Week of 10/10/08

In attempt to make these posts more regular I will be trying a different approach of simply posting links to various papers of interest, without much graphics or commentary.

• V. A. Martinez, G. Bryant and W. van Megen, “Slow Dynamics and Aging of a Colloidal Hard Sphere Glass,” Physical Review Letters 101, 135702-4 (2008)
• J. M. Brader, M. E. Cates and M. Fuchs, “First-Principles Constitutive Equation for Suspension Rheology,” Physical Review Letters 101, 138301-4 (2008)  also reviewed by
  N. Wagner, “How colloidal dispersions relax under stress,” Physics 1, 22 (2008)
• D. K. Satapathy, O. Bunk, K. Jefimovs et al., “Colloidal Monolayer Trapped near a Charged Wall: A Synchrotron X-Ray Diffraction Study,” Physical Review Letters 101, 136103-4 (2008)

3D object reconstruction from random orientations

The featured item this time is “Structure from Fleeting Illumination of Faint Spinning Objects in Flight with Application to Single Molecules” by Russell Fung and co-authors at U Wisconsin Milwaukee.

The promise of solving atomic-resolution 3D structure of biological proteins with X-ray Free Electron Lasers has several obstacles. First one is to have fast enough x-ray pulse to image molecules before it starts to undergo “Coulomb explosion”. But even that is not sufficient to produce atomic-scale structure due to low scattering cross-section of hard x-rays – so the experiment will need to be repeated many (thousands?) times to improve statistics. Luckily, protein molecules are identical, so the fundamental 3D structure of the sample could be considered the same – however, the orientation of protein molecule is going to be different each time.

There are two ways to solve the “random orientation” problem – one is to try to align the molecule, for example with a laser beam. However this has to be done with a very high precision and is difficult to achieve in practice. Another approach is to do experiment thousands of times with random orientations of molecules, catalogue all resulting projections, and then use mathematical algorithms to “fold” the projections into a unique 3D object that is consistent with all resulting projections.

Russel Fung et al. provide an algorithm that does just that – by simulating realistic conditions of 4th generation synchrotron source, X-ray Free Electron Laser, with collection of 100,000 photons, 72,000 repeated diffraction patterns from single shot experiments and scattering rates as low as 0.01 photons per pixel at large wavevectors corresponding to 1.8 Angstrom.

The result of folding using Generative Topographic Mapping for protein chignolin in random orientations is shown in figure above, for 3D movie of this reconstructed molecule see Abbas Ourmazd’s webpage at UW Milwaukee.

Recent Coherent X-ray Literature Round-Up

Electrospray approach to single-particle diffraction at XFEL facilities:

M. Bogan, W. Benner, S. Boutet et al., “Single Particle X-ray Diffractive Imaging,” Nano Letters 8, 310-316 (2008)

A study of SiN etched “logo” pattern x-ray induced destruction, similar to Chapman’s Nature Physics 2006 work (Cowboys holding hands logo, doi:10.1038/nphys461):

A. Barty, S. Boutet, M. J. Bogan et al., “Ultrafast single-shot diffraction imaging of nanoscale dynamics,” Nat Photon 2, 415-419 (2008)

Lens-less imaging of Fresnel Zone Plate using ptychography – scanning coherent diffraction – improvement in resolution and illumination function from FZP reconstruction by Rodenburg et al. PRL 98, 034801 (2007):

P. Thibault, M. Dierolf, A. Menzel et al., “High-Resolution Scanning X-ray Diffraction Microscopy,” Science321, 379-382 (2008)

Tabletop coherent soft x-ray microscopy by UColorado group – an exciting alternative to large XFEL machines:

R. L. Sandberg, C. Song, P. W. Wachulak et al., “High numerical aperture tabletop soft x-ray diffraction microscopy with 70-nm resolution,” Proceedings of the National Academy of Sciences of the United States of America 105, 24-7 (2008)

