917 research outputs found
Filtering random layering effects in imaging
Objects that are buried deep in heterogeneous media produce faint echoes which are difficult to distinguish from the backscattered field. Sensor array imaging in such media cannot work unless we filter out the backscattered echoes and enhance the coherent arrivals that carry information about the objects that we wish to image. We study such filters for imaging in strongly backscattering, finely layered media. The filters are based on a travel time transformation of the array data, the normal move-out, used frequently in connection with differential semblance velocity estimation in seismic imaging. In a previous paper [10] we showed that the filters can be used to remove coherent signals from strong plane reflectors. In this paper we show theoretically and with extensive numerical simulations that these filters, based on the normal move-out, can also remove the incoherent arrivals in the array data that are due to fine random layering in the medium. Key words. array imaging, randomly layered media, filtering
Dynamics of topological solitons in two-dimensional ferromagnets
Dynamical topological solitons are studied in classical two-dimensional
Heisenberg easy-axis ferromagnets. The properties of such solitons are treated
both analytically in the continuum limit and numerically by spin dynamics
simulations of the discrete system. Excitation of internal mode causes orbital
motion. This is confirmed by simulations.Comment: LaTeX, 15 pages, 6 figure
A 'virtuous circle' of illicit markets? Smuggling and colonial state building in the Italian interwar Dodecanese
Can illegal markets play a role in a state’s conscious strategies and efforts to establish and further order? Are there conditions under which law enforcement takes second place to wider considerations of national interest? The relationship between state policy and illicit economic activity is typically understood as an oppositional one: beyond a sense of lawlessness, illegal markets exist as a threat to the state’s economic and fiscal interests. In fact, our contemporary understanding of illegal markets is underpinned by the understanding of ‘organised crime’, which is often equally seen as inherently oppositional to a state’s claim on law and order over its territory. Contemporary discourses leave very little margin for a consideration of the relationship between state policy makers and regulators as anything other than conflictual
Homogenization of linear transport equations in a stationary ergodic setting
We study the homogenization of a linear kinetic equation which models the
evolution of the density of charged particles submitted to a highly oscillating
electric field. The electric field and the initial density are assumed to be
random and stationary. We identify the asymptotic microscopic and macroscopic
profiles of the density, and we derive formulas for these profiles when the
space dimension is equal to one.Comment: 24 page
Nonlinear waves in a cylindrical Bose-Einstein condensate
We present a complete calculation of solitary waves propagating in a steady
state with constant velocity v along a cigar-shaped Bose-Einstein trap
approximated as infinitely-long cylindrical. For sufficiently weak couplings
(densities) the main features of the calculated solitons could be captured by
effective one-dimensional (1D) models. However, for stronger couplings of
practical interest, the relevant solitary waves are found to be hybrids of
quasi-1D solitons and 3D vortex rings. An interesting hierarchy of vortex rings
occurs as the effective coupling constant is increased through a sequence of
critical values. The energy-momentum dispersion of the above structures is
shown to exhibit characteristics similar to a mode proposed sometime ago by
Lieb within a strictly 1D model, as well as some rotonlike features.Comment: 10 pages, 12 figure
Surface spin-flop and discommensuration transitions in antiferromagnets
Phase diagrams as a function of anisotropy and magnetic field are
obtained for discommensurations and surface states for an antiferromagnet in
which is parallel to the easy axis, by modeling it using the ground states
of a one-dimensional chain of classical XY spins. A surface spin-flop phase
exists for all , but the interval in over which it is stable becomes
extremely small as goes to zero. First-order transitions, separating
different surface states and ending in critical points, exist inside the
surface spin-flop region. They accumulate at a field (depending on )
significantly less than the value for a bulk spin-flop transition. For
there is no surface spin-flop phase in the strict sense;
instead, the surface restructures by, in effect, producing a discommensuration
infinitely far away in the bulk. The results are used to explain in detail the
phase transitions occurring in systems consisting of a finite, even number of
layers.Comment: Revtex 17 pages, 15 figure
Spin wave frequency shifts in exchange coupled ferromagnet/antiferromagnet structures: Application to Co/CoO
Copyright © 1997 American Institute of PhysicsCo/CoO structures have been studied almost exclusively through measurements of hysteresis, and display an enhanced and strongly temperature dependent effective in-plane anisotropy. A recent experimental study demonstrated an alternate way of investigating effects related to the coupling across the interface by measuring frequencies of long wavelength spin waves associated with the Co film. A large increase in frequency of the low frequency spin wave in the Co was observed as the temperature was lowered through the Neél temperature of CoO. We show how these frequency shifts can be understood as an effective interface anisotropy introduced by strong exchange coupling across the Co/CoO interface. This means that spin waves in the Co also include energy contributions from the larger anisotropies experienced by spins in the CoO. The theory is presented and discussed for the Co/CoO interface and other structures
Search for the Nondimerized Quantum Nematic Phase in the Spin-1 Chain
Chubukov's proposal concerning the possibility of a nondimerized quantum
nematic phase in the ground-state phase diagram of the bilinear-biquadratic
spin-1 chain is studied numerically. Our results do not support the existence
of this phase, but they rather indicate a direct transition from the
ferromagnetic into the dimerized phase.Comment: REVTEX, 14 pages +8 PostScript figure
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