Pine-tree PbS nanowires, screw dislocations and Eshelby twist

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.

Giant molecules or tiny crystals?

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.

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).

Metal-Insulator transition in VO2

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.

M. M. Qazilbash et al., “Mott Transition in VO₂ Revealed by Infrared Spectroscopy and Nano-Imaging” Science 14 December 2007: 1750-1753

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.

Monatomic Glass

This week’s item is a recent Nature paper “Vitrification of a monatomic metallic liquid” by Bhatt et. al from Angell’s group at Arizona State (Nature 448, 787-790 (16 August 2007)).

It is accompanied by News and Views article “Metal turned to Glass”, which does a better job of placing these developments in historical and scientific context than I ever could.

But very basic summary is that under pressure pure Germanium appears to be capable of vitrification – or glassy state. Previously most metallic glasses were alloys, with two or typically more than 3 components. Directional bonding due to multi-component nature of the alloy always plays a role in these cases, so the search for a simple monatomic system capable of forming a glass has been undergoing since 1960ies.

The samples appear to be too small for x-ray diffraction analysis, it would be interesting to see if large enough samples can be produced – in Ge or other monatomic metals.

3D PEEM

A quick and belated item this week is the letter by Jesson et al., from Australia on new approach to extract 3D information from what is originally a 2D PEEM images [Phys. Rev. Lett. 99, 016103 (2007), "Imaging Surface Topography Using Lloyd's Mirror in Photoemission Electron Microscopy"]

There is also this nice tutorial on PEEM at ALS website.

Another (non-research) item is the NY Times article on Eric Mazur’s teaching methods and memorizing/”cranking out” algebra vs. understanding of physical concepts. It’s a fascinating read.

Quasicrystalline Polymers

This issue’s topic is a paper by a japanese group of Kunichi Hayashida et al., Polymeric Quasicrystal: Mesoscopic Quasicrystalline Tiling in ABC Star Polymers Phys. Rev. Lett. 98, 195502 (2007).

Observations of the first quasicrystals caused much of controversy in the fields of condensed matter physics, in particular among crystallographers who ascribed to the notion that it is impossible to have aperiodic but well-ordered lattices with orientational order and 5-fold or 10-fold symmetry. Original paper by Shechtman et al. Metallic Phase with Long-Range Orientational Order and No Translational Symmetry, Phys. Rev. Lett. 53, 1951 – 1953 (1984) is over 20 years old, but a lot of interesting questions remain. To this date, majority of quasicrystals are metallic.

 

 

Kunichi Hayashida et al. report observation of first polymeric quasicrystal, using TEM and x-ray microdiffraction technique. The size of quasicrystalline grains is still relatively small – a few microns, but this means only dozens of units across, since polymer chains used here are ~50 nm long, as opposed to angstrom-sized atomic units in the case of metallic quasicrystals. But Hayashida et al. argue that thermal annealing can produce a larger sized quasicrystalline patterns.

Nanopatterned magnets, zeptoliter pipette redux

June’07 issue of Physics today features an illuminating review by Chien, Zhu & Zhu of topic of Nanopatterned magnets.

Much of it relates to the work done by nanomagnetism group at Argonne. Basic idea is that in >100nm pancake-flat (thickness ~10 nm) ferromagnetic disks often minimize magnetostatic energy by forming the whirl-like close-loop magnetization patterns known as “vortices” (not to be confused with all kinds of other vortices, such as type-II superconductors). Nanosized or more symmetric ferromagnetic particles typically form single-domain magnetization state, which is less interesting. But vortices can have some interesting dynamic properties. In elliptical magnetic pancakes equilibrium states call for two or more vortices – and even in the case of just two vortices their interactions can get quite complicated.

The same June issue of Physics Today features a review of zeptoliter pipette article mentioned on this site previously (here and here), which also mentions yours truly by name. Friso sounds skeptical, and his criticism may very well be valid – however the role of surface segregation is not easy to address in these type of nanoscopic systems.

Seamus Davis on “Atomic Scale Visualization of ‘Pseudogap’ Electronic Matter in Cuprates”

Kavli Institute on Theoretical Physics at Santa Barbara, KITP, has a great program of posting videos and slides of their numerous talks online. (You can also subscribe to their numerous RSS feeds and get them as podcasts)

This week I listened to an excellent talk by Seamus Davis of Cornell on Atomic Scale Visualization of ‘Pseudogap’ Electronic Matter in Cuprates .

This talk was a part of an on-going symposium on strongly correlated systems and cold atoms. There was also another talk by Davis a few days before, providing a nice introduction to STS and d-wave superconductivity.