Bulletin of the American Physical Society
APS March Meeting 2018
Volume 63, Number 1
Monday–Friday, March 5–9, 2018; Los Angeles, California
Session V37: Devices from 2D Materials VI - Quantum MaterialsFocus
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Sponsoring Units: DMP Chair: Grace Xing, Cornell University Room: LACC 411 |
Thursday, March 8, 2018 2:30PM - 2:42PM |
V37.00001: Deterministic activation of room-temperature quantum emitter arrays in hexagonal boron nitride Nicholas Proscia, zAV Shotan, Harishankar Jayakumar, Prithvi Reddy, Marcus Doherty, Audrius Alkauskas, Carlos Meriles, Vinod Menon Defects in hexagonal boron nitride (hBN) have recently been shown to exhibit narrow linewidth, room temperature single photon emission (SPE) which make them highly attractive for quantum technologies. However, the difficulty in control of defect formation in hBN both spectrally and spatially has been a major hurdle in further development of these SPEs for quantum technologies. Here, we demonstrate nanoscale spatial control of emitter formation in hBN through strain engineering. Our approach relies on placing the few-layer hBN film on top of a patterned substrate consisting of SiO2 nanopillars. Once transferred, the hBN drapes over the silica pillar, creating a tent-like structure where the hBN exhibits preferential quantum emission localized to the pillar edges where the hBN is subjected to large strains. The spatial control of emission and spectral homogeneity of the SPEs in hBN at room temperature offers a route forward for salable integarted quantum photonic systems and on-demand deterministic room temperature quantum emitters. |
Thursday, March 8, 2018 2:42PM - 2:54PM |
V37.00002: Electrostatic trapping of trions in transition metal dichalcogenides: towards a hybrid, optically active and gate defined quantum dot Kristiaan De Greve, Andrey Sushko, Ke Wang, Luis Jauregui, Dominik Wild, Giovanni Scuri, You Zhou, Alexander High, Philip Kim, Hongkun Park, Mikhail Lukin Optically active, two-dimensional van der Waals materials, such as the transition metal dichalcogenides (TMDCs), have recently emerged as an exciting platform for novel optoelectronic devices and device physics. Many of the attractive properties of the TMDCs can be attributed to a combination of their large excitonic binding energy and the ability to stack multiple layers together into Van der Waals heterostructures. |
Thursday, March 8, 2018 2:54PM - 3:06PM |
V37.00003: Engineering of the point defect in WSe2: a strong spin splitting of the localized defect state Chendong Zhang, Cong Wang, Feng Yang, Ming-Yang Li, Lain-Jong Li, Wang Yao, Wei Ji, Chih-Kang Shih Due to the semiconductor nature of atomically thin transition metal dichalcogenides (TMDs), it has soon been recognized that defects can play important roles in the electron transport and the optical transition in TMDs. In particular, it was demonstrated recently that the point defect (e.g. W vacancy) in WSe2 can behave as single quantum emitters, which are at the heart of the quantum optics and photonic quantum-information technologies. One of the essential steps towards the realistic application of the point defects in monolayer semiconductor is to tune the localized electronic structure trough the defect engineering. Here, we report a single potassium atom decoration of the W vacancy in monolayer WSe2. Combing the scanning microscope/spectroscopy and the first principle calculation, we unveiled the atomic and electronic structures of the K/W-vac defect. We show that the K atom behaves as a surprising p-type dopant that shifts the acceptor states upwards, and narrows the local gap by about 0.2 eV compared with the intrinsic W-vac. Most interestingly, the symmetry breaking by the K atom leads to significant spin-orbit coupling splits of the localized defect states (> 100 meV). A local magnetic moment is predicted to form when the charging effect from the substrate was excluded. |
Thursday, March 8, 2018 3:06PM - 3:42PM |
V37.00004: 2D Quantum Materials for Quantum Information Processing and Sensing Invited Speaker: Dirk Englund It was recently shown that 2D materials can be assembled into entirely new types of heterostructures, enabling optoelectronic properties that were impossible using bulk semiconductors. These atomically engineerable heterostructures hold particular promise for quantum technologies. Here, we review our recent work in two application areas. The first part of the talk focuses on chip-integrated graphene photodetectors. Depending on the different photodetection processes available in graphene, waveguide-integrated detectors can attain high response speed and responsivity [1], and they also promise single-photon resolution across a broad optical spectrum[2]. The second part of the talk focuses on light sources, including spectrally tunable thermal[3] and spectrally tunable single-photon sources[4]. |
Thursday, March 8, 2018 3:42PM - 3:54PM |
