378 research outputs found
RRS Discovery Cruise 231, 28 Feb-30 Mar 1998. BENGAL: High resolution temporal and spatial study of the BENthic biology and Geochemistry of a north-eastern Atlantic abyssal Locality
Discovery Cruise 231 was the fifth of a series of cruises within a 3-year contract (MAS3 CT950018), BENGAL, funded under the MAST III programme of the EU and running from February 1996 to January 1999. The overall objective of the contract is to monitor the influence of the seasonal sedimentation of phytodetritus on the benthic biology and chemistry of a study site on the Porcupine Abyssal Plain.Like the second BENGAL cruise (Discovery 226, March/April 1997) cruise 231 was timed to precede the current year’s input of phytodetritus expected in May/June. The intention was to obtain a series of samples and data to compare with those taken during cruise 226 and to service deployed moorings. The cruise generally experienced good weather and, with the exception of some gear failures, was very successful, achieving almost all of the intended sampling
RRS Discovery Cruise 222, Leg 2, 29 Aug-24 Sep 1996. BENGAL: High resolution temporal and spatial study of the BENthic biology and Geochemistry of a north-eastern Atlantic abyssal Locality
Discovery Cruise 222 (Leg 2) was the first of a series of cruises within a 3-year contract (MAS3 CT950018), BENGAL, funded under the MAST III programme of the EU and running from February 1996. The contract will concentrate on a single north-eastern Atlantic abyssal locality centred on 48°50’N: 16°30’W and will study in detail changes in the benthic system over one 12-month period from March 1997 to March 1998, particularly in relation to the seasonal deposition of phytodetritus.This cruise was a lead-in to this series, with the objective of obtaining baseline data from benthic and mid-water sampling gears and from both short and long-term deployed moorings/landers. It was also an opportunity to test and refine new experimental procedures in preparation for the detailed studies in 1997-98
The fine sediment conundrum; quantifying, mitigating and managing the issues
Copyright © 2017 John Wiley & Sons, Ltd. Excess fine sediment is a global cause of lotic ecosystem degradation. Despite historic interest in identifying sediment sources and quantifying instream dynamics, tackling fine sediment problems remains a key challenge for river managers and a continued focus of international research. Accordingly, a national meeting of the British Hydrological Society brought together those working on fine sediment issues at the interface of hydrology, geomorphology, and ecology. The resulting collection of papers illustrates the range of research being undertaken in this interdisciplinary research arena, by academic researchers, environmental regulators, landowners, and consultants. More specifically, the contributions highlight key methodological advancements in the identification of fine sediment sources, discuss the complexities surrounding the accurate quantification of riverbed fine sediment content, demonstrate the potential utility of faunal traits as a biological monitoring tool, and recognize the need for improved mechanistic understanding of the functional responses of riverine organisms to excess fine sediment. Understanding and mitigating the effects of fine sediment pressures remains an important and multifaceted problem that requires interdisciplinary collaborative research to deliver novel and robust management tools and sustainable solutions
Variable-range hopping in quasi-one-dimensional electron crystals
We study the effect of impurities on the ground state and the low-temperature
dc transport in a 1D chain and quasi-1D systems of many parallel chains. We
assume that strong interactions impose a short-range periodicicity of the
electron positions. The long-range order of such an electron crystal (or
equivalently, a charge-density wave) is destroyed by impurities. The 3D
array of chains behaves differently at large and at small impurity
concentrations . At large , impurities divide the chains into metallic
rods. The low-temperature conductivity is due to the variable-range hopping of
electrons between the rods. It obeys the Efros-Shklovskii (ES) law and
increases exponentially as decreases. When is small, the metallic-rod
picture of the ground state survives only in the form of rare clusters of
atypically short rods. They are the source of low-energy charge excitations. In
the bulk the charge excitations are gapped and the electron crystal is pinned
collectively. A strongly anisotropic screening of the Coulomb potential
produces an unconventional linear in energy Coulomb gap and a new law of the
variable-range hopping . remains
constant over a finite range of impurity concentrations. At smaller the
2/5-law is replaced by the Mott law, where the conductivity gets suppressed as
goes down. Thus, the overall dependence of on is nonmonotonic.
In 1D, the granular-rod picture and the ES apply at all . The conductivity
decreases exponentially with . Our theory provides a qualitative explanation
for the transport in organic charge-density wave compounds.Comment: 20 pages, 7 figures. (v1) The abstract is abridged to 24 lines. For
the full abstract, see the manuscript (v2) several changes in presentation
per referee's comments. No change in result
Anomalies of ac driven solitary waves with internal modes: Nonparametric resonances induced by parametric forces
We study the dynamics of kinks in the model subjected to a
parametric ac force, both with and without damping, as a paradigm of solitary
waves with internal modes. By using a collective coordinate approach, we find
that the parametric force has a non-parametric effect on the kink motion.
