Jens Als-Nielsen

Mean field theory, the Ginzburg criterion, and marginal dimensionality of phase transitions

By applying a real space version of the Ginzburg criterion, the role of fluctuations and thence the self‐consistency of mean field theory are assessed in a simple fashion for a variety of phase transitions. It is shown that in using this approach the concept of ’’marginal dimensionality’’ emerges in a natural way. For example, it is shown that for many homogeneous structural transformations the marginal dimensionality is two, so that mean field theory will be valid for real three‐dimensional systems. It is suggested that this simple self‐consistent approach to Landau theory should be incorporated in the teaching of elementary phase transition phenomena.

Aligned hemozoin crystals in curved clusters in malarial red blood cells revealed by nanoprobe X-ray Fe fluorescence and diffraction

The human malaria parasite Plasmodium falciparum detoxifies the heme byproduct of hemoglobin digestion in infected red blood cells by sequestration into submicron-sized hemozoin crystals. The crystal is composed of heme units interlinked to form cyclic dimers via reciprocal Fe─O (propionate) bonds. Templated hemozoin nucleation was envisaged to explain a classic observation by electron microscopy of a cluster of aligned hemozoin crystals within the parasite digestive vacuole. This dovetails with evidence that acylglycerol lipids are involved in hemozoin nucleation in vivo, and nucleation of β-hematin, the synthetic analogue of hemozoin, was consistently induced at an acylglycerol–water interface via their {100} crystal faces. In order to ascertain the nature of hemozoin nucleation in vivo, we probed the mutual orientations of hemozoin crystals in situ within RBCs using synchrotron-based X-ray nanoprobe Fe fluorescence and diffraction. The X-ray patterns indicated the presence of hemozoin clusters, each comprising several crystals aligned along their needle c axes and exposing {100} side faces to an approximately cylindrical surface, suggestive of nucleation via a common lipid layer. This experimental finding, and the associated nucleation model, are difficult to reconcile with recent reports of hemozoin formation within lipid droplets in the digestive vacuole. The diffraction results are verified by a study of the nucleation process using emerging tools of three-dimensional cellular microscopy, described in the companion paper.

The Debye–Scherrer camera at synchrotron sources: a revisit

In a powder diffraction pattern one measures the intensity of Miller-indexed Bragg peaks versus the wavevector transfer sinθ/λ. With increasing wavevector transfer the density of occurrence of Bragg peaks increases while their intensity decreases until they vanish into the background level. The lowest possible background level is that due to Compton scattering from the powder. A powder diffraction instrument has been designed and tested that yields this ideal low-background level, obtainable by having the space between sample and detector all in vacuum with the entrance window so far upstream that scattering from it is negligible. To minimize overlap of Bragg peaks the combination of fine collimation of synchrotron radiation, a thin cylindrical sample and a high-resolution imaging plate detector is taken advantage of.

Recent Applications of X Rays in Condensed Matter Physics

Since their discovery by Wilhelm Conard Rontgen, X rays have played a significant role in our lives: they make the unseen in our bodies visible; they enhance our security in air travel; they make possible nondestructive testing of a wide variety of materials. And through x‐ray diffraction and spectroscopy they make it possible to probe the order of matter at the atomic level.

Observation of a central mode in an exchange-coupled singlet-groundstate system

A central mode in the ferromagnetic phase transition of Pr3Tl is observed by inelastic and quasielastic neutron scattering. The integrated intensity has a Lorentzian wavevector dependence with a correlation range that is much larger than in ordinary ferromagnets. The softening of the exciton mode is considerably smaller than predicted by mean field theory.

