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.
This week’s item is Science Express paper “Dislocation-Driven Nanowire Growth and Eshelby Twist ” by Michael Bierman et al. (doi:10.1126/science.1157131). By growing PbS nanowires using chemical vapor deposition (CVD) they observe hyper-branched structures, similar to the pine trees with a trunk and multiple branches. The spiral growth pattern is due to the existence of a single screw 
dislocation within the trunk of the nanoscale “pine tree”. The authors test the theory of screw dislocations developed by Eshelby in 1950ies, in particular his prediction of the “Eshelby twist”, an angular twist in the lattice, with twist per unit length proportional to Burgers vector of the dislocation and inversely proportional to the radius of the structure. Because of small radii of the grown PbS nanostructures, nanowires present an excellent testing ground for expected Eshelby twist. The authors find that the fit to Eshelby theory produces Burgers vector on the order of 6 Angstroms, comparable to the expected value of a single unit cell, 5.94 Angstroms.
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.
The featured article this time is advanced online publication in Nature on Superconductivity in FeAs-based layered compounds by Takahashi et al.
This is the first publication in Science/Nature from what I am sure many to follow on this topic, with a few new publications per day appearing on arxiv.
By starting with LaOFeAs compound and doping F at oxygen sites, the
superconducting temperature of 26K is reached under atmospheric pressure. This Tc can be increased up to 43K by applying pressure (maximum of 43K at
4GPa, with the higher pressure decreasing the Tc). This is the highest
non-cuprate Tc observed, and there is already some evidence that the pairing mechanism is non-conventional.
This week we highlight a paper in Nature Physics by Nikolay Kardjilov and co-authors “Three-dimensional imaging of magnetic fields with polarized neutrons”.
3D tomography and microscopy with x-rays is nothing new. Neutrons, however, provide the advantage of strong scattering from magnetic spins - but microscopy with neutrons is limited due to lack of focusing optics, low brightness and monochromacity of neutron sources.
Kardjilov and co-authors present a new technique based on observing the rotation of spin polarization of neutrons as they travel through magnetic material. The result is a 3D view of local magnetization with 100-micron spatial resolution. This technique requires highly polarized and monochromatic neutron beams.
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.
Nature Materials has a News and Views article by Ian Robinson titled “Coherent diffraction: Giant molecules or tiny crystals?”, which reviews recent coherent electron diffraction results by Huang et al. featured here earlier. One of the interesting points made in this mini-review is the phase diagram on the left showing a transition from bulk cubic crystal to decahedral and icosahedral structures, including quasi-molten and liquid phases.
This weeks item is Nature Physics paper Random organization in periodically driven systems by Corte, Golub, Chaikin and Pine.
When you consider a dilute suspension of colloidal particles undergoing a periodic driving force - such as shear, at low density and low strain, particles far from their near neighbours will be undergoing periodic motion around the same point. However, if the two
particles are near each other, they are likely to collide during shearing, changing their relative position. These particles can collide until they become sufficiently separated and settle into reversible fluctuations around their positions - hence self-organizing behaviour.
At high values of maximum strain, or at high densities, however, the particles never cease colliding, leading to irreversible dynamics - hence dynamic phase transition above a well-defined strain threshold.
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.
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).
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).
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)
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.
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.
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”.
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.
In an article Phys. Rev. Lett. 100, 024501 (2008) Nicolas Bremond, Abdou Thiam, and Jerome Bibette are studying coalescence of the emulsion droplets (sizes on the order of tens of microns) in microfluidic channels. What they find is that coalescence of the droplets happens when the dropets begin to separate, and proceeds in a cascading “chain reaction” of sort, with an entire train of particles coalescing together, once the first pair of particles start merging.
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.
The featured article is “Recapturing and trapping single molecules with a solid-state nanopore” by Gershow, M. & Golovchenko, J. A. Nature Nanotech. 2, 775–779 (2007).
The idea behind nanopore-based DNA sequencing is relatively simple - pass a DNA molecule through a nanopore channel, sort of the same way you pass a thread through an eye of the needle. As the DNA is passing through the pore, it can be “read out” by looking for specific signatures of the four bases in, for example, electrical capacitance. Experimental implementation of this idea is of course very challenging.
Gershow and Golovchenko demonstrate that they have ability to “suck” the DNA molecule back into the pore, sufficiently long time after it left the pore. The figure above demonstrates this process schematically.
See also a News and Views by Derek Stein, as well as “Colloquium” review of DNA sequencing approaches in Review of Modern Physics paper by DiVentra and Zwolak.
There were a number of papers in the past month or so on Metal-Insulator transition in vanadium dioxide. The first work is the near-scanning infrared microscopy measurements done by Mumtaz Qazilbash et al. from Basov group at UCSD published last week in Science revealing nucleation and growth of metallic domains in vanadium dioxide films with nanoscale resolution.
Last month there was another Science paper by Peter Baum et al. from Zewail group at Caltech looking at “4D” visualization of metal-insulator transition in vanadium dioxide - involving a combination of 3D observations of Bragg peaks with electron diffraction, resolved on the femtosecond scale.
Also see this PNAS article by Baum and Zewail (PNAS | November 20, 2007 | vol. 104 | no. 47 | 18409-18414, Open access), as well as a Science perspective by Cavalleri.
Baum et al., Science 318 (5851), 788. [DOI: 10.1126/science.1147724]
And in September, there was a PRL by Kubler et al. on light-induced femtosecond MIT transition in VO2, followed by another PRL by Hilton et al. in late November on a similar topic.
A recent paper by M. Argentina et al., Phys. Rev. Lett. 99, 224503 (2007), titled Settling and Swimming of Flexible Fluid-Lubricated Foils, addresses an intriguing questions - can “carpets fly?”.
Some flat, sheet-like shaped fish, like rays and skates, can glide along the ocean bottom almost effortlessly, and the authors of this paper try to explore the parameter space and find the best conditions under which elastic sheets (or foils) have the easiest time to propagate in a Newtonian or non-Newtonian fluids.
X-ray Papers Electronic and Magnetic Materials Papers Liquids Papers Glasses Papers