Bulletin of the American Physical Society
APS March Meeting 2019
Volume 64, Number 2
Monday–Friday, March 4–8, 2019; Boston, Massachusetts
Session V14: 2D Materials (Metals, Superconductors, and Correlated Materials) -- Heterostructures and Twisted GrapheneFocus
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Sponsoring Units: DMP DCOMP Chair: Young Jun Chang, University of Seoul Room: BCEC 153C |
Thursday, March 7, 2019 2:30PM - 2:42PM |
V14.00001: Coulomb drag between graphene and LaAlO3/SrTiO3 heterostructures Qing Guo, Jianan Li, Jen-Feng Hsu, Hyungwoo Lee, Chang-Beom Eom, Patrick Irvin, Brian R D'Urso, Jeremy Levy Vertical stacking of heterostructures that combine layered materials offer new ways of combining interesting properties of dissimilar electronic materials. Over the past few years we have been integrating graphene with complex-oxide heterostructures, specifically, the LaAlO3/SrTiO3 system. Furthermore, conducting nanostructures can be written under graphene, producing interesting interactions between the two systems. Here we report Coulomb drag measurements between single-layer graphene and a conductive LaAlO3/SrTiO3 interface. The observed Coulomb drag resistance is non-monotonic and increases for temperatures below 20 K. The temperature dependence is also non-monotonic, showing a notable departure from quadratic scaling, as expected from Fermi-liquid theory . Coulomb drag experiments between the graphene and LaAlO3/SrTiO3 layers show strong coupling in both the normal and superconducting state of the LaAlO3/SrTiO3. |
Thursday, March 7, 2019 2:42PM - 2:54PM |
V14.00002: Polarity of Coulomb drag signal in carbon nanotube-graphene heterostructures Laurel Anderson, Austin K Cheng, Kenji Watanabe, Takashi Taniguchi, Philip Kim Coulomb drag is an inter-conductor interaction in which current in one conductor induces a current in a proximate conducting object. This electron-electron coupling can transfer momentum or energy between isolated conductors, creating inter-conductor electronic responses without particle exchange. Coulomb drag has been extensively studied as a probe of interactions in low-dimensional systems. We report measurements of 1D-2D drag resistance between a carbon nanotube and graphene separated by a thin boron nitride insulating layer. The carrier densities of the nanotube and graphene can be separately tuned to access different regimes of drag response. The polarity of the observed drag signal is inconsistent with inter-conductor momentum transfer, suggesting it is a possible signature of energy drag or hydrodynamic electron transport in graphene. |
Thursday, March 7, 2019 2:54PM - 3:06PM |
V14.00003: Edge State Wave-Functions probed by Tunneling Spectroscopy Taras Patlatiuk, Christian Scheller, Daniel M Hill, Yaroslav Tserkovnyak, Amir Yacoby, Loren Pfeiffer, Kenneth West, Dominik Zumbuhl Edge states have recently attracted a lot of attention due to their appearance at surfaces of topological materials and quantum Hall systems. Here we study quantum Hall edge states formed in a GaAs 2D electron gas using a tunnel-coupled quantum wire [1]. We use momentum-conserving tunneling spectroscopy to distinguish spatially overlapping states with different momenta. The tunneling conductance is proportional to the overlap between the edge state and the wire mode wave functions. Because of the varying shape of wire wave functions, their overlaps with edge states will also vary, resulting in a distinct sequence of tunneling conductance intensity patterns for different wire modes. We use self-consistent calculations to obtain the electrostatic potential at zero magnetic field and employ a Schrödinger solver to get the wave functions of the Landau level edge states and wire modes at a finite field. Conductance maps obtained using simulated wave functions agree very well with the measured data, thus confirming the calculated wave functions. |
Thursday, March 7, 2019 3:06PM - 3:18PM |
V14.00004: Signatures of Coherent Refraction of Dirac Electrons at Molecular Graphene Interfaces Morgan Brubaker, Yi-Ting Chen, Alison Day, Hari Manoharan One of the many interesting features of graphene is the light-like behavior of its electrons near the Dirac points, characterized by a massless dispersion relation. This behavior suggests the possibility of constructing devices, such as Veselago lenses, that can focus electron wavefunctions in a manner analogous to the focusing of light by optical instruments. Molecular graphene, consisting of carbon monoxide molecules arranged on a copper surface so as to confine the electrons to a honeycomb lattice, is an artificial form of graphene that can be controlled on an atomic level, and is thus an ideal candidate for investigating light-like behavior of Dirac fermions. We report experiments that probe the behavior of electrons incident upon a graphene junction through quasiparticle interference measurements of a molecular graphene system in a low-temperature, ultra-high vacuum scanning tunneling microscope. Our results provide evidence of coherent refraction of electrons passing through the junction, akin to light bending at interfaces between mismatched media. |
Thursday, March 7, 2019 3:18PM - 3:30PM |
