Damping of antiferromagnetic spin waves by valence fluctuations in the double layer perovskite YBaFe2O5

Inelastic neutron scattering experiments show that spin dynamics in the charge ordered insulating ground state of the double-layer perovskite YBaFe2O5 is well described in terms of eg superexchange interactions. Above the Verwey transition at TV = 308 K, t2g double exchange-type conduction within antiferromagnetic FeO2--BaO--FeO2 double layers proceeds by an electron hopping process that requires a spin flip of the five-fold coordinated Fe ions, costing an energy 5S^2 of approximately 0.1 eV. The hopping process disrupts near-neighbor spin correlations, leading to massive damping of zone-boundary spin waves.Comment: RevTeX, 4 pages, 4 figures, submitted to Phys. Rev. Let

Stabilization of charge ordering in La_(1/3)Sr_(2/3)FeO_(3-d) by magnetic exchange

The magnetic exchange energies in charge ordered La_(1/3)Sr_(2/3)FeO_(3-d) (LSFO) and its parent compound LaFeO_(3) (LFO) have been determined by inelastic neutron scattering. In LSFO, the measured ratio of ferromagnetic exchange between Fe3+ - Fe5+ pairs (J_F) and antiferromagnetic exchange between Fe3+ - Fe3+ pairs (J_AF) fulfills the criterion for charge ordering driven by magnetic interactions (|J_F/J_AF| > 1). The 30% reduction of J_AF as compared to LFO indicates that doped holes are delocalized, and charge ordering occurs without a dominant influence from Coulomb interactions.Comment: 18 pages, 4 color figure

Evaluation of a shoreface nourishment in De Haan: analysis of 20 years of data

In the framework of the implementation of the Master Plan for Coastal Safety and Vlaamse Baaien project, an assessment of the added value of shoreface nourishment as coastal protection measure and as alternative to classic procedures for beach nourishment maintenance will be performed. This project, entitled “Shoreface nourishments as coastal protection measure”, is carried out at Flanders Hydraulics Research with as central activity the monitoring of a pilot shoreface nourishment that will take place in 2013 in Mariakerke.The only antecedent of a shoreface nourishment in Belgium is the nourishment performed in De Haan in 1990. This document brings together results from the monitoring carried out in the 90’s as well as new results obtained from later surveys in the study area. The results is a more than twenty years analysis of the morphological evolution of the coast at the Haan. The interpretation of the results is not straight forward due to the various nourishments performed since then in neighboring areas. However, it is possible to conclude that after a local cross-and long-shore redistribution of the sand nourished in the shoreface, this sand has not been transported and therefore has remain in the system acting as sand supply for the beach.A general reduction of the background erosion has also been detected during this analysis but the fact that during the study period no severe storms occurred makes it difficult to draw a definitive conclusion in this regard.Taking into account the difficulties encountered to draw conclusions on the evolution of the beach and shoreface in De Haan during this study and the goals of the new project it is recommended that during the monitoring of the pilot shoreface nourishment to be started in 2013 no other nourishments on the beach and/or neighboring shoreface and beaches are executed

Unusual signatures of the ferromagnetic transition in the heavy Fermion compound UMn2_2Al20_{20}

Magnetic susceptibility results for single crystals of the new cubic compounds UT$_2$Al$_{20}$ (T=Mn, V, and Mo) are reported. Magnetization, specific heat, resistivity, and neutron diffraction results for a single crystal and neutron diffraction and inelastic spectra for a powder sample are reported for UMn$_2$Al$_{20}$. For T = V and Mo, temperature independent Pauli paramagnetism is observed. For UMn$_2$Al$_{20}$, a ferromagnetic transition is observed in the magnetic susceptibility at $T_c$ = 20 K. The specific heat anomaly at $T_c$ is very weak while no anomaly in the resistivity is seen at $T_c$. We discuss two possible origins for this behavior of UMn$_2$Al$_{20}$: moderately small moment itinerant ferromagnetism, or induced local moment ferromagnetism.Comment: 5 pages, 5 figures, to be published in Phys. rev.