Specifically, we find that the internal mode leads to a resonance for
frequencies of the parametric driving close to its own frequency, in which case
the energy of the system grows as well as the width of the kink. These
predictions of the collective coordinate theory are verified by numerical
simulations of the full partial differential equation. We finally compare this
kind of resonance with that obtained for non-parametric ac forces and conclude
that the effect of ac drivings on solitary waves with internal modes is exactly
the opposite of their character in the partial differential equation.Comment: To appear in Phys Rev
Defect Statistics in the Two Dimensional Complex Ginsburg-Landau Model
The statistical correlations between defects in the two dimensional complex
Ginsburg-Landau model are studied in the defect-coarsening regime. In
particular the defect-velocity probability distribution is determined and has
the same high velocity tail found for the purely dissipative time-dependent
Ginsburg-Landau (TDGL) model. The spiral arms of the defects lead to a very
different behavior for the order parameter correlation function in the scaling
regime compared to the results for the TDGL model.Comment: 24 page
Effect of stress-triaxiality on void growth in dynamic fracture of metals: a molecular dynamics study
The effect of stress-triaxiality on growth of a void in a three dimensional
single-crystal face-centered-cubic (FCC) lattice has been studied. Molecular
dynamics (MD) simulations using an embedded-atom (EAM) potential for copper
have been performed at room temperature and using strain controlling with high
strain rates ranging from 10^7/sec to 10^10/sec. Strain-rates of these
magnitudes can be studied experimentally, e.g. using shock waves induced by
laser ablation. Void growth has been simulated in three different conditions,
namely uniaxial, biaxial, and triaxial expansion. The response of the system in
the three cases have been compared in terms of the void growth rate, the
detailed void shape evolution, and the stress-strain behavior including the
development of plastic strain. Also macroscopic observables as plastic work and
porosity have been computed from the atomistic level. The stress thresholds for
void growth are found to be comparable with spall strength values determined by
dynamic fracture experiments. The conventional macroscopic assumption that the
mean plastic strain results from the growth of the void is validated. The
evolution of the system in the uniaxial case is found to exhibit four different
regimes: elastic expansion; plastic yielding, when the mean stress is nearly
constant, but the stress-triaxiality increases rapidly together with
exponential growth of the void; saturation of the stress-triaxiality; and
finally the failure.Comment: 35 figures, which are small (and blurry) due to the space
limitations; submitted (with original figures) to Physical Review B. Final
versio
The temperature-flow renormalization group and the competition between superconductivity and ferromagnetism
We derive a differential equation for the one-particle-irreducible vertex
functions of interacting fermions as a function of the temperature. Formally,
these equations correspond to a Wilsonian renormalization group scheme which
uses the temperature as an explicit scale parameter. Our novel method allows us
to analyze the competition between superconducting and various magnetic Fermi
surface instabilities in the one-loop approximation. In particular this
includes ferromagnetic fluctuations, which are difficult to treat on an equal
footing in conventional Wilsonian momentum space techniques. Applying the
scheme to the two-dimensional t-t' Hubbard model we investigate the RG flow of
the interactions at the van Hove filling with varying next-nearest neighbor
hopping t'. Starting at t'=0 we describe the evolution of the flow to strong
coupling from an antiferromagnetic nesting regime over a d-wave regime at
moderate t' to a ferromagnetic region at larger absolute values of t'. Upon
increasing the particle density in the latter regime the ferromagnetic
tendencies are cut off and the leading instability occurs in the triplet
superconducting pairing channel.Comment: 18 pages, 11 figure
Spin fluctuations in nearly magnetic metals from ab-initio dynamical spin susceptibility calculations:application to Pd and Cr95V5
We describe our theoretical formalism and computational scheme for making
ab-initio calculations of the dynamic paramagnetic spin susceptibilities of
metals and alloys at finite temperatures. Its basis is Time-Dependent Density
Functional Theory within an electronic multiple scattering, imaginary time
Green function formalism. Results receive a natural interpretation in terms of
overdamped oscillator systems making them suitable for incorporation into spin
fluctuation theories. For illustration we apply our method to the nearly
ferromagnetic metal Pd and the nearly antiferromagnetic chromium alloy Cr95V5.
We compare and contrast the spin dynamics of these two metals and in each case
identify those fluctuations with relaxation times much longer than typical
electronic `hopping times'Comment: 21 pages, 9 figures. To appear in Physical Review B (July 2000
Pairing and Density Correlations of Stripe Electrons in a Two-Dimensional Antiferromagnet
We study a one-dimensional electron liquid embedded in a 2D antiferromagnetic
insulator, and coupled to it via a weak antiferromagnetic spin exchange
interaction. We argue that this model may qualitatively capture the physics of
a single charge stripe in the cuprates on length- and time scales shorter than
those set by its fluctuation dynamics. Using a local mean-field approach we
identify the low-energy effective theory that describes the electronic spin
sector of the stripe as that of a sine-Gordon model. We determine its phases
via a perturbative renormalization group analysis. For realistic values of the
model parameters we obtain a phase characterized by enhanced spin density and
composite charge density wave correlations, coexisting with subleading triplet
and composite singlet pairing correlations. This result is shown to be
independent of the spatial orientation of the stripe on the square lattice.
Slow transverse fluctuations of the stripes tend to suppress the density
correlations, thus promoting the pairing instabilities. The largest amplitudes
for the composite instabilities appear when the stripe forms an antiphase
domain wall in the antiferromagnet. For twisted spin alignments the amplitudes
decrease and leave room for a new type of composite pairing correlation,
breaking parity but preserving time reversal symmetry.Comment: Revtex, 28 pages incl. 5 figure
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