Search results for "Stability"
showing 10 items of 3085 documents
Crushing of Interstellar Gas Clouds in Supernova Remnants: the Role of Thermal Conduction and Radiative Losses
2004
We model hydrodynamic interactions of an old supernova remnant shock wave with a small interstellar gas cloud, taking into account the effects of thermal conduction and radiative losses. In particular, we consider a representative case of a Mach 30 shock impacting on an isolated cloud with density contrast χ = 10 with respect to the ambient medium. Thermal conduction appears to be effective in suppressing the Kelvin-Helmholtz and Rayleigh-Taylor instabilities which would develop at the cloud boundaries. We demonstrate that the radiative losses play a crucial role in the dynamics of the shock-cloud interaction, dominating evolution of the shocked cloud medium.
Multiscale simulations of topological transformations in magnetic-skyrmion spin structures
2017
Magnetic Skyrmions belong to the most interesting spin structures for the development of future information technology as they have been predicted to be topologically protected. To quantify their stability, we use an innovative multiscale approach to simulating spin dynamics based on the Landau-Lifshitz-Gilbert equation. The multiscale approach overcomes the micromagnetic limitations that have hindered realistic studies using conventional techniques. We first demonstrate how the stability of a Skyrmion is influenced by the refinement of the computational mesh and reveal that conventionally employed traditional micromagnetic simulations are inadequate for this task. Furthermore, we determine…
Multi-wavelength VLBI phase-delay astrometry of a complete sample of radio sources
2007
AbstractWe report on the first global high-precision (differential phase-delay) astrometric analyses performed on a complete set of radio sources. We have observed the S5 polar cap sample, consisting of 13 quasars and BL Lac objects, with the VLBA at 8.4, 15, and 43 GHz. We have developed new algorithms to enable the use of the differential phase-delay observable in global astrometric observations. From our global analyses, we determine the relative positions between all pairs of sources with typical precisions ranging from 10 to 200 μas, depending on observing frequency and source separation. In this paper, we discuss the impact of this observable in the enhancement of the astrometric prec…
Towards asteroseismology of core-collapse supernovae with gravitational wave observations – II. Inclusion of space–time perturbations
2018
Improvements in ground-based, advanced gravitational wave (GW) detectors may allow in the near future to observe the GW signal of a nearby core-collapse supernova. For the most common type of progenitors, likely with slowly rotating cores, the dominant GW emission mechanisms are the post-bounce oscillations of the proto-neutron star (PNS) before the explosion. We present a new procedure to compute the eigenmodes of the system formed by the PNS and the stalled accretion shock in general relativity including spacetime perturbations. The new method improves on previous results by accounting for perturbations of both the lapse function and the conformal factor. We apply our analysis to two nume…
Spin stiffness of vector spin glasses
2011
Abstract We study domain-wall excitations for O ( m ) vector spin glasses in the limit m → ∞ , where the energy landscape is simplified considerably compared to XY or Heisenberg models due to the complete disappearance of metastability. Using numerical ground-state calculations and appropriate pairs of complementary boundary conditions, domain-wall defects are inserted into the systems and their excitation energies are measured. This allows us to determine the stiffness exponents for lattices of a range of spatial dimensions d = 2 , … , 7 . Compiling these results, we can finally determine the lower critical dimension of the model. The outcome is compared to estimates resulting from field-t…
Kinetics of Ordered Phases in Finite Spin Systems
1989
We study the growth of the ordered phase in a spin system of finite size suddenly brought below the transition temperature. Such a growth is driven by the instability of the mode corresponding to the largest eigenvalue of the interaction matrix. The relaxation occurs through different regimes according to whether the unstable mode has a negligible or macroscopic amplitude. One regime is characterised by dynamical scaling properties whereas in the other we can distinguish the growth to a macroscopic amplitude followed by rare transitions from one equilibrium amplitude to another. The analysis is carried out in the framework of a dynamical generalisation of the spherical model assuming non-ra…
3He neutron spin filter at ILL
1997
Abstract The strongly spin-dependent absorption of neutrons in nuclear spin polarized 3He opens the possibility to polarize neutrons over the full-spectrum of cold, thermal and epithermal neutrons. At ILL a neutron spin filter (NSF) development started which uses the direct optical pumping of metastable 3He atoms in a 3He plasma at 1 mbar. At present 0.5 bar 1 of gaseous 3He can be polarized within 1 h. Subsequent polarization-preserving compression by a two-stage compressor system enables one to prepare NSF cells of about 300 cm3 volume in the required pressure range of several bars. A remote type of operation based on relaxation times of up to 100 h has been demonstrated in a first survey…
Dynamics of wetting transitions: A time-dependent Ginzburg-Landau treatment
1987
The dynamic behavior at wetting transitions is studied for systems with short-range forces and nonconserved order parameter. From a continuum limit of a purely relaxational lattice model in mean-field approximation, a time-dependent Ginzburg-Landau equation with a time-dependent boundary condition at the surface is derived in the long wavelength approximation. The dynamics of relaxation close to stable and metastable states is treated in linear response. A divergence of the relaxation time occurs both for critical wetting and along the surface spinodal lines (in the case of first-order wetting), although the static surface layer susceptibilities χ1, χ11 stay finite at the surface spinodal i…
Spinodal decomposition in a binary polymer mixture: Dynamic self-consistent-field theory and Monte Carlo simulations
2001
We investigate how the dynamics of a single chain influences the kinetics of early stage phase separation in a symmetric binary polymer mixture. We consider quenches from the disordered phase into the region of spinodal instability. On a mean field level we approach this problem with two methods: a dynamical extension of the self consistent field theory for Gaussian chains, with the density variables evolving in time, and the method of the external potential dynamics where the effective external fields are propagated in time. Different wave vector dependencies of the kinetic coefficient are taken into account. These early stages of spinodal decomposition are also studied through Monte Carlo…
Nuclei Far from Stability and the R-Process Waiting-Point Concept
1992
The nucleosynthesis process by rapid neutron captures (the r-process) is responsible for the formation of about half of the nuclear species in nature beyond Fe. While the astrophysical site for the r-process is not yet unambiguously identified, its association with the cores of low-mass stars undergoing type II supernova (SN) events is strongly suggested (see, e.g., Refs.1,2).