Bulletin of the American Physical Society
APS March Meeting 2021
Volume 66, Number 1
Monday–Friday, March 15–19, 2021; Virtual; Time Zone: Central Daylight Time, USA
Session E46: 2D Heterostructures I: Graphene Superlattices and Moire EffectsLive
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Sponsoring Units: DCMP Chair: Bheema Lingam Chittari, Univ of Seoul |
Tuesday, March 16, 2021 8:00AM - 8:12AM Live |
E46.00001: Long-range ballistic transport of Brown-Zak fermions in graphene superlattices Julien Barrier, Piranavan Kumaravadivel, Vladimir Falko, Andre Geim, Alexey Berdyugin In quantising magnetic fields, graphene superlattices exhibit a complex fractal spectrum often referred to as the Hofstadter butterfly. It can be viewed as a collection of Landau levels that arise from quantization of Brown-Zak minibands recurring at rational (p/q) fractions of the magnetic flux quantum per superlattice unit cell. We show that, in graphene-on-boron-nitride superlattices, Brown-Zak fermions can exhibit mobilities above 106 cm2/Vs and the mean free path exceeding several micrometres. The exceptional quality allows us to show that Brown-Zak minibands are 4q times degenerate and all the degeneracies (spin, valley and mini-valley) can be lifted by exchange interactions below 1K. We also found negative bend resistance for Brown-Zak fermions at 1/q fractions for electrical probes placed as far as several micrometres apart. The latter observation highlights the fact that Brown-Zak fermions are Bloch quasiparticles propagating in high magnetic fields along straight trajectories, just like electrons in zero field. In some parts of the Hofstadter spectrum, Landau levels exhibit nonlinear and staircase-like features that cannot be explained within a single-particle picture. |
Tuesday, March 16, 2021 8:12AM - 8:24AM Live |
E46.00002: Probing the electronic structure of a nanopatterned graphene device using nanoARPES Alfred Jones, Bjarke S. Jessen, Lene Gammelgaard, Deepnarayan Biswas, Roland Koch, Chris Jozwiak, Eli Rotenberg, Aaron Bostwick, Peter Bøggild, Antti-Pekka Jauho, Cory Dean, Søren Ulstrup The lithographic patterning of the 2D sheets of graphene potentially allows for control and manipulation of the already interesting electronic properties of 2D materials, for example by inducing a variable gap at the Dirac point of graphene. However, edge disorder has until recently proven to be a limiting factor in the widespread employment of this to create novel electronic states out of 2D materials [1]. |
Tuesday, March 16, 2021 8:24AM - 8:36AM Live |
E46.00003: Charge sensing of an unconventional ferroelectric metal in bilayer graphene Sergio de la Barrera, Zhiren Zheng, Qiong Ma, Zhen Bi, Ming-hao Liu, Nannan Mao, Yang Zhang, Natasha Kiper, Kenji Watanabe, Takashi Taniguchi, Jing Kong, William A. Tisdale, Nuh Gedik, Liang Fu, Suyang Xu, Pablo Jarillo-Herrero, Raymond Ashoori Moiré superlattices in twisted and lattice-mismatched van der Waals systems have recently led to a dazzling display of correlated phenomena including superconductivity, magnetism, and correlated insulating states. Here, we show that Bernal-stacked bilayer graphene in moiré superlattices with insulating boron nitride can result in a surprising electronically-driven ferroelectric state, exhibiting dramatic and unconventional hysteresis in transport and capacitive charge sensing. This ferroelectricity manifests as a robust and switchable electric polarization that coexists with metallicity, unexpected for this well-studied system. By measuring capacitances between the bilayer graphene and the top and bottom gates separately, we find large layer-asymmetric charge compressibility arising from an electronic phase transition. Moiré structures of Bernal-stacked bilayer graphene thus offer a new paradigm for exploring emergent electronic correlations and spontaneous symmetry breaking in an ultraclean two-dimensional system. |
Tuesday, March 16, 2021 8:36AM - 8:48AM Live |
E46.00004: Observation of flat bands in tunable semiconductor artificial graphene Ziyu Liu, Lingjie Du, Ken W. West, Saeed Fallahi, Loren Pfeiffer, Michael Manfra, Aron Pinczuk Flat bands near M points in the Brillouin zone are key features of honeycomb symmetry in artificial graphene (AG) where electrons may condense into novel correlated phases. Here we report the realization of flat bands of AG in GaAs quantum well transistors where the electron density is tuned by applied back-gate voltages. Two new peaks in the low-temperature optical emission spectra are linked to the van-Hove singularity doublet of a pair of flat bands near M-points of AG. As the Fermi level is tuned to overlap the optical emission doublet, PL spectra display dramatic stability against changes in electron density that indicate interplays between key electron-electron interactions and the lattice with honeycomb symmetry [1]. |
