Bulletin of the American Physical Society
APS March Meeting 2019
Volume 64, Number 2
Monday–Friday, March 4–8, 2019; Boston, Massachusetts
Session X14: Devices from 2D Materials -- Twisted bilayersFocus
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Sponsoring Units: DMP DCMP Chair: Matthew Yankowitz, Columbia University Room: BCEC 153C |
Friday, March 8, 2019 8:00AM - 8:12AM |
X14.00001: Bilayer graphene Josephson junctions with induced spin orbit coupling Avradeep Pal, Kenji Watanabe, Takashi Taniguchi, Srijit Goswami Interfacial coupling of graphene with transition metal dichalcogenides (TMDs) have emerged as a way to induce enhanced spin orbit coupling in few layer graphene. In our experiment, bilayer Graphene is sandwiched between exfoliated WSe2 (bottom) and h-BN (top) and edge contacted by means of superconducting NbTiN. We report the first study of induced superconductivity in such systems by demonstrating the Josephson effect in a series of samples of varying junction lengths (300nm to 1300nm). All measured devices are found to be in the long junction limit. By means of bottom gating, we access a wide range of densities far away from the charge neutrality point, and trace the product as a function of Thouless Energy (Eth); thereby testing the validity of standard SNS long junction diffusive limit superconducting proximity effect scaling behavior (IcRn = αEth) in spin orbit coupled graphene systems. As expected from theory – a universal scaling of these parameters is observed for all devices, however with a severely reduced α than expected. |
Friday, March 8, 2019 8:12AM - 8:24AM |
X14.00002: Electron scattering and thermopower in twisted bilayer graphene Ting Fung Chung, Yang Xu, Lisha An, Evan Witkoske, Na (Luna) Lu, Yong Chen Twisted bilayer graphene (TBLG) with a small twist-angle is of particular interest because of its distinct energy band, where recent electrical transport experiments revealed van Hove singularities (VHSs), hybridization gaps, and correlation-induced behaviors. To better understand the effect of VHSs in TBLG, we perform transport experiments in this study. We observe that the temperature-dependent resistivity follows a power-law for carrier density between the main charge neutrality and hybridization gap. The evolution of the temperature exponent with carrier density shows a W-shaped dependence, with minima near the VHSs and maxima toward the hybridization gap. This W-shaped behavior could be attributed to different electron scattering mechanisms. We also study thermopower in TBLG, where we observe multiple sign changes in thermopower near VHSs at low temperatures. Our work may provide more information about the electronic structure and physical process in the TBLG system. |
Friday, March 8, 2019 8:24AM - 8:36AM |
X14.00003: Towards a gate-controlled valley-analyzer in bilayer graphene. Hao Chen, Jens Martin The inequivalent valleys K and K' in k-space of 2D-hexagonal materials offer a new valley-degree of freedom similar to spin and controllable via electrostatic gating. In bilayer graphene, the sign of Berry curvature in each valley is determined by the direction of the external electric field whereas the sign of Berry curvature in each valley has opposite sign. Theorists have predicted that at 1D-boundaries at which the sign of Berry curvature changes, counter-propagating valley-polarized 1D-channels emerge. Including spin and sublattice degeneracy, there are 4 quantized conduction channels in each direction. This approach provides a gate-controlled platform for valley-polarizers and analyzers, yet it is technically challenging to build such a nanoscale system. Here, we fabricate such system with four pairs of dual-split gates with optimized geometry and stacking methods. Low temperature measurements show large conductance contrast with a factor of 100 between different gate configurations. In our devices, we achieve conductivity of nearly 4 conduction channels in each valley polarized state, much closer to the ballistic limit than previous work. Ultimately, one pair can be valley polarizer and the subsequent pair could detect the valley polarization and serve as valley analyzer. |
Friday, March 8, 2019 8:36AM - 8:48AM |
