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 F41: Magnetism in 2D SystemsLive

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Sponsoring Units: DCMP Chair: Dante O'Hara, US Naval Research Lab 
Tuesday, March 16, 2021 11:30AM  11:42AM Live 
F41.00001: Magnetic proximity effect in graphene on 2D van der Waals magnets ChunChih Tseng, Kyle Hwangbo, Qianni Jiang, Kenji Watanabe, Takashi Taniguchi, JiunHaw Chu, Xiaodong Xu, Matthew Yankowitz Over the past few years, a growing family of van der Waals (vdW) materials have been discovered to host intrinsic magnetic ordering. These materials exhibit a wide array of magnetic orders in the atomicallythin limit, including both intra and interlayer ferro and antiferromagnetism. Interfacing graphene with these 2D magnets offers intriguing possibilities for realizing new phenomena stemming from the magnetic proximity effect (MPE) owing to the high quality of the layered junction. Here, we investigate the MPE in heterostructures comprising graphene on XPS_{3} (X=Ni/Fe/Mn) substrates. Our magnetotransport characterization reveals signatures of charge transfer effects in a variety of these heterostructures. We will additionally discuss ongoing efforts to fabricate pristine interfaces between graphene and the environmentallysensitive 2D magnets. 
Tuesday, March 16, 2021 11:42AM  11:54AM Live 
F41.00002: Interplay between optoelectronic response and edge magnetic coupling in graphene nanoflakes Raquel EstebanPuyuelo, Bhalchandra Pujari, Oscar Grånäs, Biplab Sanyal The long spindiffusion length, spinlifetime and excellent optical absorption coefficient of graphene provide an attractive platform for building optoelectronic devices and spinbased logic in a nanometer regime. We analyze how the size and shape of graphene nanoflakes can be used to alter their magnetic structures and optical properties, using timedependent density functional theory. As the edges of zigzag graphene nanoribbons are known to align antiferromagnetically and armchair ones are nonmagnetic, a combination of both in a single nanoflake geometry can be used to optimize the groundstate magnetic structure to tailor the exchange coupling to ferro or antiferromagnetic edge magnetism. This allows for optimizing the switching conditions between the two magnetic states, for example, tuning the energyefficiency by countering the stability of the external field's magnitude. Additionally, we show that the magnetic ordering alters the optical response of the flake. Finally, we investigate how the high harmonic generation of a graphene nanoflake depends on its magnetic configuration and the polarization of the laser field. 
Tuesday, March 16, 2021 11:54AM  12:06PM Live 
F41.00003: Imaging the Collimation and Magnetic Focusing of Dirac Electrons Debarghya Dutta, Asaf Rozen, John Birkbeck, Shahal Ilani Van der Waals heterostructures have emerged as a new playground for exploring electronoptics  the solidstate analogues of optical devices, such as lenses, beam splitters and interferometers. A fundamental building block for constructing these electronoptic devices is an electron source that can provide a narrow and collimated beam. In this work, we observe that the naturally occuring PN junction formed at a metalgraphene interface leads to a strongly collimated beam of electrons, which can be turned on/off by simply changing the doping of the graphene. We noninvasively study this collimation by imaging magnetic focusing in realspace using a scanning singleelectron transistor as an electrostatic potential probe.We observe a drastic transition of the potential landscape when the graphene doping changes sign, from featureless for electrondoped (no PN junction) to oscillating for holedoped. Additionally, we show that there is a universal relation between the measured contact resistance of the metalgraphene interface to the degree of collimation, which should be considered in future ballistic experiments. 