Tuesday, March 16, 2021 8:48AM - 9:00AM Live |
E46.00005: Photonic crystal for graphene plasmons Lin Xiong, Carlos Forsythe, Minwoo Jung, Alexander S McLeod, Yutao Li, Shuai Zhang, Yinan Dong, Song Liu, Michael Fogler, James Edgar, Gennady Shvets, Cory Dean, Dimitri N Basov Graphene surface plasmon polaritons (SPPs) are hybrid excitations of electrons and photons which can be controlled by the optical properties of graphene. Periodically varying the optical properties results in a photonic crystal for graphene SPPs. Here we utilize cryogenic near-field optical microscopy to study the band structure induced by a graphene photonic crystal consisting of a high-mobility graphene atop a patterned SiO2 dielectric layer [Xiong, Nat. Commun. 10: 4780 (2019)]. Gating through the dielectric provides a periodic field effect that spatially modulates local carrier densities [Forsythe, Nat. Nanotech. 13, 566–571 (2018)] and the propagation of plasmon polaritons through the graphene. A full plasmonic bandgap and characteristic SPP propagation properties are revealed. Selective engineering of domain wall in the middle of the photonic crystal produces localized SPP modes propagating strictly along the domain wall. These findings signify a new route towards designer-engineered band-structures to route and manipulate highly confined plasmons within high mobility graphene devices. |
Tuesday, March 16, 2021 9:00AM - 9:12AM Live |
E46.00006: Topological flat bands in tetralayer graphene on boron nitride moire superlattices Youngju Park, Bheema Lingam Chittari, Jeil Jung We show that rhombohedral four-layer graphene (4LG) nearly aligned with a hexagonal boron nitride (hBN) substrate has nearly flat low energy bands with generally non-zero valley Chern numbers. These bands are isolated even in the absence of a perpendicular electric field thanks to the opening of a primary bandgap at charge neutrality and secondary gaps near the moire Brillouin zone corners. The bandwidths are controllable through an electric field and they can become as narrow as ∼5meV when the interlayer potential differences between top and bottom layers amount up to |Δ|≈40meV. The local density of states (LDOS) analysis shows that the nearly flat band states wave functions are associated to the non-dimer low energy sublattice sites at the top or bottom layer graphene and their degree of localization is strongly gate tunable. Similar behaviors are seen in nLG/BN for n=5−8 where generally the valley Chern number of the first valence band of nLG/BN is equal to the number of graphene layers, Cν=±1 =±n. |
Tuesday, March 16, 2021 9:12AM - 9:24AM Live |
E46.00007: Topological phases in N-layer ABC-graphene boron-nitride moire superlattices David Galeano, Bheema Lingam Chittari, Youngju Park, Jin-Hua Sun, Jeil Jung Rhombohedral N = 3 trilayer graphene on hexagonal boron nitride (TG/BN) hosts gate tunable valley contrasting topological nearly flat bands that could trigger spontaneous quantum Hall phases under appropriate conditions of the valley and spin polarization. Recent experiments have reported signatures of Cν = 2 valley Chern bands at 1/4 hole filling in contrast to the predicted value of Cν = 3 when calculated within non-interacting single particle models. We discuss the low energy models for rhombohedral N-layer graphene (N=1,2,3) aligned with hexagonal boron nitride (hBN) under the influence of off-diagonal pseudo magnetic vector potential terms that alter the 120° rotational symmetry and leads to valley Chern numbers |Cν | ≤ N. Our analysis suggests that nematic order with broken rotational symmetry can lead to valley Chern bands in keeping with recent Hall conductivity observations. |
Tuesday, March 16, 2021 9:24AM - 9:36AM Live |
