Roger Pynn

Inelastic neutron scattering studies on 1D near‐Heisenberg antiferromagnets: A test of the Haldane conjecture

Our studies of the spin dynamics for two near‐Heisenberg antiferromagnets (AFMs) revealed: In CsNiCl3 (S=1) at low T≪TN we confirm the conclusions given earlier, but disagreement with the spin‐wave calculations used is found for the eigenvectors. For T>TN, the most important result is the observation that all 〈SαSα〉 (α=x,y,z) are identical, demanding another interpretation for the gap observed than the easy axis one. In CsVCl3 (S= (3)/(2) ) we could not observe any inconsistency in the microscopic parameters determined above and below TN. Although there is a striking coincidence with Haldane’s conjecture [unexplained gap for S=1 (CsNiCl3), no such gap for S= (3)/(2) (CsVCl3)], it is not clear whether this can be considered a proof for Haldane’s conjecture.

The magnetization of a grain boundary in nickel

Abstract We report the measured atomic density and magnetic moment of a nickel twist grain boundary averaged over its lateral dimensions as a function of distance from the interface plane.

Cooperative DNA binding and assembly by a bZip peptide-amphiphile

The bipartite basic zipper (bZip) GCN4 peptide, containing a leucine zipper and a basic binding region, is a well-studied transcription factor that can be rationally adapted to control dimerization or assembly. We have covalently appended alkyl tails to the C-terminus (leucine zipper terminus) of a bZip sequence, yielding mono- and dialkyl bZip peptide-amphiphiles that allowed us to investigate how molecular design can control the formation of secondary structure and self-assembled structure. We demonstrate that these peptide-amphiphiles exhibit four qualities that are representative of their modular construction. First, circular dichroism confirms that self-assembly of peptide-amphiphiles above the critical micelle concentration (CMC) results in an enhanced α-helical secondary structure as peptide head groups are confined to the assembled interface with high local concentrations. Second, the binding of the peptide-amphiphiles to DNA yields a further increase in secondary structure, where the helicity of the basic binding region is stabilized by forming native-like contacts, an “induced fit mechanism”. Third, competitive fluorescence binding assays show peptide-amphiphiles bind cooperatively to DNA well below the CMC, where DNA templates monomeric binding and hydrophobic forces promote cooperativity, but the ability of the peptide to recognize a specific DNA sequence is not retained. And fourth, SANS results demonstrate the assembly of large lamellar aggregates as peptide-amphiphiles complex with DNA, supporting a structural hypothesis in which peptide-amphiphiles bind to the DNA in a native-like ‘standing’ orientation. These designed synthetic molecular architectures are capable of hierarchical assembly making them useful as functional building blocks that may be applied to a variety of systems, including gene delivery and artificial transcription factors.

Shear cell for the study of liquid-solid interfaces by neutron scattering

A cell for examining the density profile of sheared fluids at the solid‐liquid interface by neutron reflectometry is presented. This cell has also proven valuable in examining near‐surface bulk structures in the plane perpendicular to the shear flow using small angle neutron scattering. The shear rates can be controlled by changing the volume flow through the cell over three orders of magnitude. All components of the cell are designed to be chemically inert. A temperature‐controlled environment compatible with neutron studies is also briefly described. Preliminary neutron reflectivity and small angle neutron scattering results using this cell are presented, and potential applications are discussed.

Optimization of neutron scattering instrumentation using neutron spin echo: Application to the discrimination of diffuse scattering in neutron reflectivity experiments

We describe a method, based on the neutron spin-echo technique, to achieve good resolution in many neutron-scattering experiments, without sacrificing signal intensity. By “good resolution” we do not mean the ∼10−6 energy resolution usually associated with neutron spin-echo spectrometers, but rather the level of resolution traditionally obtained by collimation or monochromatization of neutron beams, often on specialized instruments, and almost always at some penalty in measured intensity. The method we discuss allows good resolution to be achieved in any chosen direction in the vector space defined by the neutron scattering wave vector, Q, and the energy transfer, E. Although the method has general applicability to many neutron scattering measurements, we discuss in detail its application to the problem of separating diffuse scattering from specular reflection in neutron reflectometry. The technological basis of the method is the availability of thin films of magnetic or easily magnetizable material that ...