X-ray holography with 5 reference beams is obviously better than holography with 3 or 1 reference beams. How about 1,000,000 reference beams? This is what can be accomplished with uniformly redundant arrays:

S. Marchesini, S. Boutet, A. E. Sakdinawat et al., “Massively parallel X-ray holography,” Nat Photon 2, 560-563 (2008)

Coherent imaging of 80-100nm particle (in SAXS mode, similar to work by Miao group) with 5nm resolution, but done at 15 keV. Coherent fraction of the beam drops off as lambda^2, and efficiency of area x-ray detectors is substantially reduced at higher energies too. But at higher energies one can capture more of the Q range for the same solid angle defined by scattering geometry. Still, 5 nm number is better resolution that I expected – this should imply there are at least 15-20 highly visible fringes in diffraction pattern, instead of 7 or so. Maybe it’s log scale of intensity that hides extra fringes…

C. G. Schroer, P. Boye, J. M. Feldkamp et al., “Coherent X-Ray Diffraction Imaging with Nanofocused Illumination,” Physical Review Letters 101, 090801-4 (2008)

A review article on coherent x-ray diffractive imaging of small particles:

J. Miao, T. Ishikawa, Q. Shen and T. Earnest, “Extending X-ray crystallography to allow the imaging of noncrystalline materials, cells, and single protein complexes,” Annual Review of Physical Chemistry 59, 387-410 (2008)

First example of x-ray holography in hard x-ray regime. Sample preparation is quite a bit more challenging.

L. Stadler, C. Gutt, T. Autenrieth et al., “Hard X Ray Holographic Diffraction Imaging,” Physical Review Letters 100, 245503-4 (2008)

More coherent x-ray lens-less image reconstructions

New paper in Science by Pierre Thibault et al. “High-Resolution Scanning X-ray Diffraction Microscopy” Science 321, 379 (2008).

Authors use an approach identical to ptychography  to demonstrate the power of the technique by reconstructing the Fresnel Zone Plate – similar to work by Rodenburg et al., PRL 98, 034801 (2007).

John Miao and his UCLA group has used lensless imaging to reconstruct image of a single virus:

C. Song et al., “Quantitative Imaging of Single, Unstained Viruses with Coherent X-rays” arXiv:0806.2875.

And Stadler et al. “Hard X Ray Holographic Diffraction Imaging” Phys. Rev. Lett. 100, 245503 (2008) show that the x-ray holographic approach similar to the one previously used by Eisebitt et al., Nature 432, 885 (2004) works in hard-xray regime as should be expected. They cleverly used five carefully positioned nanoparticles as the negative sources of reference beam, and successfully demonstrated that letter “P” can be reconstructed, adding to an impressive alphabet of reconstructed letters and logos. While use of hard x-rays paves the road for imaging of thick speciments, it’s not clear if one could take advantage of the same principle in high-angle diffraction geometry, which is where real action is for hard x-rays.

Physics Today: Fe-based Superconductors, first dedicated synchrotron facility

May-2008 issue of Physics Today has an article on recently discovered Fe-based superconductors.

In the same issue there’s an article on evolution of a dedicated synchrotron facility, a 240 MeV storage ring in Winsconsin named Tantalus.

two PRLs on x-ray phasing

Two new PRLs are dealing with x-ray phasing.

The first paper is de Jonge et al., “Quantitative Phase Imaging with a Scanning Transmission X-Ray Microscope” Phys. Rev. Lett. 100, 163902 (2008). Typically the differential phase contrast measurements become non-trivial for thick specimens, when the adsorption and phase-wrapping effects become significant. This paper resolves this problems when differential phase contrast measurements are done in scanning transmission x-ray microscopy mode (STXM), since the solution is overconstrained, allowing to arrive at unique phase and adsorption values.

The second paper is Johnson et al., “Coherent Diffractive Imaging Using Phase Front Modifications” Phys. Rev. Lett. 100, 155503 (2008).

