Bulletin of the American Physical Society
APS March Meeting 2023
Volume 68, Number 3
Las Vegas, Nevada (March 5-10)
Virtual (March 20-22); Time Zone: Pacific Time
Session JJ07: V: Complex Structured Materials, Including Graphene II |
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Sponsoring Units: DCMP Chair: Qingming Zhang, Lanzhou University/Institute of Physics Room: Virtual Room 7 |
Monday, March 20, 2023 3:00PM - 3:12PM |
JJ07.00001: A Simple Heavy Fermion Model for Twisted Bilayer Graphene in Flux Aaron Chew, Jonah Herzog-Arbeitman, Oskar Vafek, Andrei B Bernevig The flat bands and large Coulomb scale of twisted bilayer graphene allow the low energy degrees of freedom to be written in terms of heavy fermions hybridizing with conduction electrons [1]. This work puts forward a simplified generalization of this model in flux, allowing for an analytical description of twisted bilayer graphene in magnetic field. We compare our results with a strong coupling Hofstadter calculation at rational flux. Using the recently developed gauge invariant formalism [2], we are able to reach numerically extremely small fluxes, allowing us to compare the two methods and confirm their agreement. Due to the simple nature of the heavy fermion model, we are able to give an intuitive interpretation of the spectrum of TBG in flux and provide wavefunctions for the dominant, strongly correlated Landau levels. |
Monday, March 20, 2023 3:12PM - 3:24PM |
JJ07.00002: Spectroscopy of magic-angle twisted bilayer graphene via planar tunneling Zhenyuan Zhang, Shuang Wu, Kenji Watanabe, Takashi Taniguchi, Eva Y Andrei Magic-angle twisted bilayer graphene (MATBG) exhibits a variety of quantum phases, including correlated insulating states and superconductivity, that were revealed through transport, STM/STS and inverse compressibility measurements. Here we report on results obtained with an alternative technique — planar tunneling - in which the graphene sample and a graphite probe are separated by a few-layer hBN sheet acting as a tunneling barrier. The planar tunneling junction size is adjustable and can cover length scales down to several moire periods, that are not accessible to either transport or STM devices. The ultra-thin hBN tunneling layer guarantees highly stable and non-invasive tunneling measurements. By using the planar tunneling geometry on MATBG, we observed spectroscopic features of flat bands, a cascade of phase transitions at integer fillings, the emergence of Chern insulators, and Landau levels near the charge neutrality point. The ability of the planar tunneling geometry to combine access to both the density of states and transport signals in a single device, provides a new handle for gaining insight into the physics of the interplay between strong correlations and non-trivial topology responsible for the emergence of novel correlated quantum states in MATBG. |
Monday, March 20, 2023 3:24PM - 3:36PM |
JJ07.00003: Nematic skyrmion crystals in Twisted Double Bilayer Graphene systems DEBASMITA GIRI, Dibya K Mukherjee, Herb Fertig, Arijit Kundu Two-dimensional moire systems have recently emerged as a platform that can be used to study the interplay between topology and strong correlations. In particular, twisted double bilayer graphene (TDBG) is one such system whose topological properties can be changed in a controlled manner by tuning experimental parameters such as an applied displacement field. Spin-polarized correlated gaps have been previously observed in TDBG for half-filled conduction bands. In this work, we investigate the fate of the correlated state as we dope the system away from half-filling. We show that the added electrons form a nematic skyrmion crystal for some commensurate fillings of the conduction band. We report that the stability of the crystal state strongly depends on the localization of the Berry curvature in the moire Brillouin Zone. Lastly, we extend our analysis to higher Chern number cases where we find similar low-energy solutions. |
Monday, March 20, 2023 3:36PM - 3:48PM |
JJ07.00004: SNS junction in small angle twisted bilayer graphene Ritajit Kundu, Arijit Kundu Due to the competition between van der Waals interaction and the elastic energy of the interface of a minimally twisted bilayer graphene system undergoes a smooth atomic reconstruction and forms domains of AB and BA stacked regions that are separated by solitonic boundaries. The difference in valley Chern number between the two sides of the boundary results in two ballistic modes propagating on boundaries, which are further connected via AA-stacked regions. Such a system is best described by network models of the Chalker-Coddington type, where each propagating channel forms the network's edges, and the AA stacked regions, in this case, serve as the scattering nodes. The form of the scattering matrices in each node and the corresponding link matrices are constrained by the microscopic symmetries of the system. Based on such a scattering network approach, we study transport characteristics of a superconductor-normal-superconductor (SNS) junction, where a small twist angle twisted graphene bilayer is the normal region. |
Monday, March 20, 2023 3:48PM - 4:00PM |
JJ07.00005: Emergent charge order instabilities in a moiré superlattice at fractional band filling Ahmed Abouelkomsan, Kang Yang, Ipsita Mandal, Emil J Bergholtz Recent experiments have established the flat bands of ABC stacked trilayer graphene aligned with boron nitride as a platform for ferromagnetism and strong correlations. Depending on the direction of the gate-voltage, the topology of the flat bands manifested in their Chern numbers can be either trivial or non-trivial tuning the system between two distinct physical regimes. We provide evidence of possible charge order instabilities at fractional band fillings corresponding to non-integer number of electrons per moiré unit cell. Remarkably, the charge order can be understood as a nesting instability of an emergent Fermi surface from the purely interacting problem. The competition between the emergent Fermi liquids and the charge ordered states is found to be sensitive to the filling fraction and the direction of the gate-voltage. We further propose an effective action that describes the low energy physics in order to interpret the phases analytically. Our results provide an explicit example of strong-weak coupling dualities in a condensed matter system and sheds the light on the non trivial role of particle-hole asymmetry in moiré superlattices. |
