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 E42: Topological Phenomena in 2D Materials and Physics in Moire SystemsLive
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Sponsoring Units: DCMP Chair: Ryan Muzzio |
Tuesday, March 16, 2021 8:00AM - 8:12AM Live |
E42.00001: Navier-Stokes flows meet topological electronics Eddwi Hasdeo, Edvin Idrisov, Johan Ekström, Thomas L Schmidt Electrons in metals often flow ballistically like billiard balls colliding with impurities or phonons. However, in very clean samples and at intermediate temperatures, electrons are more likely to scatter with other electrons and can thus conserve their energy and momentum for long times. In this regime, electrons flow like water governed by the celebrated Navier-Stokes equation. There are numerous transport peculiarities observed in this regime such as negative non-local resistance, charge and heat transport decoupling and conductance higher than for ballistic transport. However, so far quantum effects seem rather absent in the collective dynamics. |
Tuesday, March 16, 2021 8:12AM - 8:24AM Live |
E42.00002: Fragile topological flat bands through adatom superlattice engineering on graphene Anastasiia Skurativska, Stepan Tsirkin, Fabian Donat Natterer, Titus Neupert, Mark Fischer Magic-angle twisted bilayer graphene has received a lot of interest due to its flat bands with potentially non-trivial topology that lead to intricate correlated phases. However, control over the fabrication and thus system parameters of such devices is limited. |
Tuesday, March 16, 2021 8:24AM - 8:36AM Live |
E42.00003: Genuine non-Abelian Berry phase in inhomogeneously strained moir\'e pattern Dawei Zhai, Wang Yao Periodicity of long wavelength moir\'e patterns is often destroyed by inhomogeneous strain introduced in fabrications of van der Waals layered structures. We present a framework to describe massive Dirac fermions in such distorted moir\'e patterns of transition metal dichalcogenides homobilayers, accounting for the dynamics of layer pseudospin [1]. In decoupled bilayers, we show two causes of in-plane layer pseudospin precession: By the coupling of layer antisymmetric strain to valley magnetic moment; and by the Aharonov-Bohm (AB) effect in the SU(2) gauge potential for the case of R-type bilayer under antisymmetric strain and H-type under symmetric strain. With interlayer coupling in the moir\'e, its interplay with strain manifests as a non-Abelian gauge field. We show a genuine non-Abelian AB effect in such field, where the evolution operators for different loops are noncommutative. This provides an exciting platform to explore non-Abelian gauge field effects on electron, with remarkable tunability of the field by strain and interlayer bias. |
Tuesday, March 16, 2021 8:36AM - 8:48AM Live |
E42.00004: Plasmon modes of topological-state networks in twisted bilayer graphene Brian Vermilyea, Michael Fogler We study surface plasmons in minimally twisted bilayer graphene that develops a triangular network of partial dislocations (or AB-BA domain walls) hosting one-dimensional electronic states. We formulate a theoretical model describing propagation of plasmons along one-dimensional links and their scattering at the nodes of such a network. Our model predicts plasmonic spectra composed of multiple bands quasi-periodic in frequency. Due to the long-range Coulomb interaction between neighboring links, the scattering amplitudes have an oscillatory dependence on the node geometry. Therefore, they can be controlled by an applied mechanical strain. We discuss optical nano-imaging experiments that can verify our predictions. |
Tuesday, March 16, 2021 8:48AM - 9:00AM Live |
E42.00005: Chirality in twisted bilayer graphene Tobias Stauber, José González, Tony Low, Guillermo Gomez Santos We derive a simple formula for the real-space chirality of twisted bilayer graphene and observe a sign-change due to an emergent C6-symmetry. Surprisingly, this change occurs at roughly the same twist angle for Fermi energies up to EF~75meV, see Figure 1. Our observation thus offers a new definition of the magic angle based on a macroscopic observable which is accessible in typical transport experiments [1]. We further predict an unprecedented large local chirality for the plasmonic near-field when sufficiently screened. Twisted bilayer graphene or other twisted two-dimensional heterostructure might thus provide a novel platform to catalyze the reaction of new chiral molecules while conserving time-reversal symmetry [2]. |
Tuesday, March 16, 2021 9:00AM - 9:12AM Live |
E42.00006: Symmetry breaking, strain solitons and mechanical edge modes in two dimensional crystals Joshua Chiel, Josephine Yu, Harsh Mathur, Onuttom Narayan The bistability of AB and BA stackings of bilayer graphene provides an unusual mechanical example of spontaneous symmetry breaking. We develop a continuum elasticity model in which the basic degree of freedom is the relative displacement of the two layers. Within our model, domain walls between regions of AB and BA stacking are solitons; we are able to give a quantitative description of these domain walls in agreement with experiments. Our model also predicts a zoo of point-like defects that are analogous to vortices and some of which have been observed in experiments. We also study the dynamics of the relative displacement field and find that the associated vibrational modes are gapped in the bulk but there are gapless modes that propagate along the domain walls. These modes are reminiscent of the edge electronic modes of topological insulators. For a simpler model that may be applicable to monolayer antimonene we are able to make this connection precise by showing that the mechanical model is the square of a Dirac Hamitonian and then drawing upon the well established topological classification of Dirac Hamiltonians. We surmise that low-scale symmetry breaking, domain walls and gapless edge modes are common features of two dimensional materials, not unique to bilayer graphene. |