Tuesday, March 16, 2021 12:06PM  12:18PM Live 
F41.00004: First principle study on electronic, magnetic, and phase transition properties of 2D magnetic material CrTe_{2} Yuhang Liu, George J. de Coster, Roger Lake, Mahesh R Neupane CrTe_{2} is a novel magnetic layered transition metal dichalcogenide that has been recently synthesized in a metastable 1T bulk phase with a highcurie temperature of 310 K. While its 1T phase has been investigated both experimentally and theoretically, detailed theoretical studies on the other possible phases and phase transition are imperative. In this work, we studied the structural, electronic, and magnetic properties of 1T, 1H, and 2H phases of CrTe_{2} using density functional theory (DFT). The 1T phase of CrTe2 was found to be the ground state in both bulk and monolayer structures at low temperatures. The energy barriers of the phase transition between the 1T, 1H, and 2H phases were calculated by the Nudge Elastic Band (NEB) method. By applying the linear response method, we obtained the selfconsistent U parameters in 1T, 1H, and 2H phases of CrTe_{2}. The 2H and 1H phases were predicted to be metallic and ferromagnetic with DFT+U correction, which is in contradiction to previous theoretical studies. 
Tuesday, March 16, 2021 12:18PM  12:30PM Live 
F41.00005: Spindependent magnetotransport in bilayer WSe_{2} EnMin Shih, Qianhui Shi, Daniel Rhodes, Bumho Kim, Kenji Watanabe, Takashi Taniguchi, James Hone, Cory R Dean We report magnetotransport measurements in Hall bar and corbino geometry in the bilayer WSe_{2} van der Waals heterostructure twodimensional electron system, showing distinct behavior for electrons with spinup and spindown orientations. In WSe_{2}, several lowest LLs are spinpolarized (spinup) due to the large g factor. When the spindown levels are being filled, the conductivity is much lower than in spinup levels, and at high fields it becomes completely localized, with the longitudinal conductance approaching zero and a wide Hall plateau lasting over a large filling factor range. Also, the conductance shows opposite temperature dependence  conductance of spindown levels decreases with decreasing T (while the spinup levels show the normal behavior, i.e., increasing with decreasing T). At the Landau level crossing position, we observe the manifestation of quantum Hall ferromagnetic state which gives rise to a resistance spike inside the gap. 
Tuesday, March 16, 2021 12:30PM  12:42PM Live 
F41.00006: Magnetic Response of a Twisted Bilayer Graphene Barrier. Dario Bahamon, Guillermo Gomez Santos, Tobias Stauber Twisted bilayer graphene (TBG) has recently drawn great attention because of the wide variety of novel electronic and optical properties. Behind the new phenomena, for low twist angle, we can find the emergence of flatbands produced by the localization of the electronic wavefunction on the AAstacked regions. In this regime, a paramagnetic response to an inplane magnetic field is expected. Here, to induce the abovementioned response electrically we created a TBG barrier putting a graphene flake on top of biased graphene nanoribbon. Our results [1] show that for low twist angles the applied electric field induces the formation of a welldefined Moiré superlattice of inplane magnetic moments whose magnitude and orientation can be tuned the sourcedrain voltage. Our results are robust against lattice relaxation, lattice orientation and edge vacancies. 
Tuesday, March 16, 2021 12:42PM  12:54PM Live 
F41.00007: Magneto intersubband oscillations in twisted double bilayer graphene Petar Tomić, Folkert De Vries, Peter Rickhaus, Giulia Zheng, Elias Portoles, Thomas Ihn, Klaus Ensslin Here we investigate interlayer scattering in twisted double bilayer graphene with an intermediate twist (1.94°) angle. We focus on the decoupled regime where the wavefunctions are bilayer polarized and the device behaves as a weakly coupled double quantum well [1]. Two regimes show enhanced interlayer scattering that we observe through magneto interlayer (intersubband) oscillations [2]. In the regime around the Lifshitz transition, density and temperature dependent resistivity measurements qualitatively indicate increasing trend of interlayer electronphonon scattering towards the Lifshitz transition that is in accordance with a simple elph model. Additionally, interlayer scattering is observed at the onset of the second subband. Observed negative compressibility and the flatness of the bands suggest electronelectron or impurity scattering as the dominant scattering mechanism. Since the nature of the superconducting state in twisted graphene heterostructures remains unknown, our observations can potentially improve the understanding of the coupling mechanism of electrons in twisted graphene. 
