Bulletin of the American Physical Society
APS March Meeting 2020
Volume 65, Number 1
Monday–Friday, March 2–6, 2020; Denver, Colorado
Session J67: Correlations and Topology in Moiré SuperlatticesInvited
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Sponsoring Units: DCMP Chair: Pablo Jarillo-Herrero, Massachusetts Institute of Technology MIT Room: Four Seasons 2-3 |
Tuesday, March 3, 2020 2:30PM - 3:06PM |
J67.00001: Correlated electronic states and Fermi surface topology in flat-band graphene layers Invited Speaker: Eva Andrei Stacking two-dimensional atomic crystals, or applying a spatially periodic potential can radically change their electronic properties. In particular, it is possible to engineer conditions leading to the creation of extremely narrow energy, flat bands, where the quenched kinetic energy facilitates the emergence of correlated electronic states. This talk will highlight two examples where the electronic ground state and Fermi surface topology depend sensitively on the filling of the flat bands: stacked graphene layers1,2 with a relative twist angle close to a magic value of ~ 1o, and buckled graphene layers in which a periodically modulated pseudo-magnetic field creates flat electronic bands. |
Tuesday, March 3, 2020 3:06PM - 3:42PM |
J67.00002: Magnetism and topology in graphene-based flat minibands Invited Speaker: David Goldhaber-Gordon Until recently, flat bands were achieved as Landau levels at high magnetic field. Recently, Pablo Jarillo-Herrero of MIT and coworkers demonstrated flat minibands in graphene-based superlattices, discovering correlated insulators and superconductors at different fillings. We have now discovered dramatic magnetic states in such systems. Specifically, in magic-angle twisted bilayer graphene also aligned with a hexagonal boron nitride (hBN) cladding layer, we observe a giant anomalous Hall effect and signs of chiral edge states. This all occurs at zero magnetic field, near 3 electrons per moire cell in the conduction miniband [1]. Remarkably, the magnetization of the sample can be reversed by applying a small DC current. Although the anomalous Hall resistance is not quantized, and dissipation is significant, we suggest that the system is an incipient Chern insulator, similar to an integer quantum Hall state. In a different superlattice system, ABC-trilayer graphene aligned with hBN, again near 3 electrons per moire cell a Chern insulator emerges [2]. A magnetic field of order 100 mT is needed to quantize the anomalous hall signal. This trilayer system can be tuned in-situ to display superconductivity instead of magnetism [3]. We will discuss possible magnetic states, and complementary probes to examine which state actually emerges as the ground state in each system. |
Tuesday, March 3, 2020 3:42PM - 4:18PM |
J67.00003: Orbital magnetism and topology in moiré heterostructure Invited Speaker: Andrea Young
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Tuesday, March 3, 2020 4:18PM - 4:54PM |
J67.00004: Mapping the twist-angle disorder and unconventional Landau levels in magic angle graphene Invited Speaker: Eli Zeldov The emergence of flat electronic bands and of the strongly correlated and superconducting phases in twisted bilayer graphene crucially depends on the interlayer twist angle upon approaching the magic angle 1.1°. Although advanced fabrication methods allow alignment of graphene layers with global twist angle control of about 0.1°, little information is currently available on the distribution of the local twist angles in actual magic angle graphene (MAG) transport devices. Here we map the local variations in hBN encapsulated devices with relative precision better than 0.002° and spatial resolution of a few moiré periods. Utilizing a scanning nanoSQUID-on-tip, we attain tomographic imaging of the Landau levels in the quantum Hall state in MAG, which provides a highly sensitive probe of the charge disorder and of the local band structure determined by the local [1]. We find a correlation between the degree of twist angle disorder and the quality of the MAG transport characteristics. However, even state-of-the-art transport devices, exhibiting pronounced global MAG features, such as multiple correlated insulator states, high-quality Landau fan diagrams, and superconductivity, display significant variations in the local with a span that can be close to 0.1°. Devices may even have substantial areas where no local MAG behavior is detected, yet still display global MAG characteristics in transport. The derived maps reveal substantial gradients and a network of jumps. We show that the twist angle gradients generate large unscreened electric fields that drastically change the quantum Hall state by forming edge states in the bulk of the sample, and may also significantly affect the phase diagram of correlated and superconducting states. The findings call for exploration of band structure engineering utilizing twist-angle gradients and gate-tunable built-in planar electric fields for novel correlated phenomena and applications. |
Tuesday, March 3, 2020 4:54PM - 5:30PM |
J67.00005: Theory of flat bands, correlated insulators and superconductivity in graphene Moire heterostructures Invited Speaker: Eslam Khalaf The remarkable discovery of correlated insulating behavior and superconductivity in twisted bilayer graphene (TBG) inspired a vast theoretical and experiment effort to (i) understand the origin of this behavior including the nature of symmetry breaking and the mechanism of the superconducitivity and (ii) find similar systems which share some of its interesting properties while differing in some details leading to a wider range of possible phases. The purpose of this talk is to present some recent works contributing to the progress on these two fronts. In regards to (i), we will present an exhaustive discussion of possible mean field symmetry broken states in TBG [1]. By highlighting the existence of a hidden approximate symmetry in the model, we will identify a manifold of low-energy states whose competition is settled by small symmetry breaking terms and present numerical results supporting this argument. Furthermore, we will discuss their possible experimental signatures and comment on their compatibility with different experiments. Regarding (ii), we will discuss a related Moire system, twisted double bilayer graphene (TDB), which was recently synthesized experimentally [2] and shown to exhibit correlated insulating behavior and spin-triplet superconductivity. We will show the results of a thorough analysis of its band structure, correlated insulating state and superconducting phase which compare favorably with experimental findings [3]. |
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