Tuesday, March 16, 2021 9:12AM - 9:24AM Live |
E42.00007: Quantum Transport in Graphene-Based Heterojunctions Amir Taqieddin, N. R. Aluru Graphene nanoribbons (GNRs) with unique and tunable electronic properties are promising candidates for a variety of nanotechnology applications. Recent advances in bottom-up synthesis have enabled design of tailored 2D semiconducting graphene-based nanocircuits using 1D GNRs. In this work, we investigate the electronic structure and conductance of semiconducting strained 1D and 2D GNR heterojunctions with emergent topological states. We use first-principles density functional theory and non-equilibrium Green’s function calculations to compute the electronic band structure and conductance of the various heterojunctions. First, we show that the strength of the topological states in heterojunctions can be controlled by manipulating the band alignment of the connected GNRs through optimizing the lateral length and doping in the GNRs. Then, we investigate the current-voltage characteristics and the electronic conductance of the 1D and 2D structures under uniaxial mechanical strain. Our results show that higher current and conductance are obtained as an increasing compressive strain is applied at a given bias. The reported electronic characteristics of the strained GNR heterojunctions aim to facilitate development of efficient flexible nanoelectronics. |
Tuesday, March 16, 2021 9:24AM - 9:36AM Live |
E42.00008: Optical Properties of Twisted Bilayer Transition Metal Dichalcogenides from the Bethe-Salpeter Equation Miriam Scharnke, Martik Aghajanian, Valerio Vitale, Kemal Atalar, Arash A Mostofi, Johannes Lischner We present results for optical properties of twisted transition metal dichalcogenide homo- and heterobilayers, such as absorption spectra and exciton binding energies, and discuss their dependence on the chemical composition of the constituent monolayers and the twist angle between them. For this, we solve the Bethe-Salpeter equation based on wavefunctions and energies from tight-binding calculations that include up to sixth nearest neighbour hoppings. The screened electron-hole interaction is described by a Keldysh potential. |
Tuesday, March 16, 2021 9:36AM - 9:48AM Live |
E42.00009: Flat band topology of magic-angle graphene on a transition metal dichalcogenide Tianle Wang, Nick Bultinck, Michael Zaletel We consider twisted bilayer graphene on a transition metal dichalcogenide substrate, where proximity-induced spin-orbit coupling significantly alters the eight flat bands which occur near the magic angle. The resulting band structure features a pair of extremely flat bands across most of the mini-Brillouin zone. Further details depend sensitively on the symmetries of the heterostructure; we find semiconducting band structures when all two-fold rotations around in-plane axis are broken, and semi-metallic band structures otherwise. We calculate the Chern numbers of the different isolated bands, and identify the parameter regimes and filling factors where valley Chern insulators and topological insulators are realized. Interestingly, we find that for realistic values of the proximity-induced terms, it is possible to realize a topological insulator protected by time-reversal symmetry by doping two holes or two electrons per superlattice unit cell into the system. |
Tuesday, March 16, 2021 9:48AM - 10:00AM Live |
E42.00010: Orbital Chern insulator states in twisted monolayer-bilayer graphene and electrical switching of topological and magnetic order Hryhoriy Polshyn, Jihang Zhu, Manish Kumar, Yuxuan Zhang, Fangyuan Yang, Charles Tschirhart, Marec Serlin, Kenji Watanabe, Takashi Taniguchi, Allan MacDonald, Andrea Young We experimentally investigate narrow and topologically nontrivial moiré minibands hosted by van der Waals heterostructures consisting of a graphene monolayer rotationally faulted with respect to a Bernal-stacked bilayer. At fillings ν= 1 and 3 electrons per moiré unit cell within these bands, we observe quantized anomalous Hall effects with Rxy≈h/2e2, indicative of spontaneous polarization of the system into a single valley-projected band with Chern number C= 2. Remarkably, we also observe the evidence of symmetry broken Chern insulator states at ν= 1.5 and 3.5. At ν= 3 we find that the sign of the quantum anomalous Hall effect can be reversed via field-effect control of the chemical potential. This curious effect arises from the magnetization contribution due to topological edge states, which drive a reversal of the total magnetization and thus a switch of the favored magnetic state. Remarkably, we find that this switch is hysteretic, which we use to demonstrate non-volatile electric-field-induced reversal of the magnetic state. Voltage control of magnetic states can be used to electrically pattern nonvolatile magnetic domain structures hosting chiral edge states, with applications ranging from reconfigurable microwave circuit elements to ultra-low-power magnetic memory. |
Tuesday, March 16, 2021 10:00AM - 10:12AM Live |
