2,821 research outputs found
Some remarks on adaptive stabilization of infinite-dimensional systems
It is the purpose of this note to show that a first-order adaptive controller stabilizes a large class of infinite-dimensional systems described by strongly continous semigroups. It is assumed that the plant is minimum-phase and has invertible high-frequency gain. Knowledge of the sign of the high-frequency gain is not required
High resolution modelling of the North Icelandic Irminger Current (NIIC)
The northward inflow of Atlantic Water through Denmark Strait – the North Icelandic Irminger Current (NIIC) – is simulated with a numerical model of the North Atlantic and Arctic Ocean. The model uses the technique of adaptive grid refinement which allows a high spatial resolution (1 km horizontal, 10 m vertical) around Iceland. The model is used to assess time and space variability of volume and heat fluxes for the years 1997–2003. Passive tracers are applied to study origin and composition of NIIC water masses. <br><br> The NIIC originates from two sources: the Irminger Current, flowing as part of the sub-polar gyre in 100–500 m depth along the Reykjanes Ridge and the shallow Icelandic coastal current, flowing north-westward on the south-west Icelandic shelf. The ratio of volume flux between the deep and shallow branch is around 2:1. The NIIC continues as a warm and saline branch northward through Denmark Strait where it entrains large amounts of polar water due to the collision with the southward flowing East Greenland Current. After passing Denmark Strait, the NIIC follows the coast line eastward being an important heat source for north Icelandic waters. <br><br> At least 60% of the temporal temperature variability of north Icelandic waters is caused by the NIIC. The NIIC volume and heat transport is highly variable and depends strongly on the wind field north-east of Denmark Strait. Daily means can change from 1 Sv eastward to 2 Sv westward within a few days. Highest monthly mean transport rates occur in summer when winds from north are weak, whereas the volume flux is reduced by around 50% in winter. Summer heat flux rates can be even three times higher than in winter. The simulation also shows variability on the interannual scale. In particular weak winds from north during winter 2002/2003 combined with mild weather conditions south of Iceland led to anomalous high NIIC volume (+40%) and heat flux (+60%) rates. In this period, simulated north Icelandic water temperatures are at least 0.5 K warmer than average
„Brüderbund und Freundschaft mit den sozialistischen Ländern darf nicht spekulativ genutzt werden.“: Deutsch-polnischer Schleichhandel in Leipzig als Konsumkultur „von unten“
„Brotherhood and friendship with socialist countries must not be used for speculation“. German-Polish illicit trade in Leipzig as consumer culture „from below“
During the communist rule in the Eastern Bloc the economy of shortage influenced everyday life in the GDR and Poland in a decisive way. While facing problems of supply, people in Leipzig developed informal practices like contraband trade to fulfil their needs and wishes of consumption. Transborder encounters between the societies of GDR and Poland in the seventies and eighties fuelled an exchange of goods of shortage in Leipzig’s contraband trade structures. Practices reached from spontaneous help by acquaintances to professionalized groups of smugglers and dealers. The grade of implicitness, stability and structure of this informal phenomenon can be described as an own “culture of consumption from below”. In the atmosphere of political and social difficulties and distrust this led to a negative perception by the government bodies and by the public sphere of Leipzig alike
Software for the Mathews Stanford radio drum
AbstractThe Mathews Radio Drum is explored as a multidimensional controller for music. The techniques can be applied to any three-dimensional spatial system. Some problems are examined regarding notation and interpretation of gestures with respect to composition, and the organization of hardware and software for real-time interactive performance
Exchange interactions in transition metal oxides: The role of oxygen spin polarization
Magnetism of transition metal (TM) oxides is usually described in terms of
the Heisenberg model, with orientation-independent interactions between the
spins. However, the applicability of such a model is not fully justified for TM
oxides because spin polarization of oxygen is usually ignored. In the
conventional model based on the Anderson principle, oxygen effects are
considered as a property of the TM ion and only TM interactions are relevant.
Here, we perform a systematic comparison between two approaches for spin
polarization on oxygen in typical TM oxides. To this end, we calculate the
exchange interactions in NiO, MnO, and hematite (Fe2O3) for different magnetic
configurations using the magnetic force theorem. We consider the full spin
Hamiltonian including oxygen sites, and also derive an effective model where
the spin polarization on oxygen renormalizes the exchange interactions between
TM sites. Surprisingly, the exchange interactions in NiO depend on the magnetic
state if spin polarization on oxygen is neglected, resulting in non-Heisenberg
behavior. In contrast, the inclusion of spin polarization in NiO makes the
Heisenberg model more applicable. Just the opposite, MnO behaves as a
Heisenberg magnet when oxygen spin polarization is neglected, but shows strong
non-Heisenberg effects when spin polarization on oxygen is included. In
hematite, both models result in non-Heisenberg behavior. General applicability
of the magnetic force theorem as well as the Heisenberg model to TM oxides is
discussed.Comment: 19 pages, 2 figure
Geometric, electronic and magnetic structure of FeO clusters
Correlation between geometry, electronic structure and magnetism of solids is
both intriguing and elusive. This is particularly strongly manifested in small
clusters, where a vast number of unusual structures appear. Here, we employ
density functional theory in combination with a genetic search algorithm,
GGA and a hybrid functional to determine the structure of gas phase
FeO clusters. For FeO cation clusters we also
calculate the corresponding vibration spectra and compare them with
experiments. We successfully identify FeO, FeO,
FeO, FeO and propose structures for
FeO. Within the triangular geometric structure of
FeO a non-collinear, ferrimagnetic and ferromagnetic state are
comparable in energy. FeO and FeO are
ferrimagnetic with a residual magnetic moment of 1~\muB{} due to ionization.
FeO is ferrimagnetic due to the odd number of Fe atoms. We
compare the electronic structure with bulk magnetite and find
FeO, FeO, FeO to be mixed
valence clusters. In contrast, in FeO and FeO
all Fe are found to be trivalent.Comment: 14 pages, 21 figure
Modeling Klein tunneling and caustics of electron waves in graphene
We employ the tight-binding propagation method to study Klein tunneling and
quantum interference in large graphene systems. With this efficient numerical
scheme, we model the propagation of a wave packet through a potential barrier
and determine the tunneling probability for different incidence angles. We
consider both sharp and smooth potential barriers in n-p-n and n-n' junctions
and find good agreement with analytical and semiclassical predictions. When we
go outside the Dirac regime, we observe that sharp n-p junctions no longer show
Klein tunneling because of intervalley scattering. However, this effect can be
suppressed by considering a smooth potential. Klein tunneling holds for
potentials changing on the scale much larger than the interatomic distance.
When the energies of both the electrons and holes are above the Van Hove
singularity, we observe total reflection for both sharp and smooth potential
barriers. Furthermore, we consider caustic formation by a two-dimensional
Gaussian potential. For sufficiently broad potentials we find a good agreement
between the simulated wave density and the classical electron trajectories.Comment: 14 pages, 12 figure
Input-to-state stability of Lur’e systems
An input-to-state stability theory, which subsumes results of circle criterion type, is developed in the context of continuous-time Lur’e systems. The approach developed is inspired by the complexified Aizerman conjecture.</p
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