a pure exchange
spin-quadrupole current remains, which appears as a fundamental consequence of
Pauli's principle. We extend the real-time diagrammatic approach to efficiently
calculate the average of multi-particle spin-observables, in particular the
spin-quadrupole current. Although the paper addresses only leading order and
spin-conserving tunneling we formulate the technique for arbitrary order in an
arbitrary, spin-dependent tunnel coupling in a way that lends itself to
extension to quantum-dot spin-valve structures
Time scales for Majorana manipulation using Coulomb blockade in gate-controlled superconducting nanowires
We numerically compute the low-energy spectrum of a gate-controlled
superconducting topological nanowire segmented into two islands, each
Josephson-coupled to a bulk superconductor. This device may host two pairs of
Majorana bound states and could provide a platform for testing Majorana fusion
rules. We analyze the crossover between (i) a charge-dominated regime
utilizable for initialization and readout of Majorana bound states, (ii) a
single-island regime for dominating inter-island Majorana coupling, (iii) a
Josephson-plasmon regime for large coupling to the bulk superconductors, and
(iv) a regime of four Majorana bound states allowing for topologically
protected Majorana manipulations. From the energy spectrum, we derive
conservative estimates for the time scales of a fusion-rule testing protocol
proposed recently [arXiv:1511.05153]. We also analyze the steps needed for
basic Majorana braiding operations in branched nanowire structures
Perampanel inhibition of AMPA receptor currents in cultured hippocampal neurons.
Perampanel is an aryl substituted 2-pyridone AMPA receptor antagonist that was recently approved as a treatment for epilepsy. The drug potently inhibits AMPA receptor responses but the mode of block has not been characterized. Here the action of perampanel on AMPA receptors was investigated by whole-cell voltage-clamp recording in cultured rat hippocampal neurons. Perampanel caused a slow (τ∼1 s at 3 µM), concentration-dependent inhibition of AMPA receptor currents evoked by AMPA and kainate. The rates of block and unblock of AMPA receptor currents were 1.5×105 M-1 s-1 and 0.58 s-1, respectively. Perampanel did not affect NMDA receptor currents. The extent of block of non-desensitizing kainate-evoked currents (IC50, 0.56 µM) was similar at all kainate concentrations (3-100 µM), demonstrating a noncompetitive blocking action. Parampanel did not alter the trajectory of AMPA evoked currents indicating that it does not influence AMPA receptor desensitization. Perampanel is a selective negative allosteric AMPA receptor antagonist of high-affinity and slow blocking kinetics
Four-Majorana qubit with charge readout: dynamics and decoherence
We present a theoretical analysis of a Majorana-based qubit consisting of two
topological superconducting islands connected via a Josephson junction. The
qubit is operated by electrostatic gates which control the coupling of two of
the four Majorana zero modes. At the end of the operation, readout is performed
in the charge basis. Even though the operations are not topologically
protected, the proposed experiment can potentially shed light on the coherence
of the parity degree of freedom in Majorana devices and serve as a first step
towards topological Majorana qubits. We discuss in detail the charge-stability
diagram and its use for characterizing the parameters of the devices, including
the overlap of the Majorana edge states. We describe the multi-level spectral
properties of the system and present a detailed study of its controlled
coherent oscillations, as well as decoherence resulting from coupling to a
non-Markovian environment. In particular, we study a gate-controlled protocol
where conversion between Coulomb-blockade and transmon regimes generates
coherent oscillations of the qubit state due to the overlap of Majorana modes.
