Bulletin of the American Physical Society
APS March Meeting 2020
Volume 65, Number 1
Monday–Friday, March 2–6, 2020; Denver, Colorado
Session M65: 2D Materials: Graphene and Transition Metal Dichalcogenides |
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Sponsoring Units: DCMP Chair: Sergio de la Barrera, Massachusetts Institute of Technology MIT Room: Mile High Ballroom 4F |
Wednesday, March 4, 2020 11:15AM - 11:27AM |
M65.00001: Observation of flat bands and singularity collapse in back-gated semiconductor artificial graphene Lingjie Du, Ziyu Liu, Loren Pfeiffer, Michael Manfra, Aron Pinczuk Electronic flat bands are key predicted features in tunable artificial graphene (AG) near M points in the Brillouin zone. Such dispersionless electron flat bands could support unconventional superconducting states. Here, we report the first direct observation of flat bands in semiconductor AG transistors where the electron density is tuned by applied voltage. Doublets observed in low temperature photoluminescence (PL) display a density dependence that is linked to the van-Hove singularity doublet near M points. We find that the doublets in PL persist for a large range of electron densities, suggesting a pinning of the Fermi energy in a flat band. Interestingly, as the temperature is lowered to below 10K, the lineshape of the doublet in PL spectra shows a dramatic temperature dependence in which the higher energy peak quickly collapses. This unexpected temperature dependence suggests possible formation of electron liquid states near the flat band at low temperature. |
Wednesday, March 4, 2020 11:27AM - 11:39AM |
M65.00002: First-principle Studies of Flexoelectric Effect in Corrugated Two-dimensional Materials Yeongrok Jin, Jaekwang Lee The flexoelectricity, electrical polarization induced by a strain gradient, is one of the very exotic physical phenomena because it can induce the huge piezoelectricity even in centrosymmetric materials. Here, combining density functional theory calculation and mathematical analysis, we find that the corrugation generates the great out-of-plane polarization mediated by the flexoelectric effect even in centrosymmetric two-dimensional materials. The flexoelectricity in 2D materials is highly sensitive to the corrugation strength and tunable by adjusting the curvature at the nm scale. The correlation between curvature, flexoelectricity and resulting out of plane polarization will be discussed and associated underlying mechanisms will be introduced in detail. |
Wednesday, March 4, 2020 11:39AM - 11:51AM |
M65.00003: Electronically Engineered Nanoporous Graphene Peter Jacobse, Ryan McCurdy, Daniel Rizzo, Jingwei Jiang, Paul L Butler, Gregory Veber, Steven Louie, Felix Fischer, Michael F Crommie Recent advances in the synthesis of graphene nanoribbons using on-surface bottom-up techniques have enabled fabrication of not only atomically well-defined one-dimensional structures, but also two-dimensional structures such as nanoporous graphenes. These are particularly interesting for applications due to their sieve-like topology. Here we present a new methodology for creating covalently connected, fully conjugated two-dimensional graphene structures through the utilization of cyclopentadiene (CP) moieties. CP elements exhibit a propensity to initiate fusion between nanoribbons and result in a two-dimensional structure with well-defined interface topology. The resulting new material is analyzed using scanning tunneling microscopy (STM), scanning tunneling spectroscopy (STS), and bond-resolved scanning tunneling microscopy (BRSTM). We observe emergent interface-localized electronic states that hybridize to yield a dispersive two-dimensional band of states at an energy inside the bandgap of an isolated GNR. |
Wednesday, March 4, 2020 11:51AM - 12:03PM |
M65.00004: Band structure engineering of 2D materials using 1D superlattice Yutao Li, Scott A Dietrich, Carlos Forsythe, Shaowen Chen, Takashi Taniguchi, Kenji Watanabe, James C Hone, Cory Dean Atomically thin 2D materials such as graphene provide promising new platforms to realize synthetic band structures by superlattice (SL) patterning. We exploit our recently developed technique of dielectric superlattices to pattern graphene with a 1D superlattice potential with periodicity as low as 47nm. We observe strong anisotropy in electrical transport features along directions parallel versus perpendicular to the SL basis vector. The main and mini Dirac points, induced by the SL potential, may be “flattened” at certain strengths of SL modulation, leading to an alternating sequence of resistance maxima and minima in the direction parallel to the SL basis vector. Our results establish a multitude of possibilities of band structure engineering on a 2D material under a 1D superlattice potential with possible applications in ballistic electron optics, van der Waal FETs, and plasmonics. |
Wednesday, March 4, 2020 12:03PM - 12:15PM |
M65.00005: Periodic strain-induced magnetization in graphene over NbSe2 Antonio Manesco, Jose L. Lado, Eduardo Ribeiro, Gabrielle Weber, Durval Rodrigues Jr. Electronic correlations are known to dramatically impact the electronic properties of graphene in the presence of real or artificial gauge fields, as a consequence of the strongly enhanced density of states. Paradigmatic examples of this include the magnetically-ordered quantum Hall state in monolayer graphene, and the correlated and superconducting phases in twisted graphene multilayers. Motivated by recent experiments, here we explore the emergence of a periodic-modulated magnetization caused by epitaxial-induced strain in graphene stemming from a NbSe2 substrate. In particular, we show the emergence of a magnetic state induced by the elastic substrate-induced pseudo gauge field, its interplay with the superconducting state of NbSe2, and the potential tunability of the correlated state by means of external electric and magnetic fields. Our results put forward a hybrid graphene/NbSe2 heterostructure with tunable correlations, providing a powerful platform to explore correlated physics in hybrid van der Waals materials. |