X14.00004: Magnetotransport of twisted bilayer transition metal dichalcogenides En-Min Shih, Lei Wang, Augusto Ghiotto, Daniel A Rhodes, Cheng Tan, Abhay Pasupathy, James Hone Two-dimensional van der Waals material has become an exciting field. One of the reason is that this system provides multiple degrees of freedom, including stacking order, interlayer spacing and interlayer twisted angle, to engineer the material band structure. Manipulate these degrees of freedom has led to observation of several emergent phenomena, including the fractal quantum Hall effect, tunable Mott insulators, and unconventional superconductivity. In particular, interlayer interaction in van der Waals heterostructures at a different twisted angle could induce many exotic phenomena. How the interlayer interaction affect the electronic structure of a material is a fundamental question. In this study, we report magnetotransport of twisted bilayer transition-metal dichalcogenides (TMDs) and demonstrate how its band structure vary with twisted angle. These results broaden the application range of van der Waals heterostructure for future electronic devices. |
Friday, March 8, 2019 8:48AM - 9:00AM |
X14.00005: Charge-transfer insulation in twisted bilayer graphene. Paula Mellado, Louk Rademaker We studied the real space structure of states in twisted bilayer graphene at the "magic angle" θ = 1.08○. The flat bands close to charge neutrality are composed of a mix of "ring" and "center" orbitals around the AA stacking region. An effective model with localized orbitals is constructed, which necessarily includes more than just the four flat bands. Long-range Coulomb interaction causes a charge-transfer at half-filling of the flat bands from the "center" to the "ring" orbitals. Consequently, the Mott phase is a featureless spin-singlet paramagnet. We estimate the effective Heisenberg coupling that favors the singlet coupling to be J = 3.3 K, consistent with experimental values. The superconducting state depends on the nature of the dopants: hole-doping yields p+ip-wave whereas electron-doping yields d+id-wave pairing symmetry. |
Friday, March 8, 2019 9:00AM - 9:12AM |
X14.00006: Phases of a phenomenological model of twisted bilayer graphene John Dodaro, Steven Kivelson, Yoni Schattner, Xiao-Qi Sun, Chao Wang We propose a lattice scale two-band generalized Hubbard model as a caricature of the electronic structure of twisted bilayer graphene. Various possible broken symmetry phases can arise, including a nematic phase (which is a form of orbital ferromagnet) and an orbital-triplet spin-singlet superconducting phase. Concerning the mechanism of superconductivity -- we propose an analogy with superconductivity in alkali-doped C_{60} in which a violation of Hund's first rule plays a central role. |
Friday, March 8, 2019 9:12AM - 9:48AM |
X14.00007: Characterization of atomic scale lattice reconstruction in twisted van der Waals interfaces of layered material Invited Speaker: Philip Kim Control of the interlayer twist of van der Waals (vdW) interfaces has been widely used to engineer an artificial 2-dimensional (2D) electronic systems by the formation of a moiré superlattice. Many exotic physical phenomena occur associated with the incommensurability of the moiré superstructures where the wealth of the nontrivial topology of electronic band structures plays a key role to create exotic physical phenomena. In this presentation, we will discuss the engineered atomic scale reconstruction at twisted vdW interfaces using electron microscopy, optical spectroscopy, and electrical transport. We then will discuss emerging electronic and optoelectronic physics in the vdW interface between homojunctions. |
Friday, March 8, 2019 9:48AM - 10:00AM |
X14.00008: Transport measurements of correlated insulating states in twisted bilayer graphene Aaron Sharpe, Eli J Fox, Arthur Barnard, Joe Finney, Kenji Watanabe, Takashi Taniguchi, Marc Kastner, David Goldhaber-Gordon Twisted bilayer graphene (TBG) has emerged as a prominent platform for strongly correlated electrons. When two sheets of monolayer graphene are stacked at an angle near 1.1°, interactions are expected to break the four-fold spin and valley degeneracy resulting in correlated insulating states and superconductivity near densities corresponding to an integer number of electrons per superlattice unit cell [Cao et al., Nature 556, 43–50 (2018), Cao et al., Nature 556, 80–84 (2018)]. Here we present transport measurements studying these correlated states. |
Friday, March 8, 2019 10:00AM - 10:12AM |