We show that, in addition to fluctuations of the Majorana coupling,
considerable measurement errors may be accumulated during the conversion
intervals when electrostatic fluctuations in the superconducting islands are
present. These results are also relevant for several proposed implementations
of topological qubits which rely on readout based on charge detection
Distance Constraint Satisfaction Problems
We study the complexity of constraint satisfaction problems for templates
that are first-order definable in , the integers with
the successor relation. Assuming a widely believed conjecture from finite
domain constraint satisfaction (we require the tractability conjecture by
Bulatov, Jeavons and Krokhin in the special case of transitive finite
templates), we provide a full classification for the case that Gamma is locally
finite (i.e., the Gaifman graph of has finite degree). We show that
one of the following is true: The structure Gamma is homomorphically equivalent
to a structure with a d-modular maximum or minimum polymorphism and
can be solved in polynomial time, or is
homomorphically equivalent to a finite transitive structure, or
is NP-complete.Comment: 35 pages, 2 figure
Synoptische, diagnostische und numerische Untersuchungen der Intensitätsänderungen von tropischen Wirbelstürmen und Monsuntiefs
The intensity changes of an Australian monsoon depression and typhoons Flo (1990)
and Ed (1990) are investigated using operational analyses, diagnostic studies, and numerical
model calculations. The analyses are based on gridded data from the Australian Bureau of
Meteorology’s (BMRC’s) Tropical Analysis System, the European Centre for Medium-Range
Weather Forecasts (ECMWF) data archive, and the 1990 Tropical Cyclone Motion experiment
(TCM-90). The TCM-90 analyses are provided by the Japan Meteorological Agency and the
National Centers for Environmental Prediction and incorporate typhoon boguses of the
Geophysical Fluid Dynamics Laboratory and the Japan Meteorological Agency.
The monsoon depression developed over northwestern Australia in February 1994.
After remaining quasi-stationary for several days after its formation in the monsoon trough, the
depression drifted slowly southwestwards along the coast of Western Australia and eventually
weakened. The evolution is exemplified by time-height cross-sections of various kinematic
quantities as well as the apparent source of potential vorticity, which has been taken into
account in the PV equation for the first time. A prominent feature of these diagnostic fields is
the impact of convective and diabatic processes on the development. The structure of the
depression is very similar to those of the few monsoon depressions that have been documented
over the Indian subcontinent. The formation of the depression coincided with the passage of an
upper-level middle-latitude trough and a low-level anticyclone to the south, but the extent to
which the genesis was influenced by these disturbances is unclear.
The equation for the mean tangential wind in a storm-following cylindrical coordinate
system is applied to investigate the intensity changes of tropical cyclones and monsoon
depressions. In this equation, the symmetric, the resolved asymmetric and the subgrid-scale
contributions are fully separated. The tangential wind changes can be interpreted in terms of
absolute vorticity flux, angular momentum flux, PV flux or the Eliassen-Palm flux-divergence.
The corresponding eddy fluxes are compared, and the impact of convection on the intensity
changes as well as the interaction between a monsoon depression and upper-level troughs and
ridges is investigated. Analyses of observational data for a monsoon depression that formed
near the coast of northwestern Australia in February 1994 show that all terms in the tangential
wind equation are important, except for the drift term. These important terms include above all
the subgrid-scale terms which are presumed to be largely due to processes associated with
deep convection.
In the lower and middle troposphere, the main terms contributing to the intensification
of the monsoon depression are the radial absolute vorticity fluxes (cyclonic inward fluxes of
relative and planetary vorticity; or the corresponding terms that include the Eliassen-Palm
fluxes, the radial PV-fluxes, or the angular momentum fluxes) and the vertical advection of
tangential wind. In the upper troposphere, the cyclonic tangential wind change can be attributed
mainly to the relative vorticity fluxes (anticyclonic outward fluxes of relative vorticity),
the vertical advection of tangential wind, and the subgrid-scale contributions. The weakening
tendency of the upper-level anticyclone associated with these effects and the channelled,
widespread outflow are related to the strengthening of the monsoon depression. The inward
movement of the eddy PV-flux-maxima in the upper troposphere reflects the approach of PV
anomalies associated with passing mid-latitude troughs. We hypothesize that the convection
near the monsoon depression is enhanced in front of the approaching PV anomalies by the
reduction of static stability and by the vacuum cleaner effect, and that this influences the
intensification of the depression.
