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Notes: Neutron scattering data for dilute PdMn, PdCo, and PdFe alloys show forward peaks at small values of K, which indicate an inhomogeneous distribution of the magnetization in these alloys. These inhomogeneities indicate the existence of magnetization clouds that produce the ferromagnetism in these materials; the shape of the peaks at small K also suggest a quasielastic contribution to the neutron diffuse scattering. We, therefore, calculated the neutron scattering cross sections for these alloys using a local moment model; we also developed an analytic expression for the magnetization clouds that can be compared with experimental results. The calculations were made using Monte Carlo methods in the dilute alloy region of impurity concentration. © 1996 American Institute of Physics.

Notes: The influence of implicit finite size effects on the fluctuation in the number of particles in a subvolume is studied for a small system of hard disks with a fixed number of particles. The implicit (or anomalous) finite size effects—that arise from the use of periodic boundary conditions—are taken into account by including the periodicity of the total system into a model pair correlation function. Two pair correlation functions are considered; the accurate Percus–Yevick result and an approximation proposed by Baus and Colot that yields an excellent isothermal compressibility. Although very good agreement with canonical ensemble Monte Carlo results is obtained in both cases, it appears that the theoretical expression obtained for the fluctuation in the number of particles is rather sensitive to the thermodynamic and structural information conveyed by the pair correlation function. © 1999 American Institute of Physics.

Notes: The equilibrium structure of a hard-sphere fluid confined in a small spherical cavity is investigated. In such systems the statistical mechanical ensembles are no longer equivalent and we consider both open (grand canonical) and closed (canonical) cavities in order to analyze the effects of size and packing constraints on the density profile of the confined fluid. For systems in the grand canonical ensemble the profiles are obtained from grand canonical ensemble Monte Carlo simulations and from density functional theory. The profiles of the closed (canonical) systems are obtained by means of canonical ensemble Monte Carlo simulations. A scheme is proposed which expands the canonical ensemble density profiles in terms of grand canonical averages; this is formally a series in powers of the inverse average number of particles. By comparing canonical ensemble Monte Carlo data with the results of the expansion applied to grand canonical ensemble Monte Carlo data and to the results of density functional theory the series expansion is shown to converge very quickly in most situations, even when the cavity contains only a few particles. However, as a consequence of packing constraints, in certain situations the density profile develops a pronounced peak in the center of the cavity. Then significant differences arise between the canonical and grand canonical profiles and the convergence of the series is much slower in the central zone where the peak develops. Describing accurately the various terms in the expansion and, hence, the detailed shapes of the profiles provides a searching test of density functional approximations. We find that recent modifications of Rosenfeld's fundamental measure theory, which are designed to describe situations of low effective dimensionality, perform better than his original theory and yield accurate results for all cases except those near maximum packing. © 1998 American Institute of Physics.

Notes: Crystal structure solution by anomalous dispersion methods has been greatly facilitated using the rapidly tunable station 9.5 at the Daresbury SRS. Both SIROAS and MAD techniques, with IP data, have been used in the phasing of a brominated nucleotide and a seleno deaminase, respectively. The electron density maps in each case are interpretable. Throughput of projects could be improved upon with a better duty cycle detector. Another category of data collection is that at very high resolution. Detailed structure refinement pushes the limits of resolution and data quality. Station 9.5 has been used to collect high resolution (1.4 A(ring)) native data for the protein concanavalin A. This utilized very short wavelengths (0.7 A(ring)), the image plate, and crystal freezing. A total of 155 407 measurements from two crystals benefited from the on-line nature of the IP detector device, but a slow and quick pass are required to capture the full dynamic range of the data. There are data seen to 1.2 A(ring) and beyond for a pure Mn substituted form of the protein, but a higher intensity still is required to actually record these data. By comparison, trials at CHESS, on a multipole wiggler (station A1) with a CCD (without image intensifier) system, yield native concanavalin A data to 0.98 A(ring) and beyond. This demonstrates that the combination of yet higher intensity and the ease of use of a CCD offers worthwhile improvements; in this case an increase in the data by a factor of (1.4/0.98)3, thus at least doubling the data to parameter ratio for protein structure model refinement and potentially opening up direct structure determination of proteins of the size of concanavalin A (25 kDa).Finally, possibilities at ESRF and further detector developments, such as mosaic CCDs and scintillator coatings, offer further impetus for the field. These include more intense rapidly tunable beams for anomalous dispersion-based structure solution and "ideal'' higher resolution data collection and reactivity studies. ESRF BL19 is described; facilities on BL19 will include a system for freezing and storing crystals at cryogenic temperatures, so that data can be recorded from the same crystal on different runs. Overall, there have been tremendous strides made in this field in the last 15 years, and yet further improvements are to come. © 1995 American Institute of Physics.

Notes: The magnetic properties of a material are affected by the type of atomic order present in the sample. In particular, the atoms of one kind change their magnetic behavior depending on the composition of its first-neighbor shell. The analysis of magnetic samples by neutron diffuse scattering allows us to separate the atomic short-range order component from the magnetic component, but simultaneously we can study the correlations that affect the magnetic properties of materials. Using neutron-scattering data, we have analyzed the atomic short-range order in several samples where the host is Pd and the impurity is Mn. This analysis allows us to see the effects that this order has on the magnetic properties. © 1999 American Institute of Physics.

Notes: Pd and Pt have been shown to be highly polarizable when mixed with ferromagnets like Ni, Fe, and Co. In these alloys the total moment, or magnetization cloud, is produced by the magnetic moment plus the induced moment on the surrounding atoms. With diffuse magnetic scattering of neutrons, it is possible to measure these magnetization clouds, M(K), as Fourier transform of the moments; M(K) represents the site occupation magnetic moment correlation. We consider that the atoms of the matrix polarize around the atoms of the impurity, following the orientation of its moments and forming giant moments. To compare with experimental data, we obtained an expression for the magnetic moments assuming a local magnetic environment. We then developed an expression for the magnetization cloud that includes the total moment associated with a giant moment, M(0), determined by magnetization measurements; we used M(0)=dμ/dc, where μ is the average moment of the alloy and c the impurity concentration, to establish a relation between neutron and magnetization measurements. © 1997 American Institute of Physics.

Notes: Evidence is presented for an anuran homologue of the mammalian spinocervicothalamic system. In vitro tract-tracing experiments with biotinylated dextran amine in Xenopus laevis show that ascending spinal fibres from all levels of the spinal cord, passing via the dorsolateral funiculus, terminate in a cell area ventrolateral to the dorsal column nucleus. This cell area can be considered a possible homologue of the mammalian lateral cervical nucleus. After tracer applications to the ventral thalamus or to the torus semicircularis (both targets for somatosensory projections), the anuran lateral cervical nucleus was retrogradely labelled contralateral to the application sites. Tracer applications to the dorsolateral funiculus at the obex level and rostral spinal cord resulted in labelling of the cells of origin of the spinocervical tract. These were found, mainly ipsilaterally, in the ventral part of the dorsal horn, and were rather evenly distributed throughout the spinal cord. These data suggest the presence of an anuran homologue of the mammalian spinocervicothalamic system. A brief survey of the literature shows that such a system is much more common in vertebrates than previously thought.