Since phase is lost during the measurements, it is impossible to simply fourier-transform the coherent x-ray diffraction pattern to obtain a real-space image of an object with nanoscale resolution. There are numerous numerical approaches of phase-retrieval based on oversampling the diffraction pattern. This paper presents an alternative approach of introducing a phase plate, and deconvolving the set of phases resulting from the sample by scanning the phase object around, making the contribution from the phase plate known, and providing information on un-altered phases that would be observed if no phase plate was present. This technique is similar to ptychography, as it provides additional constraints that help arriving at unique solution in a rapidly convergent manner, except it scans the known phase plate, rather than the object being imaged.

sub-picosecond movies of nucleation dynamics

A shortlived SPPS facility is still producing papers – this week it’s the PRL paper by Aaron Lindenberg and some 28 co-authors ” X-Ray Diffuse Scattering Measurements of Nucleation Dynamics at Femtosecond Resolution” Phys. Rev. Lett. 100, 135502 (2008).

This is yet another pump-probe experiment, where pump is a femtosecond laser which ablates/melts a crystal, and a probe is a sub-picosecond x-ray pulse from SPPS. X-ray probe pulse length is still a limiting parameter in overall time resolution of such pump-probe setups. This experiment had a time resolution of 700 fs, but in the near future at XFEL facilities such as LCLS the time resolution will approach tens of femtosecond.

Lindenberg and coworkers were able to look at both high-angle and small-angle diffuse scattering resulting in ablation process in this time-resolved mode. Their data indicates presence of short-lived nanoscale voids (shown in green in the figure on the right) in the liquid state caused by the laser pulse, and these voids merge together to form larger voids over the timescale of 20 ps or so – claims supported by molecular dynamics simulations. While their data was taken in reciprocal space, by recording ensemble-averaged structure factor S(q) at various time delays from the laser pulse, in the future one could envision fully inverting the speckle patterns shown in the figure above, to obtain a real-space images of the nanoscale voids.

Droplet Coalescence

Two recent PRLs are addressing the issue of how two droplets merge into one.

Kamel Fezzaa and Yujie Wang from Argonne use ultrafast x-ray phase-contrast imaging to take sub-microsecond exposures of droplet coalescence, which is complete in just under a milisecond.

Phys. Rev. Lett. 100, 104501 (2008)

The studied liquid droplets are ~1mm (bar size in the image on the left) in size, and can be seen with <5 micron resolution using phase contrast (as opposed to adsorption contrast) using high energy 13keV x-rays. Fezzaa and Wang cleverly used the hybrid filling pattern of Advanced Photon Source, where each electron bunch produces a short x-ray pulse 472 nanoseconds long used for imaging, with each pulse separated from the next one by 3.6 microseconds. The result is a sub-microsecond “shutter time” defined by the length of each pulse, with consecutive images taken 3.6 microseconds apart.

Of particular interest in this study is the stability of torroidal air bubble formed due to air trapped by the two rapidly coalescing droplet menisci. Fezzaa and Wang show for the first time that the torroidal bubble remains trapped until some 400 microsecond after the droplets start merging.

The second recent paper on this topic of droplet coalescence is by Sara Case and Sid Nagel at University of Chicago. PRL 100, 084503 (2008)

Case and Nagel abandon the visual approach to studies of ultrafast coalescence process, and instead adopt a technique which measures the changes in conductivity across the connection between the two droplets as a function of time. When the droplets begin to coalesce, the effective resistivity is high, since it is defined primarily by the width of the narrow region where the two droplets touch each other. As they coalesce, this resistivity will drop. This technique proves to be especially useful in the timescale range from sub-microsecond to hundreds of microsecond. Case and Nagel observe a cross-over in power-law behavior for R(t) from 1/t for small t to 1/√t at large t, but do not see time-dependent fluctuations in R(t) which would be the signature of the connected menisci repeatedly disconnecting and reconnecting again.