E42.00011: Two-dimensional MX family of Dirac materials and quantum spin Hall insulators with tunable electronic and topological properties Yan-Fang Zhang, Jinbo Pan, Huta Banjade, Jie Yu, Hsin Lin, Arun Bansil, Shixuan Du, Qimin Yan A novel class of two dimensional MX (M=Be, Mg, Zn and Cd, X = Cl, Br and I) compounds with an unbalanced chemical formula was proposed by using data-driven method and first-principle calculations, exhibiting Dirac states with ultra-high Fermi velocities emerged from the interplay of incomplete charge transfer and hexagonal bipartite lattice. Intrinsically being time-reversal-invariant topological insulators when considering spin-orbital coupling, some members of this family offer exciting opportunities to host superconductivity in a highly tunable topological matrix. The electronic and topological properties are found to be highly tunable and amenable to modulation via anion-layer substitution and vertical electric field. The presence of sizable spin-orbital-coupling band gaps, ultra-high carrier mobilities, and small effective masses makes the MX family promising for electronics and spintronics applications. |
Tuesday, March 16, 2021 10:12AM - 10:24AM Live |
E42.00012: Deep Moiré Potentials in Twisted TMD heterobilayers Sara Shabani, Dorri Halbertal, Wenjing Wu, Mingxing Chen, Song Liu, James Hone, Wang Yao, Dmitri Basov, Xiaoyang Zhu, Abhay Narayan Twisted transition metal dichalcogenides (TMD) heterobilayers are a powerful platform to study interlayer excitonic states and charge confinement due to long wavelength periodic potentials produced by moiré patterns. Here we report the experimental observation of deep (>0.3 eV) moiré potentials in WSe2/MoSe2 heterobilayers using scanning tunneling microscopy (STM). We investigated the structural and electronic properties of nearly R-stacked (0 degree) and H-stacked (60 degrees) at small twist angles. Our spectroscopic measurements demonstrate a non-monotonic dependence of the moiré potential as a function of moiré wavelength in H-stacked heterobilayers. This non-monotonicity is directly linked to a drastic change in the structure of the moiré unit cell and the emergence of one-dimensional soliton domain walls at long moiré wavelength. We find that the magnitude of the moiré potential cannot be explained solely by interlayer hybridization, but is instead dominated by the three-dimensional structure and strain present within individual moiré unit cells. |
Tuesday, March 16, 2021 10:24AM - 10:36AM Live |
E42.00013: First-Principles Study on the Electronic Structure and Topological Properties of Heavy-Element-Decorated α-Graphdiyne Woochang Kim, Young Woo Choi, Hyoung Joon Choi Graphdiyne is a two-dimensional allotrope of carbon whose electronic structure resembles that of graphene. The atomic structure of graphdiyne offers a lot of possibilities for engineering its electronic structure through various kinds of functionalization. In this talk, we study the electronic structure and topological properties of heavy-element-decorated α-graphdiyne using first-principles density functional theory calculations. We obtain stable atomic structures with heavy elements adatoms while preserving the hexagonal crystal symmetry of the pristine graphdiyne and analyze the electronic structure of the system focusing on effects of the spin-orbit coupling. Then, we characterize the topologically nontrivial phase of bulk, and the robust edge states confirm this analysis. Our results suggest that heavy-element-decorated graphdiyne can offer an excellent platform for realizing topologically nontrivial phases in two dimensions. |
Tuesday, March 16, 2021 10:36AM - 10:48AM Live |
E42.00014: Electronic properties in twisted transition metal dichalcogenides in the large Moiré wavelength limit Qianhui Shi, Sara Shabani, Zhiying Wang, Wenjing Wu, Kevin WC Kwock, Xinyi Xu, Song Liu, Takashi Taniguchi, Kenji Watanabe, James Hone, Abhay Narayan, Cory Dean A flatband emerging from Moiré superlattices, first established in magic-angle twisted bilayer graphene, provides a platform for various correlated states. Twisted transition metal dicalcogendies - on the other hand - allows more tunability, as the Moiré flatbands can exist in a large range of twist angles, and the degeneracy can also be tuned by choosing different materials. Correlated electronic states have been studied in WSe2/WS2 heterobilayers and twisted bilayer WSe2, all with relatively small Moiré wavelengths. Here, we present studies on twisted WSe2/MoSe2 bilayers with Moiré wavelengths larger than 10 nm, which gives rise to an even smaller flatband width. We will discuss the observation of a continuum of insulating states and evidence for magnetism through capacitance and transport measurements. |
Tuesday, March 16, 2021 10:48AM - 11:00AM On Demand |
E42.00015: Chemical trends in homo- and hetero- twisted bilayer transition metal dichalcogenides from ab-initio tight-binding Valerio Vitale, Kemal Atalar, Arash A Mostofi, Johannes Lischner Recently, it has been shown that small-angle twisted bilayer transition metal dichalcogenides (tBL-TMDs) may exhibit rich phase diagrams as function of carrier density and temperature [1-5]. The emergence of competing electronic phases, including correlated insulating and superconducting phases, is attributed mainly to strong electron-electron interactions in the localized states associated with the flat bands near the Fermi level [6]. |
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