1
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Ge Z, Slizovskiy S, Polizogopoulos P, Joshi T, Taniguchi T, Watanabe K, Lederman D, Fal'ko VI, Velasco J. Giant orbital magnetic moments and paramagnetic shift in artificial relativistic atoms and molecules. NATURE NANOTECHNOLOGY 2023; 18:250-256. [PMID: 36879123 DOI: 10.1038/s41565-023-01327-0] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 06/30/2022] [Accepted: 01/13/2023] [Indexed: 06/18/2023]
Abstract
Materials such as graphene and topological insulators host massless Dirac fermions that enable the study of relativistic quantum phenomena. Single quantum dots and coupled quantum dots formed with massless Dirac fermions can be viewed as artificial relativistic atoms and molecules, respectively. Such structures offer a unique testbed to study atomic and molecular physics in the ultrarelativistic regime (particle speed close to the speed of light). Here we use a scanning tunnelling microscope to create and probe single and coupled electrostatically defined graphene quantum dots to unravel the magnetic-field responses of artificial relativistic nanostructures. We observe a giant orbital Zeeman splitting and orbital magnetic moment up to ~70 meV T-1 and ~600μB (μB, Bohr magneton) in single graphene quantum dots. For coupled graphene quantum dots, Aharonov-Bohm oscillations and a strong Van Vleck paramagnetic shift of ~20 meV T-2 are observed. Our findings provide fundamental insights into relativistic quantum dot states, which can be potentially leveraged for use in quantum information science.
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Affiliation(s)
- Zhehao Ge
- Department of Physics, University of California Santa Cruz, Santa Cruz, CA, USA
| | - Sergey Slizovskiy
- Department of Physics and Astronomy, University of Manchester, Manchester, UK
- National Graphene Institute, University of Manchester, Booth Street East, Manchester, UK
| | | | - Toyanath Joshi
- Department of Physics, University of California Santa Cruz, Santa Cruz, CA, USA
| | - Takashi Taniguchi
- International Center for Materials Nanoarchitectonics and National Institute for Materials Science, Tsukuba, Japan
| | - Kenji Watanabe
- Research Center for Functional Materials, National Institute for Materials Science, Tsukuba, Japan
| | - David Lederman
- Department of Physics, University of California Santa Cruz, Santa Cruz, CA, USA
| | - Vladimir I Fal'ko
- Department of Physics and Astronomy, University of Manchester, Manchester, UK.
- National Graphene Institute, University of Manchester, Booth Street East, Manchester, UK.
- Henry Royce Institute for Advanced Materials, Manchester, UK.
| | - Jairo Velasco
- Department of Physics, University of California Santa Cruz, Santa Cruz, CA, USA.
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2
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Ghandchi M, Darvish G, Moravvej-Farshi MK. Photoelectrical properties of integrated photodetectors based on bilayer graphene quantum dots with asymmetric metal contacts: a NEGF-DFT study. Phys Chem Chem Phys 2021; 24:1590-1597. [PMID: 34942635 DOI: 10.1039/d1cp04957g] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/21/2022]
Abstract
We propose investigating the electro-optical properties of photodetectors based on mono- and bilayer graphene quantum dots or nanodots (GNDs). These photodetectors consist of dissimilar metals (gold, silver and titanium) that are in contact with the GNDs. To obtain photoelectrical characteristics, we employed density functional theory to solve the non-equilibrium Green's function. Photo-responsivities and quantum efficiencies obtained for these nanostructures were far better than those for structures based on graphene nanoribbons. Among the proposed photodetectors, the best performance belonged to the bilayer structures illuminated by in-plane polarized incident light. The proposed photodetectors operate without a need for externally applied voltage and are suitable for parallel light propagation using directional couplers based on the evanescent field of incident light; hence, they have applications in optical integrated circuits.
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Affiliation(s)
- Majid Ghandchi
- Department of Electrical Engineering, Ahar Branch, Islamic Azad University, Ahar 5451116714, Iran
| | - Ghafar Darvish
- Department of Electrical Engineering, Science and Research Branch, Islamic Azad University, Tehran 1477893855, Iran.
| | - Mohammad Kazem Moravvej-Farshi
- Nano Plasmo-Photonic Research Group, Faculty of Electrical and Computer Engineering, Tarbiat Modares University, Tehran 1411713116, Iran
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3
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Zacharias M, Kelires PC. Quantum Confinement of Electron-Phonon Coupling in Graphene Quantum Dots. J Phys Chem Lett 2021; 12:9940-9946. [PMID: 34614351 DOI: 10.1021/acs.jpclett.1c02899] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 05/03/2023]
Abstract
On the basis of first-principles calculations and the special displacement method, we demonstrate the quantum confinement scaling law of the phonon-induced gap renormalization of graphene quantum dots (GQDs). We employ zigzag-edged GQDs with hydrogen passivation and embedded in hexagonal boron nitride. Our calculations for GQDs in the sub-10 nm region reveal strong quantum confinement of the zero-point renormalization ranging from 20 to 250 meV. To obtain these values we introduce a correction to the Allen-Heine theory of temperature-dependent energy levels that arises from the phonon-induced splitting of 2-fold degenerate edge states. This correction amounts to more than 50% of the gap renormalization. We also present momentum-resolved spectral functions of GQDs, which are not reported in previous contributions. Our results lay the foundation to systematically engineer temperature-dependent electronic structures of GQDs for applications in solar cells, electronic transport, and quantum computing devices.
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Affiliation(s)
- Marios Zacharias
- Research Unit for Nanostructured Materials Systems, Cyprus University of Technology, P.O. Box 50329, 3603 Limassol, Cyprus
- Department of Mechanical and Materials Science Engineering, Cyprus University of Technology, P.O. Box 50329, 3603 Limassol, Cyprus
| | - Pantelis C Kelires
- Research Unit for Nanostructured Materials Systems, Cyprus University of Technology, P.O. Box 50329, 3603 Limassol, Cyprus
- Department of Mechanical and Materials Science Engineering, Cyprus University of Technology, P.O. Box 50329, 3603 Limassol, Cyprus
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4
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Ge Z, Slizovskiy S, Joucken F, Quezada EA, Taniguchi T, Watanabe K, Fal'ko VI, Velasco J. Control of Giant Topological Magnetic Moment and Valley Splitting in Trilayer Graphene. PHYSICAL REVIEW LETTERS 2021; 127:136402. [PMID: 34623864 DOI: 10.1103/physrevlett.127.136402] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 03/28/2021] [Accepted: 08/17/2021] [Indexed: 06/13/2023]
Abstract
Bloch states of electrons in honeycomb two-dimensional crystals with multivalley band structure and broken inversion symmetry have orbital magnetic moments of a topological nature. In crystals with two degenerate valleys, a perpendicular magnetic field lifts the valley degeneracy via a Zeeman effect due to these magnetic moments, leading to magnetoelectric effects which can be leveraged for creating valleytronic devices. In this work, we demonstrate that trilayer graphene with Bernal stacking (ABA TLG), hosts topological magnetic moments with a large and widely tunable valley g factor (g_{ν}), reaching a value g_{ν}∼1050 at the extreme of the studied parametric range. The reported experiment consists in sublattice-resolved scanning tunneling spectroscopy under perpendicular electric and magnetic fields that control the TLG bands. The tunneling spectra agree very well with the results of theoretical modeling that includes the full details of the TLG tight-binding model and accounts for a quantum-dot-like potential profile formed electrostatically under the scanning tunneling microscope tip.
