A study of the adhesion of metals and non-metals in an ultra-high vacuum Final report, 12 Jun. 1965 - 1 Jul. 1966

Adhesive properties of metal, glass, ceramic, polymer, and mineral combinations in ultrahigh vacuu

Full-field structured-illumination super-resolution X-ray transmission microscopy

Modern transmission X-ray microscopy techniques provide very high resolution at low and medium X-ray energies, but suffer from a limited field-of-view. If sub-micrometre resolution is desired, their field-of-view is typically limited to less than one millimetre. Although the field-of-view increases through combining multiple images from adjacent regions of the specimen, so does the required data acquisition time. Here, we present a method for fast full-field super-resolution transmission microscopy by structured illumination of the specimen. This technique is well-suited even for hard X-ray energies above 30 keV, where efficient optics are hard to obtain. Accordingly, investigation of optically thick specimen becomes possible with our method combining a wide field-of-view spanning multiple millimetres, or even centimetres, with sub-micron resolution and hard X-ray energies

Non-universality of artificial frustrated spin systems

Magnetic frustration effects in artificial kagome arrays of nanomagnets with out-of-plane magnetization are investigated using Magnetic Force Microscopy and Monte Carlo simulations. Experimental and theoretical results are compared to those found for the artificial kagome spin ice, in which the nanomagnets have in-plane magnetization. In contrast with what has been recently reported, we demonstrate that long range (i.e. beyond nearest-neighbors) dipolar interactions between the nanomagnets cannot be neglected when describing the magnetic configurations observed after demagnetizing the arrays using a field protocol. As a consequence, there are clear limits to any universality in the behavior of these two artificial frustrated spin systems. We provide arguments to explain why these two systems show striking similarities at first sight in the development of pairwise spin correlations.Comment: 7 pages, 6 figure

Ultrasensitive 3He magnetometer for measurements of high magnetic fields

We describe a 3He magnetometer capable to measure high magnetic fields (B > 0.1 Tesla) with a relative accuracy of better than 10^-12. Our approach is based on the measurement of the free induction decay of gaseous, nuclear spin polarized 3He following a resonant radio frequency pulse excitation. The measurement sensitivity can be attributed to the long coherent spin precession time T2* being of order minutes which is achieved for spherical sample cells in the regime of motional narrowing where the disturbing influence of field inhomogeneities is strongly suppressed. The 3He gas is spin polarized in-situ using a new, non-standard variant of the metastability exchange optical pumping. We show that miniaturization helps to increase T2* further and that the measurement sensitivity is not significantly affected by temporal field fluctuations of order 10^-4.Comment: 27 pages, 7 figure

Wide range and tunable linear TMR sensor using two exchange pinned electrodes

A magnetic tunnel junction sensor is proposed, with both the detection and the reference layers pinned by IrMn. Using the differences in the blocking temperatures of the IrMn films with different thicknesses, crossed anisotropies can be induced between the detection and the reference electrodes. The pinning of the sensing electrode ensures a linear and reversible output. It also allows tuning both the sensitivity and the linear range of the sensor. The authors show that the sensitivity varies linearly with the ferromagnetic thickness of the detection electrode. It is demonstrated that an increased thickness leads to a rise of sensitivity and a reduction of the operating range

Quick X-ray microtomography using a laser-driven betatron source

Laser-driven X-ray sources are an emerging alternative to conventional X-ray tubes and synchrotron sources. We present results on microtomographic X-ray imaging of a cancellous human bone sample using synchrotron-like betatron radiation. The source is driven by a 100-TW-class titanium-sapphire laser system and delivers over $10^8$ X-ray photons per second. Compared to earlier studies, the acquisition time for an entire tomographic dataset has been reduced by more than an order of magnitude. Additionally, the reconstruction quality benefits from the use of statistical iterative reconstruction techniques. Depending on the desired resolution, tomographies are thereby acquired within minutes, which is an important milestone towards real-life applications of laser-plasma X-ray sources

Ontogenesis of the Corpora Pedunculata: Integral Relay Structures of Chemosensory Content Addressable Memory Networks of Hexapods: A Synthesis of Development and Function

