621 research outputs found
Conductance of a quantum point contact based on spin-density-functional theory
We present full quantum mechanical conductance calculations of a quantum
point contact (QPC) performed in the framework of the density functional theory
(DFT) in the local spin-density approximation (LDA). We show that a
spin-degeneracy of the conductance channels is lifted and the total conductance
exhibits a broad plateau-like feature at 0.5*2e^{2}/h. The lifting of the
spin-degeneracy is a generic feature of all studied QPC structures (both very
short and very long ones; with the lengths in the range 40<l<500 nm). The
calculated conductance also shows a hysteresis for forward- and backward sweeps
of the gate voltage. These features in the conductance can be traced to the
formation of weakly coupled quasi-bound states (magnetic impurities) inside the
QPC (also predicted in previous DFT-based studies). A comparison of obtained
results with the experimental data shows however, that while the spin-DFT based
"first-principle" calculations exhibits the spin polarization in the QPC, the
calculated conductance clearly does not reproduce the 0.7 anomaly observed in
almost all QPCs of various geometries. We critically examine major features of
the standard DFT-based approach to the conductance calculations and argue that
its inability to reproduce the 0.7 anomaly might be related to the infamous
derivative discontinuity problem of the DFT leading to spurious
self-interaction errors not corrected in the standard LDA. Our results indicate
that the formation of the magnetic impurities in the QPC might be an artefact
of the LDA when localization of charge is expected to occur. We thus argue that
an accurate description of the QPC structure would require approaches that go
beyond the standard DFT+LDA schemes.Comment: 9 pages, 5 figure
The variability of the East Sakhalin Current induced by winds over the continental shelf and slope
Long-term current measurements of the East Sakhalin Current (ESC) in the Sea of Okhotsk are analyzed using the technique of empirical orthogonal functions (EOFs) in the frequency domain. The first and second EOFs at subtidal frequencies represent motions over the continental shelf and slope, respectively, corresponding to the variability of the two cores of the intense ESC. The first EOF can be explained by the first-mode coastal trapped wave (CTW). The structure of the second EOF is similar to that of the second-mode CTW to the first approximation. According to the distribution of the cross-spectra between EOFs and the wind stress over the whole area of the Sea of Okhotsk, the first EOF is correlated with the alongshore component of the wind stress over the northern and western shelves. The distribution of the phase of the wind stress, which is correlated with the first EOF, indicates that a resonance between the CTW and wind stress drives the motion represented by the first EOF at lower frequencies. At higher frequencies the phase of the wind stress correlated with the first EOF is almost uniform in space, being consistent with the greater speed of phase propagation of the EOF compared with that for the free CTW at these frequencies. The second EOF is correlated with the wind stress curl in the central part of the Sea of Okhotsk. The motion by the second EOF is confined over the slope at lower frequencies and becomes large over the shelf at higher frequencies. This change in the structure of the second EOF is consistent with the results of the numerical experiment of the flow induced by the offshore forcing by Chapman and Brink (1987). The phase of the wind stress curl which is correlated with the second EOF changes clearly in space at some frequencies, suggesting that the motion represented by the second EOF propagates along the isobath with the coast to the right. The wind stress curl contains the wavenumber resonant with the lowest two or three modes of CTWs
Pulse Propagation in Resonant Tunneling
We consider the analytically solvable model of a Gaussian pulse tunneling
through a transmission resonance with a Breit-Wigner characteristic. The
solution allows for the identification of two opposite pulse propagation
regimes: if the resonance is broad compared to the energetic width of the
incident Gaussian pulse a weakly deformed and slightly delayed transmitted
Gaussian pulse is found. In the opposite limit of a narrow resonance the dying
out of the transmitted pulse is dominated by the slow exponential decay
characteristic of a quasi-bound state with a long life time (decaying state).
We discuss the limitation of the achievable pulse transfer rate resulting from
the slow decay. Finally, it is demonstrated that for narrow resonances a small
second component is superimposed to the exponential decay which leads to
characteristic interference oscillations.Comment: 6 pages, 4 figure
Collimated Jet or Expanding Outflow: Possible Origins of GRBs and X-Ray Flashes
We investigate the dynamics of an injected outflow propagating in a
progenitor in the context of the collapsar model for gamma-ray bursts (GRBs)
through two dimensional axisymmetric relativistic hydrodynamic simulations.
