Mapping the twist-angle disorder and unconventional Landau levels in magic angle graphene

by Mr. Aviram Uri

Weizmann Institute Of Science

at Condensed Matter Seminar

Mon, 23 Mar 2020, 11:30
Sacta-Rashi Building for Physics (54), room 207

Abstract

This seminar will be on Zoom *only* please see the details below.

The recently discovered flat electronic bands and strongly correlated and superconducting phases in magic angle twisted bilayer graphene (MATBG) [1,2] crucially depend on the interlayer twist angle. Although control of the global twist angle of about 0.1° has been demonstrated, little information is available on the distribution of the local twist angles. Utilizing a scanning nanoSQUID-on-tip [3], we attain tomographic imaging of the Landau levels in the quantum Hall state [4] and map the local twist angle variations in hBN encapsulated devices with relative precision better than 0.002° and spatial resolution of a few moiré periods. We find a correlation between the degree of disorder and the MATBG transport characteristics quality. However, even state-of-the-art devices, exhibiting correlated states, Landau fans, and superconductivity, display significant twist angle variations of up to 0.1° with substantial gradients and a network of jumps, and may contain areas with no local MATBG behavior, highlighting the importance of percolation physics. The correlated states are found to be particularly fragile with respect to twist-angle disorder. The twist angle gradients generate large gate-tunable in-plane electric fields, unscreened even in the metallic regions, which drastically alter the quantum Hall state by forming edge channels in the bulk of the sample and may significantly affect the phase diagram of the correlated and superconducting states. We establish twist angle disorder as a fundamentally new kind of disorder, calling for band structure engineering utilizing twist-angle gradients and gate-tunable built-in planar electric fields for novel correlated phenomena and applications.

1. Y. Cao, V. Fatemi, A. Demir, S. Fang, S. L. Tomarken, J. Y. Luo, J. D. Sanchez-Yamagishi, K. Watanabe, T. Taniguchi, E. Kaxiras, R. C. Ashoori, and P. Jarillo-Herrero, ''Correlated insulator behaviour at half-filling in magic-angle graphene superlattices'', Nature 556, 80–84 (2018).

2. Y. Cao, V. Fatemi, S. Fang, K. Watanabe, T. Taniguchi, E. Kaxiras, and P. Jarillo-Herrero, ''Unconventional superconductivity in magic-angle graphene superlattices'', Nature 556, 43–50 (2018).

3. 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).

4. 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'', Nat. Phys. (2019).

Dganit Meidan is inviting you to a scheduled Zoom meeting.

Topic: Condensed matter seminar - Aviram Uri
Time: Mar 23, 2020 11:30 AM Jerusalem

Join Zoom Meeting
https://zoom.us/j/628979544?pwd=Q0s4SStQQkxXRTJaWjN0Z2JsZEJhZz09

Meeting ID: 628 979 544
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Created on 20-02-2020 by Meidan, Dganit (dganit)
Updaded on 21-03-2020 by Meidan, Dganit (dganit)