Bulletin of the American Physical Society
2024 APS March Meeting
Monday–Friday, March 4–8, 2024; Minneapolis & Virtual
Session K09: Moiré Physics in Bilayer Graphene 
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Sponsoring Units: DCMP Chair: Prabhakar Misra, Howard University Room: L100J 
Tuesday, March 5, 2024 3:00PM  3:12PM 
K09.00001: Abstract Withdrawn

Tuesday, March 5, 2024 3:12PM  3:24PM 
K09.00002: Mesoscopic transport in a Chern mosaic Sayak Bhattacharjee, Julian MayMann, Yves Hon H Kwan, Trithep Devakul We study transport properties of a Chern mosaic: a regular tiling of alternating Chern domains. We demonstrate the key role of edge networks towards transport properties of such systems and study a minimal model capturing the relevant physics. Our results are relevant to experiments on moire systems such as marginally twisted bilayer graphene (TBG), magic angle TBG aligned with hexagonal boron nitride, and helical trilayer graphene. 
Tuesday, March 5, 2024 3:24PM  3:36PM 
K09.00003: Intracellular charge density texture in twisted bilayer graphene away from the chiral limit Jungho Daniel Choi, Jie Wang, Jennifer Cano The continuum model of twisted bilayer graphene is known to possess perfectly flat, analytically solvable zero modes in the chiral limit, i.e. when electron hopping at AA stacking sites is set to vanish. We previously found that these flat bands yield intracellular textures in charge density that possess more nodes at higher magic angles. In particular, we note that there are nodes with real space positions dependent on Bloch momentum, and we now show that this explains the existence of a node on the AA stacking site at the Γ point for the first magic angle, which may have implications for scanning tunnelling microscopy experiments. We also present how these electron density patterns change when tuned away from magic angles and the chiral approximation, as these nodes are no longer protected as features of the exact zero mode wave functions. 
Tuesday, March 5, 2024 3:36PM  3:48PM 
K09.00004: First Landau level in moiré graphene system at zero magnetic field Manato Fujimoto, Patrick Ledwith, Daniel E Parker, Junkai Dong, Eslam Khalaf, Ashvin Vishwanath Chern insulators, which are the lattice analogues of the quantum Hall states, provide a promising platform to explore fractional Chern insulators with partial band fillings, offering a pathway to manipulate nonAbelian excitations. Moiré materials, such as twisted bilayer graphene and twisted transition metal dichalcogenides, are ideal platform to realize fractional Chern insulators because they can be tuned to host narrow Chern bands with "vortexable" single particle wavefunctions, generalizing those of the lowest Landau level. Fractional Chern insulators were recently observed at zero magnetic field in twisted MoTe2. A natural next step is nonAbelian states, such as the MooreRead Pfaffian state, which appear at half filling of the first Landau level. In this work, we provide a sharp definition of what first Landau level quantum geometry means when there is no external magnetic field, or magnetic translation symmetry. We find moiré graphene systems that realize this geometry, and compare with trial wavefunctions that have a manifest first Landau level character. 
Tuesday, March 5, 2024 3:48PM  4:00PM 
K09.00005: Quasicrystals in Twisted Graphene on Hexagonal Boron Nitride viewed through Scanning Tunneling Microscopy and Spectroscopy Xinyuan Lai, Daniele Guerci, Guohong Li, Kenji Watanabe, Takashi Taniguchi, Justin H Wilson, Jed H Pixley, Eva Y Andrei Twisted bilayer graphene (TBG) hosts moiré patterns with C3 rotational symmetry and a twistangle dependent superperiod. Aligning TBG with hexagonal Boron Nitride (hBN) breaks the C3 symmetry and introduces a new set of moiré patterns between the hBN and the adjacent graphene layer, GBN, that coexists with the TBG moiré pattern. Commensurate TBG and GBN moiré patterns which are known to produce nontrivial band topology giving rise to an anomalous quantum Hall effect, are difficult to realize as they require very precise alignment of the layers. Surprisingly, we find that commensurate TBG/GBN exist over a much wider than expected range of twist angles, indicating a strong tendency of the lattices to relax toward a commensurate state. In most cases however the two patterns are incommensurate resulting in a quasicrystal structure. Using scanning tunneling microscopy and spectroscopy we image the coexisting TBG/GBN moiré patterns simultaneously and study their electronic properties with spatially resolved local Density of States. We find that as a function of energy and magnetic field, the electronic states transit between crystal and quasicrystal structures. The latter exhibit unexpected symmetries and new periodicity emerging from the two sets of coexistent moiré patterns. Furthermore, the quasicrystal structures are accompanied by an unexpected emergence of flat electronic bands that reveal correlation induced gaps as the Fermi energy is swept through them. 
