Bulletin of the American Physical Society
APS March Meeting 2017
Volume 62, Number 4
Monday–Friday, March 13–17, 2017; New Orleans, Louisiana
Session Y30: Properties of Multilayer 2D Materials and Heterostructures |
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Sponsoring Units: DCMP DMP Chair: Enrique Cobas, Naval Research Laboratory Room: 293 |
Friday, March 17, 2017 11:15AM - 11:27AM |
Y30.00001: Direct measurement of the band gaps in graphene on hexagonal boron nitride Suyong Jung, Hakseong Kim We have performed electron tunneling spectroscopy measurements on graphene-hexagonal boron-nitride ($h$-BN) van der Waals heterostructures. In this device scheme, single-layer graphene flake is coupled to a rotationally aligned $h$-BN substrate and a thin $h$-BN tunneling barrier is placed on top of the graphene-$h$-BN stack, which allows us to probe the electronic structures of the graphene superlattice with tunneling spectroscopy measurements as functions of external gate voltage and magnetic field. The superlattice Dirac points are identified as dips in differential conductance plots at both the electron- and hole-doped regions. We have observed that the intrinsic energy gaps are formed at both the superlattice and the main Dirac points, which are confirmed by the development of Landau levels fanning out at the edges of both energy gaps as the external magnetic field increases. We will discuss the intrinsic values and possible origins of the band gaps in graphene superlattice on $h$-BN substrates. [Preview Abstract] |
Friday, March 17, 2017 11:27AM - 11:39AM |
Y30.00002: Recovering edge states of graphene nanoislands on Ir(111) by silicon intercalations Lei Tao, Hui Chen, Yande Que, Dongfei Wang, Ruizi Zhang, Wende Xiao, Yu-yang Zhang, Shixuan Du, Sokrates Pantelides, Hong-jun Gao It has been predicted by theory that free-standing graphene nanoribbons with zigzag edges have spin-polarized edge states with a promise for applications. However, it has been widely reported that graphene nanoislands (GNIs) on metal substrates have no states that are localized at zigzag edges because of interaction with substrate electrons. Here, we demonstrate that edge states of GNIs with zigzag edges on Ir(111) can be recovered by intercalating a layer of Si atoms between GNIs and the Ir substrate. Using scanning tunneling microscopy and spectroscopy, in combination with density functional theory calculations, we show that GNIs are effectively decoupled from the Ir substrate by the intercalated Si layer, leading to the recovery of edge states that were originally suppressed by graphene-substrate interaction. We also find that edge states gradually shift to the Fermi level with increasing lateral sizes of the GNIs. In addition, theoretical calculations show that edge states of some irregular GNIs are spin-polarized, which suggests an avenue for construction of graphene-based spintronic devices. [Preview Abstract] |
Friday, March 17, 2017 11:39AM - 11:51AM |
Y30.00003: Thermoelectric Transport Across Graphene/Hexagonal Boron Nitride/Graphene Heterostructures Nirakar Poudel, Zhen Li, Stephen Cronin, Chun-chung Chen, Li Shi We report thermoelectric transport measurements across a graphene/hexagonal boron nitride (h-BN)/graphene heterostructure device. Using an AC lock-in technique, we are able to separate the thermoelectric contribution to the I--V characteristics of these important device structures. The temperature gradient is measured optically using Raman spectroscopy, which enables us to explore thermoelectric transport produced at material interfaces, across length scales of just 1--2 nm. Based on the observed thermoelectric voltage ($\Delta $V) and temperature gradient ($\Delta $T), a Seebeck coefficient of --99.3 $\mu $V/K is ascertained for the heterostructure device. The obtained Seebeck coefficient can be useful for understanding the thermoelectric component in the cross-plane I--V behaviors of emerging 2D heterostructure devices. These results provide an approach to probing thermoelectric energy conversion in two-dimensional layered heterostructures. [Preview Abstract] |
Friday, March 17, 2017 11:51AM - 12:03PM |