Keyhole Imaging, Relaxation in Nanoparticles, CDW correlations

In advanced publication of Nature Physics, Brian Abbey and colleagues present a new technique, called “Keyhole Coherent Diffractive Imaging”, which enables them to study extended objects (in similar fashion as ptychography, but based on a somewhat different geometry). The basic idea of KCDI is to combine Fresnel and Fraunhoffer imaging in a divergent (curved) wavefront by placing an imaged object behind the focal spot of a lens and use this additional information to reconstruct the wavefront. It is somewhat counter-intuitive to understand why curved wavefronts should provide a faster phase-retrieval algorithm convergence than flat wavefronts, but it’s a bit like mixing near-field and far-field techniques.

In advanced publication of Nature Materials, Urbana group lead by Jim Zuo show how application of coherent electron diffraction imaging (very similar to x-ray techniques) can reveal the relationship between the coordination number of Au atoms within a ~4 nm nanoparticle and the out-of-plane bond length. Electrons interact much stronger with matter than x-rays and one could argue that electrons are better for imaging of small nanosized objects, while x-rays are better for imaging extended micron-sized objects. Surface relaxation is a well-known phenomenon in surface science – due to reduced number of near-neighbors, atoms in the near-surface region end up with “dangling bonds” effect – and by turning these bonds inward the atoms can reduce out-of-plane atomic distance. Zuo and his group provide a very detailed analysis of surface relaxation for various facets of Au nanoparticles, as a function of near-neighbor coordination number.

And in a recent issue of Physical Review Letters, David LeBolloch’ and colleagues show that charge density wave condensate in blue bronze compound can spontaneously develop long-range correlations (up to micron-size) when the charge density waves is depinned due to applied current and is in a sliding state (imagine a particle on a washboard potential which is getting tilted).

X-ray lens-less imaging of buried nanostructures

Coherent X-ray Diffraction allows reconstruction of small particles with ~15 nanometer resolution. However, High Resolution Transmission Electron Microscopy (see current issue of MRS Bulletin for review) can achieve an atomic-scale resolution for crystalline nanoscale objects.

But since electrons interact very strongly with positively charged ions in the lattice, electron microscopy cannot see deep inside of materials. X-rays have deep penetrating power, and can provide detailed structural information about deeply buried structures. In small-angle scattering regime all of the material in the path of the x-rays will contribute to the coherent speckle. However, using the energy tunability feature of synchrotron sources, one could scan the energy across the resonant edge of a specific element, changing the effective electron density of specific atomic species. The difference between density maps will reveal the specific elemental distribution of this atomic species, since all other elements will subtract off. Changyong Song from John Miao’s Coherent X-ray Imaging group at UCLA and co-workers have demonstrated this by imaging a Bi nanostructure buried deep within Si matrix (Phys. Rev. Lett. 100, 025504 (2008) ). This microscopy technique, which combines lens-less imaging based on phase-retrieval of coherent x-ray diffraction pattern and the resonant measurements could be extended to samples containing multiple elements – repeating these measurements at various adsorption edges of different elements will produce detailed elemental maps of deeply buried structures (microns or more beneath the surface).

Dark-field X-ray Phase Contrast Imaging

This week’s featured paper is the paper by Franz Pfeiffer and colleagues at Paul Scherrer Institute in Switzerland:

Hard-X-ray dark-field imaging using a grating interferometer, Nature Materials 7, 134 – 137 (2008) .

This Nature Materials paper is related to the previous papers by the same group: Pfeiffer et al., Phase retrieval and differential phase-contrast imaging with low-brilliance X-ray sources, Nature Physics 2, 258 – 261 (2006)

as well as Pfeiffer et al., Shearing Interferometer for Quantifying the Coherence of Hard X-Ray Beams, Phys. Rev. Lett. 94, 164801 (2005).