Tuesday, March 5, 2024 4:00PM  4:12PM 
K09.00006: Negative magnetoresistance in largeangle twisted bilayer graphene Xuetao Ma, Zhaoyu Liu, Kenji Watanabe, Takashi Taniguchi, David H Cobden, JiunHaw Chu, Matthew Yankowitz Twisting two sheets of graphene can lead to a stunning array of new electronic properties. Most famously, twisted bilayer graphene (tBLG) at the magic angle of 1.1° exhibits superconductivity, orbital magnetism, and topological insulating states. At larger twist angles, the Dirac cones from the two graphene sheets become electronically decoupled owing to their large separation in momentum space. Largeangle tBLG therefore behaves approximately like monolayer graphene, but with an extra twofold layer degeneracy that can be lifted by a perpendicular electric field. Here, we report the surprising observation of negative magnetoresistance (NMR) in such samples. We observe ubiquitous NMR when the two graphene layers are doped to the same carrier type, as well as at the charge neutrality point in the special case of zero displacement field. In the doped case, the NMR persists above room temperature. These observations contrast the typical transport properties of ultraclean monolayer or Bernal bilayer graphene, necessitating a new theoretical framework for understanding their origin. 
Tuesday, March 5, 2024 4:12PM  4:24PM 
K09.00007: Twist Angle Dependent Cyclotron Orbits in Twisted Bilayer Graphene Measured with Time Domain Terahertz Spectroscopy Benjamin Mead, Spenser Talkington, Seongjun Yang, CheolJoo Kim, Eugene J Mele, Liang Wu The electronic properties of twisted bilayer graphene (TBG) have been shown to sensitively depend on the twist angle. Controlling the twist angle is capable of producing electronic states that greatly differ from those of monolayer graphene. Despite significant research interest in TBG the twist angle dependence of electronic properties hasn't been rigorously measured. Using timedomain terahertz (THz) spectroscopy, we measured the Faraday rotation on CVDgrown graphene which was then stacked to make different twistangle bilayer samples. We model the observed THz spectrum with a Drude term describing free electron like motion and a lattice polarizability to determine the effective mass and scattering rate change from small to large twist angles. 
Tuesday, March 5, 2024 4:24PM  4:36PM 
K09.00008: Unique electronic states in biaxially heterostrained graphene bilayers moirés Florie Mesple, Niels R Walet, Guy Trambly de Laissardiere, Francisco Guinea, Djordje Dosenovic, Hanako Okuno, Colin Paillet, Adrien Michon, Claude Chapelier, Vincent T Renard Twistmoiré patterns have a demonstrated potential to allow engineering of bilayers of graphene to achieve many physical properties, by affecting the distribution of electrons and their interactions, as well as topological properties of the bands. One of the more novel approaches is to use heterostrain, the relative deformations between the layers, and the physical properties are particularly sensitive to that. We will show experimental evidence that heterostrain doesn't need the twist to express, but can induce new electronic states in aligned bilayers, through the formation of biaxial moirés. We will describe how the unique relaxation of these moirés form symmetrybreaking spiralling solitons that localize electronic states even more efficiently than twist moirés. This investigation is performed down to the atomic scale using Scanning Tunneling Microcopy and Spectroscopy. We also report detailed atomisticrelaxation and tightbinding calculations that include the exact stacking of the structure and permit to explain all the rich features of this moiré. Future works will surely benefit from including uniaxial and biaxial heterostrain to manipulate the electronic and topological states of graphene bilayers. 
