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

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

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 Stevekrupin.jpg 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.

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