V14.00005: Electron pairing by remote-phonon scattering in oxide-supported graphene Donghan Shin, Massimo V Fischetti, Alexander Demkov Using first principles calculations (density functional theory) we find that placing graphene on an (111)-oriented perovskite SrTiO3 (STO) surface provides a convenient doping mechanism. Moreover, further theoretical analysis suggests that coupling of electrons in graphene to interfacial hybrid plasmon/optical modes via remote-phonon scattering may result in an effective attractive electron-electron interaction that, in turn, could lead to electron pairing and superconductivity. Specifically, we consider the graphene sheet supported by the STO and also gated. Using the full RPA dynamic polarizability of the whole system (including the hybrid mode arising from the coupling of the graphene plasmons to the optical phonons of the STO substrate and gate insulator), we estimate the superconductive transition temperature in both the weak- and the (Eliashberg) strong-coupling limits and also by solving the gap equation. |
Thursday, March 7, 2019 3:30PM - 3:42PM |
V14.00006: Magnetoelectric control of the topological phase in graphene Hiroyuki Takenaka, Shane Sandhoefner, Alexey E Kovalev, Evgeny Y Tsymbal The appearance of topological states in antiferromagnetic (AFM) materials has recently aroused considerable attention, resulting in the emergent field of topological AFM spintronics. Here, we theoretically explore the effect of voltage-controlled AFM order in chromia (Cr2O3) on the emerging topological states in graphene coupled across the Cr2O3/graphene interface. Chromia is a magnetoelectric AFM insulator exhibiting surface magnetization, intrinsically coupled to theNéel vector, which can be switched by an electric field. We find that the proximity effect leads to the broken time-reversal symmetry and the Rashba spin-orbit coupling in graphene, producing the topologically nontrivial band gaps. The resulting topological states and the quantum anomalous Hall effect in graphene are switchable by voltage with the Néel vector in chromia. Interestingly, the band gaps near the K and K’ points in graphene are different due to the staggered coupling on the two sublattices, resulting in the non-vanishing valley current and the valley-polarized anomalous Hall state. The switchable Néel vectorpotentially produces a topological phase transition in graphene. |
Thursday, March 7, 2019 3:42PM - 4:18PM |
V14.00007: Fractional Chern insulators in graphene heterostructures Invited Speaker: Eric Spanton Graphene is a highly tunable platform for studying the effects of electron-electron interactions in two dimensions. Encapsulation with a 2D dielectric (hexagonal boron nitride, hBN), and more recently the use of single-crystal graphite top and bottom gates have decreased the electronic disorder to a level suitable for the to study fragile and exotic strongly correlated states. Additionally, control of twist angle between closely-matched crystal lattices allows for unique control of electronic properties, leading to the “Hofstadter butterfly” and more recently unconventional superconductivity. I will describe the first experimental observation of a class of states in nearly aligned hBN/graphene heterostructures called fractional Chern insulators, a close relative of the fractional quantum Hall effect. In graphene, fractional Chern insulators arise in the presence of electron-electron interactions, high magnetic fields, and a long wavelength ‘moiré’ superlattice formed by close alignment between hBN and graphene lattices. Twist angle between graphene and hBN, electron density and perpendicular electric field tune the underlying single-particle bands to realize different types of fractional Chern insulators. The realization of fractional Chern insulators opens the door for the study of novel topological phase transitions and exotic defect states. |
Thursday, March 7, 2019 4:18PM - 4:30PM |
V14.00008: Normal state transport in superconducting twisted bilayer graphene Hryhoriy Polshyn, Yuxuan Zhang, Matthew Yankowitz, Shaowen Chen, Takashi Taniguchi, Kenji Watanabe, David E Graf, Cory R Dean, Andrea Young Twisted bilayer graphene (tBLG) near the flat band condition is a versatile new platform for the study of correlated physics in 2D. Resistive states have been observed at several commensurate fillings of the flat miniband, along with superconducting states near half filling. To better understand the electronic structure of this system, we study electronic transport of graphite gated superconducting tBLG devices in the normal regime. At high magnetic fields, we observe full lifting of the spin and valley degeneracy. The transitions in the splitting of this four-fold degeneracy as a function of carrier density indicate Landau level (LL) crossings, which tilted field measurements show occur between LLs with different valley polarization. Similar LL structure measured in two devices, one with twist angle θ=1.08° at ambient pressure and one at θ=1.27° and 1.33GPa, suggests that the dimensionless combination of twist angle and interlayer coupling controls the relevant details of the band structure. In addition, we find that the temperature dependence of the resistance at B=0 shows linear growth at several hundred Ohm/K in a broad range of temperatures. We discuss the implications for modeling the scattering processes in this system. |