Biochemistry of malaria parasite infected red blood cells by X-ray microscopy

Red blood cells infected by the malaria parasite Plasmodium falciparum are correlatively imaged by tomography using soft X-rays as well as by scanning hard nano-X-ray beam to obtain fluorescence maps of various elements such as S and Fe. In this way one can deduce the amount of Fe bound either in hemoglobin or in hemozoin crystals in the digestive vacuole of the malaria parasite as well as determine the hemoglobin concentrations in the cytosols of the red blood cell and of the parasite. Fluorescence map of K shows that in the parasite’s schizont stage the K concentration in the red blood cell cytosol is diminished by a factor of seven relative to a pristine red blood cell but the total amount of K in the infected red blood cell is the same as in the pristine red blood cell.

Model-independent structure factors from powder X-ray diffraction: a novel approach.

Under the experimental condition that all Bragg peaks in a powder X-ray diffraction (PXRD) pattern have the same shape, one can readily obtain the Bragg intensities without fitting any parameters. This condition is fulfilled at the P02.1 beamline at PETRA III using the seventh harmonic from a 23 mm-period undulator (60 keV) at a distance of 65 m. For grain sizes of the order of 1 µm, the Bragg peak shape in the PXRD is entirely determined by the diameter of the capillary containing the powder sample and the pixel size of the image plate detector, and consequently it is independent of the scattering angle. As an example, a diamond powder has been chosen and structure factors derived which are in accordance with those calculated from density functional theory methods of the WIEN2k package to within an accuracy that allows a detailed electron density analysis.

Order and Fluctuations in Smectic Liquid Crystals

Liquid crystals are among the most interesting condensed states of matter; they are interesting in their own right and we also expect that insights into their properties will help to understand other condensed phases that exist in nature. In all condensed phases there are thermally excited fluctuations. Depending on the spatial dimensionality along with the symmetry and range of interparticle interactions, these fluctuations can play a role of varying significance in determining properties of matter. In some materials the fluctuations play an unimportant role and the statistical mechanical calculations to understand which phases occur and their properties can be carried out simply by a mean field approximation. In other materials the fluctuations profoundly alter the material properties in the vicinity of phase transitions, and only recently have we learned how to make approximate calculations with sufficient accuracy to explain the thermodynamic behavior of these materials. This is the class of phase changes known as critical phenomena. Finally, in certain cases, the fluctuations are so important as to prevent the establishment of phases which interactions between the molecules would otherwise favor.

Unraveling heme detoxification in the malaria parasite by in situ correlative X-ray fluorescence microscopy and soft X-ray tomography

A key drug target for malaria has been the detoxification pathway of the iron-containing molecule heme, which is the toxic byproduct of hemoglobin digestion. The cornerstone of heme detoxification is its sequestration into hemozoin crystals, but how this occurs remains uncertain. We report new results of in vivo rate of heme crystallization in the malaria parasite, based on a new technique to measure element-specific concentrations at defined locations in cell ultrastructure. Specifically, a high resolution correlative combination of cryo soft X-ray tomography has been developed to obtain 3D parasite ultrastructure with cryo X-ray fluorescence microscopy to measure heme concentrations. Our results are consistent with a model for crystallization via the heme detoxification protein. Our measurements also demonstrate the presence of considerable amounts of non-crystalline heme in the digestive vacuole, which we show is most likely contained in hemoglobin. These results suggest a tight coupling between hemoglobin digestion and heme crystallization, highlighting a new link in the crystallization pathway for drug development.

Advanced methods in scanning x-ray microscopy

New developments in X-ray instrumentation and analysis have facilitated the development and improvement of various scanning X-ray microscopy techniques. In this contribution, we offer an overview of recent scanning hard X-ray microscopy measurements performed at the Swiss Light Source. We discuss scanning transmission X-ray microscopy in its transmission, phase contrast, and dark-field imaging modalities. We demonstrate how small-angle X-ray scattering analysis techniques can be used to yield additional information. If the illumination is coherent, coherent diffraction imaging techniques can be brought to bear. We discuss how, from scanning microscopy measurements, detailed measurements of the X-ray scattering distributions can be used to extract high-resolution images. These microscopy techniques with their respective imaging power can easily be combined to multimodal X-ray microscopy.