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Affiliation(s)
- Zhehao Ge
- Department of Physics, University of California, Santa Cruz, California 95064, USA
| | - Sergey Slizovskiy
- Department of Physics and Astronomy, University of Manchester, Oxford Road, Manchester M13 9PL, United Kingdom
- National Graphene Institute, University of Manchester, Booth Street East, Manchester M13 9PL, United Kingdom
| | - Frédéric Joucken
- Department of Physics, University of California, Santa Cruz, California 95064, USA
| | - Eberth A Quezada
- Department of Physics, University of California, Santa Cruz, California 95064, USA
| | - Takashi Taniguchi
- International Center for Materials Nanoarchitectronics National Institute for Materials Science, 1-1 Namiki, Tsukuba 305-0044, Japan
| | - Kenji Watanabe
- Research Center for Functional Materials National Institute for Materials Science, 1-1 Namiki, Tsukuba 305-0044, Japan
| | - Vladimir I Fal'ko
- Department of Physics and Astronomy, University of Manchester, Oxford Road, Manchester M13 9PL, United Kingdom
- National Graphene Institute, University of Manchester, Booth Street East, Manchester M13 9PL, United Kingdom
- Henry Royce Institute for Advanced Materials, Manchester M13 9PL, United Kingdom
| | - Jairo Velasco
- Department of Physics, University of California, Santa Cruz, California 95064, USA
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5
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Flat band carrier confinement in magic-angle twisted bilayer graphene. Nat Commun 2021; 12:4180. [PMID: 34234146 PMCID: PMC8263728 DOI: 10.1038/s41467-021-24480-3] [Citation(s) in RCA: 7] [Impact Index Per Article: 2.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/05/2020] [Accepted: 06/14/2021] [Indexed: 11/08/2022] Open
Abstract
Magic-angle twisted bilayer graphene has emerged as a powerful platform for studying strongly correlated electron physics, owing to its almost dispersionless low-energy bands and the ability to tune the band filling by electrostatic gating. Techniques to control the twist angle between graphene layers have led to rapid experimental progress but improving sample quality is essential for separating the delicate correlated electron physics from disorder effects. Owing to the 2D nature of the system and the relatively low carrier density, the samples are highly susceptible to small doping inhomogeneity which can drastically modify the local potential landscape. This potential disorder is distinct from the twist angle variation which has been studied elsewhere. Here, by using low temperature scanning tunneling spectroscopy and planar tunneling junction measurements, we demonstrate that flat bands in twisted bilayer graphene can amplify small doping inhomogeneity that surprisingly leads to carrier confinement, which in graphene could previously only be realized in the presence of a strong magnetic field.
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6
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Woods CR, Ares P, Nevison-Andrews H, Holwill MJ, Fabregas R, Guinea F, Geim AK, Novoselov KS, Walet NR, Fumagalli L. Charge-polarized interfacial superlattices in marginally twisted hexagonal boron nitride. Nat Commun 2021; 12:347. [PMID: 33436620 PMCID: PMC7804449 DOI: 10.1038/s41467-020-20667-2] [Citation(s) in RCA: 88] [Impact Index Per Article: 29.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/08/2020] [Accepted: 12/08/2020] [Indexed: 11/30/2022] Open
Abstract
When two-dimensional crystals are brought into close proximity, their interaction results in reconstruction of electronic spectrum and crystal structure. Such reconstruction strongly depends on the twist angle between the crystals, which has received growing attention due to interesting electronic and optical properties that arise in graphene and transitional metal dichalcogenides. Here we study two insulating crystals of hexagonal boron nitride stacked at small twist angle. Using electrostatic force microscopy, we observe ferroelectric-like domains arranged in triangular superlattices with a large surface potential. The observation is attributed to interfacial elastic deformations that result in out-of-plane dipoles formed by pairs of boron and nitrogen atoms belonging to opposite interfacial surfaces. This creates a bilayer-thick ferroelectric with oppositely polarized (BN and NB) dipoles in neighbouring domains, in agreement with our modeling. These findings open up possibilities for designing van der Waals heterostructures and offer an alternative probe to study moiré-superlattice electrostatic potentials.
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Affiliation(s)
- C R Woods
- Department of Physics & Astronomy, University of Manchester, Manchester, M13 9PL, UK.
- National Graphene Institute, University of Manchester, Manchester, M13 9PL, UK.
| | - P Ares
- Department of Physics & Astronomy, University of Manchester, Manchester, M13 9PL, UK
- National Graphene Institute, University of Manchester, Manchester, M13 9PL, UK
| | - H Nevison-Andrews
- Department of Physics & Astronomy, University of Manchester, Manchester, M13 9PL, UK
- National Graphene Institute, University of Manchester, Manchester, M13 9PL, UK
| | - M J Holwill
- Department of Physics & Astronomy, University of Manchester, Manchester, M13 9PL, UK
- National Graphene Institute, University of Manchester, Manchester, M13 9PL, UK
| | - R Fabregas
- Department of Physics & Astronomy, University of Manchester, Manchester, M13 9PL, UK
| | - F Guinea
- Imdea Nanociencia, Faraday 9, 28049, Madrid, Spain
- Donostia International Physics Center, Paseo Manuel de Lardizabal, 4, 20018, Donostia-San Sebastian, Spain
| | - A K Geim
- Department of Physics & Astronomy, University of Manchester, Manchester, M13 9PL, UK
- National Graphene Institute, University of Manchester, Manchester, M13 9PL, UK
| | - K S Novoselov
- Department of Physics & Astronomy, University of Manchester, Manchester, M13 9PL, UK
- National Graphene Institute, University of Manchester, Manchester, M13 9PL, UK
- Centre for Advanced 2D Materials, National University of Singapore, Singapore, 117546, Singapore
- Chongqing 2D Materials Institute, Liangjiang New Area, 400714, Chongqing, China
| | - N R Walet
- Department of Physics & Astronomy, University of Manchester, Manchester, M13 9PL, UK
| | - L Fumagalli
- Department of Physics & Astronomy, University of Manchester, Manchester, M13 9PL, UK.
- National Graphene Institute, University of Manchester, Manchester, M13 9PL, UK.
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7
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Sheridan E, Chen L, Li J, Guo Q, Hao S, Yu M, Eom KT, Lee H, Lee JW, Eom CB, Irvin P, Levy J. Gate-Tunable Optical Nonlinearities and Extinction in Graphene/LaAlO 3/SrTiO 3 Nanostructures. NANO LETTERS 2020; 20:6966-6973. [PMID: 32870015 DOI: 10.1021/acs.nanolett.0c01379] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/11/2023]
Abstract
We explore the ultrafast optical response of graphene subjected to intense (∼106 V/cm) local (∼10 nm) electric fields. Nanoscale gating of graphene is achieved using a voltage-biased, SrTiO3-based conductive nanowire junction "written" directly under the graphene and isolated from it by an insulating ultrathin (<2 nm) LaAlO3 barrier. Upon illumination with ultrafast visible-to-near-infrared (VIS-NIR) light pulses, the local field from the nanojunction creates a strong gate-tunable second-order nonlinearity in the graphene and produces a substantial difference-frequency (DFG) and sum-frequency generation (SFG) response detected by the nanojunction. Spectrally sharp, gate-tunable extinction features (>99.9%) are observed in the VIS-NIR and SFG spectral ranges, in parameter regimes that are positively correlated with the enhanced nonlinear response. The observed graphene-light interaction and nonlinear response are of fundamental interest and open the way for future exploitation in graphene-based optical devices such as phase shifters, modulators, and nanoscale THz sources.
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Affiliation(s)
- Erin Sheridan
- Department of Physics and Astronomy, University of Pittsburgh, Pittsburgh, Pennsylvania 15260, United States
- Pittsburgh Quantum Institute, Pittsburgh, Pennsylvania 15260, United States
| | - Lu Chen
- Department of Physics and Astronomy, University of Pittsburgh, Pittsburgh, Pennsylvania 15260, United States
- Pittsburgh Quantum Institute, Pittsburgh, Pennsylvania 15260, United States
| | - Jianan Li
- Department of Physics and Astronomy, University of Pittsburgh, Pittsburgh, Pennsylvania 15260, United States
- Pittsburgh Quantum Institute, Pittsburgh, Pennsylvania 15260, United States
| | - Qing Guo
- Department of Physics and Astronomy, University of Pittsburgh, Pittsburgh, Pennsylvania 15260, United States
- Pittsburgh Quantum Institute, Pittsburgh, Pennsylvania 15260, United States
| | - Shan Hao
- Department of Physics and Astronomy, University of Pittsburgh, Pittsburgh, Pennsylvania 15260, United States
- Pittsburgh Quantum Institute, Pittsburgh, Pennsylvania 15260, United States
| | - Muqing Yu
- Department of Physics and Astronomy, University of Pittsburgh, Pittsburgh, Pennsylvania 15260, United States
- Pittsburgh Quantum Institute, Pittsburgh, Pennsylvania 15260, United States
| | - Ki-Tae Eom
- Department of Materials Science and Engineering, University of Wisconsin-Madison, Madison, Wisconsin 53076, United States
| | - Hyungwoo Lee
- Department of Materials Science and Engineering, University of Wisconsin-Madison, Madison, Wisconsin 53076, United States
| | - Jung-Woo Lee
- Department of Materials Science and Engineering, University of Wisconsin-Madison, Madison, Wisconsin 53076, United States
| | - Chang-Beom Eom
- Department of Materials Science and Engineering, University of Wisconsin-Madison, Madison, Wisconsin 53076, United States
| | - Patrick Irvin
- Department of Physics and Astronomy, University of Pittsburgh, Pittsburgh, Pennsylvania 15260, United States
- Pittsburgh Quantum Institute, Pittsburgh, Pennsylvania 15260, United States
| | - Jeremy Levy
- Department of Physics and Astronomy, University of Pittsburgh, Pittsburgh, Pennsylvania 15260, United States
- Pittsburgh Quantum Institute, Pittsburgh, Pennsylvania 15260, United States
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8
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Zhang Y, Su Y, He L. Local Berry Phase Signatures of Bilayer Graphene in Intervalley Quantum Interference. PHYSICAL REVIEW LETTERS 2020; 125:116804. [PMID: 32976000 DOI: 10.1103/physrevlett.125.116804] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 05/13/2020] [Accepted: 08/20/2020] [Indexed: 06/11/2023]
Abstract
Chiral quasiparticles in Bernal-stacked bilayer graphene have valley-contrasting Berry phases of ±2π. This nontrivial topological structure, associated with the pseudospin winding along a closed Fermi surface, is responsible for various novel electronic properties. Here we show that the quantum interference due to intervalley scattering induced by single-atom vacancies or impurities provides insights into the topological nature of the bilayer graphene. The scattered chiral quasiparticles between distinct valleys with opposite chirality undergo a rotation of pseudospin that results in the Friedel oscillation with wavefront dislocations. The number of dislocations reflects the information about pseudospin texture and hence can be used to measure the Berry phase. As demonstrated both experimentally and theoretically, the Friedel oscillation, depending on the single-atom vacancy or impurity at different sublattices, can exhibit N=4, 2, or 0 additional wavefronts, characterizing the 2π Berry phase of the bilayer graphene. Our results provide a comprehensive study of the intervalley quantum interference in bilayer graphene and can be extended to multilayer graphene, shedding light on the pseudospin physics.
