4He films on graphite studied by neutron scattering

The properties of ⁴He films adsorbed on graphite have been studied by neutron scattering. In particular excitations of the commensurate phase of the monolayer are discussed. The first two adsorbed layers are solid and the next ones stay liquid. At the boundaries of the superfluid film excitations could be studied. Also the phonons, maxon and rotons of the film are investigated. An explanation of the lower density of the very thin films compared to bulk ⁴He is given.Presented by H. J. Lauter.     

The advantages of elliptic curve cryptography for wireless security

This article provides an overview of elliptic curves and their use in cryptography. The focus is on the performance advantages to be obtained in the wireless environment by using elliptic curve cryptography instead of a traditional cryptosystem like RSA. Specific applications to secure messaging and identity-based encryption are discussed.     

Neuroimaging and the trimodal brain: applications for developmental communication neuroscience

New details generated by neuroimaging and other methods for studying the neural correlates of behavior have led to the formulation of a new model of brain organization, the Trimodal Brain. The model incorporates current biochemical; anatomical, and physiological concepts of prenatal brain growth and combines them with findings on right/left functional asymmetries. It provides a means of relating a wide range of human behaviors and clinical states according to a common base of neural organization, and stresses the utility of an organizational rather than a lesion model for understanding developmental communication disorders.     

Cryptographic Hash Functions from Expander Graphs

We propose constructing provable collision resistant hash functions from expander graphs in which finding cycles is hard. As examples, we investigate two specific families of optimal expander graphs for provable collision resistant hash function constructions: the families of Ramanujan graphs constructed by Lubotzky-Phillips-Sarnak and Pizer respectively. When the hash function is constructed from one of Pizer’s Ramanujan graphs, (the set of supersingular elliptic curves over with ℓ-isogenies, ℓ a prime different from p), then collision resistance follows from hardness of computing isogenies between supersingular elliptic curves. For the LPS graphs, the underlying hard problem is a representation problem in group theory. Constructing our hash functions from optimal expander graphs implies that the outputs closely approximate the uniform distribution. This property is useful for arguing that the output is indistinguishable from random sequences of bits. We estimate the cost per bit to compute these hash functions, and we implement our hash function for several members of the Pizer and LPS graph families and give actual timings.     

Current rating of multicore cables

An algorithm to calculate the current-carrying capability of multicore cables is presented. The method takes into account the presence of air gaps and metallic conductors in the cable bundle. Under an assumption of a uniform loss density, analytical expressions, which take into account conduction inside the bundle and convection and radiation outside it, are developed. The method is illustrated with a numerical example of a telecommunication cable with 96 stranded cores. Experimental results are also reported for the same cable.     

An Interpretation of Dispersionless Modes in Neutron Diffraction Experiments on Helium Films

We analyze the dynamic structure function S(q, ) for neutron scattering off helium films in the direction perpendicular to the substrate. Dispersionlessmodes that have been found in neutron diffraction experiments on helium films adsorbed on graphite powder (H. J. Lauter, H. Godfrin, and H. Wiechert, Excitations in ⁴ He films, in Proceedings of the Second International Conference on Phonon Physics, World Scientific, 1985) are interpreted as standing waves or resonances of the helium liquid perpendicular to the substrate. Our interpretation is supported by microscopic calculations of the ⁴He dynamic structure function.     

In situ polarized (3)He system for the Magnetism Reflectometer at the Spallation Neutron Source

We report on the in situ polarized (3)He neutron polarization analyzer developed for the time-of-flight Magnetism Reflectometer at the Spallation Neutron Source at Oak Ridge National Laboratory. Using the spin exchange optical pumping method, we achieved a (3)He polarization of 76% ± 1% and maintained it for the entire three-day duration of the test experiment. Based on transmission measurements with unpolarized neutrons, we show that the average analyzing efficiency of the (3)He system is 98% for the neutron wavelength band of 2-5?A. Using a highly polarized incident neutron beam produced by a supermirror bender polarizer, we obtained a flipping ratio of >100 with a transmission of 25% for polarized neutrons, averaged over the wavelength band of 2-5?A. After the cell was depolarized for transmission measurements, it was reproducibly polarized and this performance was maintained for three weeks. A high quality polarization analysis experiment was performed on a reference sample of Fe∕Cr multilayer with strong spin-flip off-specular scattering. Using a combination of the position sensitive detector, time-of-flight method, and the excellent parameters of the (3)He cell, the polarization analysis of the two-dimensional maps of reflected, refracted, and off-specular scattered intensity above and below the horizon were obtained, simultaneously.