Initially, we locally inject an outflow near the center of a progenitor. We
calculate 25 models, in total, by fixing its total input energy to be 10^{51}
ergs s^{-1} and radius of the injected outflow to be cm while
varying its bulk Lorentz factor, , and its specific
internal energy, . The injected outflow propagates
in the progenitor and drives a large-scale outflow or jet. We find a smooth but
dramatic transition from a collimated jet to an expanding outflow among
calculated models. The maximum Lorentz factor is, on the other hand, sensitive
to both of and ; roughly . Our finding will explain a smooth transition between the
GRBs, X-ray rich GRBs (XRRs) and X-ray Flashes (XRFs) by the same model but
with different values.Comment: Comments 51 pages, 21 figures. accepted for publication in ApJ high
resolution version is available at
http://www.mpa-garching.mpg.de/~mizuta/COLLAPSAR/collapsar.htm
On the Lagrangian Dynamics of Atmospheric Zonal Jets and the Permeability of the Stratospheric Polar Vortex
The Lagrangian dynamics of zonal jets in the atmosphere are considered, with
particular attention paid to explaining why, under commonly encountered
conditions, zonal jets serve as barriers to meridional transport. The velocity
field is assumed to be two-dimensional and incompressible, and composed of a
steady zonal flow with an isolated maximum (a zonal jet) on which two or more
travelling Rossby waves are superimposed. The associated Lagrangian motion is
studied with the aid of KAM (Kolmogorov--Arnold--Moser) theory, including
nontrivial extensions of well-known results. These extensions include
applicability of the theory when the usual statements of nondegeneracy are
violated, and applicability of the theory to multiply periodic systems,
including the absence of Arnold diffusion in such systems. These results,
together with numerical simulations based on a model system, provide an
explanation of the mechanism by which zonal jets serve as barriers to
meridional transport of passive tracers under commonly encountered conditions.
Causes for the breakdown of such a barrier are discussed. It is argued that a
barrier of this type accounts for the sharp boundary of the Antarctic ozone
hole at the perimeter of the stratospheric polar vortex in the austral spring.Comment: Submitted to Journal of the Atmospheric Science
The sensitivity of Euro-Atlantic regimes to model horizontal resolution
There is growing evidence that the atmospheric dynamics of the Euro-Atlantic sector during winter is driven in part by the presence of quasi-persistent regimes. However, general circulation models typically struggle to simulate these with, for example, an overly weakly persistent blocking regime. Previous studies have showed that increased horizontal resolution can improve the regime structure of a model but have so far only considered a single model with only one ensemble member at each resolution, leaving open the possibility that this may be either coincidental or model dependent. We show that the improvement in regime structure due to increased resolution is robust across multiple models with multiple ensemble members. However, while the high-resolution models have notably more tightly clustered data, other aspects of the regimes may not necessarily improve and are also subject to a large amount of sampling variability that typically requires at least three ensemble members to surmount
Cold Dark Matter and b --> s gamma in the Horava-Witten Theory
The minimal supersymmetric standard model with complete, partial or no Yukawa
unification and radiative electroweak breaking with boundary conditions from
the Horava-Witten theory is considered. The parameters are restricted by
constraining the lightest sparticle relic abundance by cold dark matter
considerations and requiring the b-quark mass after supersymmetric corrections
and the branching ratio of b --> s gamma to be compatible with data. Complete
Yukawa unification can be excluded. Also, t-b Yukawa unification is strongly
disfavored since it requires almost degenerate lightest and next-to-lightest
sparticle masses. However, the b-tau or no Yukawa unification cases avoid this
degeneracy. The latter with mu<0 is the most natural case. The lightest
sparticle mass, in this case, can be as low as about 77 GeV.Comment: 16 pages including 4 figures, Revtex, version to appear in Phys.
Lett.
The Hydrodynamics of Gamma-Ray Burst Remnants
This paper reports on the results of a numerical investigation designed to
address how the initially anisotropic appearance of a GRB remnant is modified
by the character of the circumburst medium and by the possible presence of an
accompanying supernova (SN). Axisymmetric hydrodynamical calculations of light,
impulsive jets propagating in both uniform and inhomogeneous external media are
presented, which show that the resulting dynamics of their remnants since the
onset of the non-relativistic phase is different from the standard self-similar
solutions. Because massive star progenitors are expected to have their close-in
surroundings modified by the progenitor winds, we consider both free winds and
shocked winds as possible external media for GRB remnant evolution. Abundant
confirmation is provided here of the important notion that the morphology and
visibility of GRB remnants are determined largely by their circumstellar
environments. For this reason, their detectability is highly biased in favor of
those with massive star progenitors; although, in this class of models, the
beamed component may be difficult to identify because the GRB ejecta is
eventually swept up by the accompanying SN. The number density of asymmetric
GRB remnants in the local Universe could be, however, far larger if they expand
in a tenuous interstellar medium, as expected for some short GRB progenitor
models. In these sources, the late size of the observable, asymmetric remnant
could extend over a wide, possibly resolvable angle and may be easier to
constrain directly.Comment: 10 pages, 12 figures, emulateapj style, submitted to Ap
Light Higgsino in Heavy Gravitino Scenario with Successful Electroweak Symmetry Breaking
We consider, in the context of the minimal supersymmetric standard model, the
case where the gravitino weighs 10^6 GeV or more, which is preferred by various
cosmological difficulties associated with unstable gravitinos. Despite the
large Higgs mixing parameter B together with the little hierarchy to other soft
supersymmetry breaking masses, a light higgsino with an electroweak scale mass
leads to successful electroweak symmetry breaking, at the price of fine-tuning
the higgsino mixing mu parameter. Furthermore the light higgsinos produced at
the decays of gravitinos can constitute the dark matter of the universe. The
heavy squark mass spectrum of O(10^4) GeV can increase the Higgs boson mass to
about 125 GeV or higher.Comment: 13 pages, 3 figures; v2: version to appear in JHE
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