Tuesday, March 5, 2024 4:36PM  4:48PM 
K09.00009: Tunable photon absorption and Rayleigh response in twisted bilayer graphene Disha Arora, Deepanshu Aggarwal, Sankalpa Ghosh, Rohit Narula We investigate the polarizationcontrolled absorption characteristics of twisted bilayer graphene (tBLG) subject to one and twophoton absorption. We find distinct peaks in onephoton absorption coefficient as a function of the excitation energy, corresponding to the van Hove singularities of tBLG. These peaks shift to higher energies as the rotation angle between the graphene layers is increased, even reaching the visible spectrum for higher twist angles. When subjected to two photons, introducing a twist between the two graphene layers enhances the magnitude of absorption coefficient α_{2} by ~ 6 orders of magnitude, which is more pronounced in the visible region for higher θ. We explore various polarization configurations for the two photons independently and determine the condition under which α_{2} becomes extremal. Additionally, we study the response of emitted photons through polarizationcontrolled Rayleigh scattering in tBLG. Compared to SLG, the integrated Rayleigh intensity (IRI) is strongly enhanced for small theta for parallel polarization (PP). For crosspolarization (CP), it exhibits a markedly complex behavior suggestive of strong interference effects mediated by the optical matrix elements. At small twist, the ratio R_{A}= IRI for PP/CP by ~1300 times viz a viz SLG. Measured as a function of the incoming energy, R_{A }exhibits a characteristic evolution as θ reduces, thus providing a unique fingerprint of the prevailing twist angle which would be interesting to verify experimentally. 
Tuesday, March 5, 2024 4:48PM  5:00PM 
K09.00010: Extended linearinT resistivity due to electronphason scattering in moiré superlattices Hector Ochoa, Rafael M Fernandes We show that scattering of electrons off of phason modes in incommensurate moiré superlattices, such as twisted bilayer graphene, can give rise to a large, linearinT resistivity down to very low temperatures. This mechanism contains features common to other two mechanisms usually invoked to explain linearinT resistivity: phonons and quantum critical fluctuations. Indeed, while phasons are similar to acoustic phonons, they result from a global invariance of the free energy that does not correspond to a microscopic symmetry of the Hamiltonian, hence their dynamics are not protected by a local conservation law. Consequently, phasons are generically overdamped at long wavelengths, like quantum critical fluctuations in metals. The associated transfer of spectral weight to low energies makes phason scattering a very efficient channel for entropy production at low temperatures. In particular, the resistivity remains linear down to a newly identified scale T* that can be lower than the BlochGrüneisen temperature, below which a quadraticinT behavior emerges. Phasons should also dominate other thermodynamic and transport properties at low temperatures, such as the specific heat and the thermal conductivity. 
Tuesday, March 5, 2024 5:00PM  5:12PM 
K09.00011: Structural relaxation effects on the lowenergy electronic structure of twisted bilayer graphene ChiRuei Pan, Martin Callsen, WeiEn Tseng, MeiYin Chou The structural relaxation and electronic band structure of the moire pattern formed by twisted bilayer graphene (TBG) with a wide range of twist angles are calculated by planewavebased and linearscaling localizedorbitbased firstprinciples methods. We provide the relaxation patterns for the interlayer separation and the inplane displacements (with respect to the ideal TBG structure) covering the whole moiré pattern. Our results show that as the twist angle becomes small, one single graphene sheet undergoes a gradual transition from a nearly flat to a corrugated layer due to different layer interactions in different regions of the moiré pattern. The inplane displacement exhibits a vortexlike pattern, and the displacement direction is reversed from the AA to AB stacking region. Overall, the atomic relaxation leads to the shrinkage of the AA stacking area in favor of the more energetically stable AB/BA stacking domains. For the electronic band structure, we find that the energy states at the Γ point above and below the four flat bands near the Fermi level split further away after structural relaxation. At larger twist angles, the splitting is mainly contributed by the outofplane relaxation. Approaching the magic angle, the inplane and outofplane relaxation have nearly equal contributions to the splitting. However, at the magic angle, the splitting actually becomes smaller for vertical relaxations. Our calculations emphasize the necessity of building a largescale moiré supercell to explore the full relaxation in each degree of freedom. 
