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Exploring the link between crystal defects and nonaffine displacement fluctuations

Research output: Contribution to journalArticle

Pankaj Popli, Sayantani Kayal, Peter Kurt Sollich, Surajit Sengupta

Original languageEnglish
Publication statusAccepted/In press - 28 Aug 2019


  • EG12096

    EG12096.pdf, 5.63 MB, application/pdf


King's Authors


We generalize, and then use, a recently introduced formalism to study thermal fluctuations of atomic displacements in several two and three dimensional crystals. We study both close packed as well as open crystals with multi atom bases. Atomic displacement fluctuations in a solid, once coarse- grained over some neighborhood may be decomposed into two mutually orthogonal components. In any dimension d there are always d^2 affine displacements representing local strains and rotations of the ideal reference configuration. In addition, there exists a number of non-affine localized displacement modes that cannot be represented as strains or rotations. The number of these modes depends on d and the size of the coarse graining region. All thermodynamic averages and correlation functions concerning the affine and non-affine displacements may be computed within a harmonic theory. We show that for compact crystals, such as the square and triangular in d = 2 and the simple, body-centered and face-centered cubic crystals in d = 3, a single set of d−fold degenerate modes always dominate the non-affine sub-space and are separated from the rest by a large gap. These modes may be identified with specific precursor configurations that lead to lattice defects. In open crystals, such as the honeycomb and kagome lattices, there is no prominent gap although soft non-affine modes continue to be associated with known floppy modes representing localized defects. Higher order coupling between affine and non-affine components of the displacements quantify the tendency of the lattice to be destroyed by large homogeneous strains. We show that this coupling is larger by almost an order of magnitude for open lattices as compared to compact ones. Deformation mechanisms such as lattice slips and stacking faults in close packed crystals can also be understood within this framework. The qualitative features of these conclusions are expected to be independent of the details of the atomic interactions.

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