**January 19 2011**

**New article: Phys. Rev. Lett. 106, 010401**

Quantum Zigzag Transition in Ion Chains

Efrat Shimshoni, Giovanna Morigi and Shmuel Fishman.

Phys. Rev. Lett. 106, 010401 (2011).

A string of trapped ions at zero temperature exhibits a structural phase transition to a zigzag structure, tuned by reducing the transverse trap potential or the interparticle distance. The transition is driven by transverse, short wavelength vibrational modes. We argue that this is a quantum phase transition, which can be experimentally realized and probed. Indeed, by means of a mapping to the Ising model in a transverse field, we estimate the quantum critical point in terms of the system parameters, and find a finite, measurable deviation from the critical point predicted by the classical theory. A measurement procedure is suggested which can probe the effects of quantum fluctuations at criticality. These results can be extended to describe the transverse instability of ultracold polar molecules in a one-dimensional optical lattice.