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G. Schubert

Publications and source records attributed to G. Schubert.

13 recordsLinked to original sources
arXiv2020

A Long-lived Sharp Disruption on the Lower Clouds of Venus

Planetary-scale waves are thought to play a role in powering the yet-unexplained atmospheric superrotation of Venus. Puzzlingly, while Kelvin, Rossby and stationary waves manifest at the upper clouds (65--70 km), no planetary-scale waves or stationary patterns have been reported in the intervening level of the lower clouds (48--55 km), although the latter are probably Lee waves. Using observations by the Akatsuki orbiter and ground-based telescopes, we show that the lower clouds follow a regular cycle punctuated between 30$^{\circ}$N--40$^{\circ}$S by a sharp discontinuity or disruption with potential implications to Venus's general circulation and thermal structure. This disruption exhibits a westward rotation period of $\sim$4.9 days faster than winds at this level ($\sim$6-day period), alters clouds' properties and aerosols, and remains coherent during weeks. Past observations reveal its recurrent nature since at least 1983, and numerical simulations show that a nonlinear Kelvin wave reproduces many of its properties.

astro-ph.EP↗
arXiv2013

Effects of disorder and contacts on transport through graphene nanoribbons

We study the transport of charge carriers through finite graphene structures. The use of numerical exact kernel polynomial and Green function techniques allows us to treat actual sized samples beyond the Dirac-cone approximation. Particularly we investigate disordered nanoribbons, normal-conductor/graphene interfaces and normal-conductor/graphene/normal-conductor junctions with a focus on the behavior of the local density of states, single-particle spectral function, optical conductivity and conductance. We demonstrate that the contacts and bulk disorder will have a major impact on the electronic properties of graphene-based devices.

cond-mat.mes-hall↗
arXiv2010

Behavior of test particles in the plasma sheath upon local bias and plasma switching

Equilibrating gravitation by electric forces, microparticles can be confined in the plasma sheath above suitably biased local electrodes.Their position depends on the detailed structure of the plasma sheath and on the charge that the particles acquire in the surrounding plasma, that is by the electron and ion currents towards it. Bias switching experiments reveal how the charge and equilibrium position of the microparticle change upon altered sheath conditions. We observe similar particle behaviors also in the afterglow of the discharge for a persisting bias voltage on the electrode: damped oscillation into a new equilibrium or (accelerated) fall according to the bias.

physics.plasm-ph↗
arXiv2009

Comparative study of semiclassical approaches to quantum dynamics

Quantum states can be described equivalently by density matrices, Wigner functions or quantum tomograms. We analyze the accuracy and performance of three related semiclassical approaches to quantum dynamics, in particular with respect to their numerical implementation. As test cases, we consider the time evolution of Gaussian wave packets in different one-dimensional geometries, whereby tunneling, resonance and anharmonicity effects are taken into account. The results and methods are benchmarked against an exact quantum mechanical treatment of the system, which is based on a highly efficient Chebyshev expansion technique of the time evolution operator.

quant-ph↗
arXiv2009

Heavy Element Enrichment of a Jupiter-mass Protoplanet as a Function of Orbital Location

We calculate heavy element enrichment in a Jupiter-mass protoplanet formed by disk instability at various radial distances from the star, considering different disk masses and surface density distributions. Although the available mass for accretion increases with radial distance (a) for disk solid surface density (sigma) functions sigma=sigma_0*a^(-alpha) with alpha < 2, the accretion timescale is significantly longer at larger radial distances. Efficient accretion is limited to the first ~ 1E5 years of planetary evolution, when the planet is extended and before gap opening and type II migration take place. The accreted mass is calculated for disk masses of 0.01, 0.05 and 0.1 M_sun with alpha = 1/2, 1, and 3/2. We show that a Jupiter-mass protoplanet can accrete 1 to 110 M_earth of heavy elements, depending on the disk properties. Our results explain the large variation in heavy element enrichment found in extra-solar giant planets. Since higher disk surface density is found to lead to larger heavy element enrichment, our model results are consistent with the correlation between heavy element enrichment and stellar metallicity. Our calculations also suggest that Jupiter could have formed at a larger radial distance than its current location while still accreting the mass of heavy elements predicted by interior models. We conclude that in the disk instability model the final composition of a giant planet is strongly determined by its formation environment. The heavy element abundance of a giant planet does not discriminate between its origin by either disk instability or core accretion.