Y30.00004: Improvement of I$_{on}$/I$_{off}$ for $h$-BN encapsulated bilayer graphene by graphite local back gate electrode Teerayut Uwanno, Takashi Taniguchi, Kenji Watanabe, Kosuke Nagashio The critical issue for bilayer graphene (BLG) devices is low I$_{on}$/I$_{off}$ even at the band gap of 0.3eV. Band gap in BLG can be formed by creating potential difference between the two layers of BLG. This can be done by applying external electric field perpendicularly to BLG to induce different carrier densities in the two layers. Due to such origin, the spatial uniformity of band gap in the channel is quite sensitive to charge inhomogeneity in BLG. In order to apply electric field of 3V/nm to open the maximum band gap of 0.3eV, high-$k$ gate stack has been utilized so far. However, oxide dielectrics usually have large charge inhomogeneity causing in-plane potential fluctuation in BLG channel. Due to surface flatness and small charge inhomogeneity, $h$-BN has been used as dielectrics to achieve high quality graphene devices, however, I$_{on}$/I$_{off\, }$for BLG/$h$-BN heterostuctures has not been reported yet. In this study, we used graphite as local back gate electrode to BLG encapsulated with $h$-BN. This resulted in much higher I$_{on}$/I$_{off}$, indicating the importance of screening of charge inhomogeneity from SiO$_{2}$ substrate surface by local graphite back gate electrode. [Preview Abstract] |
Friday, March 17, 2017 12:03PM - 12:15PM |
Y30.00005: Designer 2D bandstructures by superlattice patterning of van der Waals materials Carlos Forsythe, Pilkyung Moon, Mikito Koshino, Takashi Taniguchi, Kenji Watanabe, Philip Kim, Cory Dean Nanopatterning two-dimensional electron systems with long wavelength periodic potentials has long been used as a controllable way to modify the intrinsic electronic bandstructure of crystal lattices. In graphene, the combination of an extremely high mobility, linear energy dispersion, and direct access to the two-dimensional electron gas, provides an ideal platform with greater versatility than possible with conventional materials. However, efforts so far have been limited by the difficulty in patterning structures at the necessary small length scales while maintaining a high degree of fidelity over micron-sized device channels. Here we present transport data from graphene devices under superlattice potentials realized by dielectric engineering at the nano-scale. We observe clear evidence of both Brillouin zone folding and Hofstadter's fractal quantum Hall effect. This system is highly tunable, admitting a wide range of lattice symmetries and strength. Transport response is compared against theoretical modelling for a number of different superlattice symmetries, showing excellent agreement with theoretical modeling. This device architecture represents a new 2DEG system, the limitations of which are only starting to be explored. [Preview Abstract] |
Friday, March 17, 2017 12:15PM - 12:27PM |
Y30.00006: Modeling artificial graphene in Si/SiGe hetrostructures Leon Maurer, John King Gamble, Jonathan Moussa, Lisa Tracy, Shih-Hsien Huang, Yen Chuang, Jiun-Yun Li, Chih-Wen Liu, Tzu-Ming Lu Artificial graphene is a synthetic material made using a nanostructure with identical 2D potential wells arranged in a honeycomb lattice. Unlike normal graphene, the properties of artificial graphene can be controlled by changing the nanostructure geometry and adjusting applied voltages. We perform a theoretical study of artificial graphene formed from a 2D electron gas (2DEG) in Si/SiGe and Ge/SiGe heterostructures by a metal honeycomb gate and a global top gate. While many models of artificial graphene assume a simple form for the potential landscape in the 2DEG, we instead calculate the potential landscape for actual devices with a range of bias voltages and geometries. This allows us to find the resulting bandstructure and calculate transport parameters, which we compare directly to experimental results. Sandia is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the US Department of Energy’s National Nuclear Security Administration under Contract No. DE-AC04-94AL85000. This work was funded by the Laboratory Directed Research and Development Program. The work at NTU was supported by the Ministry of Science and Technology (103-2622-E-002-031 and 103-2112-M- 002-002-MY3). [Preview Abstract] |
Friday, March 17, 2017 12:27PM - 12:39PM |
