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 N40: 2D Moiré Materials: Electronic and Correlated PhenomenaFocus
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Sponsoring Units: DMP Chair: Ying Wang, University of Wisconsin - Madison; You Zhou, University of Maryland College Park Room: Room 232 |
Wednesday, March 8, 2023 11:30AM - 12:06PM |
N40.00001: Coherent Spin-Lattice Coupling in van der Waals Magnetic Crystals Invited Speaker: Prashant Padmanabhan The enormous potential of van der Waals (vdW) magnets in nanoscale information storage and spintronic technologies has stimulated a great deal of research into their equilibrium magnetic and structural properties. Nevertheless, comparatively little is known about their behavior under non-equilibrium conditions or the nature of magneto-structural coupling in this material class on its intrinsic ultrafast timescale. This talk will discuss our recent studies of non-equilibrium lattice and spin dynamics in vdW magnets using ultrafast spectroscopy. We will begin with CrI3, where we observe strong light-driven coherent lattice oscillations, commensurate with phonon modes of A1g symmetry. Intriguingly, the amplitude of the c-axis A1g mode shows a strong pump helicity dependence below the magnetic ordering temperature. The correlation between magnetic order and vibrational amplitude of this mode is indicative of a strong coherent coupling between the spin and lattice degrees of freedom. Our ab initio and dynamic simulations suggest that this coupling originates from the modulation of exchange interactions through the distortion of the lattice. We will then discuss our recent results involving spin-phonon coupling in exfoliated flakes of Mn3Si2Te6. Here, we observe a sudden temperature-dependent shift in the frequency of a coherent phonon mode, excited via impulsive stimulated Raman scattering, below the ferrimagnetic ordering temperature. This suggests that long-range spin order can significantly stiffen the crystal lattice through an analogous exchange-mediated mechanism. Our work highlights the strong interplay between the lattice and spin structure in layered magnetic materials, providing crucial insight for the realization of the next generation of high-speed nanoscale magneto-optic technologies. |
Wednesday, March 8, 2023 12:06PM - 12:18PM |
N40.00002: A Quantum Ruler for Topology and Quantum Geometry in Moiré Superlattices: Part 1/3 Paul Haney, Marlou R Slot, Yulia Maximenko, Sungmin Kim, Daniel T Walkup, En-Min Shih, Dilek Yildiz, Kenji Watanabe, Takashi Taniguchi, Yafis Barlas, Nikolai Zhitenev, Fereshte Ghahari, Joseph A Stroscio A material's response to magnetic fields provides a wealth of information about its properties, from the shape of its Fermi surface to detailed information about the topological properties of its electronic structure. Moiré materials offer a particularly interesting case study in this response, owing to their large lattice constant. We theoretically analyze the response of twisted double bilayer graphene to magnetic fields using recently developed formalism, which includes higher order corrections in magnetic field. The first-order correction is derived from the gate-tunable valley-contrasting Berry curvature. The second-order correction is governed by the magnetic susceptibility, which is shown to play in important role in this system. Our calculations reveal the role of the moiré superlattice in these strong higher-order responses. |
Wednesday, March 8, 2023 12:18PM - 12:30PM |
N40.00003: A Quantum Ruler for Topology and Quantum Geometry in Moiré Superlattices: Part 2/3 Yulia Maximenko, Marlou R Slot, Paul Haney, Sungmin Kim, Daniel T Walkup, En-Min Shih, Dilek Yildiz, Kenji Watanabe, Takashi Taniguchi, Yafis Barlas, Nikolai Zhitenev, Fereshte Ghahari Kermani, Joseph A Stroscio Flat and narrow band physics in moiré quantum matter (MQM) has proven to be extremely rich with new emergent quantum phases. The topological properties of the eigenstates of the moiré Hamiltonian are critical for establishing the quantum phase of the system. While the emergence of non-trivial Chern numbers has been observed, it is important to characterize the quantum geometry in detail including Berry curvature and less known quantum metric effects throughout the bands. Using a local probe, we employ magnetic oscillations as a “ruler” for quantum geometry in small-angle twisted double bilayer graphene (TDBG). Part 2: We perform gate-tuned scanning tunneling spectroscopy of the narrow moiré minibands in TDBG in magnetic fields up to B = 15 T to fully map the bands with varying displacement fields. The high-resolution Landau level spectra reveal tunable electron- and hole-like pockets that deviate significantly in their magnetic response from the semiclassical model. We demonstrate that these effects result from the quantum geometry in MQM and are specific to the typical MQM superlattice length scale. |
Wednesday, March 8, 2023 12:30PM - 12:42PM |