A novel experimental procedure for removing ambiguity from the interpretation of neutron and x‐ray reflectivity measurements: ‘‘Speckle holography’’

The analysis of neutron or x‐ray reflectivity data to obtain density profiles close to surfaces is akin to the notorious phaseless Fourier problem, well known in many fields such as crystallography. It is a difficult problem because a highly nonlinear transform relates the density profile to the data; this results in the existence of several very different solutions, which are also hard to find. A novel experimental procedure is presented, the analogue of astronomical speckle holography, which is designed to eliminate the ambiguity problems inherent in traditional reflectivity measurements. The theoretical basis of this procedure is explained, and it is illustrated with a simple example using both simulated and real experimental data.

Neutron spin echo scattering angle measurement (SESAME)

We describe experiments in which the neutron spin echo technique is used to measure neutron scattering angles. We have implemented the technique, dubbed spin echo scattering angle measurement (SESAME), using thin films of Permalloy electrodeposited on silicon wafers as sources of the magnetic fields within which neutron spins precess. With 30‐μm-thick films we resolve neutron scattering angles to about 0.02° with neutrons of 4.66A wavelength. This allows us to probe correlation lengths up to 200nm in an application to small angle neutron scattering. We also demonstrate that SESAME can be used to separate specular and diffuse neutron reflection from surfaces at grazing incidence. In both of these cases, SESAME can make measurements at higher neutron intensity than is available with conventional methods because the angular resolution achieved is independent of the divergence of the neutron beam. Finally, we discuss the conditions under which SESAME might be used to probe in-plane structure in thin films and...

Neutron scattering instrumentation at reactor based installations

During the past decade neutron scattering techniques have been applied to an increasingly wide range of scientific problems. Concurrently, a number of substantial improvements of neutron scattering instrumentation have occurred to stimulate this trend. In this article several such developments which have occurred at reactor‐based installations are described. Individual spectrometer components which are discussed in some detail include: neutron‐optical devices such as guide tubes, supermirrors and multilayer systems; neutron monochromators with optimum reflectivity, mosaic and focusing characteristics; position‐sensitive detectors of several types; and equipment required for neutron polarization analysis. Several novel spectrometers which have enhanced the role of neutron scattering during the past ten years are also described. These include spectrometers for small‐angle scattering, backscattering, and neutron spin echo. An extensive bibliography is included which covers both early and more recent developm...

Neutron Scattering—A Non-destructive Microscope for Seeing Inside Matter

How can we determine the relative positions and motions of atoms in a bulk sample of solid or liquid? Somehow we need to be able to see inside the sample with a suitable magnifying glass. It turns out that neutrons provide us with this capability. They have no charge, and their electric dipole moment is either zero or too small to measure. For these reasons, neutrons can penetrate matter far better than charged particles. Furthermore, neutrons interact with atoms via nuclear rather than electrical forces, and nuclear forces are very short range—on the order of a few femtometers (i.e., a few times 10−15 m). Thus, as far as the neutron is concerned, solid matter is not very dense because the size of a scattering center (i.e., a nucleus) is typically 100,000 times smaller than the distance between centers. As a consequence, neutrons can travel large distances through most materials without being scattered or absorbed. The attenuation, or decrease in intensity, of a beam of neutrons by aluminum, for example, is about 1% per millimeter compared with 99% per millimeter or more for X-rays. Figure 2.1 illustrates just how easily neutrons penetrate various materials compared with X-rays or electrons. Like so many other things in life, the neutron’s penetrating power is a doubleedged sword. On the plus side, the neutron can penetrate deep within a sample even if it first has to pass through a container (such as would be required for a liquid or powder sample, for example, or if the sample had to be maintained at low temperature or high pressure). The corollary is that neutrons are only weakly scattered once they do penetrate. To make matters worse, available neutron beams have inherently low intensities. X-ray instruments at synchrotron sources can provide fluxes of 1018 photons per second per square millimeter compared with 104 neutrons per second per square millimeter in the same energy bandwidth even at the most powerful continuous neutron sources. The combination of weak interactions and low fluxes make neutron scattering a signal-limited technique, which is practiced only because it provides information about the structure of materials that cannot be obtained in simpler, less expen-

On the structure of liquid alkali metals

The authors calculate a structure factor for the liquid alkali metals assuming that the soft core of the potential plays the dominant structure-determining role. The structure factor is calculated in an analytic form suitable for fitting to experimental data. They obtain good agreement with existing measurements on Na, K and Rb.