Past Event

Hector Ochoa, Columbia University (Special Seminar)

February 19, 2021
9:30 AM - 10:30 AM

Title: "Carbon with a twist: electrons and phonons in moiré graphene bilayers"


Sliding and twisting the layers of Van der Waals materials give rise to superstructures with new physics, such as the plethora of strongly correlated phenomena (superconductivity included) observed in twisted bilayer graphene [1]. In this talk, I will review our current understanding of how electrons behave in this emergent superstructures and present my work on twisted bilayer graphene, where vibrational modes of the lattice strongly impact the electronics due to their geometrical amplification in the moiré beating pattern. I will argue that the long-wavelength dynamics of these generically incommensurate structures are dominated by new collective modes: phasons [2]. These modes correspond to coherent superpositions of optical phonons describing the sliding motion of stacking domain walls separating regions of partial interlayer commensuration. I will discuss various physical consequences of this observation, including the widespread presence of structural disorder in the form of strain accumulated between the layers. At neutrality, these tensions favor orbital magnetic order with charged vortex excitations [3], which may explain some of the disparities observed in transport measurements in different devices.

[1] Y. Cao et al., Nature 556, 43; ibid., 80 (2018).

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

[3] H. Ochoa, Phys. Rev. B 102, 201107 (2020).