Thursday, March 7, 2019 4:30PM - 4:42PM |
V14.00009: Fabrication and measurement procedures for magic angle graphene devices Oriol Rubies-Bigordà, Yuan Cao, Daniel Rodan Legrain, Valla Fatemi, Shiang Fang, Kenji Watanabe, Takashi Taniguchi, Efthimios Kaxiras, Pablo Jarillo-Herrero Understanding the behavior of strongly correlated materials, such as unconventional superconductors, has been a challenge for decades. Graphene has turned out to be a new platform to study these phenomena. In particular, we have observed unconventional superconductivity and correlated insulating states by stacking two graphene layers and twisting them at a small angle, of about 1.1°. In this talk, we will focus on the fabrication procedures leading to reproducibly achieving high quality and low disorder ‘magic angle’ twisted bilayer graphene devices, with maximum critical temperatures over 2K and critical fields over 100mT. Such achievements with graphene further contribute to an emerging field, commonly referred to as ‘twistronics’, where other 2d –materials can be twisted to give rise to new correlated systems. |
Thursday, March 7, 2019 4:42PM - 4:54PM |
V14.00010: Systematic analysis of superconductivity in magic-angle twister bilayer graphene nanodevices Daniel Rodan Legrain, Yuan Cao, Oriol Rubies-Bigorda, Valla Fatemi, Shiang Fang, Kenji Watanabe, Takashi Taniguchi, Efthimios Kaxiras, Pablo Jarillo-Herrero Superconductivity and correlated insulating states have recently been observed in ‘magic-angle’ twisted bilayer graphene heterostructures at twist angles close to 1.1 degrees, featuring nearly-flat bands owing to strong interlayer coupling. We here perform a systematic study of superconductivity and correlated phenomena in more than 10 different superconducting devices with angles comprised between 0.99 degrees and 1.19 degrees. Angle dependence of critical temperature and field, together with the interplay between superconductivity and insulating states, are examined. Higher quality devices allow us to extend the analysis to superconductivity when doping the twisted bilayer system with electrons. This 2D superconductor, based on graphene moiré superlattices, offers a uniquely tunable, clean and relatively simple platform for the investigation of strongly correlated physics. |
Thursday, March 7, 2019 4:54PM - 5:06PM |
V14.00011: Transport Properties of a Few-layer Rhombohedral-stacked Graphene Yanmeng Shi, Yaping Yang, Servet Canozdemir, Seok-Kyun Son, Shuigang Xu, Jun Yin, Sergey Slizovskiy, Artem Mishchenko Recently, two-dimensional systems that host flat bands have been extensively investigated experimentally and theoretically, due to the intriguing correlation effects as a result of large density of states. In a few-layer rhombohedral-stacked graphene, the dispersionless flat band localised at the outer surfaces is a promising candidate to realise such strong correlation physics. In this talk, I will present our preliminary data on the electrical transport in a few-layer rhombohedral-stacked graphene. We found that a gap is opened at the surfaces when a displacement field exceeds a threshold value. In a magnetic field we found robust quantum oscillations suggesting the high electronic quality of the observed flat-band surface states. |
Thursday, March 7, 2019 5:06PM - 5:18PM |
V14.00012: Quantum Hall effect in a monolayer graphene magnetized by an antiferromagnet Yingying Wu, Gen Yin, Lei Pan, Alexander Grutter, Kang L. Wang We experimentally demonstrate the quantum Hall effect in a monolayer graphene magnetized by an underlying layer of an antiferromagnet. When interfacing with the AFM, the π bond formed by the p orbitals of carbon is found to sufficiently overlap with the Cr atoms in the AFM layer, and therefore experiences a sizeable Hund's-rule exchange energy. Through field-cooling, the change in the antiferromagnetic order modifies the spin splitting, and thereby shifts quantum Hall plateau, as well as the SdH oscillation. This transport behavior is then used to quantitatively estimate the spin split energy ( about several hundreds of meV ). |
Thursday, March 7, 2019 5:18PM - 5:30PM |
V14.00013: Anomalous drag in double bilayer graphene quantum-Hall superfluids Allan MacDonald, Ming Xie Semiconductor double-layers in the quantum Hall regime tend to have superfluid exciton condensate ground states when the total filling factor is an odd integer, provided that the Landau orbitals at the Fermi level in the two layers have the same orbital character. Since the $N=0$ Landau level of bilayer graphene contains states with both $n=0$ and $n=1$ orbital character, the physics of double bilayers falls outside previously studied cases. We show that the superfluid phase stiffness vanishes in double bilayer graphene when $n=0$ and $n=1$ orbitals states are degenerate in one of the layers, even though the gap for charged excitations remain large, and speculate that this property is behind the recent discovery of strong anomalous drag near a $n=0/1$ degeneracy point. |
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