Flat bands and magic angles in twisted card-deck heterostructures

Flat bands and magic angles in twisted card-deck heterostructures
Alex Kruchkov, Harvard University
Picture of Alex Kruchkov
Date and time: Thu, Sep 26, 2019 - 11:30am
Refreshments at 11:15am
Location: Lederle 1033
Category: Condensed Matter Seminar
Abstract:

Van der Waals heterostructures remind a layered deck of cards which can be used for tricks if properly twisted. Recently, correlated insulation and unconventional superconductivity were observed in twisted bilayer graphene (TBG) at the so-called magic angle [1,2]. The magic angle is defined as a twist at which the electronic band structure of TBG reveals remarkably flat (dispersionless) electronic bands at charge neutrality. These bands can be further tuned towards absolutely flat, that reveal a remarkable mathematical connection to wave functions of the lowest Landau level in Quantum Hall Effect on torus [3]. In this ideal setting, the perfectly flat bands give rise to an infinite set of magic angles with robust periodicity. One natural extension of this model are flat bands and magic angles in twist-alternate graphene multilayers [4]. A peculiar example is twisted trilayer graphene, where the flat bands can coexist with Dirac cones [4,5]. In this talk, I will also address the twisted double bilayer graphene [6], for which the superconductivity and spin-polarized insulation were recently reported [7]. Finally, we discuss twisted transition metal dichalcogenides with interesting flat band physics in wide range of twist angles.
Refs:
[1] Cao, Fatemi, Fang, Watanabe, Taniguchi, Kaxiras, Jarillo-Herrero, “Unconventional
superconductivity in magic-angle graphene superlattices”, Nature 556, 43 (2018)
[2] ] Cao, Fatemi, Demir, Fang, Tomarken, Luo, Sanchez-Yamagishi et al, "Correlated Insulator
Behaviour at Half-Filling in Magic Angle Graphene Superlattices," Nature 556, 80 (2018)
[3] Tarnopolsky, Kruchkov, Vishwanath, “Origin of magic angles in twisted bilayer graphene”,
PRL, 122, 106405 (2019).
[4] Khalaf, Kruchkov, Tarnopolsky, Vishwanath, “Magic Angle Hierarchy in Twisted Graphene
Multilayers”, PRB 100, 085109 (2019).
[5] Carr, Li, Zhu, Kaxiras, Sachdev, Kruchkov, “Coexistence of ultraheavy and ultrarelativistic
Dirac quasiparticles in sandwiched graphene”, arXiv1907.00952.
[6] Haddadi, Wu, Kruchkov, Yazyev, “Moiré flat bands in twisted double bilayer graphene”,
arXiv: 1906.00623.
[7] Liu, Hao, Khalaf, Lee, Watanabe, Taniguchi, Vishwanath, Kim , “Spin-polarized Correlated
Insulator and Superconductor in Twisted Double Bilayer Graphene”, arXiv:1903.08130.