8,083 research outputs found
Heavy cosmic strings
We argue that cosmic strings with high winding numbers generally form in
first-order gauge symmetry breaking phase transitions, and we demonstrate this
using computer simulations. These strings are heavier than single-winding
strings and therefore more easily observable. Their cosmological evolution may
also be very different.Comment: 4 pages, updated to match the published versio
The dehydration, rehydration and tectonic setting of greenstone belts in a portion of the northern Kaapvaal Craton, South Africa
High-grade gneiss terranes and low-grade granite-greenstone terranes are well known in several Archaean domains. The geological relationship between these different crustal regions, however, is still controversial. One school of thought favors fundamental genetic differences between high-grade and low-grade terranes while others argue for a depth-controlled crustal evolution. The detailed examination of well-exposed Archaean terranes at different metamorphic grades, therefore, is not only an important source of information about the crustal levels exposed, but also is critical to the understanding of the possible tectonic and metamorphic evolution of greenstone belts with time. Three South African greenstone belts are compared
The LDEF ultra heavy cosmic ray experiment
The LDEF Ultra Heavy Cosmic Ray Experiment (UHCRE) used 16 side viewing LDEF trays giving a total geometry factor for high energy cosmic rays of 30 sq m sr. The total exposure factor was 170 sq m sr y. The experiment is based on a modular array of 192 solid state nuclear track detector stacks, mounted in sets of four in 48 pressure vessels. The extended duration of the LDEF mission has resulted in a greatly enhanced potential scientific yield from the UHCRE. Initial scanning results indicate that at least 1800 cosmic ray nuclei with Z greater than 65 were collected, including the world's first statistically significant sample of actinides. Post flight work to date and the current status of the experiment are reviewed
Determination of the micromagnetic parameters in (Ga,Mn)As using domain theory
The magnetic domain structure and magnetic properties of a ferromagnetic
(Ga,Mn)As epilayer with perpendicular magnetic easy-axis are investigated. We
show that, despite strong hysteresis, domain theory at thermodynamical
equilibrium can be used to determine the micromagnetic parameters. Combining
magneto-optical Kerr microscopy, magnetometry and ferromagnetic resonance
measurements, we obtain the characteristic parameter for magnetic domains
, the domain wall width and specific energy, and the spin stiffness
constant as a function of temperature. The nucleation barrier for magnetization
reversal and the Walker breakdown velocity for field-driven domain wall
propagation are also estimated
Two contrasting metamorphosed ultramafic-mafic complexes from greenstone belts, the northern Kaapvaal Craton and their significance in Archaen tectonics
The character of Archaean ultramafic-mafic complexes can, given their prominance in greenstone belts, provide critical clues to help deduce the tectonic setting of these belts. Here are described two contrasting, metamorphosed, ultramafic-mafic complexes, the first a partially serpentinized dunitic body with associated chromite from Lemoenfontein, one of several peridotitic bodies occurring as discrete lenses and pods in granulite facies gneisses of the northern Kaapvaal craton. The second, the Rooiwater complex is a major layered igneous body, now metamorphosed in the amphibolite facies, but without pervasive deformation, which crops out in the northern Murchison greenstone belt. The Lemoenfontein chromites and associated ultramafic rocks are lithologically and chemically similar to their Phanerozoic equivalents of ophiolitic origin, interpreted as obducted oceanic crust. The Lemoenfontein complex is a remnant of Archaean oceanic material. In contrast, the Rooiwater complex is, despite the lack of exposed intrusive contacts, similar to layered igneous complexes such as Ushushwana or Bushveld. These complexes are intrusive in continental environments. It is concluded that contrasting ultramafic-mafic complexes represent a heterogeneity in greenstone belts with either oceanic or continental environments involved
Investigation of the Domain Wall Fermion Approach to Chiral Gauge Theories on the Lattice
We investigate a recent proposal to construct chiral gauge theories on the
lattice using domain wall fermions. We restrict ourselves to the finite volume
case, in which two domain walls are present, with modes of opposite chirality
on each of them. We couple the chiral fermions on only one of the domain walls
to a gauge field. In order to preserve gauge invariance, we have to add a
scalar field, which gives rise to additional light mirror fermion and scalar
modes. We argue that in an anomaly free model these extra modes would decouple
if our model possesses a so-called strong coupling symmetric phase. However,
our numerical results indicate that such a phase most probably does not exist.