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Affiliation(s)
- Yu Zhang
- Center for Advanced Quantum Studies, Department of Physics, Beijing Normal University, Beijing 100875, People's Republic of China
| | - Ying Su
- Theoretical Division, T-4 and CNLS, Los Alamos National Laboratory, Los Alamos, New Mexico 87545, USA
| | - Lin He
- Center for Advanced Quantum Studies, Department of Physics, Beijing Normal University, Beijing 100875, People's Republic of China
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9
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Ng KSH, Voisin B, Johnson BC, McCallum JC, Salfi J, Rogge S. Scanned Single-Electron Probe inside a Silicon Electronic Device. ACS NANO 2020; 14:9449-9455. [PMID: 32510926 DOI: 10.1021/acsnano.0c00736] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/11/2023]
Abstract
Solid-state devices can be fabricated at the atomic scale, with applications ranging from classical logic to current standards and quantum technologies. Although it is very desirable to probe these devices and the quantum states they host at the atomic scale, typical methods rely on long-ranged capacitive interactions, making this difficult. Here, we probe a silicon electronic device at the atomic scale using a localized electronic quantum dot induced directly within the device at a desired location, using the biased tip of a low-temperature scanning tunneling microscope. We demonstrate control over short-ranged tunnel coupling interactions of the quantum dot with the device's source reservoir using sub-nanometer position control of the tip and the quantum dot energy level using a voltage applied to the device's gate reservoir. Despite the ∼1 nm proximity of the quantum dot to the metallic tip, we find that the gate provides sufficient capacitance to enable a high degree of electric control. Combined with atomic-scale imaging, we use the quantum dot to probe applied electric fields and charge in individual defects in the device. This capability is expected to aid in the understanding of atomic-scale devices and the quantum states realized in them.
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Affiliation(s)
- Kevin S H Ng
- Centre for Quantum Computation and Communication Technology, School of Physics, The University of New South Wales, Sydney, New South Wales 2052, Australia
- 5. Physikalisches Institut and Center for Integrated Quantum Science and Technology, Universität Stuttgart, Pfaffenwaldring 57, 70569 Stuttgart, Germany
| | - Benoit Voisin
- Centre for Quantum Computation and Communication Technology, School of Physics, The University of New South Wales, Sydney, New South Wales 2052, Australia
| | - Brett C Johnson
- Centre for Quantum Computation and Communication Technology, School of Physics, University of Melbourne, Melbourne, Victoria 3010, Australia
| | - Jeffrey C McCallum
- Centre for Quantum Computation and Communication Technology, School of Physics, University of Melbourne, Melbourne, Victoria 3010, Australia
| | - Joe Salfi
- Centre for Quantum Computation and Communication Technology, School of Physics, The University of New South Wales, Sydney, New South Wales 2052, Australia
- Department of Electrical and Computer Engineering, University of British Columbia, Vancouver, BC V6T 1Z4, Canada
| | - Sven Rogge
- Centre for Quantum Computation and Communication Technology, School of Physics, The University of New South Wales, Sydney, New South Wales 2052, Australia
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10
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Keren I, Dvir T, Zalic A, Iluz A, LeBoeuf D, Watanabe K, Taniguchi T, Steinberg H. Quantum-dot assisted spectroscopy of degeneracy-lifted Landau levels in graphene. Nat Commun 2020; 11:3408. [PMID: 32641683 PMCID: PMC7343833 DOI: 10.1038/s41467-020-17225-1] [Citation(s) in RCA: 6] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Abstract] [Key Words] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/30/2020] [Accepted: 06/17/2020] [Indexed: 11/09/2022] Open
Abstract
Energy spectroscopy of strongly interacting phases requires probes which minimize screening while retaining spectral resolution and local sensitivity. Here, we demonstrate that such probes can be realized using atomic sized quantum dots bound to defects in hexagonal Boron Nitride tunnel barriers, placed at nanometric distance from graphene. With dot energies capacitively tuned by a planar graphite electrode, dot-assisted tunneling becomes highly sensitive to the graphene excitation spectrum. The spectra track the onset of degeneracy lifting with magnetic field at the ground state, and at unoccupied excited states, revealing symmetry-broken gaps which develop steeply with magnetic field - corresponding to Landé g factors as high as 160. Measured up to B = 33 T, spectra exhibit a primary energy split between spin-polarized excited states, and a secondary spin-dependent valley-split. Our results show that defect dots probe the spectra while minimizing local screening, and are thus exceptionally sensitive to interacting states.
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Affiliation(s)
- Itai Keren
- Racah Institute of Physics, The Hebrew University, 91904, Jerusalem, Israel
| | - Tom Dvir
- Racah Institute of Physics, The Hebrew University, 91904, Jerusalem, Israel
| | - Ayelet Zalic
- Racah Institute of Physics, The Hebrew University, 91904, Jerusalem, Israel
| | - Amir Iluz
- Racah Institute of Physics, The Hebrew University, 91904, Jerusalem, Israel
| | - David LeBoeuf
- LNCMI, Centre National de la Recherche Scientifique, EMFL, Université Grenoble Alpes, INSA Toulouse, Université Toulouse Paul Sabatier, Grenoble, France
| | - Kenji Watanabe
- National Institute for Material Science, 1-1 Namiki, Tsukaba, 305-0044, Japan
| | - Takashi Taniguchi
- National Institute for Material Science, 1-1 Namiki, Tsukaba, 305-0044, Japan
| | - Hadar Steinberg
- Racah Institute of Physics, The Hebrew University, 91904, Jerusalem, Israel.
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11
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Quezada-López EA, Ge Z, Taniguchi T, Watanabe K, Joucken F, Velasco J. Comprehensive Electrostatic Modeling of Exposed Quantum Dots in Graphene/Hexagonal Boron Nitride Heterostructures. NANOMATERIALS (BASEL, SWITZERLAND) 2020; 10:E1154. [PMID: 32545525 PMCID: PMC7353366 DOI: 10.3390/nano10061154] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [Key Words] [Grants] [Track Full Text] [Download PDF] [Figures] [Subscribe] [Scholar Register] [Received: 05/11/2020] [Revised: 06/05/2020] [Accepted: 06/06/2020] [Indexed: 11/22/2022]
Abstract
Recent experimental advancements have enabled the creation of tunable localized electrostatic potentials in graphene/hexagonal boron nitride (hBN) heterostructures without concealing the graphene surface. These potentials corral graphene electrons yielding systems akin to electrostatically defined quantum dots (QDs). The spectroscopic characterization of these exposed QDs with the scanning tunneling microscope (STM) revealed intriguing resonances that are consistent with a tunneling probability of 100% across the QD walls. This effect, known as Klein tunneling, is emblematic of relativistic particles, underscoring the uniqueness of these graphene QDs. Despite the advancements with electrostatically defined graphene QDs, a complete understanding of their spectroscopic features still remains elusive. In this study, we address this lapse in knowledge by comprehensively considering the electrostatic environment of exposed graphene QDs. We then implement these considerations into tight binding calculations to enable simulations of the graphene QD local density of states. We find that the inclusion of the STM tip's electrostatics in conjunction with that of the underlying hBN charges reproduces all of the experimentally resolved spectroscopic features. Our work provides an effective approach for modeling the electrostatics of exposed graphene QDs. The methods discussed here can be applied to other electrostatically defined QD systems that are also exposed.
