Detecting symmetry breaking in magic angle graphene using scanning tunneling microscopy

Abstract

A growing body of experimental work suggests that magic angle twisted bilayer graphene exhibits a "cascade" of spontaneous symmetry breaking transitions, sparking interest in the potential relationship between symmetry-breaking and superconductivity. However, it has proven difficult to find experimental probes which can unambiguously identify the nature of the symmetry breaking.Here we show how atomically-resolved scanning tunneling microscopy can be used as a fingerprint of symmetry breaking order. By analyzing the pattern of sublattice polarization and "Kekulé" distortions in small magnetic fields, order parameters for each of the most competitive symmetry-breaking states can be identified. In particular, we show that the "Kramers intervalley coherent state," which theoretical work predicts to be the ground state at even integer fillings, shows a Kekulé distortion which emerges only in a magnetic field.

Skyrmions in twisted bilayer graphene: stability, pairing, and crystallization

“…KD has been observed in the related context of QHFM within the lowest LL of monolayer graphene [103][104][105] , including the imaging of an individual valley skyrmion 104 . We caution that KD in TBG has also been predicted for IKS order in the presence of a small amount of strain 20,102 .…”

“…Measuring pseudospin textures at even integer filling is trickier since experimental techniques are not able to directly couple to the valley degree of freedom. IVC generally leads to √ 3 × √ 3 spatial order at the microscopic graphene scale, but the T -symmetry of the KIVC insulator means that it does not exhibit a Kekulé density distortion (KD) 101,102 . Paired skyrmions preserve T and hence do not give rise to KD 102 .…”

“…IVC generally leads to √ 3 × √ 3 spatial order at the microscopic graphene scale, but the T -symmetry of the KIVC insulator means that it does not exhibit a Kekulé density distortion (KD) 101,102 . Paired skyrmions preserve T and hence do not give rise to KD 102 . They may still leave a dipole-shaped fingerprint in sublattice polarization within regions where the state is locally in the valley Hall configuration, i.e.…”

Anderson's theorem for correlated insulating states in twisted bilayer graphene

“…A prime candidate for understanding the correlated insulating phases, which occur near even integer fillings of the moiré flat bands are Kramers intervalley coherent (K-IVC) states [18][19][20][21][22][23][24]. These states exhibit a pattern of magnetization currents, which triple the graphene unit cell, thereby breaking the lattice translation symmetry as well as time reversal.…”