The use of shearing inteferometer, which to x-rays look like series of micron-sized “combs” or diffraction gratings, allows imaging of milimeter-sized objects using the differential phase contrast, rather than adsorption, as a contrast mechanism. These are the techniques that can be adopted using rather primitive “highly incoherent” in-house x-ray sources – such as x-ray tubes and rotating anodes, and therefore do not require a trip to a synchrotron.

Xraying Nanoparticles in Ball-Lightning and Fuel Injection jets

Observations of ball-lightning – long-lived (2 to 50 seconds) bright fireballs size of baseball to beach ball – have been observed for centuries. Previous theoretical work ascribes the longevity of the ball lightnings to the slow oxidation process of silicon, forming nanoparticle networks. Now, Mitchell et al. Phys. Rev. Lett. 100, 065001 (2008) have created artificial ball-lightning using localized microwaves and have studied them in-situ with synchrotron radiation (using small-angle x-ray scattering), proving that the fireballs do indeed contain nanoparticles with sizes of the order 50 nm.

Small Angle X-ray Scattering, or SAXS, has been used to all kinds of samples – liquid, vapor or solid, but this may be the first time this technique was applied to plasma.

Meanwhile, Wang et al. (Nature Physics, advanced publication) performed an ultra-fast time resolved study of morphology of optically dense jets from fuel injector nozzle. The achieved microsecond temportal resolution is due to application of time-resolved full-field phase contrast imaging. Unlike typical radiography (such as used at the dentist’s office) that is sensitive to the mass density (adsorption) of material through intensity measurements, phase contrast measurements rely on phase changes.

For more details on phase contrast imaging see Wilkins et al., Nature 384, 335 – 338 (28 November 1996)

Techniques using visible light scattering are suffering from problems due to multiple scattering from various interfaces of jet droplets – ironically, these interfaces are precisely what serves as a contrast mechanism in the x-ray phase contrast imaging technique used by Wang et. al in this study.

X-raying Sandcastles

This week the featured article is “Morphological clues to wet granular pile stability” by M. Scheel (advanced Nature Materials publication).

As every child playing on the beach quickly learns, there’s a magic degree of “wetness” of the sand to create most robust sand-castles – too dry and the sand structure doesn’t hold with individual sand grains falling apart (water acts like a cohesive element), and if sand gets too wet, the structure easily collapses under its own weight.

Using high energy x-ray microtomography Scheel of Max Planck institute and coworkers obtained detailed 3D images of grain configuration, allowing them to map out a phase diagram of wet granular matter (like sand) as a function of liquid content. The microscopic local particle-particle configuration changes dramatically as the wetness is varied, and macroscopic mechanical properties, such as yield stress and tensile strength, can be derived from the local packing of grains.

Update (2/25/08): this article is now featured on cover of Nature Materials

It is also accompanied by the News and View item by Arshad Kudrolli “Granular Matter: Sticky sand”.

Nanoparticle Self-assembly with DNA

Image on the left is the cover of Jan. 31 issue of Nature.

Anyone who took high school chemistry has played with “sticks and balls” models of molecules or crystalline atoms. There are magnetic toy sets which allow kids to assemble their own version of crystals.

A similar feat, but on nanoscale, was accomplished by two groups – one at Brookhaven (Nykypanchuk et. al  Nature 451, 549-552 (2008)) and another at Northwestern (Park et. al, Nature 451, 553-556 (2008)).

The basic idea behind these two experiments is to graft different strands of DNA molecules onto particles, creating two “species” of particles with complimentary pairs of DNA strands. These two different types of strands can merge together into a double helix at high temperatures, making a strong connection between particles of the opposite species. The result is a cubic structure, similar to ion salts.

The nanoparticle crystals held together by DNA molecules are rather fragile, and most of volume is occupied by water. The typical size of the unit cell is on the order of 35-50 nm, with particle size just 10 nm in size. The nanoparticles therefore occupy only a tiny fraction of the total crystal volume, with density of such “fluffy” crystals less than milk foam in your latte.