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Affiliation(s)
- Eberth A. Quezada-López
- Department of Physics, University of California, Santa Cruz, CA 95064, USA; (E.A.Q.-L.); (Z.G.); (F.J.)
| | - Zhehao Ge
- Department of Physics, University of California, Santa Cruz, CA 95064, USA; (E.A.Q.-L.); (Z.G.); (F.J.)
| | - Takashi Taniguchi
- International Center for Materials Nanoarchitectronics National Institute for Materials Science, 1-1 Namiki, Tsukuba 305-0044, Japan;
| | - Kenji Watanabe
- Research Center for Functional Materials National Institute for Materials Science, 1-1 Namiki, Tsukuba 305-0044, Japan;
| | - Frédéric Joucken
- Department of Physics, University of California, Santa Cruz, CA 95064, USA; (E.A.Q.-L.); (Z.G.); (F.J.)
| | - Jairo Velasco
- Department of Physics, University of California, Santa Cruz, CA 95064, USA; (E.A.Q.-L.); (Z.G.); (F.J.)
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12
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Le TL, Nguyen VL. Quantitative study of electronic whispering gallery modes in electrostatic-potential induced circular graphene junctions. JOURNAL OF PHYSICS. CONDENSED MATTER : AN INSTITUTE OF PHYSICS JOURNAL 2020; 32:255502. [PMID: 32126539 DOI: 10.1088/1361-648x/ab7c16] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/24/2023]
Abstract
Electronic whispering gallery modes (EWGMs) have been recently observed in several circular graphene junctions, pn and pp', created in scanning tunneling microscopy experiments. By computing the local density of states within the Dirac-Weyl formalism for massless fermions we demonstrate that the EWGMs may really be emerged in any type of the electrostatic-potential induced circular graphene junctions, including uni-junctions (e.g. np- or pp'-junctions) as well as bipolar-junctions (e.g. pnp-heterojunctions). Surprisingly, quantitative analyses show that for all the EWGMs identified (regardless of junction types) the quality (Q) factors seem to be ≤102, very small compared to those in ordinary optical whispering gallery modes microresonators, while the corresponding mode radii may tunably be in nanometer-scale. Our theoretical results are in good agreement with existent experimental data, putting a question to the application potential of the EWGMs identified.
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Affiliation(s)
- T Lien Le
- University of Science and Technology of Hanoi, Vietnam Academy of Science and Technology, 18 Hoang Quoc Viet, Hanoi, Vietnam
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13
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Liu YW, Hou Z, Li SY, Sun QF, He L. Movable Valley Switch Driven by Berry Phase in Bilayer-Graphene Resonators. PHYSICAL REVIEW LETTERS 2020; 124:166801. [PMID: 32383950 DOI: 10.1103/physrevlett.124.166801] [Citation(s) in RCA: 6] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 10/09/2019] [Revised: 01/14/2020] [Accepted: 04/03/2020] [Indexed: 06/11/2023]
Abstract
Berry phase, the geometric phase accumulated over a closed loop in parameter space during an adiabatic cyclic evolution, has been demonstrated to play an important role in many quantum systems since its discovery. In gapped Bernal bilayer graphene, the Berry phase can be continuously tuned from zero to 2π, which offers a unique opportunity to explore the tunable Berry phase on physical phenomena. Here, we report experimental observation of Berry-phase-induced valley splitting and crossing in movable bilayer-graphene p-n junction resonators. In our experiment, the resonators are generated by combining the electric field of a scanning tunneling microscope tip with the gap of bilayer graphene. A perpendicular magnetic field changes the Berry phase of the confined bound states in the resonators from zero to 2π continuously and leads to the Berry phase difference for the two inequivalent valleys in the bilayer graphene. As a consequence, we observe giant valley splitting and unusual valley crossing of the lowest bound states. Our results indicate that the bilayer-graphene resonators can be used to manipulate the valley degree of freedom in valleytronics.
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Affiliation(s)
- Yi-Wen Liu
- Center for Advanced Quantum Studies, Department of Physics, Beijing Normal University, Beijing 100875, People's Republic of China
| | - Zhe Hou
- International Center for Quantum Materials, School of Physics, Peking University, Beijing 100871, China
| | - Si-Yu Li
- Center for Advanced Quantum Studies, Department of Physics, Beijing Normal University, Beijing 100875, People's Republic of China
| | - Qing-Feng Sun
- International Center for Quantum Materials, School of Physics, Peking University, Beijing 100871, China
- Collaborative Innovation Center of Quantum Matter, Beijing 100871, China
- Beijing Academy of Quantum Information Sciences, West Boulevard No. 3, No. 10 Xibeiwang East Road, Haidian District, Beijing 100193, China
| | - Lin He
- Center for Advanced Quantum Studies, Department of Physics, Beijing Normal University, Beijing 100875, People's Republic of China
- State Key Laboratory of Functional Materials for Informatics, Shanghai Institute of Microsystem and Information Technology, Chinese Academy of Sciences, 865 Changning Road, Shanghai 200050, People's Republic of China
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14
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Li SY, Su Y, Ren YN, He L. Valley Polarization and Inversion in Strained Graphene via Pseudo-Landau Levels, Valley Splitting of Real Landau Levels, and Confined States. PHYSICAL REVIEW LETTERS 2020; 124:106802. [PMID: 32216392 DOI: 10.1103/physrevlett.124.106802] [Citation(s) in RCA: 20] [Impact Index Per Article: 5.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 09/16/2019] [Revised: 12/04/2019] [Accepted: 02/19/2020] [Indexed: 06/10/2023]
Abstract
It is quite easy to control spin polarization and the spin direction of a system via magnetic fields. However, there is no such direct and efficient way to manipulate the valley pseudospin degree of freedom. Here, we demonstrate experimentally that it is possible to realize valley polarization and valley inversion in graphene by using both strain-induced pseudomagnetic fields and real magnetic fields. Pseudomagnetic fields, which are quite different from real magnetic fields, point in opposite directions at the two distinct valleys of graphene. Therefore, the coexistence of pseudomagnetic fields and real magnetic fields leads to imbalanced effective magnetic fields at two distinct valleys of graphene. This allows us to control the valley in graphene as conveniently as the electron spin. In this work, we report a consistent observation of valley polarization and inversion in strained graphene via pseudo-Landau levels, splitting of real Landau levels, and valley splitting of confined states using scanning tunneling spectroscopy. Our results highlight a pathway to valleytronics in strained graphene-based platforms.
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Affiliation(s)
- Si-Yu Li
- Center for Advanced Quantum Studies, Department of Physics, Beijing Normal University, Beijing 100875, People's Republic of China
| | - Ying Su
- Theoretical Division, T-4 and CNLS, Los Alamos National Laboratory, Los Alamos, New Mexico 87545, USA
| | - Ya-Ning Ren
- Center for Advanced Quantum Studies, Department of Physics, Beijing Normal University, Beijing 100875, People's Republic of China
| | - Lin He
- Center for Advanced Quantum Studies, Department of Physics, Beijing Normal University, Beijing 100875, People's Republic of China
- State Key Laboratory of Functional Materials for Informatics, Shanghai Institute of Microsystem and Information Technology, Chinese Academy of Sciences, 865 Changning Road, Shanghai 200050, People's Republic of China
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15
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Chen H, Zhou P, Liu J, Qiao J, Oezyilmaz B, Martin J. Gate controlled valley polarizer in bilayer graphene. Nat Commun 2020; 11:1202. [PMID: 32139694 PMCID: PMC7058031 DOI: 10.1038/s41467-020-15117-y] [Citation(s) in RCA: 16] [Impact Index Per Article: 4.0] [Reference Citation Analysis] [Abstract] [Key Words] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/28/2019] [Accepted: 02/19/2020] [Indexed: 11/19/2022] Open
Abstract
Sign reversal of Berry curvature across two oppositely gated regions in bilayer graphene can give rise to counter-propagating 1D channels with opposite valley indices. Considering spin and sub-lattice degeneracy, there are four quantized conduction channels in each direction. Previous experimental work on gate-controlled valley polarizer achieved good contrast only in the presence of an external magnetic field. Yet, with increasing magnetic field the ungated regions of bilayer graphene will transit into the quantum Hall regime, limiting the applications of valley-polarized electrons. Here we present improved performance of a gate-controlled valley polarizer through optimized device geometry and stacking method. Electrical measurements show up to two orders of magnitude difference in conductance between the valley-polarized state and gapped states. The valley-polarized state displays conductance of nearly 4e2/h and produces contrast in a subsequent valley analyzer configuration. These results pave the way to further experiments on valley-polarized electrons in zero magnetic field.
