Bulletin of the American Physical Society
APS March Meeting 2022
Volume 67, Number 3
Monday–Friday, March 14–18, 2022; Chicago
Session T67: Scanning Probe Microscopy of 2D MaterialsFocus Recordings Available
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Sponsoring Units: DMP Chair: Christopher Gutiérrez, University of California Los Angeles Room: Hyatt Regency Hotel -Hyde Park |
Thursday, March 17, 2022 11:30AM - 12:06PM |
T67.00001: High-resolution Landau level spectroscopy in small-angle twisted double bilayer graphene Invited Speaker: Marlou R Slot The wealth of correlated phases in twisted van der Waals multilayers has propelled the quest for novel many-body phenomena in tunable solid-state platforms. While the field recently started off with superconductivity and correlated insulating states in magic-angle twisted bilayer graphene, the attention has increasingly turned to systems containing more layers. The in-depth investigation of novel correlated and topological phases in these materials requires a nanoscale approach, forgoing the influence of twist-angle disorder. We present a combination of high-resolution gate-tuned STM, AFM, and KPFM measurements at temperatures down to 10 mK and at magnetic fields up to 15 T to locally investigate van der Waals materials. Here, we focus on small-angle twisted double bilayer graphene. A rich, layer-dependent local density of states featuring narrow bands is revealed, sensitively tunable by the electric and magnetic field. Landau levels emerge out of the valence and conduction narrow bands upon application of a perpendicular magnetic field. Comparing to theory, we show the Landau levels originate from different symmetry points in the band structure where electron and hole pockets reside and are tuned by the displacement field. Spatial Landau level spectroscopy measurements visualize the nanoscale localization of the non-uniform Landau levels in the moiré potential, including C3-symmetry breaking at particular carrier densities. The large and tunable parameter space available for spatially resolved spectroscopy offers a detailed insight in the Landau level behavior and correlated physics in twisted double bilayer graphene. |
Thursday, March 17, 2022 12:06PM - 12:18PM |
T67.00002: On-chip single electron transistors as a thermodynamic probe of van der Waals materials Patrick R Forrester, Andrew T Pierce, Seung Hwan Lee, Shaowen Chen, Yonglong Xie, Andrei J Levin, Yuan Cao, Kenji Watanabe, Takashi Taniguchi, Amir Yacoby The single electron transistor (SET) is an exceptionally sensitive electrometer. It can measure important thermodynamic properties, such as the electronic compressibility and the chemical potential, which can be used to extract entropy and charge gaps in solid state systems. SETs have proven particularly powerful in characterizing two-dimensional electron gasses by, for example, identifying elusive fractional quantum hall states, measuring the thermodynamics of magnons in graphene, and recently observing fractional Chern insulators in magic-angle twisted bilayer graphene. By fabricating SETs directly on a sample, one can straightforwardly integrate this measurement capability into ultra-low temperature and high magnetic field environments. In this talk, I will discuss fabrication of these sensors on van der Waals heterostructures. I will also present compressibility data using on-chip aluminum SETs to study graphene in the quantum hall regime and discuss the relevance of this technique to elucidating open questions regarding the thermodynamics of magnon excitations in this system. |
Thursday, March 17, 2022 12:18PM - 12:30PM |
T67.00003: Scanning Probe Characterization and Classification over Defective WS2 and Au {111}. John C Thomas, Antonio Rossi, Darian Smalley, Luca Francaviglia, Zhuohang Yu, Tianyi Zhang, Shalini Kumari, Joshua A Robinson, Mauricio Terrones, Masahiro Ishigami, Eli Rotenberg, Edward S Barnard, Archana Raja, Ed K Wong, D. Frank Ogletree, Marcus Noack, Alex Weber-Bargioni Point defect identification in two-dimensional materials enables an understanding of the local environment within a given system, where scanning probe microscopy that takes advantage of hyperspectral tunneling bias spectroscopy acquisition can both image and identify the atomic and electronic landscape. Transition metal dichalcogenides (TMDs) have gained substantial interest for a variety of unique properties in its monolayer form such as serving as a host substrate for photo- and spin- active functionalization and showing promise in tunable band gap control. Here dense spectroscopic volume is collected autonomously via Gaussian process regression, where convolutional neural networks are used in tandem for defect identification and subsequent feedback. Monolayer semiconductor is explored on sulfur vacancies within tungsten disulfide (WS2), to provide hyperspectral insight into available sulfur-substitution sites within a TMD that is combined with spectral confirmation on the Au{111} herringbone reconstruction for both tip state verification and local fingerprinting. Additionally, we delve into similar investigations of absorbed metal impurities onto pristine W2 with scanning tunneling microscopy and spectroscopy. |
Thursday, March 17, 2022 12:30PM - 12:42PM |
