23 research outputs found
Historical and recent processes shaping the geographic range of a rocky intertidal gastropod: phylogeography, ecology, and habitat availability
Factors shaping the geographic range of a species can be identified when phylogeographic patterns are combined with data on contemporary and historical geographic distribution, range-wide abundance, habitat / food availability and through comparisons with co-distributed taxa. Here, we evaluate range dynamism and phylogeography of the rocky intertidal gastropod Mexacanthina lugubris lugubris across its geographic range - the Pacific coast of the Baja peninsula and southern California. We sequenced mitochondrial DNA (CO1) from ten populations and compliment these data with museum records, habitat availability and range-wide field surveys of the distribution and abundance of M. l. lugubris and its primary prey (the barnacle Chthamalus fissus). The geographic range of M. l. lugubris can be characterized by three different events in its history: an old sundering in the mid-peninsular region of Baja (~ 417,000 years ago) and more recent northern range expansion and southern range contraction. The mid-peninsular break is shared with many terrestrial and marine species, although M. l. lugubris represents the first mollusc to show it. This common break is often attributed to a hypothesized ancient seaway bisecting the peninsula, but for M. l. lugubris it may result from large habitat gaps in the southern clade. Northern clade populations, particularly near the historical northern limit (prior to the 1970ās) have high local abundances and reside in a region with plentiful food and habitat ā which makes its northern range conducive to expansion. The observed southern range contraction may result from the opposite scenario, with little food or habitat nearby. Our study highlights the importance of taking an integrative approach to understanding the processes that shape the geographic range of a species via combining range-wide phylogeography data with temporal geographic distributions and spatial patterns of habitat / food availability
Quantum-fluctuation-induced repelling interaction of quantum string between walls
Quantum string, which was brought into discussion recently as a model for the
stripe phase in doped cuprates, is simulated by means of the
density-matrix-renormalization-group method. String collides with adjacent
neighbors, as it wonders, owing to quantum zero-point fluctuations. The energy
cost due to the collisions is our main concern. Embedding a quantum string
between rigid walls with separation d, we found that for sufficiently large d,
collision-induced energy cost obeys the formula \sim exp (- A d^alpha) with
alpha=0.808(1), and string's mean fluctuation width grows logarithmically \sim
log d. Those results are not understood in terms of conventional picture that
the string is `disordered,' and only the short-wave-length fluctuations
contribute to collisions. Rather, our results support a recent proposal that
owing to collisions, short-wave-length fluctuations are suppressed, but
instead, long-wave-length fluctuations become significant. This mechanism would
be responsible for stabilizing the stripe phase
Structural, Electronic, and Magnetic Properties of MnO
We calculate the structural, electronic, and magnetic properties of MnO from
first principles, using the full-potential linearized augmented planewave
method, with both local-density and generalized-gradient approximations to
exchange and correlation. We find the ground state to be of rhombohedrally
distorted B1 structure with compression along the [111] direction,
antiferromagnetic with type-II ordering, and insulating, consistent with
experiment. We show that the distortion can be understood in terms of a
Heisenberg model with distance dependent nearest-neighbor and
next-nearest-neighbor couplings determined from first principles. Finally, we
show that magnetic ordering can induce significant charge anisotropy, and give
predictions for electric field gradients in the ground-state rhombohedrally
distorted structure.Comment: Submitted to Physical Review B. Replaced: regenerated figures to
resolve font problems in automatically generated pd
Quantum magnetism in two dimensions: From semi-classical N\'eel order to magnetic disorder
This is a review of ground-state features of the s=1/2 Heisenberg
antiferromagnet on two-dimensional lattices. A central issue is the interplay
of lattice topology (e.g. coordination number, non-equivalent nearest-neighbor
bonds, geometric frustration) and quantum fluctuations and their impact on
possible long-range order. This article presents a unified summary of all 11
two-dimensional uniform Archimedean lattices which include e.g. the square,
triangular and kagome lattice. We find that the ground state of the spin-1/2
Heisenberg antiferromagnet is likely to be semi-classically ordered in most
cases. However, the interplay of geometric frustration and quantum fluctuations
gives rise to a quantum paramagnetic ground state without semi-classical
long-range order on two lattices which are precisely those among the 11 uniform
Archimedean lattices with a highly degenerate ground state in the classical
limit. The first one is the famous kagome lattice where many low-lying singlet
excitations are known to arise in the spin gap. The second lattice is called
star lattice and has a clear gap to all excitations.
Modification of certain bonds leads to quantum phase transitions which are
also discussed briefly. Furthermore, we discuss the magnetization process of
the Heisenberg antiferromagnet on the 11 Archimedean lattices, focusing on
anomalies like plateaus and a magnetization jump just below the saturation
field. As an illustration we discuss the two-dimensional Shastry-Sutherland
model which is used to describe SrCu2(BO3)2.Comment: This is now the complete 72-page preprint version of the 2004 review
article. This version corrects two further typographic errors (three total
with respect to the published version), see page 2 for detail
Three-body fragmentation dynamics of amidogen and methylene radicals via dissociative recombination
Contains fulltext :
33159.pdf (publisher's version ) (Open Access)We report the investigation into the three-body fragmentation of the triatomic molecules NH2 and CH2 via the process of dissociative recombination (DR). Recently reported analysis of experiments into the DR of the similar system H2O indicated that the DR process is violent, involving a large degree of geometrical change and energy distribution from the initial attachment of the free electron by the parent ion to the final dissociation step. Comparison of data from NH2 and CH2 with that of H2O gives a similar picture of the DR process, though there are significant differences in the results, not only in the branching fraction between the expected decay channels, but also in the distribution of the available reaction energy over the product hydrogen atoms as well as in the molecular geometry at the point of dissociation