V37.00005: Spin-Split Impurity States in Manganese-Doped Single-Layer Molybdenum Disulfide Caleb Zerger, Alex Contryman, Hong Li, Xiaolin Zheng, Hari Manoharan Manganese-doped single-layer molybdenum disulfide (Mn-MoS2) has been predicted to be a two-dimensional dilute magnetic semiconductor (DMS). We present scanning tunneling microscopy and spectroscopy data on Mn-MoS2, in which we identify multiple impurity states associated with manganese dopants. Tip induced band bending causes ring features to appear around impurities in energy-dependent spectral maps, with some impurities exhibiting multiple rings at different energies. By quantifying the number, energies, and sizes of these charging rings, we are able to associate these impurity states with several impurity incorporation sites. By correlating this data with density functional theory calculations, we find evidence of specific impurity sites with a large energy splitting between up and down spin states. In addition to various impurity types, we identify local electronic signatures of manganese acting as both a disulfur substitutional dopant and as a molybdenum substitutional dopant, the latter a necessary precursor to the two-dimensional DMS state. |
Thursday, March 8, 2018 3:54PM - 4:06PM |
V37.00006: Transport Spectroscopy of Sublattice-Resolved Resonant Scattering in Hydrogen-Doped Bilayer Graphene Tiancong Zhu, Jyoti Katoch, Denis Kochan, Simranjeet Singh, Jaroslav Fabian, Roland Kawakami Two-dimensional materials are atomically thin membranes with extreme surface sensitivity, enabling unprecedented tuning of their electronic, magnetic and spintronic properties via surface modification. In particular, hydrogenation has emerged as a powerful technique to alter the properties of graphene, which is predicted to induce band-gap, spin-orbit coupling, as well as magnetism. |
Thursday, March 8, 2018 4:06PM - 4:18PM |
V37.00007: Controllable quantum devices in bilayer graphene Klaus Ensslin, Hiske Overweg, Marius Eich, Peter Rickhaus, Thomas Ihn Quantum dots in graphene have been mostly realized by etching leading to localized states at the disordered edges. [1] It is well known that in bilayer graphene a vertical electric field opens a band gap. [2] Gap-enabled electrostatic confinement has been used with limited success to define quantum devices [3] because of leakage currents thought to occur at the physical sample edges [4]. Here we fabricate electrostatically tunable barriers on bilayer graphene devices with a graphite back gate. We measure pinch-off resistances exceeding GOhms and observe quantized conductance plateaus in one-dimensional constrictions. [5] With suitable gate geometries, few-carrier hole and electron quantum dots can be electrostatically defined. We control the occupation of quantum dots with single holes and electrons. Four-fold level bunching is observed in Coulomb blockade spectroscopy which is related to valley and spin states. The magnetic field dependenct conductance allows us to investigate orbital and spin/valley effects. |
Thursday, March 8, 2018 4:18PM - 4:30PM |
V37.00008: Sign Reversal of Electron-Hole Coulomb Drag in Double Graphene Bilayers Mohammad Zarenia, David Neilson, Alex Hamilton, Francois Peeters Closely coupled two-dimensional electron-hole sheets are attracting great interest as they should generate novel quantum phases driven by the strong Coulomb attractions between the sheets. Coulomb drag of carriers in one sheet by carriers moving in the other is a powerful tool to study Fermi liquid properties and identify the formation of novel phases. Two independent Coulomb drag experiments on electron-hole sheets in graphene double bilayers have reported an unexplained and puzzling sign reversal of the Coulomb drag signal. We demonstrate that this unusual effect can be explained by the multiband character of bilayer graphene and the temperature dependence of effective mass at low densities caused by the electron-electron interactions. Our theory produces excellent agreement with the observed structure in the Coulomb drag resistance, capturing the key features of the recent experiments over the full reported range of temperatures. |
Thursday, March 8, 2018 4:30PM - 4:42PM |
V37.00009: Structural study of atomic relaxation and commensurate transition in twisted bilayer graphene Rebecca Engelke, Hyobin Yoo, Kuan Zhang, Paul Cazeaux, Suk Hyun Sung, Robert Hovden, Adam Tsen, Takashi Taniguchi, Kenji Watanabe, Gyu-Chul Yi, Miyoung Kim, Mitchell Luskin, Ellad Tadmor, Philip Kim At small twist angles, artificially stacked bilayer graphene is known to relax into a structure of periodic, commensurate domains with discommensurate domain walls, providing the material with a superlattice potential and a network of topologically-protected conduction paths. Dark-field transmission electron microscopy has been used to visualize these domains and make inferences about the structure. We supplement this information with quantitative analysis of electron diffraction patterns in comparison with simulations, to gain new insight into the details of the commensurate structure and its twist-angle dependence. |
Thursday, March 8, 2018 4:42PM - 4:54PM |
