Nanoscale imaging of quantum Hall edge currents in graphene and mapping the twist angle disorder in magic angle bilayer graphene
Speaker
Aviram Uri
Weizmann Institute
Abstract
The recently predicted topological magnetoelectric effect [1] is a fundamental attribute of topological states of matter with broken time reversal symmetry, and is the underlying mechanism of the quantum Hall effect. Using a scanning SQUID-on-tip [2] ultrasensitive nanoscale magnetometer, we directly image the equilibrium currents of individual quantum Hall edge states in graphene for the first time [3]. We reveal that the edge states, which are commonly assumed to carry only a chiral downstream current, in fact carry a pair of counter-propagating currents [4]. The topological downstream current in the incompressible region is counterbalanced by a heretofore unobserved non-topological upstream current flowing in the adjacent compressible region. We then apply the same technique to the recently discovered magic angle twisted bilayer graphene (MATBG). We attain tomographic imaging of the Landau levels [5] providing a sensitive probe of the local band structure and twist angle 𝜃. We map local 𝜃 _variations with relative precision better than 0.002° and spatial resolution of a few moiré periods. We find that devices exhibiting high-quality global MATBG features including superconductivity, display significant 𝜃 _variations with a span of ~0.1° and may even have substantial areas where no local MATBG behavior is detected, highlighting the importance of percolation physics. We reveal substantial gradients and a network of jumps in 𝜃. Twist angle gradients are shown to generate large gate-tunable electric fields that drastically change the quantum Hall state by forming edge states in the bulk of the sample. Twist angle disorder is shown to be a new type of disorder, fundamentally different from charge disorder.
1. X.-L. Qi, T. L. Hughes, and S.-C. Zhang, ''Topological field theory of time-reversal invariant insulators'', Phys. Rev. B 78, 195424 (2008).
2. D. Vasyukov, Y. Anahory, L. Embon, D. Halbertal, J. Cuppens, L. Neeman, A. Finkler, Y. Segev, Y. Myasoedov, M. L. Rappaport, M. E. Huber, and E. Zeldov, ''A scanning superconducting quantum interference device with single electron spin sensitivity'', Nat. Nanotechnol. 8, 639–644 (2013).
3. A. Uri, Y. Kim, K. Bagani, C. K. Lewandowski, S. Grover, N. Auerbach, E. O. Lachman, Y. Myasoedov, T. Taniguchi, K. Watanabe, J. Smet, and E. Zeldov, ''Nanoscale imaging of equilibrium quantum Hall edge currents and of the magnetic monopole response in graphene'', arXiv:1908.02466, Nat. Phys in press.
4. M. R. Geller and G. Vignale, ''Currents in the compressible and incompressible regions of the two-dimensional electron gas'', Phys. Rev. B 50, 11714–11722 (1994).
5. A. Uri, S. Grover, Y. Cao, J. A. Crosse, K. Bagani, D. Rodan-Legrain, Y. Myasoedov, K. Watanabe, T. Taniguchi, P. Moon, M. Koshino, P. Jarillo-Herrero, and E. Zeldov, ''Mapping the twist angle and unconventional Landau levels in magic angle graphene'', arXiv:1908.04595, under review in Nature.
Bio
Aviram received his undergraduate degree from Tel Aviv University and is currently a doctoral candidate in the group of Eli Zeldov at Weizmann Institute of Science, where he works on scanning nanoscale SQUID on a tip microscopy.