As a check on the robustness of the findings, the results using BMRC analyses are
compared with those using ECMWF analyses. Despite the high diurnal variability, the patterns
of the daily-mean symmetric, resolved-asymmetric and unresolved-asymmetric contributions
agree relatively well, although the figures from the higher resolution ECMWF analyses show
more details with more distinct peaks.
The intensity changes of typhoons Flo and Ed, which were strongly influenced by deep
convection, have been investigated using the Europa- and the Deutschland-Modell of the
Deutscher Wetterdienst. The sensitivity of model forecasts to different initial conditions,
typhoon boguses, model resolution and physical parameterizations has been examined.
The typhoons Flo and Ed developed east of Guam and drifted northwest and westward,
respectively. During the observation period, they were embedded in the monsoon trough and
were about 1700 km apart. After 15 September 1990, Flo intensified rapidly to supertyphoon
strength and recurved towards Japan. Before weakening over the coast of Vietnam, Ed undertook
an unusual, southwestward drift.
The effects of the large-scale and subgrid-scale asymmetries on the intensity of Typhoon
Flo were not significant despite the nearby TUTT-cell and could not explain the intense
deepening phase after 15 September. An inward or outward drift of maximal or minimal
absolute eddy-vorticity fluxes was not analysed. The predictions of the asymmetry effects give
useful hints about the kind and strength of the effects independently of the grid-resolution and
the typhoon bogus incorporated in the start analysis. As in the case of the Australian monsoon
depression, the vertical advection of momentum and the subgrid-scale effects are not
negligible.
For the two typhoons, the Europa- and the Deutschland-Modell showed a modest forecast
skill for intensity change. The initial data fields, the incorporated boguses, the model
resolution, and the physical parameterizations have a significant influence on the intensity
forecasts. The boundary values barely affect the intensity prediction. Compared to operational
forecast models, the two models have good skill in predicting typhoon tracks. The track
prediction is nearly independent of the grid resolution and the cumulus, radiation and diffusion
parameterization applied. Variations in track arise primarily from different boundary conditions
and initial analyses including the typhoon boguses used. The model resolution, the initial
analysis used, and the typhoon bogus incorporated have nearly no influence on the distribution
and size of the rain areas. However, the typhoon bogus used and the model resolution strongly
affect the amount of core region precipitation. The intensity of the model typhoons is
correlated with the precipitation intensity and the strength of convection. Averaged over the
entire typhoon area, the differences in the amount of rain for the two model resolutions is
small, however.Die Intensitätsänderungen eines australischen Monsuntiefs und der Taifune Flo (1990)
und Ed (1990) wurden mit Hilfe von operationellen Analysen, diagnostischen Studien und
numerischen Modellrechnungen untersucht. Die Analysen basierten auf Gitterdaten vom
tropischen Analysesystem des australischen Wetterbüros (BMRC), vom Datenarchiv des
Europäischen Zentrums für mittelfristige Wettervorhersage (ECMWF) und vom Experiment
zur Zugbahn tropischer Zyklone von 1990 (TCM-90). Die TCM-90-Analysen wurden vom
Japanischen Wetterdienst (JMA) und vom Nationalen Zentrum für Umweltvorhersage (NCEP)
erstellt und enthielten Taifunkeimzellen des Geophysikalischen Labors für Fließdynamik
(GFDL) und des Japanischen Wetterdienstes.
Das Monsuntief entwickelte sich im Februar 1994 über Nordwestaustralien. Nach seiner
Entstehung im Monsuntrog verweilte es für mehrere Tage fast stationär über King Sound.