V37.00010: Atomic Scale Relaxation at the van der Waals Interface in Twisted Bilayer Graphene Hyobin Yoo, Kuan Zhang, Rebecca Engelke, Paul Cazeaux, Suk Hyun Sung, Robert Hovden, Adam Tsen, Takashi Taniguchi, Kenji Watanabe, Gyu-Chul Yi, Miyoung Kim, Mitchell Luskin, Ellad Tadmor, Philip Kim Adjusting stacking angle in two-dimensional van der Waals (vdW) atomic layers allows engineering physical properties of the heterostructures. Rotational lattice mismatch creates additional quasiperiodic structures described by moiré patterns, modifying the electronic band structures. More interestingly, the quasiperiodic moiré structures can be reconstructed into commensurate domain structures as a result of interplay between vdW interaction energy and elastic energy at the interface. The commensurate domains are topologically nontrivial in the presence of vertical displacement field, providing a network of one-dimensional transport modes along their boundaries. In this talk, we present transmission electron microscopy results to discuss the microstructures of twisted bilayer graphene. Electrical transport behavior with controlled vertical displacement field and carrier density was investigated to understand the effect of commensurate domains and their boundaries on the charge transport behavior. |
Thursday, March 8, 2018 4:54PM - 5:06PM |
V37.00011: Twist Dependent Resonant Tunneling in WSe2-based Heterostructures Kyounghwan Kim, Hema Movva, Gregory Burg, Stefano Larentis, Yimeng Wang, Takashi Taniguchi, Kenji Watanabe, Leonard Register, Emanuel Tutuc We investigate interlayer tunneling in heterostructures consisting of two tungsten diselenide (WSe2) monolayers, separated by hexagonal boron nitride (hBN). In samples where the two WSe2 monolayers are rotationally aligned, we observe resonant tunneling, manifested by a large interlayer tunneling conductance (σIL) and negative differential resistance near zero interlayer bias (VIL), which stem from energy and momentum conserving tunneling. The strong spin-orbit coupling in monolayer WSe2 splits the valence band and leads to coupled spin-valley degrees of freedom. To probe the role of spin-valley in the interlayer tunneling current-voltage characteristics, we investigate two WSe2 layers separated by hBN with 0° and 180° relative twist between the WSe2 layers. In the 0°-twist samples, a large σIL peak appears at VIL = 0 V due to alignment of the like-valley-bands, i.e., the K valleys of the two WSe2 layers. In the 180°-twist samples, in which the K and K’ valleys in opposite layers are aligned in momentum space, we observe a negligible σIL at VIL = 0 V. Spin-valley conserved interlayer tunneling occurs when the like-valley-bands in the two layers are aligned, manifested as a large σIL at |VIL| = ±0.5 V. |
Thursday, March 8, 2018 5:06PM - 5:18PM |
V37.00012: Spectroscopy of single-molecule magnets using graphene quantum dots A El Fatimy, Petr Neugebauer, Luke St. Marie, Jakub Hruby, Byoung Don Kong, Anthony Boyd, Rachael Myers-Ward, Ivan Nemec, Kevin Daniels, Anindya Nath, Dominik Bloos, Joris van Slageren, D. Kurt Gaskill, Paola Barbara Graphene’s properties as a gapless semiconductor with small heat capacity make it an ideal material for broadband hot-electron bolometers. We recently showed that graphene nanostructured into quantum dots yields bolometers with extraordinary performance [1, 2] and we are now testing these bolometers in applications requiring high sensitivity, such as spectroscopy of single molecule magnets (SMMs). SMMs can be placed directly on the graphene bolometers allowing spectroscopy studies of small amounts of these materials, as opposed to standard measurement techniques using large pellets or crystals of SMMs. We present preliminary spectroscopy studies on Mn and Co based SMMs. |
Thursday, March 8, 2018 5:18PM - 5:30PM |
V37.00013: Direct Observation of Dirac-Electron Wedding Cakes in Graphene Quantum Dots Christopher Gutierrez, Daniel Walkup, Fereshte Ghahari, Cyprian Lewandowski, Joaquin Rodriguez Nieva, Takashi Taniguchi, Kenji Watanabe, Leonid Levitov, Nikolai Zhitenev, Joseph Stroscio The ability to apply local nanometer scale gate potentials in graphene heterostructures has recently enabled the creation of quantum dot (QD) resonators inside circular p-n junction rings, where one has detailed control of electron orbits by means of spatial and magnetic confinement. In this talk, we provide the first spatial visualization of the intricate evolution of the energy spectrum, and corresponding wavefunctions, as we condense the QD atomic-like shell states into quantized Landau levels (LLs) in a circular graphene QD. The resonator states are observed to condense into LLs beginning in the resonator center and then progressing to become edge states at the QD boundaries. The formation of compressible and incompressible rings due to Coulomb interactions inside the resonator are observed in detailed spectroscopic mapping at higher magnetic fields where the screened potential develops a prominent wedding cake-like structure. Surprisingly, the inclusion of electron interactions are found to be necessary to describe even very weak field behavior, demonstrating the importance of electron interactions in confined graphene systems. |
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