N40.00004: A Quantum Ruler for Topology and Quantum Geometry in Moiré Superlattices: Part 3/3 Marlou R Slot, Yulia Maximenko, Paul Haney, Sungmin Kim, Daniel T Walkup, En-Min Shih, Dilek Yildiz, Kenji Watanabe, Takashi Taniguchi, Yafis Barlas, Nikolai Zhitenev, Fereshte Ghahari Kermani, Joseph A Stroscio Flat and narrow band physics in moiré quantum matter (MQM) has proven to be extremely rich with new emergent quantum phases. The topological properties of the eigenstates of the moiré Hamiltonian are critical for establishing the quantum phase of the system. While the emergence of non-trivial Chern numbers has been observed, it is important to characterize the quantum geometry in detail including Berry curvature and less known quantum metric effects throughout the bands. Using a local probe, we employ magnetic oscillations as a “ruler” for quantum geometry in small-angle twisted double bilayer graphene (TDBG). Part 3: The experimentally observed magnetic response in TDBG deviates strongly from the semiclassical Onsager relation. We use the expanded Onsager relation to capture the quantum geometric effects. The first-order correction in B, interpreted as orbital magnetic moment, manifests as valley splitting of Landau levels. The second-order correction—orbital magnetic susceptibility—is anomalously large and exceeds the first order for certain displacement fields. We show that this breakdown of the original Onsager relation is unique to the superlattice constants typical for MQM. |
Wednesday, March 8, 2023 12:42PM - 12:54PM |
N40.00005: Electron-phonon coupling in twisted bilayer transition metal dichalcogenides using a hybrid classical/quantum mechanical approach Kemal Atalar, Shinjan Mandal, Manish Jain, Johannes C Lischner, Arash A Mostofi Incorporating the rotational degree of freedom into 2D heterostructures brought about a plethora of emergent physics including unconventional transport and superconductivity. To engineer twisted bilayer devices with desirable transport properties, studying the twist angle and layer dependence of electron-phonon coupling (EPC) is crucial. EPC in twisted bilayers of transition metal dichalcogenides (TMDs), where tentative signatures of superconductivity [1] are observed, remains largely unexplored. |
Wednesday, March 8, 2023 12:54PM - 1:06PM |
N40.00006: Corrugation-induced electronic effects in twisted bilayer graphene Tawfiqur Rakib, Pascal Pochet, Elif Ertekin, Harley T Johnson Relative rotation between the layers of bilayer graphene creates a moiré superlattice with a periodicity that is inversely related to the interlayer twist angle. The interlayer twist angle significantly alters the electronic band structure of the twisted bilayer graphene (tBLG), forming extremely flat bands near the Fermi level at a “magic-angle” of 1.08°. However, the electronic properties in tBLG are extremely sensitive to small structural deformation. Flat bands are observed in rigid tBLG without any lattice relaxation, but this fails to explain the appearance of pseudogap states and spectral weight transfer typically observed in magic-angle tBLG experiments. Here, we show that a large amplitude corrugation, or out-of-plane deformation mode, is thermodynamically stable in low twist angle tBLG due to the competition between in-plane elastic stiffness and out-of-plane bending stiffness. The large corrugation in tBLG alters the band structure, leading to partially filled states and broken symmetry in a narrow range of twist angles near the magic-angle. Our analysis also demonstrates spectral weight transfer in the charge distribution, accompanied by pseudogap states in the magic-angle tBLG with large amplitude corrugation. The large corrugation is consistent with the observed vibrational spectra of tBLG as a function of twist angle. Our observation of large corrugation provides an exciting platform to explore deformation-induced correlated physics in tBLG. |
Wednesday, March 8, 2023 1:06PM - 1:18PM |
N40.00007: Twist angle engineering of proximity exchange in graphene/Cr2Ge2Te6 bilayers Jaroslav Fabian, Klaus Zollner
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Wednesday, March 8, 2023 1:18PM - 1:30PM |
N40.00008: Direct observation of Γ moiré bands in twisted bilayer WSe2 Gianmarco Gatti, Louk Rademaker, Anna Tamai, Tobias A de Jong, Felix Baumberger Recent transport experiments revealed a correlated insulating phase and quantum criticality points in twisted transition metal dichalcogenides (TMDs) that were predicted to host flat moire´ mini-bands. Despite these exciting experimental observations and theoretical predictions, no direct measurements of the band structure of twisted TMD are available to date. We report here for the first time on the direct observation of flat band in twisted TMDs investigating 57° twisted bilayer WSe2 by micro-focused angle-resolved photoemission spectroscopy. We resolve multiple moire´ mini-bands with strongly reduced dispersion and significant mini-gaps. By comparison with effective continuum band structure models, we attribute the origin of the flat states to a moderate moire´ potential of ≈ 50 meV emerging from the stacking of the two semiconducting layers. Our results establish a reference for future theoretical and experimental studies of the moire´ physics in twisted TMDs.