Wednesday, March 4, 2020 12:15PM - 12:27PM |
M65.00006: Strain tunable magnetotransport study of ABC trilayer graphene/h-BN moiré superlattices Chuankun Liu, Ryuichi Tsuchikawa, Jameson G Berg, Vikram V Deshpande The number of atomic layers changes the electronic band structure of graphene dramatically. While monolayer graphene hosts linear bands with massless fermions, trilayer ABC-stacked graphene exhibits a very flat quartic band at the K point and is a particularly suitable platform for strongly correlated physics. When ABC trilayer graphene is aligned with the h-BN layer, the addition of the moiré superlattice can create narrow minibands, and low energy bands can become flat and isolated near charge neutrality. Mott insulating states and unconventional superconductivity have been observed in ABC-TLG/h-BN heterostructures. Perpendicular electric field and strain are important ways to tune these bands. In our work, we have fabricated ABC-TLG/h-BN aligned heterostructures with dual graphite gates, on flexible substrates to apply both perpendicular electric field and intralayer strain onto the device. We study the responses of low energy bands by performing magnetotransport measurements at low temperatures. |
Wednesday, March 4, 2020 12:27PM - 12:39PM |
M65.00007: Interaction-driven broken symmetry states in moiré vdWs heterostructure Zhiren Zheng, Thao H Dinh, Kenji Watanabe, Takashi Taniguchi, Suyang Xu, Nuh Gedik, Qiong Ma, Pablo Jarillo-Herrero The freedom of stacking different layers of 2D van der Waals (vdW) materials with an arbitrary twist angle opens up tremendous opportunities in studying emergent properties that are not accessible in the natural crystal form. In particular, the recently discovered magic-angle twisted bilayer graphene shows that the twist angle and moiré potential can substantially enhance electron correlations for spontaneously broken symmetry states and turn semi-metallic graphene into insulator, superconductor, and quantum anomalous Hall insulator at will. With the right system and careful design, interaction-driven broken symmetry states can have many other manifestations that are not only fundamentally intriguing but also invaluable to new device applications. Here, we report on our design and experimental exploration of a multiferroic system based on moiré vdW heterostructures. Our study will open the door to a new class of 2D multiferroics, which has a potential implementation in many important applications, such as memristive devices for neuromorphic computing. |
Wednesday, March 4, 2020 12:39PM - 12:51PM |
M65.00008: Optical study of electric field tunable 2D semiconductor moiré superlattices Lizhong Li, Yanhao Tang, Kin Fai Mak, Jie Shan Moiré superlattices based on graphene or transition metal dichalcogenide (TMD) heterostructures provide an interesting platform to explore emerging quantum phases in the strong correlation regime. The large electron mass in monolayer TMDs and the lack of magic angle requirement make TMD moiré heterostructures a particularly robust platform for studying problems in strong electron correlation physics[1]. For instance, it has been recently shown that WSe2/WS2 moiré superlattices can simulate the physics of the triangular lattice Hubbard model [2]. Whereas electrostatic doping can continuously tune the filling factor of the system, a vertical displacement field could also effectively tune the relative band alignment of the heterostructure and other important parameters of this strongly correlated system. In this talk, I will discuss our recent optical study on electric field tunable TMD moiré superlattices and the search for novel quantum phases of matter. |
Wednesday, March 4, 2020 12:51PM - 1:03PM |
M65.00009: Charge transfer in transition metal dichalcogenide superlattices and moiré quantum chemistry Yang Zhang, Noah Yuan, Liang Fu We theoretically investigate the band structure and interaction effects in moiré superlattices of transition metal dichalcogenides and find the Mott and charge-transfer insulating phases around half filling. The charge transfer length of a charge-transfer insulator can be several nanometers, and the magnetic susceptibility of the Mott insulator is also worked out as a probe for the antiferromagnetic ground state. This highly tunable system provides an ideal platform to simulate diatomic solids under strong interactions. |
Wednesday, March 4, 2020 1:03PM - 1:15PM |
