Synopsis: Turning one way

Molecular dynamics simulations unveil an example of dynamical symmetry breaking at the nanoscale.
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If you take a clamped string and pluck it within a plane, you would expect it to oscillate in that plane. But if the amplitude of the vibrating string is large enough, the plane in which it moves will actually start to rotate. This is an example of dynamical chiral symmetry breaking: even though the force and the string are left-right symmetric, the motion of the string develops a definite chirality in its sense of rotation.

Not merely the stuff of shaking strings, chiral symmetry breaking is of fundamental interest in particle physics and condensed matter. In a paper appearing in Physical Review Letters, Xiaohua Zhang, Giuseppe Santoro, Ugo Tartaglino, and Erio Tosatti at SISSA, the ICTP, and the Democritos National Laboratory in Trieste, Italy, have uncovered an example of dynamical symmetry breaking at the nanoscale. With molecular dynamics simulations, they show that when two coaxial nanotubes slide lengthwise at high velocities against each other, the whole system, if free to do so, would in certain conditions begin to rotate.

Friction between the sliding nanotubes, which is peaked at certain high velocities [1], excites phonon modes in the radial direction and couples the rotational and vertical motions of the coaxial nanotubes nonlinearly. Normally, the phonon modes carry angular momentum with as much right as left chirality, but under extreme frictional excitation one chirality prevails, and a net angular momentum is stirred in the process. Although the physics of dynamical symmetry breaking in the sliding nanotubes is similar to that of the oscillating string, Zhang et al. have identified several features that are unique to this nanoscale system. – Jessica Thomas

[1] P. Tangney, Phys. Rev. Lett. 97, 195901 (2005).


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