858 research outputs found
Mass mortality and extraterrestrial impacts
The discovery of iridium enrichment at the Cretaceous/Tertiary boundary resulted in formulation of hypothesis of a cometary or asteroid impact as the cause of the biological extinctions at this boundary. Subsequent discoveries of geochemical anomalies at major stratigraphic boundaries like the Precambrian/Cambrian, Permian/Triassic, Middle/Late Jurassic, resulted in the application of similar extraterrestrial impact theories to explain biological changes at these boundaries. Until recently the major physical evidence, as is the location of the impact crater site, to test the impact induced biological extinction was lacking. The diameter of such a crater would be in the range of 60 to 100 km. The recent discovery of the first impact crater in the ocean provide the first opportunity to test the above theory. The crater, named Montagnais and located on the outer shelf off Nova Scotia, Canada, has a minimum diameter of 42 km, with some evidence to a diameter of more than 60 km. At the Montagnais impact site, micropaleontological analysis of the uppermost 80 m of the fall-back breccia represented by a mixture of pre-impact sediments and basement rocks which fills the crater and of the basal 50 m of post-impact marine sediments which overly the impact deposits, revealed presence of diversified foraminiferal and nannoplankton assemblages. The sediments which are intercalated within the uppermost part of the fall-back breccia, had to be deposited before the meteorite impact. The post-impact deposits were laid down almost immediately after the impact as also supported by the micropaleontological data. In conclusion, micropaleontological studies of sediments from the first submarine impact crater site identified in the ocean did not reveal any mass extinction or significant biological changes at the impact site or in the proximal deep ocean basin
Electronic structure of unidirectional superlattices in crossed electric and magnetic fields and related terahertz oscillations
We have studied Bloch electrons in a perfect unidirectional superlattice
subject to crossed electric and magnetic fields, where the magnetic field is
oriented ``in-plane'', i.e. in parallel to the sample plane. Two orientation of
the electric field are considered. It is shown that the magnetic field
suppresses the intersubband tunneling of the Zener type, but does not change
the frequency of Bloch oscillations, if the electric field is oriented
perpendicularly to both the sample plane and the magnetic field. The electric
field applied in-plane (but perpendicularly to the magnetic field) yields the
step-like electron energy spectrum, corresponding to the magnetic-field-tunable
oscillations alternative to the Bloch ones.Comment: 7 pages, 1 figure, accepted for publication in Phys. Rev.
Vortex Plasma in a Superconducting Film with Magnetic Dots
We consider a superconducting film, placed upon a magnetic dot array.
Magnetic moments of the dots are normal to the film and randomly oriented. We
determine how the concentration of the vortices in the film depends on the
magnetic moment of a dot at low temperatures. The concentration of the
vortices, bound to the dots, is proportional to the density of the dots and
depends on the magnetization of a dot in a step-like way. The concentration of
the unbound vortices oscillates about a value, proportional to the magnetic
moment of the dots. The period of the oscillations is equal to the width of a
step in the concentration of the bound vortices.Comment: RevTeX, 4 page
Local density approximation for a perturbative equation of state
The knowledge of a series expansion of the equation of state provides a deep
insight into the physical nature of a quantum system. Starting from a generic
``perturbative'' equation of state of a homogeneous ultracold gas we make
predictions for the properties of the gas in the presence of harmonic
confinement. The local density approximation is used to obtain the chemical
potential, total and release energies, Thomas-Fermi size and density profile of
a trapped system in three-, two-, and one- dimensional geometries. The
frequencies of the lowest breathing modes are calculated using scaling and
sum-rule approaches and could be used in an experiment as a high precision tool
for obtaining the expansion terms of the equation of state. The derived
formalism is applied to dilute Bose and Fermi gases in different dimensions and
to integrable one-dimensional models. Physical meaning of expansion terms in a
number of systems is discussed.Comment: 3 Figure
Spin relaxation at the singlet-triplet crossing in a quantum dot
We study spin relaxation in a two-electron quantum dot in the vicinity of the
singlet-triplet crossing. The spin relaxation occurs due to a combined effect
of the spin-orbit, Zeeman, and electron-phonon interactions. The
singlet-triplet relaxation rates exhibit strong variations as a function of the
singlet-triplet splitting. We show that the Coulomb interaction between the
electrons has two competing effects on the singlet-triplet spin relaxation. One
effect is to enhance the relative strength of spin-orbit coupling in the
quantum dot, resulting in larger spin-orbit splittings and thus in a stronger
coupling of spin to charge. The other effect is to make the charge density
profiles of the singlet and triplet look similar to each other, thus
diminishing the ability of charge environments to discriminate between singlet
and triplet states. We thus find essentially different channels of
singlet-triplet relaxation for the case of strong and weak Coulomb interaction.