M65.00010: Stokes spectroscopy applied to monolayer MoS2 photoluminescence at room temperature; different helicities observed for A-exciton and A-trion Ehsan Zolghadr, Sourav Garg, Patrick Kung, Newton Martins Barbosa Neto, Paulo T Araujo Strong electron-electron and/or electron-hole Coulomb interactions play important roles in optical transitions in monolayer semiconducting transition metal dichalcogenides (TMDCs). These optical transitions reflect the behaviors of excitons and/or trions. Here, we present our investigation of room temperature Stokes spectroscopy applied to the photoluminescence emitted by a large-area MoS2 monolayer grown by CVD technique standing on a SiO2 substrate. Stokes spectroscopy allows for simultaneous measurement of any helicity, which is advantageous compared to the traditional helicity-resolved photoluminescence measurement. Our results show that the PL emission presents significant amount of circularly polarized light when non-resonantly excited with linearly polarized light at 532 nm. Analyzing the Stokes spectra leads to a deconvolution in which the A-exciton and A-trion peaks have opposite helicities. While the exact valley polarization dynamics and selection rule mechanisms that leads to the optical properties still need to be clarified, this study opens the door for a better understanding of the room temperature exciton and trion physics on monolayer MoS2, an ideal candidate for industrial atomically thin opto-electronic devices such as solar cells and photodetectors. |
Wednesday, March 4, 2020 1:15PM - 1:27PM |
M65.00011: Studying Transition Metal Dichalcogenide Superlattices with nano-ARPES Conrad Stansbury, Iqbal Utama, Claudia Fatuzzo, Chris Jozwiak, Aaron Bostwick, José Avila, Eli Rotenberg, Feng Wang, Alessandra Lanzara Two-dimensional layered materials, especially monolayer transition metal dichalcogenides, have demonstrated lasting impact as a platform for combining and tuning electronic and optical behaviors on the nanoscale. The realization that moiré superlattices in these systems provide tunable control over correlations has sparked renewed interest through analogy to the family of high-temperature superconductors and in light of new phenomena such as moiré excitons. As a complement to modeling, a bottom-up understanding of these phenomena and interlayer hybridization motivates direct measurements of the electronic structures of the host materials. Nano-ARPES enables these experiments on clean, mechanically-exfoliated heterostructures where the impact of substrate and superlattice interaction on the electrons can be fully explored. In this talk, I show how this technique can address basic questions of electronic and structural order in a few model systems: heterobilayer interfaces, and in the interface of TMD monolayers with a substrate. Among the rich phenomena present at the interface, we will focus especially on the presence or absence of hybridization effects in short and long wavelength superlattices. |
Wednesday, March 4, 2020 1:27PM - 1:39PM |
M65.00012: Tip-enhanced nano-imaging of 2D alloy with gradual concentration profile Hana Hrim, Sharad Ambardar, Dmitri v Voronine Systematic optical studies of 2D MoxW1-xS2 alloys, with 0.05≤x≤0.45 grown by chemical vapor deposition (CVD) on Si/SiO2 were carried out using Raman and photoluminescence (PL) spectroscopy using 532 nm laser. Atomic force microscopy (AFM) imaging showed the correlation with the optical images. We demonstrate nano-optical imaging using tip-enhanced photoluminescence (TEPL) as a function of the composition x for obtaining the higher spatial resolution beyond the diffraction limit. Quantum plasmonic quenching of the TEPL signals was observed for larger x revealing the tunneling electron injection. In order to overcome the problem of excitation efficiency we used a 405 nm laser for TEPL which showed efficient excitation of different x components. Tip-enhanced Raman spectroscopy (TERS) confirms the alloy composition at the nanoscale. |
Wednesday, March 4, 2020 1:39PM - 1:51PM |
M65.00013: Clean 2D superconductivity in a bulk van der Waals superlattice Aravind Devarakonda, Hisashi Inoue, Shiang Fang, Cigdem Ozsoy Keskinbora, Takehito Suzuki, Markus Kriener, Liang Fu, Efthimios Kaxiras, David Charles Bell, Joseph G Checkelsky Recent interest in bulk van der Waals systems has been driven by the search for exotic physics in thin flakes exfoliated from bulk crystals and, more broadly, the desire to add novel functionalities to those available in the van der Waals heterostructure toolkit. This top down approach has proved successful in unveiling novel phenomena, the MX2 transition metal dichalcogenides being one such case where both flakes and heterostructures have been used to uncover and design exotic ground states, respectively. However, such devices have intrinsic limitations that can impede powerful experimental probes or prevent the realization of exotic physics in the first place due to disorder introduced through the fabrication process. Here, we report experimental evidence for a possible solution in the form of a bulk single crystal, van der Waals superlattice that simultaneously exhibits high cleanliness and two-dimensional superconductivity. |
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M65.00014: Algorithm on the absolute formation energy of edges in transition metal dichalcogenides Chuen Keung Sin, Junyi Zhu Equilibrium shapes are the keys to fully understand the growth dynamics of quasi 2D structures, such as transition metal dichalcogenides. However, the intrinsic complexity of the quasi 2D nature near the edge makes it challenging to formulate successful algorithm with good accuracies. As a result, the theoretical investigations on the growth thermodynamics and kinetics are limited. To solve the fundamental problem, we proposed a new calculation scheme to calculate the formation energy of edges in monolayer MoS2. With the help of electron counting model, we found the proper passivation scheme of pseudo hydrogen on the edge of ribbons and wedges and obtained semiconducting nature with excellent accuracies. |
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