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1. Dirac points for twisted bilayer graphene with in-plane magnetic field

   https://doi.org/10.4171/JST/504  

   Becker, Simon; Zworski, Maciej (June 2024, Journal of Spectral Theory)

   We study Dirac points of the chiral model of twisted bilayer graphene (TBG) with constant in-plane magnetic field. The striking feature of the chiral model is the presence of perfectly flat bands atmagic anglesof twisting. The Dirac points for zero magnetic field and non-magic angles of twisting are fixed at high symmetry pointsKandK'in the Brillouin zone, with\Gammadenoting the remaining high symmetry point. For a fixed small constant in-plane magnetic field, we show that as the angle of twisting varies between magic angles, the Dirac points move betweenK,K'points and the\Gammapoint. In particular, near magic angles, the Dirac points are located near the\Gammapoint. For special directions of the magnetic field, we show that the Dirac points move, as the twisting angle varies, along straight lines and bifurcate orthogonally at distinguished points. At the bifurcation points, the linear dispersion relation of the merging Dirac points disappears and exhibit a quadratic band crossing point (QBCP). The results are illustrated by links to animations suggesting interesting additional structure. 

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2. Fine Structure of Flat Bands in a Chiral Model of Magic Angles

   https://doi.org/10.1007/s00023-024-01478-3  

   Becker, Simon; Humbert, Tristan; Zworski, Maciej (October 2024, Annales Henri Poincaré)

   Abstract We analyse symmetries of Bloch eigenfunctions at magic angles for the Tarnopolsky–Kruchkov–Vishwanath chiral model of the twisted bilayer graphene (TBG) following the framework introduced by Becker–Embree–Wittsten–Zworski. We show that vanishing of the first Bloch eigenvalue away from the Dirac points implies its vanishing at all momenta, that is, the existence of a flat band. We also show how the multiplicity of the flat band is related to the nodal set of the Bloch eigenfunctions. We conclude with two numerical observations about the structure of flat bands. 

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3. Interacting Twisted Bilayer Graphene with Systematic Modeling of Structural Relaxation

   https://doi.org/10.1088/2516-1075/adeb25  

   Kong, Tianyu; Watson, Alexander_B; Luskin, Mitchell; Stubbs, Kevin (July 2025, Electronic Structure)

   Abstract Twisted bilayer graphene (TBG) has drawn significant interest due to recent experiments which show that TBG can exhibit strongly correlated behavior such as the superconducting and correlated insulator phases. Much of the theoretical work on TBG has been based on analysis of the Bistritzer-MacDonald model which includes a phenomenological parameter to account for lattice relaxation. In this work, we use a newly developed continuum model which systematically accounts for the effects of structural relaxation. In particular, we model structural relaxation by coupling linear elasticity to a stacking energy that penalizes disregistry. We compare the impact of the two relaxation models on the corresponding many-body model by defining an interacting model projected to the flat bands. We perform tests at charge neutrality at both the Hartree-Fock and Coupled Cluster Singles and Doubles (CCSD) level of theory and find the systematic relaxation model gives quantitative differences from the simplified relaxation model. 

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4. Strong electron–phonon coupling in magic-angle twisted bilayer graphene

   https://doi.org/10.1038/s41586-024-08227-w  

   Chen, Cheng; Nuckolls, Kevin P; Ding, Shuhan; Miao, Wangqian; Wong, Dillon; Oh, Myungchul; Lee, Ryan L; He, Shanmei; Peng, Cheng; Pei, Ding; et al (December 2024, Nature)

   Abstract The unusual properties of superconductivity in magic-angle twisted bilayer graphene (MATBG) have sparked considerable research interest^1–13. However, despite the dedication of intensive experimental efforts and the proposal of several possible pairing mechanisms^14–24, the origin of its superconductivity remains elusive. Here, by utilizing angle-resolved photoemission spectroscopy with micrometre spatial resolution, we reveal flat-band replicas in superconducting MATBG, where MATBG is unaligned with its hexagonal boron nitride substrate¹¹. These replicas show uniform energy spacing, approximately 150 ± 15 meV apart, indicative of strong electron–boson coupling. Strikingly, these replicas are absent in non-superconducting twisted bilayer graphene (TBG) systems, either when MATBG is aligned to hexagonal boron nitride or when TBG deviates from the magic angle. Calculations suggest that the formation of these flat-band replicas in superconducting MATBG are attributed to the strong coupling between flat-band electrons and an optical phonon mode at the graphene K point, facilitated by intervalley scattering. These findings, although they do not necessarily put electron–phonon coupling as the main driving force for the superconductivity in MATBG, unravel the electronic structure inherent in superconducting MATBG, thereby providing crucial information for understanding the unusual electronic landscape from which its superconductivity is derived. 

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5. The BGOOD experiment at ELSA, exotic structures in the light quark sector?

   https://doi.org/10.1051/epjconf/202429101004  

   Jude, Thomas; Braghieri, Alessandro; Cole, Philip; Eisner, Daniel; Di_Salvo, Rachele; Fantini, Alessia; Figueiredo, Antonio_Joao Clara; Freyermuth, Oliver; Fromberger, Frank; Ghio, Francesco; et al (January 2024, EPJ Web of Conferences)

   Lalik, R (Ed.)

   The BGOOD photoproduction experiment accesses forward meson angles and low momentum exchange kinematics in theudssector, which may be sensitive to molecular-like hadron structure. Recent results are presented, including strangeness photoproduction at forward meson angles, andπ⁰π⁰co herent photoproduction off the deuteron. 

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