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Affiliation(s)
- Hao Chen
- Centre for Advanced 2D Materials, National University of Singapore, 6 Science Drive 2, 117546, Singapore, Singapore
- Department of Physics, National University of Singapore, 2 Science Drive 3, 117551, Singapore, Singapore
| | - Pinjia Zhou
- Centre for Advanced 2D Materials, National University of Singapore, 6 Science Drive 2, 117546, Singapore, Singapore
| | - Jiawei Liu
- Centre for Advanced 2D Materials, National University of Singapore, 6 Science Drive 2, 117546, Singapore, Singapore
- Department of Physics, National University of Singapore, 2 Science Drive 3, 117551, Singapore, Singapore
| | - Jiabin Qiao
- Centre for Advanced 2D Materials, National University of Singapore, 6 Science Drive 2, 117546, Singapore, Singapore
| | - Barbaros Oezyilmaz
- Centre for Advanced 2D Materials, National University of Singapore, 6 Science Drive 2, 117546, Singapore, Singapore
- Department of Physics, National University of Singapore, 2 Science Drive 3, 117551, Singapore, Singapore
| | - Jens Martin
- Centre for Advanced 2D Materials, National University of Singapore, 6 Science Drive 2, 117546, Singapore, Singapore.
- Department of Physics, National University of Singapore, 2 Science Drive 3, 117551, Singapore, Singapore.
- Leibniz Institut für Kristallzüchtung, Max-Born-Strasse 2, 12489, Berlin, Germany.
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16
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Walkup D, Ghahari F, Gutiérrez C, Watanabe K, Taniguchi T, Zhitenev NB, Stroscio JA. Tuning single-electron charging and interactions between compressible Landau level islands in graphene. PHYSICAL REVIEW. B 2020; 101:https://doi.org/10.1103/physrevb.101.035428. [PMID: 33134655 PMCID: PMC7594164] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Grants] [Subscribe] [Scholar Register] [Indexed: 06/11/2023]
Abstract
Interacting and tunable quantum dots (QDs) have been extensively exploited in condensed matter physics and quantum information science. Using a low-temperature scanning tunneling microscope (STM), we both create and directly image a new type of coupled QD system in graphene, a highly interacting quantum relativistic system with tunable density. Using detailed scanning tunneling spectroscopy (STS) measurements, we show that Landau quantization inside a potential well enables novel electron confinement via the incompressible strips between partially filled Landau levels (LLs), forming isolated and concentric LL QDs. By changing the charge density and the magnetic field we can tune continuously between single- and double-concentric LL QD systems within the same potential well. In the concentric QD regime, single-electron charging peaks of the two dots intersect, displaying a characteristic avoidance pattern. At moderate fields, we observe an unconventional avoidance pattern that differs significantly from that observed in capacitively coupled double-QD systems. We find that we can reproduce in detail this anomalous avoidance pattern within the framework of the electrostatic double-QD model by replacing the capacitive interdot coupling with a phenomenological charge-counting system in which charges in the inner concentric dot are counted in the total charge of both islands. The emergence of such strange forms of interdot coupling in a single potential well, together with the ease of producing such charge pockets in graphene and other two-dimensional (2D) materials, reveals an intriguing testbed for the confinement of 2D electrons in customizable potentials.
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Affiliation(s)
- Daniel Walkup
- Physical Measurement Laboratory, National Institute of Standards and Technology, Gaithersburg, Maryland 20899, USA
- Maryland NanoCenter, University of Maryland, College Park, Maryland 20742, USA
| | - Fereshte Ghahari
- Physical Measurement Laboratory, National Institute of Standards and Technology, Gaithersburg, Maryland 20899, USA
- Maryland NanoCenter, University of Maryland, College Park, Maryland 20742, USA
| | - Christopher Gutiérrez
- Physical Measurement Laboratory, National Institute of Standards and Technology, Gaithersburg, Maryland 20899, USA
- Maryland NanoCenter, University of Maryland, College Park, Maryland 20742, USA
| | - Kenji Watanabe
- National Institute for Materials Science, Tsukuba, Ibaraki 305-0044, Japan
| | - Takashi Taniguchi
- National Institute for Materials Science, Tsukuba, Ibaraki 305-0044, Japan
| | - Nikolai B. Zhitenev
- Physical Measurement Laboratory, National Institute of Standards and Technology, Gaithersburg, Maryland 20899, USA
| | - Joseph A. Stroscio
- Physical Measurement Laboratory, National Institute of Standards and Technology, Gaithersburg, Maryland 20899, USA
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17
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Rosati R, Lengers F, Reiter DE, Kuhn T. Effective detection of spatio-temporal carrier dynamics by carrier capture. JOURNAL OF PHYSICS. CONDENSED MATTER : AN INSTITUTE OF PHYSICS JOURNAL 2019; 31:28LT01. [PMID: 30965286 DOI: 10.1088/1361-648x/ab17a8] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/09/2023]
Abstract
The spatio-temporal dynamics of electrons moving in a 2D plane is challenging to detect when the required resolution shrinks simultaneously to nanometer length and subpicosecond time scale. We propose a detection scheme relying on phonon-induced carrier capture from 2D unbound states into the bound states of an embedded quantum dot. This capture process happens locally and here we explore if this locality is sufficient to use the carrier capture process as detection of the ultrafast diffraction of electrons from an obstacle in the 2D plane. As an example we consider an electronic wave packet traveling in a semiconducting monolayer of the transition metal dichalcogenide MoSe2, and we study the scattering-induced dynamics using a single particle Lindblad approach. Our results offer a new way to high resolution detection of the spatio-temporal carrier dynamics.
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Affiliation(s)
- R Rosati
- Institut für Festkörpertheorie, Universität Münster, Wilhelm-Klemm-Str. 10, 48149 Münster, Germany. Department of Physics, Chalmers University of Technology, SE-412 96 Gothenburg, Sweden
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18
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Wang JIJ, Rodan-Legrain D, Bretheau L, Campbell DL, Kannan B, Kim D, Kjaergaard M, Krantz P, Samach GO, Yan F, Yoder JL, Watanabe K, Taniguchi T, Orlando TP, Gustavsson S, Jarillo-Herrero P, Oliver WD. Coherent control of a hybrid superconducting circuit made with graphene-based van der Waals heterostructures. NATURE NANOTECHNOLOGY 2019; 14:120-125. [PMID: 30598526 DOI: 10.1038/s41565-018-0329-2] [Citation(s) in RCA: 44] [Impact Index Per Article: 8.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 08/30/2018] [Accepted: 11/19/2018] [Indexed: 06/09/2023]
Abstract
Quantum coherence and control is foundational to the science and engineering of quantum systems1,2. In van der Waals materials, the collective coherent behaviour of carriers has been probed successfully by transport measurements3-6. However, temporal coherence and control, as exemplified by manipulating a single quantum degree of freedom, remains to be verified. Here we demonstrate such coherence and control of a superconducting circuit incorporating graphene-based Josephson junctions. Furthermore, we show that this device can be operated as a voltage-tunable transmon qubit7-9, whose spectrum reflects the electronic properties of massless Dirac fermions travelling ballistically4,5. In addition to the potential for advancing extensible quantum computing technology, our results represent a new approach to studying van der Waals materials using microwave photons in coherent quantum circuits.
