Pseudo-Goldstone modes and order-by-disorder

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• Jeffrey G. Rau (University of Windsor)
• Thursday 11 May 2023, 14:00-15:00
• TCM Seminar Room.

If you have a question about this talk, please contact Bo Peng.

In systems with competing interactions, continuous degeneracies can appear which are accidental, in that they are not related to any symmetry of the Hamiltonian. Accordingly, the “pseudo”-Goldstone modes associated with these degeneracies are also unprotected. Indeed, through a process known as “order-by-disorder” fluctuations can lift the degeneracy and induce a gap for these modes. In this talk, I will discuss how these pseudo-Goldstone gaps arise in frustrated magnets when the fluctuations are (i) quantum mechanical and (ii) thermal in origin. I will show that the gap can be exactly computed at leading order using an effective picture where the soft modes move in a potential generated by the fluctuation-induced free energy. Using explicit interacting spin-wave calculations and direct numerical simulation, I will illustrate and validate this framework for several key models of order-by-disorder. Finally, I will discuss some implications for candidate materials exhibiting order-by-disorder.

This talk is part of the Theory of Condensed Matter series.

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