Y30.00007: Anomalous Magnetic Response of Two-Dimensional Materials Valeri Kotov, Sanghita Sengupta, Oleg Sushkov The magnetic response of atomically thin materials with Dirac spectrum and spin-orbit interactions can show strong dependence on electron-electron interactions. While graphene itself has very small spin-orbit coupling, various 2D compounds "beyond graphene" are good candidates to exhibit the strong interplay between spin-orbit and Coulomb interactions. Materials in this class include dichalcogenides (such as MoS$_2$ and WSe$_2$), silicene, germanene, as well as 2D topological insulators described by the Kane-Mele model. We present a unified theory for their in-plane magnetic field response leading to "anomalous", i.e. electron interaction-dependent g-factors. Therefore our predictions can be used to construct unique magnetic probes with high sensitivity to electron correlations. [Preview Abstract] |
Friday, March 17, 2017 12:39PM - 12:51PM |
Y30.00008: Electrostatic doping and hybrid carriers in graphene on a polar SrTiO$_{\mathrm{3}}$ (111) surface: theoretical investigation Donghan Shin, Alexander A. Demkov Doping graphene layers presents a difficult practical and fundamental problem. We consider theoretically, the possibility of electrostatic doping of graphene by the intrinsic field of a polar substrate. By way of example, density functional theory calculations are carried out for a graphene sheet placed on the (111)-oriented perovskite SrTiO$_{\mathrm{3}}$ surface. We find that the Fermi surface moves well below the Dirac point of graphene, resulting simultaneously in a fast conducting channel in graphene, and a slow, large effective mass channel in the oxide surface. Electrostatic gating may allow one to explore peculiar states that, through the ``no-crossing'' reminiscent of polaritons, would represent a hybrid carrier that exists simultaneously in both materials. Importantly, in addition to the field doping, we identify a more ``obvious'' mechanism of doping through the contact potential difference, which may have wider applications in the doping of two-dimensional materials. [Preview Abstract] |
Friday, March 17, 2017 12:51PM - 1:03PM |
Y30.00009: Real Space Bose-Einstein Condensation of Optical Phonons in Intercalated van der Waals Heterostructures Igor Altfeder, Andrey Voevodin, Michael Check, Sarah Eichfeld, Joshua Robinson, Alexander Balatsky Using quantum tunneling of electrons into vibrating surface atoms, phonon oscillations can be observed on the atomic scale. Here we present scanning tunneling microscopy study of coherent quasi-bound states produced by almost dispersionless optical phonons in intercalated van der Waals heterostructures. Our results show that the effective radius of quasi-bound states, the real-space distribution of phonon standing wave amplitudes, the scattering phase shifts, and the non-linear intermode coupling strongly depend on the presence of defect-induced scattering resonance. The coherence of these quasi-bound states arises from phase- and frequency-synchronized dynamics of all phonon modes, and indicates the formation of quantum many-body condensate of optical phonons around resonant defects. We found that increasing the strength of the scattering resonance causes the increase of the condensate droplet area without affecting its density. The observed phenomenon represents the real-space analogue of non-equilibrium phonon Bose-Einstein condensation. [Preview Abstract] |
Friday, March 17, 2017 1:03PM - 1:15PM |
Y30.00010: Broken sublattice symmetry states in Bernal stacked multilayer graphene Chiho Yoon, Yunsu Jang, Jeil Jung, Hongki Min We analyze the ordered phases of Bernal stacked multilayer graphene in the presence of interaction induced band gaps due to sublattice symmetry breaking potentials, whose solutions can be analyzed in terms of light-mass and heavy-mass pseudospin doublets which have the same Chern numbers but opposite charge polarization directions. The application of a perpendicular external electric field reveals an effective Hund's rule for the ordering of the sublattice pseudospin doublets in a tetralayer, while a similar but more complex phase diagram develops with increasing layer number. [Preview Abstract] |
Friday, March 17, 2017 1:15PM - 1:27PM |