astro-ph.EP↗
arXiv2008

Particles as probes for complex plasmas in front of biased surfaces

An interesting aspect in the research of complex (dusty) plasmas is the experimental study of the interaction of micro-particles with the surrounding plasma for diagnostic purposes. Local electric fields can be determined from the behaviour of particles in the plasma, e.g. particles may serve as electrostatic probes. Since in many cases of applications in plasma technology it is of great interest to describe the electric field conditions in front of floating or biased surfaces, the confinement and behaviour of test particles is studied in front of floating walls inserted into a plasma as well as in front of additionally biased surfaces. For the latter case, the behaviour of particles in front of an adaptive electrode, which allows for an efficient confinement and manipulation of the grains, has been experimentally studied in dependence on the discharge parameters and on different bias conditions of the electrode. The effect of the partially biased surface (dc, rf) on the charged micro-particles has been investigated by particle falling experiments. In addition to the experiments we also investigate the particle behaviour numerically by molecular dynamics, in combination with a fluid and particle-in-cell description of the plasma.

physics.plasm-ph↗
arXiv2005

Optical absorption and activated transport in polaronic systems

We present exact results for the optical response in the one-dimensional Holstein model. In particular, by means of a refined kernel polynomial method, we calculate the ac and dc electrical conductivities at finite temperatures for a wide parameter range of electron phonon interaction. We analyze the deviations from the results of standard small polaron theory in the intermediate coupling regime and discuss non-adiabaticity effects in detail.

cond-mat.str-el↗
arXiv2005

Optical response of many-polaron systems

Exact results for the density of states and the ac conductivity of the spinless Holstein model at finite carrier density are obtained combining Lanczos and kernel polynomial methods.

cond-mat.str-el↗
arXiv2000

Spherical, Oscillatory $α^2$-Dynamo Induced by Magnetic Coupling Between a Fluid Shell and an Inner Electrically Conducting Core: Relevance to the Solar Dynamo

A two-layer spherical $α^2$-dynamo model consisting of an inner electrically conducting core (magnetic diffusivity $λ_i$ and radius $r_i$) with $α= 0$ surrounded by an electrically conducting spherical shell (magnetic diffusivity $λ_o$ and radius $r_o$) with a constant $α$ is shown to exhibit oscillatory behavior for values of $β= λ_i/λ_o$ and $r_i/r_o$ relevant to the solar dynamo. Time-dependent dynamo solutions require $r_i/r_o \geq 0.55$ and $β\leq O(1)$. For the Sun, $r_i/r_o$ is about 0.8 and $β\approx 10^{-3}$. The time scale of the oscillations matches the 22 year period of the sunspot cycle for $λ_0 = O(10^2 km^2 s^{-1}$). It is unnecessary to hypothesize an $αω$-dynamo to obtain oscillatory dynamo solutions; an $α^2$-dynamo suffices provided the spherical shell region of dynamo action lies above a large, less magnetically diffusive core, as is the case for the solar dynamo.

astro-ph↗
arXiv1995

Radio Science Investigation on a Mercury Orbiter Mission

We review the results from {\it Mariner 10} regarding Mercury's gravity field and the results from radar ranging regarding topography. We discuss the implications of improving these results, including a determination of the polar component, as well as the opportunity to perform relativistic gravity tests with a future {\it Mercury Orbiter}. With a spacecraft placed in orbit with periherm at 400 km altitude, apherm at 16,800 km, period 13.45 hr and latitude of periherm at +30 deg, one can expect a significant improvement in our knowledge of Mercury's gravity field and geophysical properties. The 2000 Plus mission that evolved during the European Space Agency (ESA) {\it Mercury Orbiter} assessment study can provide a global gravity field complete through the 25th degree and order in spherical harmonics. If after completion of the main mission, the periherm could be lowered to 200 km altitude, the gravity field could be extended to 50th degree and order. We discuss the possibility that a search for a Hermean ionosphere could be performed during the mission phases featuring Earth occultations. Because of its relatively large eccentricity and close proximity to the Sun, Mercury's orbital motion provides one of the best solar-system tests of general relativity. Consequently, we emphasize the number of feasible relativistic gravity tests that can be performed within the context of the parameterized post-Newtonian formalism - a useful framework for testing modern gravitational theories. We pointed out that current results on relativistic precession of Mercury's perihelion are uncertain by 0.5 %, and we discuss the expected improvement using {\it Mercury Orbiter}. We discuss the importance of {\it Mercury Orbiter} for setting limits on a possible time variation in the

astro-ph↗