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.

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.

Why Ice is Slippery, Snowflakes and other regular December items

December issue of Physics Today often features snow or ice related articles. A lot of them have to do with arguably the most misunderstood topic in physics – why ice is slippery? Generation of scientists were taught that skating on ice is possible due to lowering of melting point of ice under pressure, but this explanation is in fact wrong – one of the main reasons is premelting, existence of quasi-liquid layer coating the surface of a solid well below the melting point.

South Pole explorer Scott knew of this over a hundred years ago – below about 30 deg Centigrade ice ceases to be frictionless and skiing feels similar to skiing over rough sand. And the idea of premelting dates back to Faraday, who was arguing over origins of premelting with none other than James Thomson.

Here are some papers

J. Dash, “Between two and three dimensions” Physics Today, Volume 38, Issue 12, December 1985, pp.26-35

R. Rosenberg, “Why is Ice Slippery” Phys. Today Vol. 58 Issue 12 December 2005, p. 50

Y. Furukawa and J. S. Wettlaufer “Snow and Ice Crystals”, Phys. Today 70, December 2007, p. 70.

Gallery of snowcrystals

Explaining Ice: The Answers are Slippery, New York Times, Feb. 21, 2006

Dash, J.G., A.W. Rempel and J.S. Wettlaufer, “The physics of premelted ice and its geophysical consequences”, Reviews of Modern Physics, 78(3), 695-741, 2006

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.

Si and Ge nanowire growth papers

This week’s journal club items (delayed by a few days due to travel) are two recent papers by IBM group on Vapor-Liquid-Solid growth of silicon and germanium nanowires, using properties of AuSi and AuGe low-temperature eutectic alloys.

The first one is a Nature letter by Hannon et al., investigating migration of Au in formation of Si nanories.

The second one (by the same group) is a Science report by Kadambaka et al., on Ge nanowire growth below eutectic point, via vapor-solid-solid growth mechanism, which allows for control of rate and in some cases shape of the nanoparticles.

Bonus item: another closely related paper on this subject is already mentioned previous work by Peter and Eli Sutter on production and manipulation of “zeptoliter” droplets of AuGe in April issue of Nature Materials.