Danach zog es langsam südwestwärts entlang der westaustralischen Küste und schwächte sich
schließlich über dem Meer ab. Diese Monsuntiefentwicklung wurde an Hand von Zeit-Höhen-
Diagrammen verschiedener kinematischer Größen und an Hand der PV-Quelle, die erstmals in
der PV-Gleichung berücksichtigt wurde, veranschaulicht. Charakteristisch für diese diagnostischen
Felder war der deutlich erkennbare Einfluß der konvektiven und diabatischen Prozesse
auf die Monsuntiefentwicklung. Die Strukturen des australischen Monsuntiefs waren sehr
ähnlich zu den Strukturen bei indischen Monsuntiefs. Die Entstehung des Monsuntiefs fand zur
gleichen Zeit statt wie der Vorbeizug eines Höhentrogs und eines Antizyklons in der unteren
Troposphäre südlich des Tiefs. In wie weit die Entstehung durch diese beiden Wetterstörungen
beeinflußt wurde, konnte nicht endgültig geklärt werden.
Zur Untersuchung der Intensitätsänderungen von tropischen Wirbelstürmen und Monsuntiefs
wurde die allgemeine Gleichung der mittleren, tangentialen Windgeschwindigkeit in
wirbelfolgenden Zylinderkoordinaten verwendet. In dieser Gleichung lassen sich die symmetrischen,
die aufgelöst-asymmetrischen und die unaufgelöst-asymmetrischen (subskaligen) Beiträge
vollständig voneinander trennen. Mit ihr können die Tangentialwindänderungen in Abhängigkeit
von den Größen Fluß der absoluten Vorticity, Drehimpulsfluß, PV-Fluß oder Eliassen-
Palm-Flußdivergenz angegeben werden. Die jeweiligen Eddyflüsse dieser Größen wurden
miteinander verglichen. Die Auswirkungen der Konvektion auf die Intensitätsänderungen
sowie die Wechselwirkungen eines Monsuntiefs mit Trögen und Rücken der oberen Troposphäre
wurden untersucht. Diagnostische Analysen aus Beobachtungsdaten vom australischen
Monsuntief zeigten, daß alle Terme der Tangentialwindgleichung mit Ausnahme des Verlagerungsterms
für die Intensitätsänderungen von tropischen Wirbelstürmen und Monsuntiefs von
Bedeutung waren. Dies schloß insbesondere die subskaligen Terme mit ein, deren Beiträge
vornehmlich von Prozessen in Zusammenhang mit hochreichender Konvektion herrührten.
In der unteren und mittleren Troposphäre war die zyklonale Intensivierung des Monsuntiefs
vor allem auf die Flüsse der absoluten Vorticity (einwärts gerichtete, zyklonale Flüsse
der relativen und planetaren Vorticity bzw. den entsprechenden Termen des Eliassen-Palm-
Flusses, des radialen PV-Flusses oder des Drehimpulsflusses) und die vertikale Advektion des
Tangentialwindes zurückzuführen. In der oberen Troposphäre erfolgte die zyklonale Tangentialwindänderung
vorwiegend aufgrund des Flusses der relativen Vorticity (des auswärts
gerichteten, antizyklonalen Flusses der relativer Vorticity), der vertikalen Advektion des Tangentialwindes
und der subskaligen Effekte. Die entsprechende Tendenz zur Abschwächung des
Antizyklons in der oberen Troposphäre und die kanalisierte, weit ausgedehnte Ausflußströmung
waren deutliche Anzeichen für die einsetzende Verstärkung des Monsuntiefs. Das
Einwärtswandern der Maxima des PV-Flusses durch Eddies in der oberen Troposphäre
spiegelte die Annäherung von PV-Anomalien in Zusammenhang mit vorbeiziehenden Höhentrögen
wider. Die Konvektion des Monsuntiefs verstärkte sich vor den herannahenden PVAnomalien
aufgrund der reduzierten, statischen Stabilität und aufgrund des Staubsaugereffekts.
Auch das Monsuntief konnte sich dadurch intensivieren.