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Affiliation(s)
- Joel I-Jan Wang
- Research Laboratory of Electronics, Massachusetts Institute of Technology, Cambridge, MA, USA.
| | - Daniel Rodan-Legrain
- Department of Physics, Massachusetts Institute of Technology, Cambridge, MA, USA
| | - Landry Bretheau
- Laboratoire des Solides Irradiés, Ecole Polytechnique, CNRS, CEA, Palaiseau, France
| | - Daniel L Campbell
- Research Laboratory of Electronics, Massachusetts Institute of Technology, Cambridge, MA, USA
| | - Bharath Kannan
- Research Laboratory of Electronics, Massachusetts Institute of Technology, Cambridge, MA, USA
- Department of Electrical Engineering and Computer Science, Massachusetts Institute of Technology, Cambridge, MA, USA
| | - David Kim
- Massachusetts Institute of Technology (MIT) Lincoln Laboratory, Lexington, MA, USA
| | - Morten Kjaergaard
- Research Laboratory of Electronics, Massachusetts Institute of Technology, Cambridge, MA, USA
| | - Philip Krantz
- Research Laboratory of Electronics, Massachusetts Institute of Technology, Cambridge, MA, USA
| | - Gabriel O Samach
- Department of Electrical Engineering and Computer Science, Massachusetts Institute of Technology, Cambridge, MA, USA
- Massachusetts Institute of Technology (MIT) Lincoln Laboratory, Lexington, MA, USA
| | - Fei Yan
- Research Laboratory of Electronics, Massachusetts Institute of Technology, Cambridge, MA, USA
| | - Jonilyn L Yoder
- Massachusetts Institute of Technology (MIT) Lincoln Laboratory, Lexington, MA, USA
| | - Kenji Watanabe
- Advanced Materials Laboratory, National Institute for Materials Science, Tsukuba, Japan
| | - Takashi Taniguchi
- Advanced Materials Laboratory, National Institute for Materials Science, Tsukuba, Japan
| | - Terry P Orlando
- Research Laboratory of Electronics, Massachusetts Institute of Technology, Cambridge, MA, USA
- Department of Electrical Engineering and Computer Science, Massachusetts Institute of Technology, Cambridge, MA, USA
| | - Simon Gustavsson
- Research Laboratory of Electronics, Massachusetts Institute of Technology, Cambridge, MA, USA
| | | | - William D Oliver
- Research Laboratory of Electronics, Massachusetts Institute of Technology, Cambridge, MA, USA.
- Department of Physics, Massachusetts Institute of Technology, Cambridge, MA, USA.
- Massachusetts Institute of Technology (MIT) Lincoln Laboratory, Lexington, MA, USA.
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19
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Luo W, Naseri A, Sirker J, Chakraborty T. Unique Spin Vortices and Topological Charges in Quantum Dots with Spin-orbit Couplings. Sci Rep 2019; 9:672. [PMID: 30679442 PMCID: PMC6345826 DOI: 10.1038/s41598-018-35837-y] [Citation(s) in RCA: 6] [Impact Index Per Article: 1.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/16/2018] [Accepted: 11/06/2018] [Indexed: 11/11/2022] Open
Abstract
Spin textures of one or two electrons in a quantum dot with Rashba or Dresselhaus spin-orbit couplings reveal several intriguing properties. We show here that even at the single-electron level stable spin vortices with tunable topological charges exist. These topological textures appear in the ground state of the dots. The textures are stabilized by time-reversal symmetry breaking and are robust against the eccentricity of the dot. The topological charge is directly related to the sign of the z component of the spin in a large dot, allowing a direct probe of its topological properties. This would clearly pave the way to possible future topological spintronics. The phenomenon of spin vortices persists for the interacting two-electron dot in the presence of a magnetic field.
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Affiliation(s)
- Wenchen Luo
- Department of Physics, School of Physics and Electronics, Central South University, Changsha, Hunan, 410083, P. R. China
- Department of Physics and Astronomy, University of Manitoba, Winnipeg, R3T 2N2, Canada
| | - Amin Naseri
- Department of Physics and Astronomy, University of Manitoba, Winnipeg, R3T 2N2, Canada
| | - Jesko Sirker
- Department of Physics and Astronomy, University of Manitoba, Winnipeg, R3T 2N2, Canada.
| | - Tapash Chakraborty
- Department of Physics and Astronomy, University of Manitoba, Winnipeg, R3T 2N2, Canada
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20
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Gutiérrez C, Walkup D, Ghahari F, Lewandowski C, Rodriguez-Nieva JF, Watanabe K, Taniguchi T, Levitov LS, Zhitenev NB, Stroscio JA. Interaction-driven quantum Hall wedding cake-like structures in graphene quantum dots. Science 2018; 361:789-794. [PMID: 30139870 DOI: 10.1126/science.aar2014] [Citation(s) in RCA: 35] [Impact Index Per Article: 5.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/12/2017] [Accepted: 06/15/2018] [Indexed: 01/29/2023]
Abstract
Quantum-relativistic matter is ubiquitous in nature; however, it is notoriously difficult to probe. The ease with which external electric and magnetic fields can be introduced in graphene opens a door to creating a tabletop prototype of strongly confined relativistic matter. Here, through a detailed spectroscopic mapping, we directly visualize the interplay between spatial and magnetic confinement in a circular graphene resonator as atomic-like shell states condense into Landau levels. We directly observe the development of a "wedding cake"-like structure of concentric regions of compressible-incompressible quantum Hall states, a signature of electron interactions in the system. Solid-state experiments can, therefore, yield insights into the behavior of quantum-relativistic matter under extreme conditions.
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Affiliation(s)
- Christopher Gutiérrez
- Center for Nanoscale Science and Technology, National Institute of Standards and Technology, Gaithersburg, MD 20899, USA.,Maryland NanoCenter, University of Maryland, College Park, MD 20742, USA
| | - Daniel Walkup
- Center for Nanoscale Science and Technology, National Institute of Standards and Technology, Gaithersburg, MD 20899, USA.,Maryland NanoCenter, University of Maryland, College Park, MD 20742, USA
| | - Fereshte Ghahari
- Center for Nanoscale Science and Technology, National Institute of Standards and Technology, Gaithersburg, MD 20899, USA.,Maryland NanoCenter, University of Maryland, College Park, MD 20742, USA
| | - Cyprian Lewandowski
- Department of Physics, Massachusetts Institute of Technology, Cambridge, MA 02139, USA
| | | | - Kenji Watanabe
- National Institute for Materials Science, Tsukuba, Ibaraki 305-0044, Japan
| | - Takashi Taniguchi
- National Institute for Materials Science, Tsukuba, Ibaraki 305-0044, Japan
| | - Leonid S Levitov
- Department of Physics, Massachusetts Institute of Technology, Cambridge, MA 02139, USA
| | - Nikolai B Zhitenev
- Center for Nanoscale Science and Technology, National Institute of Standards and Technology, Gaithersburg, MD 20899, USA
| | - Joseph A Stroscio
- Center for Nanoscale Science and Technology, National Institute of Standards and Technology, Gaithersburg, MD 20899, USA.
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21
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Graphene quantum dots (GQDs) and its derivatives for multifarious photocatalysis and photoelectrocatalysis. Catal Today 2018. [DOI: 10.1016/j.cattod.2018.01.005] [Citation(s) in RCA: 100] [Impact Index Per Article: 16.7] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/20/2022]
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22
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Abdullah HM, Bahlouli H, Peeters FM, Van Duppen B. Confined states in graphene quantum blisters. JOURNAL OF PHYSICS. CONDENSED MATTER : AN INSTITUTE OF PHYSICS JOURNAL 2018; 30:385301. [PMID: 30102244 DOI: 10.1088/1361-648x/aad9c7] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/08/2023]
Abstract
Bilayer graphene samples may exhibit regions where the two layers are locally delaminated forming a so-called quantum blister in the graphene sheet. Electron and hole states can be confined in this graphene quantum blisters (GQB) by applying a global electrostatic bias. We scrutinize the electronic properties of these confined states under the variation of interlayer bias, coupling, and blister's size. The spectra display strong anti-crossings due to the coupling of the confined states on upper and lower layers inside the blister. These spectra are layer localized where the respective confined states reside on either layer or equally distributed. For finite angular momentum, this layer localization can be at the edge of the blister and corresponds to degenerate modes of opposite momenta. Furthermore, the energy levels in GQB exhibit electron-hole symmetry that is sensitive to the electrostatic bias. Finally, we demonstrate that confinement in GQB persists even in the presence of a variation in the inter-layer coupling.
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Affiliation(s)
- H M Abdullah
- Department of Physics, King Fahd University of Petroleum and Minerals, 31261 Dhahran, Saudi Arabia. Saudi Center for Theoretical Physics, PO Box 32741, Jeddah 21438, Saudi Arabia. Department of Physics, University of Antwerp, Groenenborgerlaan 171, B-2020 Antwerp, Belgium
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23
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Velasco J, Lee J, Wong D, Kahn S, Tsai HZ, Costello J, Umeda T, Taniguchi T, Watanabe K, Zettl A, Wang F, Crommie MF. Visualization and Control of Single-Electron Charging in Bilayer Graphene Quantum Dots. NANO LETTERS 2018; 18:5104-5110. [PMID: 30035544 DOI: 10.1021/acs.nanolett.8b01972] [Citation(s) in RCA: 9] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/24/2023]
Abstract
Graphene p-n junctions provide an ideal platform for investigating novel behavior at the boundary between electronics and optics that arise from massless Dirac Fermions, such as whispering gallery modes and Veselago lensing. Bilayer graphene also hosts Dirac Fermions, but they differ from single-layer graphene charge carriers because they are massive, can be gapped by an applied perpendicular electric field, and have very different pseudospin selection rules across a p-n junction. Novel phenomena predicted for these massive Dirac Fermions at p-n junctions include anti-Klein tunneling, oscillatory Zener tunneling, and electron cloaked states. Despite these predictions there has been little experimental focus on the microscopic spatial behavior of massive Dirac Fermions in the presence of p-n junctions. Here we report the experimental manipulation and characterization of massive Dirac Fermions within bilayer graphene quantum dots defined by circular p-n junctions through the use of scanning tunneling microscopy-based (STM) methods. Our p-n junctions are created via a flexible technique that enables realization of exposed quantum dots in bilayer graphene/hBN heterostructures. These quantum dots exhibit sharp spectroscopic resonances that disperse in energy as a function of applied gate voltage. Spatial maps of these features show prominent concentric rings with diameters that can be tuned by an electrostatic gate. This behavior is explained by single-electron charging of localized states that arise from the quantum confinement of massive Dirac Fermions within our exposed bilayer graphene quantum dots.