Near-Field X-ray Speckle

There is a brand new paper that appeared today in Nature Physics by Cerbino and co-workers that describes a new x-ray coherent technique based on observations of Near-Field Speckle pattern.

Typically x-ray speckles (or visible light speckles) are observed in Fraunhoffer, or far-field diffraction regime, in which parallel beam approximation can be applied.

The other extreme regime is the near-field (aka Fresnel) geometry, where the detector is placed in the relative vicinity of the sample. This regime is often ignored by scientists because of the complicated scattering patterns caused by interference between scattered and transmitted beams.

However, recently it was shown that Fresnel geometry has some advantages – both in terms of performing lensless imaging microscopy: curved wavefronts, resulting from for example focusing Fresnel Zone Plate optics, result in faster convergence of lensless imaging algorythms – see this paper by Williams et al. Phys. Rev. Lett. 97, 025506 (2006) - and now in terms of using near-field x-ray speckle for X-ray Photon Correlation Spectroscopy.

The Near-Field Speckle setup is limited to relatively small Q-range – the example used in the featured paper by Cerbino et. al is covering ultra-small angle scattering range of Q<0.001 inverse nanometers, corresponding to lengthscales on the order of 10 microns. While such low angles are difficult to access with far-field hard x-ray speckle, the same lengthscales can in principle be reached with visible light (laser) speckle – dynamic light scattering techniques. However, one big advantage of x-rays here is their penetrating
ability and no complications due to multiple scattering effects. Therefore, Near Field X-ray Speckle has a lot of potential for use in non-transparent materials and thick specimens where multiple light scattering effects make laser-based measurements difficult.

Secrets of universal random close packing, plus wedging the phase diagrams

Two quick highlights – one from soft matter and another from hard condensed matter.

First highlight is a PRL with an intriguing title ” Why is Random Close Packing Reproducible?” by Randall Kamien and Andrea Liu.

In very simple terms, the random close packing – jammed configuration where each sphere is confined by its neighbours – is invariably at .64 of volume filling. The ordered hexagonally close packed can achieve higher ratios, of about 0.74, and one would think that the special number of 0.64 can be derived and/or proven mathematically – not so.

Kamien and Liu show that 0.64 is the number for which the number of available configuration states disappears (goes to zero).

The second highlight is another PRL by Oleg Krupin, Eli Rotenberg and Steve Kevan “Controlling the Magnetic Ground State in Cr1-xVx Films”.

Here they used a very clever “wedge” design by making a wedge of CrV material, where the thickness varies along one direction and composition (V doping) varies along the perpendicular direction. This allows mapping out the magnetic phase diagram (C-SDW, I-SDW AFM phase vs. Paramagnetic phase) as a function of temperature, thickness of the film and V composition without having to change the samples.

Sudoku, folding proteins and coherent diffraction

…all of these problems have a lot in common – they involve exhaustive search with a huge number of parameters, accompanied by a similarly large number of constraints, or rules.

The picture on the left is from the cover illustration to PNAS issue back in January of this year, accompanied by the paper by Elser et al., “Searching with iterated maps” PNAS 104, 418 (2007).

One of the key applications is in coherent x-ray imaging – coherent diffraction patterns are fourier transforms of real-space density of an object, but due to phase problem (loss of phases during the measurements of x-ray intensities) one can’t simply perform inverse fourier transform to get back to real-space image. But oversampling – measuring intensities at at least twice the spatial frequency of the object – can solve the phase problem by bringing the number of equations equal to the number of unknown variables again. One can solve for phases by considering restraints, or rules, imposed by such experiments – for example, intensities are known (while phases are not), real space densities are real positive numbers, typically contained within a finite volume (support). There may be more elaborate restraints.

By using an approach based on differential map algorithm one can alternate performing simple projections in real and reciprocal space, using the restraints mentioned above, alternated with Fourier transforms to go from real space to reciprocal space and back.

This approach can be applied to other problems – from finding the local minimum in energy landscape for protein folding, packing and tiling probelms to finding a unique solution for sudoku problems, jumble and other puzzles.