T67.00004: Measuring In-Plane Electrostatic Potentials In 2D Systems and Correlating them with Electronic Structure Wyatt A Behn, Zachary J Krebs, Keenan J Smith, Kenji Watanabe, Takashi Taniguchi, Victor W Brar Accurately mapping the local electrostatic potential of 2D materials is critical in understanding how the electronic structure changes due to a perturbing potential. In-plane potentials produced by superlattices and defects are of particular importance given the effects they may have on transport properties and on the formation of localized states. However, characterizing the electrostatic potential with local probes (including STM and AFM) is challenging for 2D materials as these measurements are typically invasive, locally doping the material as they probe it. Kelvin Probe Force Microscopy (KPFM) does not induce local perturbations but suffers from inaccuracies due to the contributions of long-range electrostatic fields. We present a calibration scheme for KPFM that allows for the potential of 2D materials to be obtained accurately and with nanometer resolution. This new methodology is used to reconstruct the potential profile of electrostatically defined quantum dots in graphene/hexagonal boron nitride heterostructures. Experimentally extracted energy level spacings measured by scanning tunneling spectroscopy compare well to reconstructed KPFM data. This approach allows for the response of quasiparticles as a function of carrier concentration to be determined in a self-consistent way. |
Thursday, March 17, 2022 12:42PM - 12:54PM |
T67.00005: Imaging the Breaking of Electrostatic Dams in Graphene for Ballistic and Viscous Fluids Zachary Krebs, Wyatt A Behn, Victor W Brar, Songci Li, Keenan J Smith, Takashi Taniguchi, Kenji Watanabe Under special conditions the conduction of electrical current through a material can mimic the flow of a viscous fluid. This so-called 'hydrodynamic' regime occurs when the rate of momentum-conserving collisions between charge carriers is larger than the momentum-relaxing rate due to other scattering sources. Ultraclean graphene has emerged as an ideal platform for studying hydrodynamic flow, leading to the possibility of directly imaging fluid-like behaviors with high-resolution experimental probes. In this work, we use scanning tunneling potentiometry to resolve the nanometer-scale flow of electrons in graphene as they pass through channels defined by smooth and tunable p-n junction barriers. We observe that as the sample temperature and channel widths are increased, the electronic flow undergoes a Knudsen-to-Gurzhi transition from a ballistic to viscous regime characterized by a channel conductance that exceeds the ballistic limit, as well as suppressed charge accumulation against the barriers. Our results are successfully modelled by finite-element simulations of two-dimensional viscous current flow, and they illustrate how electronic fluid flow evolves with carrier density, channel width, and temperature. |
Thursday, March 17, 2022 12:54PM - 1:06PM |
T67.00006: Inverse Layer Dependence of Friction on Re-doped MoS2 Revealed by Atomic Force Microscopy Mehmet Z Baykara, Ogulcan Acikgoz, Enrique Guerrero, Alper Yanilmaz, Omur E Dagdeviren, Cem Celebi, David A Strubbe The possibility to tune the nanoscale frictional properties of two-dimensional (2D) materials via chemical doping is of considerable scientific and technological interest. Here, we present the results of atomic-force-microscopy-based friction measurements on Re-doped molybdenum disulfide (MoS2) [arXiv:2007.05805]. In stark contrast to the seemingly universal observation of decreasing friction with increasing number of layers on 2D materials, friction on Re-doped MoS2 exhibits an anomalous, i.e. inverse, dependency on the number of layers. Raman spectroscopy measurements reveal signatures of Re intercalation, providing clues regarding the physical mechanisms that result in this remarkable observation (see abstract of Strubbe et al.). |
Thursday, March 17, 2022 1:06PM - 1:18PM |
T67.00007: Mechanism of inverse layer-dependence of friction in Re-doped MoS2: DFT study of elastic stiffening, frictional forces, and Raman spectroscopy David A Strubbe, Enrique Guerrero, Ogulcan Acikgoz, Alper Yanilmaz, Omur E Dagdeviren, Cem Çelebi, Mehmet Z Baykara Atomic-force microscopy (AFM) on 2D materials typically shows a reduction in frictional forces as the number of layers increases. This is because the forces are mostly due to out-of-plane surface deformation by the tip (i.e. puckering); adding more layers stiffens the surface and reduces puckering. However, Re-doped MoS2 has an increase in friction with number of layers (see abstract of Baykara et al.; arXiv:2007.05805). We explain these observations by showing with DFT that Re dopants intercalated between MoS2 layers significantly increase the out-of-plane elastic modulus, which decreases the puckering effect for a given number of layers. This stiffening effect is diluted as the number of layers increases, because when the Re concentration is low, most regions probed by the AFM have only one dopant. The experiments cannot be explained by a simpler picture involving just an unpuckered surface, because the friction actually decreases with number of layers (saturating after four). Experimental Raman spectra are more consistent with calculations for Re tetrahedrally intercalated as opposed to substituting for Mo (which leads to a much weaker stiffening effect). These results provide a framework for understanding the effects of chemical doping on the mechanics of 2D materials. |
Thursday, March 17, 2022 1:18PM - 1:54PM |