---- Note: 9 Postscript figures are appended as uuencoded compressed tar file.Comment: 27p. Latex; UCSD/PTH 93-28, Wash. U. HEP/93-6
Theory of the anomalous Hall effect from the Kubo formula and the Dirac equation
A model to treat the anomalous Hall effect is developed. Based on the Kubo
formalism and on the Dirac equation, this model allows the simultaneous
calculation of the skew-scattering and side-jump contributions to the anomalous
Hall conductivity. The continuity and the consistency with the
weak-relativistic limit described by the Pauli Hamiltonian is shown. For both
approaches, Dirac and Pauli, the Feynman diagrams, which lead to the
skew-scattering and the side-jump contributions, are underlined. In order to
illustrate this method, we apply it to a particular case: a ferromagnetic bulk
compound in the limit of weak-scattering and free-electrons approximation.
Explicit expressions for the anomalous Hall conductivity for both
skew-scattering and side-jump mechanisms are obtained. Within this model, the
recently predicted ''spin Hall effect'' appears naturally
Probable late Messinian tsunamiites near Monte Dei Corvi, Italy, and the Nijar Basin, Spain: expected architecture of offshore tsunami deposits
Three distinct, 30- to 80-cm-thick, graded, multilayered, coarse-grained sandstone layers, intercalated in the late Messinian mudstones of the Colombacci formation in Lago Mare facies of the Trave section are interpreted as tsunamiites (Ts1-Ts3). Each of these layers is sheet-like and could be followed along strike over several tens of meters. The lower two layers (Ts1-Ts2) occur in the lower part of the Colombacci formation and the third (Ts3) just below a conspicuous white "colombacci" limestone near the top of the formation. The three sandstone layers represent unique sedimentary events within the 120-m-thick San Donato-Colombacci mudstones, which contain many thin, fine-grained, possibly storm-related turbidites. Each of the three clastic layers is overall graded and strongly cross-bedded. A single layer consists of a stack of several graded sublayers that are eroded into the underlying mudstones and into each other. Absence of hummocky cross-stratification (HCS) indicates that the layers are not produced during a large, catastrophic storm event. Current ripples such as dm-sized trough cross-beds suggest strong, prolonged, unidirectional currents, capable of carrying coarse conglomeratic sands. Climbing ripples in middle-fine sand units indicate a high suspension load settling under waning current strength. Each of the Ts1-Ts3 beds satisfies a combination of criteria, described in this paper, that allow interpretation as a tsunamiite in an offshore environment. Tsunamiite Ts2 is underlain by a 15-cm-thick meshwork of synsedimentary fissures, filled with coarse sand. Ground movements induced by strong earthquakes probably caused these crevasses. The uniqueness of each layer, the erosion of the base of each of the sublayers into underlying mudstones and previously deposited sublayer and the consistent stacking of graded sandstone beds within each of the three layers, underlain by earthquake-produced fissures, strongly point to deposition by traction currents produced by the surges of a large tsunami event, triggered by a large vertical fault movements. Vertical fault displacements most likely occurred along the thrust faults like the Sibilline thrust at the SW of the Laga foreland basin, which were active at late Messinian times. A series of cyclic graded turbidites, underlain by seismically induced sand-filled fissures in the Late Messinian Feos formation in SE Spain, are interpreted as tsunamiites produced by a tsunami or seiche. © 2011 The Author(s)
Direct calorimetric measurements of isothermal entropy change on single crystal W-type hexaferrites at the spin reorientation transition
We report on the magnetic field induced isothermal entropy change, \Delta
s(Ha, T), of W-type ferrite with CoZn substitution. Entropy measurements are
performed by direct calorimetry. Single crystals of the composition
BaCoZnFeO, prepared by the flux method, are measured at
different fixed temperatures under an applied field perpendicular and parallel
to the c axis. At 296 K one deduces a value of K = 8.7 \times 10^{4} J
m for the first anisotropy constant, which is in good agreement with the
literature. The spin reorientation transition temperature is estimated to take
place between 200 and 220 K
Metallic properties of magnesium point contacts
We present an experimental and theoretical study of the conductance and
stability of Mg atomic-sized contacts. Using Mechanically Controllable Break
Junctions (MCBJ), we have observed that the room temperature conductance
histograms exhibit a series of peaks, which suggests the existence of a shell
effect. Its periodicity, however, cannot be simply explained in terms of either
an atomic or electronic shell effect. We have also found that at room
temperature, contacts of the diameter of a single atom are absent. A possible
interpretation could be the occurrence of a metal-to-insulator transition as
the contact radius is reduced, in analogy with what it is known in the context
of Mg clusters. However, our first principle calculations show that while an
infinite linear chain can be insulating, Mg wires with larger atomic
coordinations, as in realistic atomic contacts, are alwaysmetallic. Finally, at
liquid helium temperature our measurements show that the conductance histogram
is dominated by a pronounced peak at the quantum of conductance. This is in
good agreement with our calculations based on a tight-binding model that
indicate that the conductance of a Mg one-atom contact is dominated by a single
fully open conduction channel.Comment: 14 pages, 5 figure
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