Use of extended and prepared reference objects in experimental Fourier transform X-ray holography

The use of one or more gold nanoballs as reference objects for Fourier Transform holography (FTH) is analysed using experimental soft X-ray diffraction from objects consisting of separated clusters of these balls. The holograms are deconvoluted against ball reference objects to invert to images, in combination with a Wiener filter to control noise. A resolution of ~30nm, smaller than one ball, is obtained even if a large cluster of balls is used as the reference, giving the best resolution yet obtained by X-ray FTH. Methods of dealing with missing data due to a beamstop are discussed. Practical prepared objects which satisfy the FTH condition are suggested, and methods of forming them described.Comment: 7 pages, 2 figures, submitted to Applied Physics Letter

Strain mapping on gold thin film buckling and siliconblistering

Stress/Strain fields associated with thin film buckling induced by compressive stresses or blistering due to the presence of gas bubbles underneath single crystal surfaces are difficult to measure owing to the microscale dimensions of these structures. In this work, we show that micro Scanning X-ray diffraction is a well suited technique for mapping the strain/stress tensor of these damaged structures

Dynamics of liquid 4He in Vycor

We have measured the dynamic structure factor of liquid 4He in Vycor using neutron inelastic scattering. Well-defined phonon-roton (p-r) excitations are observed in the superfluid phase for all wave vectors 0.3 < Q < 2.15. The p-r energies and lifetimes at low temperature (T = 0.5 K) and their temperature dependence are the same as in bulk liquid 4He. However, the weight of the single p-r component does not scale with the superfluid fraction (SF) as it does in the bulk. In particular, we observe a p-r excitation between T_c = 1.952 K, where SF = 0, and T_(lambda)=2.172 K of the bulk. This suggests, if the p-r excitation intensity scales with the Bose condensate, that there is a separation of the Bose-Einstein condensation temperature and the superfluid transition temperature T_c of 4He in Vycor. We also observe a two-dimensional layer mode near the roton wave vector. Its dispersion is consistent with specific heat and SF measurements and with layer modes observed on graphite surfaces.Comment: 3 pages, 4 figure

Element-specific spin and orbital momentum dynamics of Fe/Gdmultilayers

The role of orbital magnetism in the laser-induced demagnetization of Fe/Gd multilayers was investigated using time-resolved X-ray magnetic circular dichroism at 2-ps time resolution given by an x-ray streak camera. An ultrafast transfer of angular momentum from the spin via the orbital momentum to the lattice was observed which was characterized by rapidly thermalizing spin and orbital momenta. Strong interlayer exchange coupling between Fe and Gd led to a simultaneous demagnetization of both layers

Status of the LBNL normal-conducting CW VHF electron photo-gun

The fabrication and installation at the Lawrence Berkeley National Laboratory of a high-brightness high-repetition rate photo-gun, based on a normal conducting 187 MHz (VHF) RF cavity operating in CW mode, is in an advanced phase. The cavity will generate an electric field at the cathode plane of ∼ 20 MV/m to accelerate the electron bunches up to ∼ 750 keV, with peak current, energy spread and transverse emittance suitable for FEL and ERL applications. The gun vacuum system has been designed for achieving pressures compatible with the use of "delicate" high quantum efficiency semiconductor cathodes to generate up to a nC bunches at MHz repetition rate with present laser technology. Several photo-cathode/laser systems are under consideration, and in particular photo-cathodes based on K CsSb are being developed for the gun and have already achieved a QE of 8% at 532 nm wavelength, or close to 20% including the Schottky barrier lowering. The cathode will be operated by a μJ fiber laser in conjunction with refractive transverse beam shaping to create a flat top transverse profile, as well as a birefringent pulse stacker to create a flat top temporal profile. The present status and the plan for future activities are presented.