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Wednesday, March 8, 2023 1:30PM - 1:42PM |
N40.00009: One-dimensional moiré physics and chemistry in heterostrained bilayer graphene Gabriel R Schleder, Michele Pizzochero, Efthimios Kaxiras Moiré physics in bilayer graphene has rapidly emerged as a promising platform to investigate correlated electronic phases. However, the formation of interlayer moiré patterns has thus far been restricted to the introduction of twist angles. Here, using classical potentials and first-principles calculations, we propose heterostrained bilayer graphene as a novel platform to access twist-angle-free moiré physics. First, we demonstrate that heterostrain of appropriate strength can lead to the formation of flat bands in bilayer graphene. Next, we show that the heterostrain-driven lattice relaxation leads to a spatially modulated reactivity towards hydrogenation, establishing the basis for moiré-driven chemistry. Our results can be readily generalized to other layered materials such as transition metal dichalcogenides. |
Wednesday, March 8, 2023 1:42PM - 1:54PM |
N40.00010: Engineering Topology in Massive Dirac Fermions Nishchay Suri, Chong Wang, Benjamin M Hunt, Di Xiao We engineer topology in massive Dirac fermions in transition-metal dichalcogenide (TMD) monolayer and bilayer materials using an artificial superlattice potential that can be created with contemporary experimental techniques such as with LaAlO$_3$/SrTiO$_3$ interface. We use symmetry analysis to analyze band inversions to provide the Chern number $mathcal C$ for the valence band as a function of tunable potential parameter space for a class of $C_4$ and $C_3$ symmetric potentials. We present a novel method to engineer Chern number $mathcal C=2$ for the valence band and show that the applied potential must have a scalar together with a non-scalar periodic part. We discover that externally applied potential allows the possibility of non-trivial topological transitions that cannot be achieved by naturally occurring moir'e patterns produced by a twist or lattice mismatch. We finally discuss the experimental implementation of our work which allows for the possibility to realize the quantum Spin Hall effect (QSHE), quantum Hall effect (QHE), and even exotic non-Abelian anyons in the fractional quantum Hall effect (FQHE). |
Wednesday, March 8, 2023 1:54PM - 2:06PM |
N40.00011: Correlated insulating states in a multi-layer TMD heterostructure with two moiré interfaces Qiran Wu, Dongxue Chen, Yuze Meng, Zhen Lian, Xiong Huang, Thinh Truong, Lei Ma, Li Yan, Xinyue Chen, Takashi Taniguchi, Kenji Watanabe, Yongtao Cui, Sufei Shi In moiré superlattices of transitional metal dichalcogenides (TMD), the enhanced Coulomb interaction gives rise to rich correlated electronic states such as Mott insulators and generalized Wigner crystals. When two moiré superlattices of nearly identical periodicities are stacked together, interlayer hopping is expected, which adds a new degree of freedom for the study of correlated many-body physics. In this talk, we study angle-aligned monolayer WS2/multilayer WSe2/monolayer WS2 heterostructure in a dual gate geometry and employ microwave impedance microscopy to probe the formation of insulating states. By applying a vertical electric field to tune the carrier distribution between the top and bottom moiré superlattices, we observe that certain insulating states disappear and reappear as a function of electric field, indicating possible competition and interaction between the two moiré superlattices. |
Wednesday, March 8, 2023 2:06PM - 2:18PM |
N40.00012: Engineering magnetism in a 2D magnet from first principles Kaichen Xie, Xiaowei Zhang, Di Xiao, Ting Cao Among the family of two-dimensional (2D) materials, 2D van der Waals (vdW) magnetic semiconductors exhibit many novel properties resulting from the weak yet tunable interlayer magnetic interaction, which adds an entirely new magnetic degree of freedom to vdW interfacial engineering. As a prototypical 2D vdW magnetic semiconductor, layered CrSBr is ferromagnetic within each layer, and antiferromagnetically coupled between neighboring layers. In this talk, we demonstrate using first-principles calculations that a variety of tuning knobs, such as external magnetic field, strain, pressure, doping, and stacking configurations, can act as control knobs to engineer electronic and excitonic properties by changing the underlying magnetism in this material. We further map these tuning knobs onto a high-dimensional parameter space, in which the variables can be continuous or categorical. Our work establishes a systematic approach to engineer magnetic phases in layered magnetic semiconductors, which opens up opportunities for spintronic and memory devices based on 2D magnetic materials. |
Wednesday, March 8, 2023 2:18PM - 2:30PM |
N40.00013: Kekule Moiré Heterostructure Yusen Ye, Chong Wang, Xiaowei Zhang, Jimin Qian, Di Xiao, Ting Cao
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