Soft heaps and clumpy crystals

摘要

Imagine the Rome metro at rush hour: passengers are squeezed into close contact with one another. But there is a physical limit beyond which they cannot go, because their bodies cannot occupy the same space. This common experience has an equivalent at the molecular scale, in what physicists call excluded volume - strong repulsive forces, of quantum-mechanical origin, that prevent atoms from occupying the same space. Because of this phenomenon, dense arrangements of atoms and molecules result in solids that have lattice structures, in which each particle excludes neighbours from its site in the lattice. It is therefore surprising to read Lenz and colleagues' paper1 in Physical Review Letters, which reports that the packing of soft particles may result in unusual crystals in which each lattice site is occupied not by a single particle, but by clumps of particles.