Tuesday, March 16, 2021 12:54PM  1:06PM Live 
F41.00008: Twisted bilayer WSe_{2 }(I): Band topology, Hubbard model, Heisenberg model, and DzyaloshinskiiMoriya interaction Haining Pan, Fengcheng Wu We present a theoretical study of singleparticle and manybody properties of twisted bilayer WSe_{2}. For singleparticle physics, we calculate the band topological phase diagram and electron local density of states (LDOS), which are found to be correlated. By comparing our theoretical LDOS with those measured by scanning tunneling microscopy, we comment on the possible topological nature of the first moiré valence band. For manybody physics, we construct a generalized Hubbard model on a triangular lattice based on the calculated singleparticle moiré bands. We show that a layer potential difference, arising, for example, from an applied electric field, can drastically change the noninteracting moiré bands, tune the spinorbit coupling in the Hubbard model, control the charge excitation gap of the Mott insulator at half filling, and generate an effective DzyaloshinskiiMoriya interaction in the effective Heisenberg model for the Mott insulator. Our theoretical results agree with transport experiments on the same system in several key aspects, and establish twisted bilayer WSe2 as a highly tunable system for studying and simulating strongly correlated phenomena in the Hubbard model. 
Tuesday, March 16, 2021 1:06PM  1:18PM Live 
F41.00009: Magnetoinfrared spectroscopy of nonpolar epigraphene Tianhao Zhao, Yuxuan Jiang, Yiran Hu, Yue Hu, Grant H Nunn, Mykhaylo Ozerov, Dmitry Smirnov, lei ma, Claire berger, Walter de Heer, Zhigang Jiang We report on a magnetoinfrared spectroscopy study of the new generation epigraphene grown on a nonpolar facet of SiC. We observe interband Landau level (LL) transitions that can be well described by a massless Dirac fermion model with a Fermi velocity of 1 x 10^{6} m/s. The transitions remain visible as the magnetic field is down to 0.25 T, indicating that the carrier density is no greater than 3.6 x 10^{10} cm^{2} as expected for chargeneutral nonpolar epigraphene. When the samples are grown thicker, LL transition splittings are spotted at high magnetic fields. Failing to explain the splittings with electronhole asymmetry, we suggest that it could arise from the (twist) bilayer epigraphene components, which we will discuss with a numerical twistbilayer model. 
Tuesday, March 16, 2021 1:18PM  1:30PM Live 
F41.00010: Control of Giant Orbital Magnetic Moment and Valley Splitting in Bernal Stacked Trilayer Graphene Zhehao Ge, Sergey Slizovskiy, Frederic Joucken, Eberth A Quezada, Takashi Taniguchi, Kenji Watanabe, Vladimir Falko, Jairo Velasco Jr. The valley degree of freedom in twodimensional (2D) materials is promising for applications in quantum technologies such as information storage, processing, and reading. In general, however, the valley degree of freedom in 2D materials is difficult to control because valleys are often degenerate or possess different energies that are indistinguishable experimentally. Therefore, an important step toward the realization of valleybased quantum information technology is the capability and control of valley splitting. In this regard, Bernal stacked trilayer graphene (ABATLG) is promising because it has two valleys that can be configured to host giant opposite nonzero orbital magnetic moments. In this talk, I will show our recent scanning tunneling spectroscopy (STS) study of ABATLG with magnetic field and back gate modulation. By applying an outofplane magnetic field we couple to the orbital magnetic moments in ABATLG and realize giant valley splitting. Subsequently, by modulating the back gate voltage in our devices we can tune the valley splitting by a factor of 2 at a constant magnetic field. 
Tuesday, March 16, 2021 1:30PM  1:42PM Live 
F41.00011: Frustrated magnetism in a cyclacene crystal Ricardo Ortiz Cano, Juan Carlos SanchoGarcía, Joaquín FernándezRossier We consider a triangular lattice of short carbon nanorings (cyclacenes). In the single molecule limit, each unit has 2 topologically protected zero modes. As a result, we could expect the low energy bands of the cyclacene crystal to be formed by such zero modes. When Coulomb repulsion is added into this picture, the low energy physics of this system should be similar to a Hubbard model of two weakly coupled triangular lattices. In the strong coupling limit, every ring should host 2 localized electrons, correlated antiferromagnetically with each other, and with the first neighbour ciclacene's, producing antiferromagnetically coupled frustrated S=1/2 triangular lattices. We carry out DFT calculations that confirm the main features of this scenario and show a noncollinear 120 antiferromagnetic phase ground state. Our results show a bottomup route to engineer correlated electronic phases with narrow bands in carbonbased crystals, complementary to the topbottom twisted bilayer approach. 