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Affiliation(s)
- Jairo Velasco
- Department of Physics , University of California , Berkeley , California 94720 , United States
- Department of Physics , University of California , Santa Cruz , California 95064 , United States
| | - Juwon Lee
- Department of Physics , University of California , Berkeley , California 94720 , United States
| | - Dillon Wong
- Department of Physics , University of California , Berkeley , California 94720 , United States
| | - Salman Kahn
- Department of Physics , University of California , Berkeley , California 94720 , United States
| | - Hsin-Zon Tsai
- Department of Physics , University of California , Berkeley , California 94720 , United States
| | - Joseph Costello
- Department of Physics , University of California , Berkeley , California 94720 , United States
| | - Torben Umeda
- Department of Physics , University of California , Berkeley , California 94720 , United States
| | - Takashi Taniguchi
- National Institute for Materials Science , 1-1 Namiki , Tsukuba , 305-0044 , Japan
| | - Kenji Watanabe
- National Institute for Materials Science , 1-1 Namiki , Tsukuba , 305-0044 , Japan
| | - Alex Zettl
- Department of Physics , University of California , Berkeley , California 94720 , United States
- Materials Sciences Division , Lawrence Berkeley National Laboratory , Berkeley , California 94720 , United States
- Kavli Energy NanoSciences Institute at the University of California , Berkeley and the Lawrence Berkeley National Laboratory , Berkeley , California 94720 , United States
| | - Feng Wang
- Department of Physics , University of California , Berkeley , California 94720 , United States
- Materials Sciences Division , Lawrence Berkeley National Laboratory , Berkeley , California 94720 , United States
- Kavli Energy NanoSciences Institute at the University of California , Berkeley and the Lawrence Berkeley National Laboratory , Berkeley , California 94720 , United States
| | - Michael F Crommie
- Department of Physics , University of California , Berkeley , California 94720 , United States
- Materials Sciences Division , Lawrence Berkeley National Laboratory , Berkeley , California 94720 , United States
- Kavli Energy NanoSciences Institute at the University of California , Berkeley and the Lawrence Berkeley National Laboratory , Berkeley , California 94720 , United States
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24
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Freitag NM, Reisch T, Chizhova LA, Nemes-Incze P, Holl C, Woods CR, Gorbachev RV, Cao Y, Geim AK, Novoselov KS, Burgdörfer J, Libisch F, Morgenstern M. Large tunable valley splitting in edge-free graphene quantum dots on boron nitride. NATURE NANOTECHNOLOGY 2018; 13:392-397. [PMID: 29556008 DOI: 10.1038/s41565-018-0080-8] [Citation(s) in RCA: 7] [Impact Index Per Article: 1.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 08/28/2017] [Accepted: 01/25/2018] [Indexed: 06/08/2023]
Abstract
Coherent manipulation of the binary degrees of freedom is at the heart of modern quantum technologies. Graphene offers two binary degrees: the electron spin and the valley. Efficient spin control has been demonstrated in many solid-state systems, whereas exploitation of the valley has only recently been started, albeit without control at the single-electron level. Here, we show that van der Waals stacking of graphene onto hexagonal boron nitride offers a natural platform for valley control. We use a graphene quantum dot induced by the tip of a scanning tunnelling microscope and demonstrate valley splitting that is tunable from -5 to +10 meV (including valley inversion) by sub-10-nm displacements of the quantum dot position. This boosts the range of controlled valley splitting by about one order of magnitude. The tunable inversion of spin and valley states should enable coherent superposition of these degrees of freedom as a first step towards graphene-based qubits.
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Affiliation(s)
- Nils M Freitag
- II. Institute of Physics B, JARA-FIT, RWTH Aachen University, Aachen, Germany
| | - Tobias Reisch
- Institute for Theoretical Physics, TU Wien, Vienna, Austria
| | | | - Péter Nemes-Incze
- II. Institute of Physics B, JARA-FIT, RWTH Aachen University, Aachen, Germany
- Centre for Energy Research, Institute of Technical Physics and Materials Science, Budapest, Hungary
| | - Christian Holl
- II. Institute of Physics B, JARA-FIT, RWTH Aachen University, Aachen, Germany
| | - Colin R Woods
- School of Physics & Astronomy, University of Manchester, Manchester, UK
| | - Roman V Gorbachev
- School of Physics & Astronomy, University of Manchester, Manchester, UK
| | - Yang Cao
- School of Physics & Astronomy, University of Manchester, Manchester, UK
| | - Andre K Geim
- School of Physics & Astronomy, University of Manchester, Manchester, UK
| | | | | | | | - Markus Morgenstern
- II. Institute of Physics B, JARA-FIT, RWTH Aachen University, Aachen, Germany.
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25
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Krishna Kumar R, Chen X, Auton GH, Mishchenko A, Bandurin DA, Morozov SV, Cao Y, Khestanova E, Ben Shalom M, Kretinin AV, Novoselov KS, Eaves L, Grigorieva IV, Ponomarenko LA, Fal'ko VI, Geim AK. High-temperature quantum oscillations caused by recurring Bloch states in graphene superlattices. Science 2018; 357:181-184. [PMID: 28706067 DOI: 10.1126/science.aal3357] [Citation(s) in RCA: 51] [Impact Index Per Article: 8.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/03/2016] [Accepted: 06/09/2017] [Indexed: 11/03/2022]
Abstract
Cyclotron motion of charge carriers in metals and semiconductors leads to Landau quantization and magneto-oscillatory behavior in their properties. Cryogenic temperatures are usually required to observe these oscillations. We show that graphene superlattices support a different type of quantum oscillation that does not rely on Landau quantization. The oscillations are extremely robust and persist well above room temperature in magnetic fields of only a few tesla. We attribute this phenomenon to repetitive changes in the electronic structure of superlattices such that charge carriers experience effectively no magnetic field at simple fractions of the flux quantum per superlattice unit cell. Our work hints at unexplored physics in Hofstadter butterfly systems at high temperatures.
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Affiliation(s)
- R Krishna Kumar
- School of Physics and Astronomy, University of Manchester, Manchester M13 9PL, UK.,National Graphene Institute, University of Manchester, Manchester M13 9PL, UK.,Department of Physics, University of Lancaster, Lancaster LA1 4YW, UK
| | - X Chen
- National Graphene Institute, University of Manchester, Manchester M13 9PL, UK
| | - G H Auton
- National Graphene Institute, University of Manchester, Manchester M13 9PL, UK
| | - A Mishchenko
- School of Physics and Astronomy, University of Manchester, Manchester M13 9PL, UK
| | - D A Bandurin
- School of Physics and Astronomy, University of Manchester, Manchester M13 9PL, UK
| | - S V Morozov
- Institute of Microelectronics Technology and High Purity Materials, Russian Academy of Sciences, Chernogolovka 142432, Russia.,National University of Science and Technology (MISiS), Moscow 119049, Russia
| | - Y Cao
- National Graphene Institute, University of Manchester, Manchester M13 9PL, UK
| | - E Khestanova
- School of Physics and Astronomy, University of Manchester, Manchester M13 9PL, UK
| | - M Ben Shalom
- School of Physics and Astronomy, University of Manchester, Manchester M13 9PL, UK
| | - A V Kretinin
- National Graphene Institute, University of Manchester, Manchester M13 9PL, UK.,School of Materials, University of Manchester, Manchester M13 9PL, UK
| | - K S Novoselov
- National Graphene Institute, University of Manchester, Manchester M13 9PL, UK
| | - L Eaves
- National Graphene Institute, University of Manchester, Manchester M13 9PL, UK.,School of Physics and Astronomy, University of Nottingham, Nottingham NG7 2RD, UK
| | - I V Grigorieva
- School of Physics and Astronomy, University of Manchester, Manchester M13 9PL, UK
| | - L A Ponomarenko
- Department of Physics, University of Lancaster, Lancaster LA1 4YW, UK
| | - V I Fal'ko
- School of Physics and Astronomy, University of Manchester, Manchester M13 9PL, UK. .,National Graphene Institute, University of Manchester, Manchester M13 9PL, UK
| | - A K Geim
- School of Physics and Astronomy, University of Manchester, Manchester M13 9PL, UK. .,National Graphene Institute, University of Manchester, Manchester M13 9PL, UK
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26
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Li R, Wang X, Li Z, Zhu H, Liu J. Folic acid-functionalized graphene quantum dots with tunable fluorescence emission for cancer cell imaging and optical detection of Hg2+. NEW J CHEM 2018. [DOI: 10.1039/c7nj05052f] [Citation(s) in RCA: 23] [Impact Index Per Article: 3.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/21/2022]
Abstract
Functional groups may alter the optical and electrical characteristics of graphene quantum dots and lead to unusual properties and related applications.