T67.00008: Electrons and Phonons in Low-Angle Twisted Bilayer Graphene Invited Speaker: Ado Jorio de Vasconcelos A low twist angle between the two stacked crystal networks in bilayer graphene enables self-organized lattice reconstruction with the formation of a periodic domain. This superlattice modulates the vibrational and electronic structures, imposing new rules for electron-phonon coupling and the eventual observation of strong correlation and superconductivity. In this talk we show direct optical images of the crystal superlattice in reconstructed twisted bilayer graphene, generated by the inelastic scattering of light in a nano-Raman spectroscope, working in the tip-enhanced Raman Spectroscopy (TERS) domain. The observation of the crystallographic structure with visible light is made possible due to lattice dynamics localization, the images resembling spectral variations caused by the presence of strain solitons and topological points. The results are rationalized by a nearly-free-phonon model and electronic calculations that highlight the relevance of solitons and topological points, particularly pronounced for structures with small twist angles, and the effect of electron-phonon interaction. The latest is also studied with date dependent doping selecting the available Raman scattering paths. The results shed linght in the physics of strongly correlated systems in twisted bilayer graphene. |
Thursday, March 17, 2022 1:54PM - 2:06PM |
T67.00009: Sparse Sampling and Nonlinear Spectroscopy for Fast Quasiparticle Interference Imaging Fabian D Natterer, Berk Zengin, Jens Oppliger, Danyang Liu, Lorena Niggli, Tohru Kurosawa Quasiparticle Interference (QPI) Imaging with the scanning tunneling microscope (STM) is becoming an important tool in the investigation of advanced quantum materials. It provides insight into the band-structure for experimental conditions that do not permit the use of angular resolved photoemission spectroscopy, such as in high magnetic fields, at ultra-low temperatures, or on small field-effect devices. However, QPI imaging is also very slow because it requires the measurements of a massive amount of energy dependent local density of states (LDOS) that can occupy an STM for weeks. We demonstrate orders of magnitude faster imaging by our combination of sparse sampling and parallel spectroscopy. Sparse sampling reduces the number of required LDOS measurements and parallel spectroscopy accelerates the individual point spectrum. These two methods provide substantial speed enhancements individually but make QPI imaging vastly faster when used in combination because of their multiplicative dependence. As an introduction we measure the QPI of Au(111) and Bi2212 at a thousandfold faster mapping and show the straightforward implementation into existing SPM systems without the need for recabling of one's apparatus. |
Thursday, March 17, 2022 2:06PM - 2:18PM |
T67.00010: Critical peeling scaling laws of tethered graphene nanoribbons Andrea Silva, Andrea Vanossi, Erio Tosatti Peeling of surface-deposited nanostructures is commonly executed by AFM pickup and lifting. Its theoretical description has important differences from, e.g., Kendall's theory for macroscopic peeling of adsorbed films [1], because unlike the macroscopic case, many nanostructures are superlubrical sliders. An appropriate theory was recently put forward by Gigli el al. [2] for superlubric graphene nanoribbons (GNRs) on gold [3,4]. After an initial bending builds up, the system reaches a steady regime, where the peeling angle is π/2 and the curvature is fixed. In that regime, a lifting amount $h$ of the tip produces no advancement of the detachment front, and a simple retraction of the free tail end, opposite to Kendall’s limit, where the detachment point advances and the tail end stands still. A third intriguing situation is expected to arise when the nanoribbon, albeit structurally lubric, does not have a freely moving tail, which may instead be thethered by construction or by accident. Here we show, both analytically and by realistic simulations, novel nontrivial exponents exhibited in this case, that are absent in the previous cases. As the tip is lifted, the peeling force increases as $h^{1/3}$ and the lifting angle asymptotically drops like $h^{−1/3)$. As the detachment front advances and the tethered tail retracts, the adsorbed fraction shrinks as $h^{4/3}$. These exponents appear to prepare the final total detachment as a critical point, where the entire ribbon eventually hangs suspended between the tip and tethering spring. |
Thursday, March 17, 2022 2:18PM - 2:30PM |
T67.00011: Factors influencing the calculation of friction of two-dimensional materials using the Lennard-Jones potential Donghyeon Moon, Suenne Kim To understand friction at the nanoscale, either short-range chemical interactions or long-range van der Waals interactions have been considered depending on the surface termination states of sliding bodies. Lennard-Jones (LJ) potential has been used to explain the frictional characteristics of two-dimensional (2D) materials in molecular dynamics simulations studies. LJ potential includes the van der Waals interactions and well describes the essential features of interactions between atoms and molecules. Two particles do not interact at infinity, repel each other when they get very close, and attract when they are at moderate distances. When applying the LJ potential in realistic systems, an approximation is made for convenience: the LJ potential is defined as zero outside the cut-off radius. The cut-off radius in various systems generally uses a value of 2.5σ, where σ corresponds to the distance at which the LJ potential energy between atoms becomes zero. |
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