Element-specific spin and orbital momentum dynamics of Fe/Gd multilayers

The role of orbital magnetism in the laser-induced demagnetization of Fe/Gd multilayers was investigated using time-resolved X-ray magnetic circular dichroism at 2-ps time resolution given by an xray streak camera. An ultrafast transfer of angular momentum from the spin via the orbital momentum to the lattice was observed which was characterized by rapidly thermalizing spin and orbital momenta. Strong interlayer exchange coupling between Fe and Gd led to a simultaneous demagnetization of both layers. 1 Author to whom correspondence should be addressed; electronic mail: [email protected]. 2 Ultrafast magnetic storage and processing is founded on our ability to control magnetism on picosecond and femtosecond time scales. Magnetic phase transitions conserve the total angular momentum and usually involve the crystal lattice as a quasi-infinite reservoir of angular momentum. A prototypical ultrafast magnetic phenomenon is the demagnetization after excitation by an intense laser pulse The Fe/Gd multilayer consists of two metals of very different electronic structure. Fe has exchange-split 3d spin bands which intersect the Fermi surface, allowing both low-energy spin-flip (Stoner) and spin wave excitations (magnons). The spin momentum dominates the total angular momentum while the orbital momentum is quenched by the strong ligand field and only partially restored by the spin-orbit interaction. The coupling of the orbital momentum to the anisotropic ligand field enables the flow of angular momentum from the spin system to the lattice during the demagnetization. A direct photon-driven exchange of spin and orbital momentum as proposed by Hübner 3 Early experiments on Gd suggested a slow laser-induced demagnetization in tens of picoseconds Our experiments were performed on a stack of 20 alternating 0.5-nm Fe and Gd layers grown on top of a 200-nm Al heat sink, protected by a thin Al cap layer, and supported by a 100 nm silicon nitride membrane. At and above room temperature the easy magnetization direction was out-of-plane. The thin layers were antiferromagnetically coupled with a common Curie temperature of about 230°C. In order to separate the transient dynamics of the Fe 3d and Gd 4f spin and orbital momenta in the Fe/Gd multilayer, we extended time-resolved XMCD [12, 13] into a laser pump -x-ray probe technique. XMCD has the unique ability to separate and quantify spin and orbital momenta with element specificity Also, XMCD avoids laser pump-induced state-filling effects since the spin-dependent band occupation is determined by recording the absorption cross-section for circularly polarized x-rays, exciting electrons from a spin-orbit-split core level into the valence states. Integration over the absorption resonances accounts for all unoccupied states. The demagnetization dynamics was initiated by heating the sample above the Curie temperature with 60-fs (full width at half maximum (FWHM)) long 800-nm laser pulses at an intensity of 20 mJ/cm 2 and 5 kHz, and probed with 60-ps (FWHM) x-ray pulses from the elliptically polarizing undulator beamline 4.0 at the Advanced Light Source Two representative streaked x-ray pulses are shown in Starting from the integrated transient Fe L 3,2 and Gd M 4,5 dichroism, the transient spin momentum m s (t) = &lt;S z &gt; and orbital momentum m l (t) = &lt;L z &gt; were determined by using sum rules orbital momentum m l (t) before and after t=0 is zero-within experimental errors-in agreement with our expectation that the Gd 4f demagnetization occurs indirectly via exchange with Gd 5d states. The Fe 3d orbital momentum m l (t) decays simultaneously with the Fe spin momentum m s (t). This can be seen more clearly when 5 m l (t) and m s (t) are normalized to their values before t=0, The ultrafast dynamics in Fe/Gd is a true demagnetization as angular momentum is transferred from spin and orbital momentum to the lattice, which acts as a sink. The demagnetization is not primarily the result of a rearrangement of angular momentum between spin and orbit, which would be visible as a change in the orbital to spin momentum ratio. Note that a partial demagnetization in a non-equilibrium situation may be possible without coupling to the lattice because of the different g-factors of electron spin and orbital momenta. It is clear that the Fe spin-orbit interaction does not constitute a bottleneck in the demagnetization of Fe/Gd because the Fe spin and orbital momenta are in or close to equilibrium. The slow dynamics in Fe/Gd, compared to N

Maritime labour, transnational political trajectories and decolonisation from below: the opposition to the 1935 British Shipping Assistance Act

