833 research outputs found
Hydrographic conditions along the western Iberian margin during marine isotope stage 2
The surface water hydrography along the western Iberian margin, as part of the North Atlantic's eastern boundary upwelling system, consists of a complex, seasonally variable system of equatorward and poleward surface and subsurface currents and seasonal upwelling. Not much information exists to ascertain if the modern current and productivity patterns subsisted under glacial climate conditions, such as during marine isotope stage (MIS) 2, and how North Atlantic meltwater events, especially Heinrich events, affected them. To help answer these questions we are combining stable isotope records of surface to subsurface dwelling planktonic foraminifer species with sea surface temperature and export productivity data for four cores distributed along the western and southwestern Iberian margin (MD95-2040, MD95-2041, MD99-2336, and MD99-2339). The records reveals that with the exception of the Heinrich events and Greenland Stadial (GS) 4 hydrographic conditions along the western Iberian margin were not much different from the present. During the Last Glacial Maximum (LGM), subtropical surface and subsurface waters penetrated poleward to at least 40.6°N (site MD95-2040). Export productivity was, in general, high on the western margin during the LGM and low in the central Gulf of Cadiz, in agreement with the modern situation. During the Heinrich events and GS 4, on the other hand, productivity was high in the Gulf of Cadiz and suppressed in the upwelling regions along the western margin where a strong halocline inhibited upwelling. Heinrich event 1 had the strongest impact on the hydrography and productivity off Iberia and was the only period when subarctic surface waters were recorded in the central Gulf of Cadiz. South of Lisbon (39°N), the impact of the other Heinrich events was diminished, and not all of them led to a significant cooling in the surface waters. Thus, climatic impacts of Heinrich events highly varied with latitude and the prevailing hydrographic conditions in this region
Circular Orbits in Einstein-Gauss-Bonnet Gravity
The stability under radial and vertical perturbations of circular orbits
associated to particles orbiting a spherically symmetric center of attraction
is study in the context of the n-dimensional: Newtonian theory of gravitation,
Einstein's general relativity, and Einstein-Gauss-Bonnet theory of gravitation.
The presence of a cosmological constant is also considered. We find that this
constant as well as the Gauss-Bonnet coupling constant are crucial to have
stability for .Comment: 11 pages, 4 figs, RevTex, Phys. Rev. D, in pres
Finite Temperature DMRG Investigation of the Spin-Peierls Transition in CuGeO
We present a numerical study of thermodynamical properties of dimerized
frustrated Heisenberg chains down to extremely low temperatures with
applications to CuGeO. A variant of the finite temperature density matrix
renormalization group (DMRG) allows the study of the dimerized phase previously
unaccessible to ab initio calculations. We investigate static dimerized systems
as well as the instability of the quantum chain towards lattice dimerization.
The crossover from a quadratic response in the free energy to the distortion
field at finite temperature to nonanalytic behavior at zero temperature is
studied quantitatively. Various physical quantities are derived and compared
with experimental data for CuGeO such as magnetic dimerization, critical
temperature, susceptibility and entropy.Comment: LaTeX, 5 pages, 5 eps figures include
Excitations of the field-induced soliton lattice in CuGeO3
Here we report the first inelastic neutron scattering study of the magnetic
excitations in the incommensurate phase of a spin-Peierls material. The results
on CuGeO3 provide direct evidence of a finite excitation gap, two sharp
magnetic excitation branches and a very low-lying excitation which is
identified as a phason mode, the Goldstone mode of the incommensurate soliton
lattice.Comment: 5 pages, revtex, 4 figures (*.eps), win-zippe
The record of a high-energy event in a mud entrapment on the inner shelf off the Guadiana river
Recent environmental changes associated with high-energy events and human impacts were investigated in a mud entrapment confined in the paleo-Guadiana incised valley. Those changes were recorded in a gravity core during the last 2500 years. An erosional event seems to have occurred at ca. 500 cal yr BP but it is not clear how much sediment was removed. This event was followed by an increase in river discharges until ca. 465 cal yr BP while the benthic foraminiferal faunas were dominated by species associated with shallow-water sandy sediments. Upward, sedimentological and benthic foraminiferal variations indicated environmental changes, promoted by variable sediment supplies to the shelf.info:eu-repo/semantics/publishedVersio
Spin-Peierls transition of the first order in S=1 antiferromagnetic Heisenberg chains
We investigate a one-dimensional S=1 antiferromagnetic Heisenberg model
coupled to a lattice distortion by a quantum Monte Carlo method. Investigating
the ground state energy of the static bond-alternating chain, we find that the
instability to a dimerized chain depends on the value of the spin-phonon
coupling, unlike the case of S=1/2. The spin state is the dimer state or the
uniform Haldane state depending on whether the lattice distorts or not,
respectively. At an intermediate value of the spin-phonon coupling, we find the
first-order transition between the two states. We also find the coexistence of
the two states.Comment: 7 pages, 12 eps figures embedded in the text; corrected typos,
replaced figure
Temperature Dependence of Spin and Bond Ordering in a Spin-Peierls System
We investigate thermodynamic properties of a one-dimensional S=1/2
antiferromagnetic Heisenberg model coupled to a lattice distortion by a quantum
Monte Carlo method. In particular we study how spin and lattice dimerize as a
function of the temperature, which gives a fundamental process of the
spin-Peierls transition in higher dimensions. The degree of freedom of the
lattice is taken into account adiabatically and the thermal distribution of the
lattice distortion is obtained by the thermal bath algorithm. We find that the
dimerization develops as the temperature decreases and it converges to the
value of the dimerization of the ground state at T=0. Furthermore we find that
the coupling constants of spins fluctuate quite largly at high temperature and
there thermodynamic properties deviate from those of the uniform chain. Doping
of non-magnetic impurities causes cut of the chain into short chains with open
boundary. We investigate thermodynamic properties of open chains taking
relaxation of the lattice into consideration. We find that strong bonds locate
at the edges and a defect of the bond alternation appears in the chain with odd
number of sites, which causes enhancement of the staggered magnetic order. We
find a spreaded staggered structure which indicates that the defect moves
diffusively in the chain even at very low temperature.Comment: 7 pages, 17 figures; added comments on section 2 and 3, corrected
typo
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