Zur Überprüfung der Zuverlässigkeit der Ergebnisse wurden die diagnostischen Felder,
berechnet aus den BMRC-Analysen, mit jenen Feldern verglichen, die aus den ECMWFAnalysen
bestimmt wurden. Die Strukturen der über einen Tag gemittelten symmetrischen,
aufgelöst-asymmetrischen und unaufgelöst-asymmetrischen Beiträge stimmten trotz der hohen
täglichen Variationen verhältnismäßig gut überein. Die Abbildungen der ECMWF-Analysen
zeigten jedoch aufgrund des geringeren Gitterabstands des Analysemodells mehr Details mit
ausgeprägteren Extrema.
Die Intensitätsänderungen der Taifune Flo und Ed wurden mit dem Europa- und
Deutschlandmodell des Deutschen Wetterdienstes untersucht. Auch diese Taifune wurden
stark von hochreichender Konvektion beeinflußt. Die Sensitivität der Modellvorhersagen auf
unterschiedliche Anfangsbedingungen, Taifunkeimzellen, Modellauflösungen und physikalische
Parameterisierungen wurde überprüft und die großskaligen und subskaligen Asymmetrieeffekte
berechnet.
Die beiden Taifune Flo und Ed entstanden östlich von Guam und zogen in nordwestliche
bzw. westliche Richtung. Während des Beobachtungszeitraums waren sie etwa 1700 km
voneinander entfernt in einen Monsuntrog eingebettet. Ab dem 15. September 1990 entwickelte
sich Flo sehr schnell zu einem Supertaifun und zog nach dem Abdrehen Richtung Japan.
Bevor sich Ed nahe der vietnamesischen Küste abschwächte, vollführte er eine unübliche,
südwestliche Zugbewegung.
Die Auswirkungen der großskaligen und subskaligen Asymmetrien auf die Intensität des
Taifuns Flo waren trotz der benachbarten TUTT-Zelle nicht groß und konnten die starke
Eintiefungsphase ab dem 15. September nicht erklären. Auch wurde kein Einwärts- oder Auswärtswandern
von maximalen oder minimalen Flüssen der absoluten Vorticity durch Eddies
analysiert. Die Vorhersagen der Asymmetrieeffekte gaben unabhängig von der Gitterauflösung
und der in der Startanalyse verwendeten Taifunkeimzelle verwertbare Hinweise über die Art
und Stärke dieser Asymmetrieeffekte. Wie beim australischen Monsuntief waren die subskaligen
Effekte und die vertikale Advektion von Impuls nicht vernachlässigbar.
Das Europa- und das Deutschlandmodell zeigten für beide Taifune ein mäßiges Vorhersagevermögen
in Bezug auf Intensitätsänderungen. Die Ausgangsdatenfelder, die eingepflanzten
Keimzellen, die Modellauflösung und die physikalischen Parameterisierungen hatten
einen signifikanten Einfluß auf die Intensitätsvorhersagen. Die Randwerte wirkten sich hingegen
kaum auf die Intensitätsprognosen aus. Verglichen mit operationellen Vorhersagemodellen
besaßen die beiden Modelle ein gutes Vorhersagevermögen für Zugbahnen. Die Zugbahnvorhersagen
waren nahezu unabhängig von der Gitterauflösung und der verwendeten Cumulus-,
Strahlungs- und Diffusionsparameterisierung. Abweichungen in der Zugbahn entstanden vor
allem durch unterschiedliche Randbedingungen und Ausgangsanalysen, einschließlich der verwendeten
Taifunkeimzellen. Die Modellauflösung, die verwendete Ausgangsanalyse und die
eingepflanzte Keimzelle hatten kaum einen Einfluß auf die Verteilung und Größe der Regengebiete.
Die verwendete Keimzelle und die Modellauflösung wirkten sich hingegen merklich
auf die kernnahen Niederschlagsmengen aus. Die Intensität der Modelltaifune war mit der
Stärke der kernnahen Niederschläge und der Stärke der Konvektion korreliert. Über das gesamte
Taifungebiet gemittelt waren die Unterschiede in der Regenmenge bei unterschiedlicher
Modellauflösung hingegen gering
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