Finally, for the linear in momentum Dresselhaus and Rashba spin-orbit
interactions, we calculate the singlet-triplet relaxation rates to leading
order in the spin-orbit interaction, and find that they are proportional to the
second power of the Zeeman energy, in agreement with recent experiments on
triplet-to-singlet relaxation in quantum dots.Comment: 29 pages, 14 figures, 1 tabl
Finite size effects in quantum field theories with boundary from scattering data
We derive a relation between leading finite size corrections for a 1+1
dimensional quantum field theory on a strip and scattering data, which is very
similar in spirit to the approach pioneered by Luscher for periodic boundary
conditions. The consistency of the results is tested both analytically and
numerically using thermodynamic Bethe Ansatz, Destri-de Vega nonlinear integral
equation and classical field theory techniques. We present strong evidence that
the relation between the boundary state and the reflection factor one-particle
couplings, noticed earlier by Dorey et al. in the case of the Lee-Yang model
extends to any boundary quantum field theory in 1+1 dimensions.Comment: 24 pages, 1 eps figure. Clarifying comments and a reference adde
Local Geometry of the Fermi Surface and Magnetoacoustic Responce of Two-Dimensional Electron Systems in Strong Magnetic Fields
A semiclassical theory for magnetotrasport in a quantum Hall system near
filling factor based on the Composite Fermions physical picture is
used to analyze the effect of local flattening of the Composite Fermion Fermi
surface (CF-FS) upon magnetoacoustic oscllations. We report on calculations of
the velocity shift and attenuation of a surface acoustic wave (SAW) which
travels above the two-dimensional electron system, and we show that local
geometry of the CF-FS could give rise to noticeable changes in the magnitude
and phase of the oscillations. We predict these changes to be revealed in
experiments, and to be used in further studies of the shape and symmetries of
the CF-FS. Main conclusions reported here could be applied to analyze
magnetotransport in quantum Hall systems at higher filling factors provided the Fermi-liquid-like state of the system.Comment: 7 pages, 2 figure
Large distance behaviour of light cone operator product in perturbative and nonperturbative QCD regimes
We evaluate the coordinate space dependence of the matrix elements of the
commutator of the electromagnetic and gluon currents in the vicinity of the
light-cone but at large distances within the parton model, DGLAP, the
resummation approaches to the small x behaviour of DIS processes, and for the
Unitarity Bound. We find that an increase of the commutator with relative
distance as is the generic property of QCD
at small but fixed space-time interval in perturbative and
nonperturbative QCD regimes. We explain that the factor follows within the
dipole model (QCD factorization theorem) from the properties of Lorents
transformation. The increase of disappeares at central impact parameters
if cross section of DIS may achieve the Unitarity Limit. We argue that such
long range forces are hardly consistent with thermodynamic equilibrium while a
Unitarity Limit may signal equilibration. Possible implications of this new
long range interaction are briefly discussed.Comment: 23 page
Nemesis Reconsidered
The hypothesis of a companion object (Nemesis) orbiting the Sun was motivated
by the claim of a terrestrial extinction periodicity, thought to be mediated by
comet showers. The orbit of a distant companion to the Sun is expected to be
perturbed by the Galactic tidal field and encounters with passing stars, which
will induce variation in the period. We examine the evidence for the previously
proposed periodicity, using two modern, greatly improved paleontological
datasets of fossil biodiversity. We find that there is a narrow peak at 27 My
in the cross-spectrum of extinction intensity time series between these
independent datasets. This periodicity extends over a time period nearly twice
that for which it was originally noted. An excess of extinction events are
associated with this periodicity at 99% confidence. In this sense we confirm
the originally noted feature in the time series for extinction. However, we
find that it displays extremely regular timing for about 0.5 Gy. The regularity
of the timing compared with earlier calculations of orbital perturbation would
seem to exclude the Nemesis hypothesis as a causal factor.Comment: 10 pages, 2 figures, accepted for publication in Monthly Notices of
the Royal Astronomical Societ
Orthogonal Linear Combinations of Gaussian Type Orbitals
The set of Gaussian Type Orbitals g(n1,n2,n3) of order (n+1)(n+2)/2, of
common n=n1+n2+n3<=7, common center and exponential, is customized to define a
set of 2n+1 linear combinations t(n,m) (-n<=m<=n) such that each t(n,m) depends
on the azimuthal and polar angle of the spherical coordinate system like the
real or imaginary part of the associated Spherical Harmonic. (Results cover
both Hermite and Cartesian Gaussian Type Orbitals.) Overlap, kinetic energy and
Coulomb energy matrix elements are presented for generalized basis functions of
the type r^s*t(n,m) (s=0,2,4,...). In addition, normalization integrals int
|g(n1,n2,n3)|d^3r are calculated up to n=7 and normalization integrals int
|r^s*t(n,m)|d^3r up to n=5.Comment: 13 pages, no figures, REVTeX4. Corrected eqs. (23) and (C4
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