# Bulletin of the American Physical Society

# Mid-Atlantic Section Fall Meeting 2020

## Volume 65, Number 20

## Friday–Sunday, December 4–6, 2020; Virtual

## Session E05: Quantum Materials on Nanoscale II |
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Chair: Alexey Kuzmenko, University of Geneva |

Saturday, December 5, 2020 11:30AM - 12:06PM |
E05.00001: Moir\'{e} metrology of energy landscapes in van der Waals heterostructures Invited Speaker: Dorri Halbertal The emerging field of twistronics, which harnesses the twist angle between two-dimensional materials, has revolutionized quantum materials research. The twist angle induced superlattice offers means to control topology and strong correlations -- topics of great interest in contemporary quantum physics. At the small twist limit, and particularly under strain, as atomic relaxation prevails, the emergent moir\'{e} superlattice encodes elusive insights into the local interlayer interaction. In this work we introduce moir\'{e} metrology as a combined experiment-theory framework to probe the stacking energy landscape of bilayer structures at the 0.1 meV/atom scale, outperforming the gold-standard of quantum chemistry. Through studying the shapes of moir\'{e} domains with numerous nano-imaging techniques, and correlating with multi-scale modelling, we assess and refine first-principle models for the interlayer interaction. We document the prowess of moir\'{e} metrology for three representative twisted systems: bilayer graphene, double bilayer graphene and H-stacked MoSe$_{\mathrm{2}}$/WSe$_{\mathrm{2}}$. Moir\'{e} metrology establishes sought after experimental benchmarks for interlayer interaction, thus enabling accurate modelling of twisted multilayers. [Preview Abstract] |

Saturday, December 5, 2020 12:06PM - 12:42PM |
E05.00002: Controlling exotic Dirac cones with moire superlattices Invited Speaker: Jedediah Pixley We discuss the effects of a moire superlattice on two-dimensional Dirac cones on the surface of a topological insulator and in nodal superconductors. First, we will present our recent theory to describe twisting a single, anomalous Dirac cone on the surface of a three-dimensional topological insulator (3D TI). Distinct from twisted bilayer graphene, twisting the surface of a 3D TI cannot open miniband gaps, and instead satellite Dirac cones emerge that can greatly renormalize the low energy excitations. This renormalization of the surface Dirac cone produces a greatly enhanced surface density of states that can lead to correlated Hartree-Fock like instabilities on the surface of a 3D TI. We demonstrate the success of this theory by comparing to exact lattice model simulations and ab-initio calculations of a superlattice potential on the surface of Bi$_{\mathrm{2}}$Se$_{\mathrm{3}}$. Second, the theory of twisting 2D nodal superconductors will be presented. It is demonstrated that the Bogoliubov-De Gennes quasiparticle velocity can vanish at a magic-angle where correlated symmetry broken states (within the superconducting phase) emerge. By applying an interlayer current we demonstrate that the magic-angle gives rise to a topological superconductor with a quantized thermal Hall effect and gapless thermal currents on the boundary. The value of the magic-angle in a variety of putative nodal superconductors will be presented. [Preview Abstract] |

Saturday, December 5, 2020 12:42PM - 1:18PM |
E05.00003: Charge-Transfer Plasmon Polaritons at Graphene/$\alpha $-RuCl$_{\mathrm{3}}$ Interfaces Invited Speaker: Daniel Rizzo The fundamental opto-electronic properties of two-dimensional (2D) materials can be tailored based on their nanoscale charge environment. While electrostatic doping offers a means of wholesale tuning of 2D charge densities, the minimum size of charge features is limited by fields fringing through relatively thick gate insulators. Conversely, charge transfer at the interface of two atomically-thin layers with different work functions should not be subject to such limitations. Specifically, the large work function of $\alpha $-RuCl$_{\mathrm{3}}$ (6.1 eV) makes it an ideal 2D electron acceptor. In our study, we exploit this behavior to generate charge-transfer plasmon polaritons (CPPs) in graphene/$\alpha $-RuCl$_{\mathrm{3}}$ heterostructures. Using infrared near-field optical microscopy we measure the CPP dispersion, yielding a quantitative measure of the graphene Fermi energy (\textasciitilde 0.6 eV) and thus the charge exchanged between $\alpha $-RuCl$_{\mathrm{3}}$ and graphene (\textasciitilde 2.7x10$^{\mathrm{13}}$ cm$^{\mathrm{-2}})$. Concurrently, we observe dispersive edge modes and internal ``circular'' CPPs which reveal sharp (\textless 50 nm) changes in the graphene optical conductivity that correspond to nanoscale modulations in the graphene doping level. Further analysis of the CPP losses implies the presence of emergent optical conductivity in the doped interfacial layer of $\alpha $-RuCl$_{\mathrm{3}}$ and suggests that it no longer possesses a Mott insulating ground state. Our results demonstrate that using high work function materials such as $\alpha $-RuCl$_{\mathrm{3}}$ in Van der Waals heterostructures presents new opportunities for controlling the local charge carrier density of graphene and other 2D materials on nanometer length scales in excess of what can be achieved with an external gate. [Preview Abstract] |

Saturday, December 5, 2020 1:18PM - 1:30PM |
E05.00004: Modeling surface electronic profile in plasmonic excitations by an effective film approach Jiantao Kong Classical macroscopic Maxwell equations with conventional boundary conditions at material interfaces are not sufficient when describing surface plasmons with very short wavelengths. Especially, when size of the system approaches nanometer range or below, the non-abrupt surface electronic profile would be important or dominant. The spilled-out free electrons effectively form a film on the surface of metal. We show that this film could be properly modeled with a dielectric function [1], utilizing Feibelman's d-function formalism [2]. This method can be easily implemented in computational electrodynamics numerical packages, with quantum mechanical surface plasmonic effects covered, exact to first order. References: [1] Kong, Shvonski and Kempa, Physical Review B 97, 165423 [2] Feibelman, Progress in Surface Science 12, 287 [Preview Abstract] |

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