For more info see this wikipedia page or slides of talk by Veit Elser from his Cornell webpage.

Quasi-Forbidden Bragg peaks from soft matter

This week’s item is a rather technical Nature Materials paper by Forster et al., “Order causes secondary Bragg peaks in soft materials”[Nature Materials 6, 888 - 893 (2007)].

Atomic crystals can often be well-ordered, meaning that the correlation length on which “perfect” atomic order exist can extend over many thousands (or even millions) unit cells. Grain boundaries, dislocations and other defects are a common cause of breaking the perfectly ordered chain of atoms.

Soft materials – liquid crystals, colloids, mesoporous materials etc. – typically consist of fairly large unit cells and it is more difficult to get these materials as well-ordered as atomic crystals. All atoms are identical, but colloidal solutions, for example, are often fairly polydisperse, and therefore crystallize with some difficulty – if at all. It is no surprise that the correlation lengths – especially when expressed in unit cells – is far shorter in soft matter, compared to atomic crystals, such as Si or Pb.

Correlation lengthscales can be determined via Debye-Scherrer formalism that relates width of the x-ray or neutron scattering peak to the typical coherent domain size within the sample.

Forster et al. address the issue of finite correlation lengths by analysis of secondary “forbidden” (or quasi-forbidden) Bragg reflections. For example, for a perfect body-centered cubic lattice 001 reflection does not exist – only (002), (011) and other indices that add up to an even number. But once you introduce some disorder, these forbidden peaks become “alive”, since destructive interference responsible for precisely canceling out contributions to these forbidden reflections becomes somewhat faulty.

Surprisingly enough, people haven’t dealt much with ordered, but only over short-range distances materials, at least not to the extent of coming up with sophisticated treatment of intensities of these secondary Bragg peaks that can answer questions like: is the material truly homogeneous but has a lot of disorder, or is it “patchy”? Forster’s paper represents a key step in dealing with these important issues.

Glass on cooling OR heating

This week’s item is a paper that just appeared on arxiv.org by Simon Mochrie’s Yale group and Argonne collaborators on a unique situation that occurs in a liquid that becomes a glass upon cooling OR heating: X. Lu et al., “How a liquid becomes a glass both on cooling and on heating” arxiv.org/0708.3663v1

Typically we think of glasses that vitrify when the temperature is (suddenly) lowered. The unique situation explored in this paper is a situation where at high temperature system forms a glass dominated by repulsive interactions, and at low temperatures due to attractive interactions.

By tracking the glass transition using coherent x-ray scattering techniques (XPCS in this case) the authors look in great detail at the logarithmically decaying slow fluctuations in the same system under very different circumstances – and therefore able to study not a single but two glass transitions with either repulsive or attractive interactions.

Lensless x-ray imaging with tabletop sources

This week’s item is the PRL paper by Sandberg et al., “Lensless Diffractive Imaging Using Tabletop Coherent High-Harmonic Soft-X-Ray Beams” Phys. Rev. Lett. 99, 098103 (2007).

It is essentially a collaboration between JILA/Colorado groups of Henry Kapteyn and Margaret Murnane specializing in coherent soft x-ray tabletop sources, and John Miao’s UCLA group which specializes in lensless imaging, as well as folks from LBNL. The fact that these are very “soft” x-rays – wavelengths of 29 nm means that the spatial resolution is still rather limited (on the order of 100-200nm), compared to synchrotron sources such as ALS or APS, but on the flip side they can do measurements in their in-house lab, instead of relying on scarce synchrotron beamtime, which is a huge benefit. Traditional x-ray sources such as rotating anode and fixed tube can typically operate in hard x-ray range (8keV and higher), but the energy is also fixed and the sources are very incoherent.

I am looking forward to the times when the tabletop sources can become cheap and commonplace enough for other groups to do their characterization using these and other coherent x-ray scattering probes.