Monday, March 20, 2023 4:00PM - 4:12PM |
JJ07.00006: Probing the Phonon Spectrum of Twisted Bilayer Graphene with Machine Learning Methods Dilara ickecan, Erdi Ata Bleda, Dogan Erbahar In recent years, there has been a growing interest in twisted bilayer structures, where two layers of single atom thick sheets are rotated with respect to each other around their normal axes, due to their unusual properties they exhibit contrary to their untwisted counterparts. The most significant development in this domain has been reported in 2018 where twisted bilayer graphene turned into a superconductor under a "magic angle" rotation. The nature of this odd behavior is still under debate; however, the traditional explanation of superconductivity relates the phenomenon to electron-phonon coupling so it is worth to study the properties of phonon spectrum of twisted structures as a function of rotation angle. Many other applications beyond superconductivity could possibly benefit from the understanding gained from these studies. While the emerging effects are most probably associated with moiré lattices-the new type of periodicity appearing in twisted structures; the relationship of the superlattice parameters to rotation angle is highly non-linear in nature and it is very challenging to build a model which will relate the effect of moiré lattices on vibrational properties. Unsurprisingly, very little work has been done in literature on this topic so far. In this work we use a novel approach to theoretically investigate the phonon dispersion of twisted bilayer graphene landscape combining molecular dynamic simulations with machine learning methods. |
Monday, March 20, 2023 4:12PM - 4:24PM |
JJ07.00007: Floquet engineering of energy bandgaps and titled states in 1T'-MoS2 using circularly polarized dressing field Godfrey Gumbs, Andrii Iurov, Liubov Zhemchuzhna, Danhong Huang, Kathy Blaise, Chinedu Ejiogu We have investigated the low-energy electronic states in 1T'-MoS2 in the presence of a high-frequency off-resonance dressing field with circular polarization. Our results include the derivation of a set of closed-form analytical expressions for the energy dispersion relations of the obtained dressed states, as well as an analysis of the modification of their bad gaps and tilting due to the presence of the external electromagnetic field. In particular, we have found that in contrast to all previously analyzed Dirac materials, the effect of circularly polarized light is not simply creating or an increase of the existing band gap. This type of dressing field is also known for a transition into a new electronic state with broken time-reversal symmetry and a finite Chern number. |
Monday, March 20, 2023 4:24PM - 4:36PM |
JJ07.00008: Heterostrain tuning of Moire superlattice in Bernal-stacked bilayer graphene Daniel Potemkin, Xu Du Moire superlattices formed by twisting two-dimensional crystals attract great research interest in recent years, exhibiting novel quantum transport properties and strongly correlated behaviors. Since the electronic properties of two-dimensional materials are sensitive to the lattice structure and symmetry, mechanical strain stands out as a promising tuning knob for the Moire superlattices. A particularly interesting approach is to create "Moire without a twist" through the heterostrain, where different strains are applied to the two atomic layers in a bi-layer stack, resulting in different lattice constants and hence Moire superlattice. In this work we explore the potential formation of a strain-tunable Moire superlattice by applying such heterostrain strain on a hBN encapsulated bilayer graphene field effect device. We characterize the gate-dependent charge transport properties while changing the heterostrain at low temperatures and in strong magnetic fields, which allows us to infer the formation of a Moire superlattice which modifies the band structure of intrinsic Bernal-stacked bilayer graphene. |
Monday, March 20, 2023 4:36PM - 4:48PM |
JJ07.00009: Behavior of Excited States in 2H and 3R Bilayer WSe2 Kathleen M McCreary, Madeleine Phillips, Hsun jen Chuang, Darshana Wickramaratne, Matthew R Rosenberger, C Stephen Hellberg, Berend T Jonker Transition metal dichalcogenide bilayers exhibit improved stability and higher carrier mobility compared to their monolayer counterparts, and may be attractive for a variety of applications. Both 2H and 3R bilayers are energetically stable and are expected to exhibit semiconducting behavior. However, 2H has received the bulk of attention due to its ready availability in the form of mechanically exfoliated flakes. Here, we detail the energies and temperature dependent behaviors of the ground and the first excited excitonic states in both 2H and 3R WSe2 bilayers. Samples are obtained through chemical vapor deposition, encapsulated with hBN, and reflectance contrast (RC) is measured to identify 1s and 2s excitonic states. At cryogenic temperatures, a splitting of approximately 17 meV is experimentally observed in both the 1s and 2s states of 3R bilayers. This splitting is consistent with our DFT calculations and is due to lack of inversion symmetry, with the two peaks corresponding to distinct excitonic transitions in the upper and lower layers of the 3R WSe2. As temperatures increase, excitonic states broaden and RC intensity decreases, preventing detection of 2s states above 100 K. The 1s state is evident at all temperatures between 4 K and 300 K, and splitting of the 1s state in 3R samples is detectable to approximately 250 K. This work provides much needed insight into bilayer systems and demonstrates that interlayer interactions are strong enough to significantly modify the optical properties in WSe2 samples. |
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