Scientific support regarding hydrodynamics and sand transport in the coastal zone: evaluation of XBeach for long term cross-shore modelling

This report investigates the capabilities of XBeach for long term profile development. Typically cross-shore profile models are just applied to model storm impacts. Within the study an attempt is made to go beyond these common applications by focusing on medium (1 year) to long term applications (10 years). Potential applications include the evaluation of long term profile stability, such as in Knokke, the evolution of a beach or shoreface nourishment, and ultimately the evaluation of coastal retreat due to sea level rise. To this end, firstly the physical processes controlling the profile evolution are identified, secondly the implementation of these processes in XBeach and in Delft3D is reviewed, and finally the capabilities of XBeach are evaluated based on modelling exercises. The study shows promising results and highlights the current bottlenecks to be addressed

Scientific support regarding hydrodynamics and sand transport in the coastal zone: literature and data review coastal zone Zeebrugge - Zwin

In this report the available literature and data relevant for the coastal zone between Zeebrugge and the Zwin is summarised.Since the extension of the harbour of Zeebrugge (finalised in 1986) a lot of bathymetrical data are available. With these data the evolution around Zeebrugge (including the Western part, area around the harbour breakwaters, Baai van Heist, the beaches of Knokke and the Appelzak gully) can be reconstructed. But also before this extension some data are available, such that the difference pre and post extension can be compared.Other data and literature are also described, including numerical modelling of currents and morphology, detailed hydrodynamic and sediment transport measuring campaigns and analysis of the sediment composition.In front of Knokke-Heist the beaches are eroding due to the presence of the Appelzak-gully. The Appelzak shows a long term landward movement. However the movement and development of the gully seems to slow down during the last 10 years. Although confirmation is needed (both based on further trend analysis and numerical modelling), this might suggest that big scale measures to reduce erosion, might be less effective Possibilities for the slow down are the more equilibrium state of the system after the extension of the harbour of Zeebrugge and the influence of the large nourishments in the last decade .Consequently, also the erosion in front of Knokke-Heist is clearly slowing down during the last decade

Rotational dynamics in the plastic-crystal phase of ethanol: Relevance for understanding the dynamics during the structural glass transition

The reorientational dynamics within the rotationally disordered cubic plastic phase of solid ethanol is investigated by means of the concurrent use of computer molecular dynamics and quasielastic neutron scattering. Motions involving widely different time scales are shown to take place above the calorimetric "glass transition" which is centered at Tg≈97 K. These correspond to well-defined reorientations belonging to the cubic point group. The dynamics of this solid exhibits features remarkably close to those of the supercooled liquid that can exist at the same temperature. Such similitude of dynamic behavior serves to provide some clues for the understanding of the nature of molecular motions at temperatures close to the canonical liquid→glass transitio

Scientific support regarding hydrodynamics and sand transport in the coastal zone: calibration of a Long term morphological model of the Belgian shelf

Longshore sediment transport in the surf zone plays a considerable role in the long-term morphological behaviour of the Belgian coast. Based on previous efforts (Zimmermann et al., 2013b; Zimmermann et al., 2013), this report firstly presents a 2D numerical model which covers almost the entire Belgian coast from Nieuwpoort to the Zwin. With the MorMerge online-parallel approach the time horizon for modelling long term morphology in the surf zone was successfully increased to 10 years. Compared with the measurement data, most major morphological changes in the surf zone are well captured qualitatively by the 2D numerical model with an exception of the sedimentation in the Baai van Heist. The modelled sedimentation/erosion is further quantitatively compared with beach accretion/erosion trends of the last 25 years reported in Houthuys (2012). This model still shows a reasonable agreement and its quality is discussed. The sedimentation in the Baai van Heist is specially investigated by an updated 2D and later a new 3D model. For a further validation of the model used in this study, another 3 models OKNO, Zeebrugge and MU-HEIST are introduced and their results are compared with that of the model used in this study. After a series of sensitivity tests a roughness map is employed to update the 2D model, but the updated 2D model seems to be still unable to reproduce the sedimentation in the Baai van Heist properly, especially from the quantitative point of view. To avoid the Gallapatti time scale, a new 3D model is built up, and driven by boundary condition of full spring-neap tides rather than representative ones imposed in the 2D model. With the technique of time-varying MorFac and multiple sediment fractions, the 3D model shows a quite good potential to reproduce the sedimentation in the Baai van Heist. Based on the promising result produced by the 3D model, additional work is proposed for further investigation and research in the future