X14.00009: Dynamic band structure in twisted bilayer graphene near the magic angle Jinhai Mao, Yuhang Jiang, Xinyuan Lai, Kenji Watanabe, Takashi Taniguchi, Eva Y. Andrei A twist between the orientations of superposed 2D crystal structures leads to a moiré superstructure which can significantly alter their electronic properties. In the case of superposed graphene layers with small twist angle, the energy band flattens resulting in a slow-down of the charge carriers until, at certain “magic angles”, they come to a complete halt. In this regime, electron-electron interactions become strong leading to the emergence of new correlated phases and topologically protected ballistic channels. Using STM on gated twisted bilayer graphene samples, we study the band structure and the emergence of correlated states as a function of doping and twist-angle near the first magic angle. Far from the magic angle, we observe two van hove singularity (VHS) peaks whose separation allows us to extract the interlayer coupling. Contrary to expectation we find that the interlayer hopping is a function of the twist angle and doping. Upon approaching the magic angle the VHS merge into a single flat band which straddles the charge neutrality point. We observe a dynamic reconstruction of the band structure with gap-like features at certain fillings and a corresponding unique spatial reconstruction of the electronic wave-function. |
Friday, March 8, 2019 10:12AM - 10:24AM |
X14.00010: Effect of vacancies on the electronic and optical properties on twisted bilayer graphene Francisco Culchac, Rodrigo Capaz We systematically study the effects of a random distribution of vacancies on the electronic and optical properties of twisted bilayer graphene (TBLG), using a tight-binding method. We study the density of states and optical absorption for different concentrations of vacancies and different rotation angle between the layers. |
Friday, March 8, 2019 10:24AM - 10:36AM |
X14.00011: Scanning tunneling microscopy of van Hove singularities in small angle twisted bilayer graphene Jeannette Kemmer, Youngjoon Choi, Harpreet Singh Arora, Robert Polski, Yiran Zhang, Hechen Ren, Kenji Watanabe, Takashi Taniguchi, Stevan Nadj-Perge Electronic properties of stacked van der Waals bilayers critically depend on the twist angle between the monolayers. A famous example of this effect is the observation of superconductivity and strongly correlated insulating phases in twisted bilayer graphene (TBG) where the two graphene monolayers are twisted by the so-called magic angle (~1.1°). This system supports electronic bands that are almost completely non-dispersive (flat), which gives rise to strong electronic correlation effects. We use scanning tunneling microscopy and spectroscopy to track the positions of van Hove singularities originating from the flat bands in gated TBG for the range of twist angles from 1° to 3°. We find that close to magic angle values the van Hove singularities are strongly modified when the flat bands cross the Fermi level. |
Friday, March 8, 2019 10:36AM - 10:48AM |
X14.00012: Pressure Induced Compression of Flatbands in Twisted Bilayer Graphene Bheema Lingam Chittari, Nicolas Leconte, Srivani Javvaji, Jeil Jung We investigate the bandwidth compression due to out of plane pressure of the moire flatbands near charge neutrality in twisted bilayer graphene for a continuous range of small rotation angles of up to ∼2.5°. The flatband bandwidth minima angles are found to grow linearly with interlayer coupling ω and decrease with Fermi velocity. Application of moderate pressure values of up to 2.5 GPa achievable through a hydraulic press should allow accessing a flatband for angles as large as ∼1.5° instead of ∼1° at zero pressure. This reduction of the moiré pattern length for larger twist angle implies an increase of the effective Coulomb interaction scale per moire cell by about 50% and enhance roughly by a factor of ∼2 the elastic energy that resists the commensuration strains due to the moire pattern. Application of pressure will hence notably facilitate the device preparation efforts required for exploring the ordered phases near magic angle flatbands. We also discuss the phase diagram of flatbands in other hybrid Dirac materials. |
Friday, March 8, 2019 10:48AM - 11:00AM |
X14.00013: Optical measurements of twisted bilayer transition metal dichalcogenides Augusto Ghiotto, Lei Wang, En-Min Shih, Daniel A Rhodes, Cheng Tan, Abhay Pasupathy, James Hone, Cory R Dean Two-dimensional Van der Waals material has become an exciting field. One of the reason is that |
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