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“Phase” fluctuations of moiré superlattices

Event Type
Department of Physics
190 ESB
Jun 20, 2022 - Jun 21, 2022   10:00 am   5:00 pm
Hector Ochoa De Eguileor Romillo
Marjorie Gamel
Originating Calendar
Physics - Departmental Events

In this talk I will argue that the long-wavelength lattice dynamics of moiré patterns is more akin to an amorphous solid than to a crystal. The reason is the existence of soft collective modes, called phasons, associated with the sliding motion of one layer with respect to the other [1]. At finite temperature this motion necessarily involves friction, microscopically, due to anharmonic coupling with optical phonons. Consequently, phasons are overdamped at low frequencies, reflecting that in this regime the moiré pattern relaxes via internal diffusive processes rather than by collective oscillations. This reflects, more broadly, that the "acoustic phonons" of the moiré superlattice are not protected by a conservation law [2]. I will discuss physical consequences of this observation for twisted bilayer graphene, including the widespread presence of “twist angle disorder”. This is governed by the collective pinning of stacking domain walls caused by lattice relaxation. The phason spectrum acquires then a gap, which displays a universal dependence on the twist-angle variance.


[1] H. Ochoa, Phys. Rev. B 100, 155426 (2019).

[2] H. Ochoa and R. M. Fernandes, Phys. Rev. Lett. 128, 065901 (2022).


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