Y30.00011: Photoexcitation cascade and quantum-relativistic jet formation in graphene Cyprian Lewandowski, Leonid Levitov Interactions between ultra-relativistic particles can lead to striking behavior in which a high-energy particle creates showers of softer particles characterized by a collimated angular distribution aligned with the particle velocity. These showers, known as jets, are a generic phenomenon relevant for all quantum cascades of linearly dispersing particles. This talk will discuss jets formed upon photoexcitation in graphene, which due to its linear dispersion provides an appealing medium for exploring quantum-relativistic phenomena. We will study the cascade generated by carrier-carrier collisions in photon absorption, wherein a single photon creates an electron-hole (e-h) excitation that decays producing multiple near-collinear secondary e-h excitations. We will argue that the cascade can occur through an off-shell mechanism such that all the particles and holes involved reside outside the energy-momentum dispersion manifold, relieving the bottleneck arising in the on-shell process due to energy and momentum conservation. The characteristics of the jets such as the angular and energy distribution of the particles will be discussed. Photogenerated jets provide an interesting setting to investigate the carrier-carrier collision processes in graphene and other Dirac materials. [Preview Abstract] |
Friday, March 17, 2017 1:27PM - 1:39PM |
Y30.00012: Coupled electron-hole bilayer graphene sheets: Superfluidity, Charge Density Waves, and Coupled Wigner Crystals Mohammad Zarenia, Francois Peeters, David Neilson The juxtaposition of superconducting and charge density wave (CDW) phases that is often observed in connection with High-Temperature Superconductors, is attracting considerable attention. In these systems, the crystal lattice provides a polarizable background, needed to drive the CDW phase. We report on a different system that exhibits the association of superfluid and CDW phases, but in which the polarizable background is uniform. Our system consists of two coupled two-dimensional bilayers of graphene, one bilayer containing electrons and the other holes interacting through the long range Coulomb interaction. To account for the inter-layer correlation energy accurately, we introduce a new approach which is based on the random phase approximation at high densities and interpolation between the weakly- and strongly-interacting regimes. We determine the zero temperature phase diagram in which the two control parameters are the equal electron and hole densities and the thickness of the insulating barrier separating the two bilayers. We find in addition to an electron-hole superfluid and a one-dimensional CDW phases that there exist also a coupled electron-hole Wigner crystal. The structure of the crystal background plays no role in determining the phase diagram. [Preview Abstract] |
Friday, March 17, 2017 1:39PM - 1:51PM |
Y30.00013: Confined States and Tunnelling in Gated Graphene Nanoribbons E, Guilleminot, L. Meza-Montes Graphene Quantum Dots (GQDs) are promising candidates for the development of quantum information processors. We propose a scheme to determine electronic states of GQDs as defined by voltage gates applied to armchair graphene nanoribbons. Using transfer matrix method based on the set of solutions proposed by Burkard $et \ al.$ [1], we study confined states of double wells and the transmission of electrons through double barrier systems. Comparison with previous results for systems on the graphene sheet shows good agreement. Confined states of a double well turn out to be very sensitive to deformation of the potential profile, showing strong localization of the electron for asymmetric systems, which also depends on the considered state. Spikes of high transmission appeared for periodic values of the incident angle of the electron travelling through a double barrier and disappear as the systems approaches to a single barrier as one barrier vanishes. We remark effects not shown in usual semiconductor heterostructures. \\ $[1]$ B. Trauzettel $et \ al.$, Nature Physics 3, 192 (2007). [Preview Abstract] |
Friday, March 17, 2017 1:51PM - 2:03PM |
Y30.00014: Semiclassical Boltzmann transport theory for multi-Weyl semimetals Hongki Min, Sanghyun Park, Seungchan Woo, E. J. Mele Multi-Weyl semimetals are a new type of Weyl semimetals which have linear dispersion along one symmetry direction and anisotropic non-linear dispersion along the remaining two directions with a topological charge larger than one. Using the semiclassical Boltzmann transport theory fully incorporating the anisotropy of the system, we calculate DC conductivity as a function of density and temperature, and obtain its characteristic density and temperature dependence derived from the topological band structure of the system. [Preview Abstract] |
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