Biological memory is the temporal storage of information as a function of evolution. Several mechanisms have evolved by which memory can be stored. There are two components involved in the storage of memory in metazoan organisms. Innate memory is strictly teleonomically determined, and hence, depends on the phylogenic predisposition of an organisms' ontogenesis. 'Learned' memory is, in contrast, strictly ontogenically determined and, hence, influenced by the organisms environment. Whilst strictly ontogenic determined memory is stored in the spatial arrangement of nerve cells, phylogenic memory is stored in the sequential arrangement of the four components of the DNA. Accordingly, ontogenic memory is lost in subsequent generations, whereas phylogenic memory is passed on and recalled during the course of evolution. Insects are among the best understood organisms. The fruit fly Drosophila melanogaster, for instance, has been widely used as a model to unravel the genetic components of development. Most of the genes that are involved in this process are known. Other insect species have been physiologically and behaviourally well researched. By assembling the information derived from the latest research on Drosophila melanogaster and other insect species, I have made the attempt to characterise the different components of molecular memory formation (hereafter referred to as mnemogenesis) in insects. Chemosensory memory pathways of Drosophila are composed of at least two different entities: the morphogenic fields such as the peripheral and the central nervous system. I have concluded that during the ontogenesis of the Drosophila chemosensory memory pathways, genes are active that function as modules during this process. Most of the genes which mediate this process are not strictly employed during the morphogenesis of the chemosensory memory pathways. However, they are redeployed to a large extent during development of other germ layers and morphogenic fields, as well. Only certain key genes, which expression is initiated by the several coinciding morphogenic signals, determine the specificity of the different components of the chemosensory memory pathways. Hence, the specificity of the chemosensory memory pathways of Drosophila is determined by the temporally and spatially distinct expression of genes, in addition to the modification of their products. Whilst stage and cell specific gene expression is primarily regulated on the level of chromosome structure and transcriptional activity, the specific function of genes that are expressed in the different regions during different stages of ontogenesis is generated by messenger ribonucleic acid and protein processing. The morphogenic cascades are probably frozen down once the chemosensory memory pathways have reached the state of maturity. The mature insect has maintained the ability to employ some components of the developmental cascade to modulate its memory in response to environmental stimuli. Imaginal chemosensory memory pathways comprise at least four levels. Chemosensory receptor (level I) cells receive environmental information. Projection neurones (level II) reduce the background noise and transfer the information to diverging memory structures, in addition to the control centres (levels III/i and III/ii). Whereas memory structures modulate chemosensory information, the control centres feed this modulated information into output fibres that link the chemosensory memory networks with the premotor fibres (level IV). The function of the memory structures, which in insects are called the corpora pedunculata, is to compare input information to the information stored intrinsically in these organs. The information that is stored intrinsic to these structures is able to modulate the behaviour of an signal, which exits the chemosensory pathways via the premotor neurones. It has been postulated that the modulation of this information depends on the synaptic configuration within the corpora pedunculata. Hence, the synaptic arrangement is thought to underlie the modulation of the information transfer within the chemosensory memory networks. Long term memory is associated with the alteration of this synaptic configuration, which in turn requires the activity of several genetic circuits. Intriguingly, these genetic circuits are probably identical to those employed during axonogenesis, in addition to other morphogenic events

Controlling shot noise in double-barrier magnetic tunnel junctions

We demonstrate that shot noise in Fe/MgO/Fe/MgO/Fe double-barrier magnetic tunnel junctions is determined by the relative magnetic configuration of the junction and also by the asymmetry of the barriers. The proposed theoretical model, based on sequential tunneling through the system and including spin relaxation, successfully accounts for the experimental observations for bias voltages below 0.5V, where the influence of quantum well states is negligible. A weak enhancement of conductance and shot noise, observed at some voltages (especially above 0.5V), indicates the formation of quantum well states in the middle magnetic layer. The observed results open up new perspectives for a reliable magnetic control of the most fundamental noise in spintronic structures.Comment: 8 pages, 4 figure