This paper uses a discussion of struggles over attempts by the National Union of Seamen to exclude seafarers form the maritime labour market in the inter-war period to contribute todebates at the intersection of maritime spaces and transnational labour geographies (cf Balachandran, 2012, Hogsbjerg, 2013). Through a focus on struggles over the British Shipping Assistance Act of 1935 it explores some of the transnational dynamics through which racialized forms of trade unionism were contested. I argue that the political trajectories, solidarities and spaces of organising constructed through the alliances which were produced to oppose the effects of the Act shaped articulations of ‘decolonisation from below’ (James, 2015). Engaging with the political trajectories and activity of activists from organisaions like the Colonial Seamen’s Association can open up both new ways of understanding the spatial politics of decolonisation and new accounts of who or how such processes were articulated and contested. The paper concludes by arguing that engagement with these struggles can help assert the importance of forms of subaltern agency in shaping processes of decolonisation

Surface and bulk contribution to Cu(111) quantum efficiency

The quantum efficiency (QE) of Cu(111) is measured for different impinging light angles with photon energies just above the work function. We observe that the vectorial photoelectric effect, an enhancement of the QE due to illumination with light with an electric vector perpendicular to the sample surface, is stronger in the more surface sensitive regime. This can be explained by a contribution to photoemission due to the variation in the electromagnetic potential at the surface. The contributions of bulk and surface electrons can then be determined

Antimicrobial resistance among migrants in Europe: a systematic review and meta-analysis

BACKGROUND: Rates of antimicrobial resistance (AMR) are rising globally and there is concern that increased migration is contributing to the burden of antibiotic resistance in Europe. However, the effect of migration on the burden of AMR in Europe has not yet been comprehensively examined. Therefore, we did a systematic review and meta-analysis to identify and synthesise data for AMR carriage or infection in migrants to Europe to examine differences in patterns of AMR across migrant groups and in different settings. METHODS: For this systematic review and meta-analysis, we searched MEDLINE, Embase, PubMed, and Scopus with no language restrictions from Jan 1, 2000, to Jan 18, 2017, for primary data from observational studies reporting antibacterial resistance in common bacterial pathogens among migrants to 21 European Union-15 and European Economic Area countries. To be eligible for inclusion, studies had to report data on carriage or infection with laboratory-confirmed antibiotic-resistant organisms in migrant populations. We extracted data from eligible studies and assessed quality using piloted, standardised forms. We did not examine drug resistance in tuberculosis and excluded articles solely reporting on this parameter. We also excluded articles in which migrant status was determined by ethnicity, country of birth of participants' parents, or was not defined, and articles in which data were not disaggregated by migrant status. Outcomes were carriage of or infection with antibiotic-resistant organisms. We used random-effects models to calculate the pooled prevalence of each outcome. The study protocol is registered with PROSPERO, number CRD42016043681. FINDINGS: We identified 2274 articles, of which 23 observational studies reporting on antibiotic resistance in 2319 migrants were included. The pooled prevalence of any AMR carriage or AMR infection in migrants was 25·4% (95% CI 19·1-31·8; I2 =98%), including meticillin-resistant Staphylococcus aureus (7·8%, 4·8-10·7; I2 =92%) and antibiotic-resistant Gram-negative bacteria (27·2%, 17·6-36·8; I2 =94%). The pooled prevalence of any AMR carriage or infection was higher in refugees and asylum seekers (33·0%, 18·3-47·6; I2 =98%) than in other migrant groups (6·6%, 1·8-11·3; I2 =92%). The pooled prevalence of antibiotic-resistant organisms was slightly higher in high-migrant community settings (33·1%, 11·1-55·1; I2 =96%) than in migrants in hospitals (24·3%, 16·1-32·6; I2 =98%). We did not find evidence of high rates of transmission of AMR from migrant to host populations. INTERPRETATION: Migrants are exposed to conditions favouring the emergence of drug resistance during transit and in host countries in Europe. Increased antibiotic resistance among refugees and asylum seekers and in high-migrant community settings (such as refugee camps and detention facilities) highlights the need for improved living conditions, access to health care, and initiatives to facilitate detection of and appropriate high-quality treatment for antibiotic-resistant infections during transit and in host countries. Protocols for the prevention and control of infection and for antibiotic surveillance need to be integrated in all aspects of health care, which should be accessible for all migrant groups, and should target determinants of AMR before, during, and after migration. FUNDING: UK National Institute for Health Research Imperial Biomedical Research Centre, Imperial College Healthcare Charity, the Wellcome Trust, and UK National Institute for Health Research Health Protection Research Unit in Healthcare-associated Infections and Antimictobial Resistance at Imperial College London