E46.00008: Strain tunable magnetotransport study of graphene/h-BN heterostructures Chuankun Liu, Ryuichi Tsuchikawa, Jameson G Berg, Vikram V Deshpande Graphene is a single atomic layer of carbon atoms in a hexagonal lattice, which has attracted plenty of research attention since its discovery. Graphene with different layers hosts unique varieties of electronic band structures. In the monolayer graphene, it hosts a relativistic Dirac spectrum with linear energy-momentum dispersion. The energy dispersion of ABA trilayer graphene is a combination of linear dispersion of monolayer graphene and quadratic dispersion of bilayer graphene. On the other hand, the energy dispersion of ABC trilayer graphene is a very flat cubic relation. When twist graphene and graphene or graphene and h-BN by a small angle, a long wave-length moiré superlattice results. New strongly correlated physics phenomena show up at specific angles. Besides the perpendicular electric field, strain is another way to modify the band structures. In our work, we have fabricated graphene/h-BN heterostructures with graphite gate on flexible substrates to apply both perpendicular electric field and intralayer strain onto the device. We study the preliminary responses of low energy bands by performing magnetotransport measurements at low temperatures. |
Tuesday, March 16, 2021 9:36AM - 9:48AM Live |
E46.00009: Transparent mirror effect in twist-angle-disordered bilayer graphene Sandeep Joy, Saad Khalid, Brian Skinner Motivated by recent observations of spatial fluctuations of twist angle in twisted bilayer graphene (TBG) samples, we investigate the effect of such twist angle fluctuations on the transport of Dirac electrons. Variations in twist angle produce spatial modulation of the Fermi velocity as well as a random gauge field. We consider a quasi-one-dimensional model of the disorder, which has a direct analogy with the propagation of light in a medium with a random refractive index. In such situations, Anderson localization of light leads to an exponential decay of transmitted intensity, known as the "transparent mirror effect." In the case of Dirac electrons, however, we show that the localization length depends strongly on the angle of incidence and diverges at a nonzero "Brewster angle". This divergence leads to a power-law decay of the transmission when averaged over incidence angles. Our results have direct implications for the conductivity and the Fano factor of TBG samples. They also suggest a mechanism for disorder-induced collimation, valley filtration, and energy filtration of Dirac electron beams, so that TBG offers a promising new platform for Dirac fermion optics. |
Tuesday, March 16, 2021 9:48AM - 10:00AM Live |
E46.00010: Fragile Insulator and Electronic Nematicity in a Graphene Moiré System* Lei Chen, Haoyu Hu, Qimiao Si Graphene heterostructures, which can host narrow moiré electron bands that amplify the correlation effect, represent a new setting to study the organizing principles of strongly correlated systems. In such correlated moiré systems, an insulating state is a prominent feature of the phase diagram which may hold the key to understanding the basic physics. Here [1] we advance the notion of a fragile insulator, a correlation-driven insulating state that is on the verge of a delocalization transition into a bad metal. Using a realistic multiorbital Hubbard model as a prototype for narrow band moiré systems, we realize such a fragile insulator and demonstrate a nematic order in this state as well as in the nearby bad metal regime. Our results provide a natural understanding of the observed electronic anisotropy [2] as well as the behavior when the insulator is tuned into a bad metal. We propose the fragile insulator and the accompanying bad metal as competing states at integer fillings that analogously anchor the overall phase diagram of the correlated moiré systems and beyond. |
Tuesday, March 16, 2021 10:00AM - 10:12AM Live |
E46.00011: Photocurrent spectroscopy study of the gap opening in graphene/hBN moiré superlattice Tianyi Han, Lei Wang, Kenji Watanabe, Takashi Taniguchi, Long Ju Moiré superlattice composed of van der Waals materials has recently been explored as a promising platform for the study and engineering of electron correlation and topological phenomena. Graphene/hBN moiré superlattice, one of the simplest two-dimensional moiré systems, serves as a great example to understand the effect of moiré potential. In this system, the semi-metallic graphene will open up a gap at the charge neutrality point because of the symmetry breaking induced by the moiré superlattice, which has been extensively investigated by transport measurement previously. Here, I will present our study of the gap opening in graphene/hBN moiré system by photocurrent spectroscopy. Several spectroscopic approaches to accurately determine the gap may refresh our understanding of the gap opening, and give us more insights on the various exotic phenomena in the moiré systems. |
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