Tuesday, March 16, 2021 1:42PM  1:54PM Live 
F41.00012: Magnetoexcitons in TransitionMetal Dichalcogenides Monolayers and DoubleLayer Heterostructures Anastasia Spiridonova, Roman Kezerashvili We study direct magnetoexcitons in transitionmetal dichalcogenides (TMDC) monolayers and indirect magnetoexcitons in doublelayer TMDC heterostructures encapsulated by hBN. The formations of direct and indirect magnetoexcitons occur in the presence of perpendicular to the layers magnetic field. We calculate the binding energies of 1s, 2s, 3s, and 4s Rydberg states of magnetoexcitons by numerical solution of the Schrödinger equation using both RytovaKeldysh and Coulomb potentials, and electron and hole masses obtained in the framework of density functional theory. We report the energy contribution from the magnetic field to the binding energies and diamagnetic coefficients for Rydberg states. It is demonstrated that the binding energies of direct and indirect magnetoexcitons can be tuned by applying the external magnetic field. Our calculations show that the choice of the interaction potentials has a significant effect on the binding energies of magnetoexcitons and the diamagnetic coefficients. 
Tuesday, March 16, 2021 1:54PM  2:06PM Live 
F41.00013: Experimental detection of graphene's singular orbital diamagnetism at the Dirac point. Jorge Vallejo Bustamante, Meydi Ferrier, Sophie Gueron, Helene Bouchiat The electronic properties of Graphene have been intensively investigated over the last decade, and signatures of the remarkable features of its Dirac spectrum have been displayed using transport and spectroscopy experiments. In contrast, the orbital magnetism of graphene, which is probably the most fundamental signature of graphene’s characteristic Berry phase, has not yet been measured at the level of a single flake. In particular, the striking prediction of a divergent diamagnetic response at zero doping calls for an experimental test. 
Tuesday, March 16, 2021 2:06PM  2:18PM Live 
F41.00014: Strong intervalley scattering in twisted bilayer graphene revealed by hightemperature magnetooscillations Isabelle Y Phinney, Denis Bandurin, Clement Collignon, Kenji Watanabe, Takashi Taniguchi, Pablo JarilloHerrero Superlatticeinduced modulation of twodimensional materials can give rise to a host of surprising yet counterintuitive effects and impact electron transport in a variety of nontrivial ways. Twisted bilayer graphene (TBG) provides an exceptional platform in which to study these superlatticeenabled transport effects, as exemplified by numerous experiments revealing many intriguing interactiondriven phenomena in this system [1]. Integral to understanding transport properties of TBG systems is a knowledge of the electron scattering mechanisms that govern momentum relaxation in the superlattice. We show that at small twist angles, unlike in monolayer graphene, intervalley electron scattering plays a critical role in momentum relaxation. This scattering process reveals itself in hightemperature magnetooscillations [2], which allow us to estimate the intervalley scattering rate and determine the quasiparticle lifetime in smallangle TBG. 
Tuesday, March 16, 2021 2:18PM  2:30PM Live 
F41.00015: Doping dependence of lowenergy band dispersions and magnetic properties of twisted bilayer graphene YoSep Cho, Young Woo Choi, Hyoung Joon Choi Magicangle twisted bilayer graphene (TBG) has drawn great attention for its correlated electronic phases appearing at different doping concentrations. To understand such phases, lowenergy band dispersions need to be calculated accurately as a function of doping. However, the large number of atoms in the moiré supercell has limited theoretical calculations of doping effects on electronic and magnetic properties. Here, we present an efficient method to calculate electronic and magnetic properties of electron and holedoped TBGs based on results of density functional theory calculations of undoped TBG. With this method, we investigate doping and temperature dependences of electronic and magnetic properties of doped TBGs. We discuss effects of doping on electrostatic screening, lowenergy band dispersions, and magnetization of TBG. 
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