Tuesday, March 5, 2024 5:12PM  5:24PM 
K09.00012: Filaments of critical states in quasiperiodic twisted bilayer graphene models Xinghai Zhang, Justin H Wilson, Matthew S Foster Twisted bilayer graphene (TBLG), hosting superconductivity and other quantum phases, is an ideal platform for studying the interplay of correlations, flat bands, and wave functions. The lowenergy physics of TBLG can be described by the BistritzerMacDonald (BM) model, wherein Dirac electrons tunnel between layers via a periodic matrix potential. Ignoring subleading AA interlayer tunneling, the chiral BM model takes the same form as Dirac surface states of class CI topological superconductors. The latter can exhibit spectrumwide quantum criticality (SWQC) in the presence of disorder [1]. Although TBLG is microscopically quasiperiodic at a generic twist angle, the BM model elegantly evades this complication. Nevertheless, quasiperiodicity can still arise from the boron nitride substrate. Moreover, twisted trilayer graphene is intrinsically quasiperiodic when mirror symmetry is broken, and superconductivity was recently discovered in a quasiperiodic trilayer system [2]. Here, we study the lowenergy effective theory incorporating two incommensurate BM matrix potentials. We find “filaments” of critical states in the plane of the two potentials. We will discuss this in the context of disorderinduced SWQC and its implications on the correlated phases in TBLG and other moiré materials. 
Tuesday, March 5, 2024 5:24PM  5:36PM 
K09.00013: Image minitwistedangle bilayer graphene by magneto scanning nearfield optical microscopy Wenjun Zheng, Ran Jing, Zijian Zhou, Mengkun Liu The application of magneto scanning nearfield optical microscopy (mSNOM) has yielded critical insights into the excitation of Dirac magnetoexcitons (DiME) on monolayer graphene, contingent upon the photon energy matching the Landau level at magnetic field [1]. The emergence of a moiré superlattice, a consequence of the angular overlap of two graphene layers, alters the electronic and plasmonic properties. In this talk, we image a twist bilayer graphene (TBG) with a mini twist angle (~0.5 degrees) with magnetic fields up to 7 Tesla. As the moiré superlattice matches the magnetic length, it exerts a profound modulation of DiME and the magneto nanophotocurrent. 
Tuesday, March 5, 2024 5:36PM  5:48PM 
K09.00014: Inducing Flat Chern Bands in Bilayer Graphene Exposed to a Superlattice Potential Invited Speaker: Daniel Seleznev It was recently argued that Bernal stacked bilayer graphene exposed to a 2D superlattice potential exhibits a variety of intriguing behaviors [1]. Chief among them is the appearance of flat Chern bands – with the possibility of Chern numbers C such that C>1 – that are favorable to the appearance of fractional Chern insulator states. Here, we explore extensions of the model of Ghorashi et al. [1] to find additional means of inducing flat Chern bands. First, we endow the superlattice potential with a tunable phase shift. The phase controls the locations and numbers of maxima and minima of the potential within the superlattice unit cell, while also allowing us to keep track of the changes in Chern numbers of the lowenergy bands. Finally, we study the effects of outofplane orbital magnetic fields featuring zero as well as single flux quanta in the superlattice unit cell. 
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