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Affiliation(s)
- Ruiyi Li
- Key Laboratory of Food Colloids and Biotechnology
- Ministry of Education
- School of Chemical and Material Engineering
- Jiangnan University
- Wuxi 214122
| | - Xuan Wang
- Key Laboratory of Food Colloids and Biotechnology
- Ministry of Education
- School of Chemical and Material Engineering
- Jiangnan University
- Wuxi 214122
| | - Zaijun Li
- Key Laboratory of Food Colloids and Biotechnology
- Ministry of Education
- School of Chemical and Material Engineering
- Jiangnan University
- Wuxi 214122
| | - Haiyan Zhu
- Key Laboratory of Food Colloids and Biotechnology
- Ministry of Education
- School of Chemical and Material Engineering
- Jiangnan University
- Wuxi 214122
| | - Junkang Liu
- Key Laboratory of Food Colloids and Biotechnology
- Ministry of Education
- School of Chemical and Material Engineering
- Jiangnan University
- Wuxi 214122
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27
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Liebmann M, Bindel JR, Pezzotta M, Becker S, Muckel F, Johnsen T, Saunus C, Ast CR, Morgenstern M. An ultrahigh-vacuum cryostat for simultaneous scanning tunneling microscopy and magneto-transport measurements down to 400 mK. THE REVIEW OF SCIENTIFIC INSTRUMENTS 2017; 88:123707. [PMID: 29289196 DOI: 10.1063/1.4999555] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/07/2023]
Abstract
We present the design and calibration measurements of a scanning tunneling microscope setup in a 3He ultrahigh-vacuum cryostat operating at 400 mK with a hold time of 10 days. With 2.70 m in height and 4.70 m free space needed for assembly, the cryostat fits in a one-story lab building. The microscope features optical access, an xy table, in situ tip and sample exchange, and enough contacts to facilitate atomic force microscopy in tuning fork operation and simultaneous magneto-transport measurements on the sample. Hence, it enables scanning tunneling spectroscopy on microstructured samples which are tuned into preselected transport regimes. A superconducting magnet provides a perpendicular field of up to 14 T. The vertical noise of the scanning tunneling microscope amounts to 1 pmrms within a 700 Hz bandwidth. Tunneling spectroscopy using one superconducting electrode revealed an energy resolution of 120 μeV. Data on tip-sample Josephson contacts yield an even smaller feature size of 60 μeV, implying that the system operates close to the physical noise limit.
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Affiliation(s)
- Marcus Liebmann
- II. Institute of Physics B and JARA-FIT, RWTH Aachen University, 52074 Aachen, Germany
| | - Jan Raphael Bindel
- II. Institute of Physics B and JARA-FIT, RWTH Aachen University, 52074 Aachen, Germany
| | - Mike Pezzotta
- II. Institute of Physics B and JARA-FIT, RWTH Aachen University, 52074 Aachen, Germany
| | - Stefan Becker
- II. Institute of Physics B and JARA-FIT, RWTH Aachen University, 52074 Aachen, Germany
| | - Florian Muckel
- II. Institute of Physics B and JARA-FIT, RWTH Aachen University, 52074 Aachen, Germany
| | - Tjorven Johnsen
- II. Institute of Physics B and JARA-FIT, RWTH Aachen University, 52074 Aachen, Germany
| | - Christian Saunus
- II. Institute of Physics B and JARA-FIT, RWTH Aachen University, 52074 Aachen, Germany
| | - Christian R Ast
- Max-Planck-Institut für Festkörperforschung, 70569 Stuttgart, Germany
| | - Markus Morgenstern
- II. Institute of Physics B and JARA-FIT, RWTH Aachen University, 52074 Aachen, Germany
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28
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Chau Nguyen H, Nguyen NTT, Lien Nguyen V. On the density of states of circular graphene quantum dots. JOURNAL OF PHYSICS. CONDENSED MATTER : AN INSTITUTE OF PHYSICS JOURNAL 2017; 29:405301. [PMID: 28695841 DOI: 10.1088/1361-648x/aa7efd] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/07/2023]
Abstract
We suggest a simple approach to calculate the local density of states that effectively applies to any structure created by an axially symmetric potential on a continuous graphene sheet such as circular graphene quantum dots or rings. Calculations performed for the graphene quantum dot studied in a recent scanning tunneling microscopy measurement (Gutierrez et al 2016 Nat. Phys. 12 1069-75) show an excellent experimental-theoretical agreement.
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Affiliation(s)
- H Chau Nguyen
- Max-Planck-Institut für Physik komplexer Systeme, Nöthnitzer Straße 38, D-01187 Dresden, Germany
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29
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Zhang ZZ, Song XX, Luo G, Deng GW, Mosallanejad V, Taniguchi T, Watanabe K, Li HO, Cao G, Guo GC, Nori F, Guo GP. Electrotunable artificial molecules based on van der Waals heterostructures. SCIENCE ADVANCES 2017; 3:e1701699. [PMID: 29062893 PMCID: PMC5650488 DOI: 10.1126/sciadv.1701699] [Citation(s) in RCA: 20] [Impact Index Per Article: 2.9] [Reference Citation Analysis] [Abstract] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Received: 05/22/2017] [Accepted: 09/25/2017] [Indexed: 06/02/2023]
Abstract
Quantum confinement has made it possible to detect and manipulate single-electron charge and spin states. The recent focus on two-dimensional (2D) materials has attracted significant interests on possible applications to quantum devices, including detecting and manipulating either single-electron charging behavior or spin and valley degrees of freedom. However, the most popular model systems, consisting of tunable double-quantum-dot molecules, are still extremely difficult to realize in these materials. We show that an artificial molecule can be reversibly formed in atomically thin MoS2 sandwiched in hexagonal boron nitride, with each artificial atom controlled separately by electrostatic gating. The extracted values for coupling energies at different regimes indicate a single-electron transport behavior, with the coupling strength between the quantum dots tuned monotonically. Moreover, in the low-density regime, we observe a decrease of the conductance with magnetic field, suggesting the observation of Coulomb blockade weak anti-localization. Our experiments demonstrate for the first time the realization of an artificial quantum-dot molecule in a gated MoS2 van der Waals heterostructure, which could be used to investigate spin-valley physics. The compatibility with large-scale production, gate controllability, electron-hole bipolarity, and new quantum degrees of freedom in the family of 2D materials opens new possibilities for quantum electronics and its applications.
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Affiliation(s)
- Zhuo-Zhi Zhang
- CAS Key Laboratory of Quantum Information, University of Science and Technology of China, Hefei, Anhui 230026, China
- Synergetic Innovation Center of Quantum Information and Quantum Physics, University of Science and Technology of China, Hefei, Anhui 230026, China
| | - Xiang-Xiang Song
- CAS Key Laboratory of Quantum Information, University of Science and Technology of China, Hefei, Anhui 230026, China
- Synergetic Innovation Center of Quantum Information and Quantum Physics, University of Science and Technology of China, Hefei, Anhui 230026, China
| | - Gang Luo
- CAS Key Laboratory of Quantum Information, University of Science and Technology of China, Hefei, Anhui 230026, China
- Synergetic Innovation Center of Quantum Information and Quantum Physics, University of Science and Technology of China, Hefei, Anhui 230026, China
| | - Guang-Wei Deng
- CAS Key Laboratory of Quantum Information, University of Science and Technology of China, Hefei, Anhui 230026, China
- Synergetic Innovation Center of Quantum Information and Quantum Physics, University of Science and Technology of China, Hefei, Anhui 230026, China
| | - Vahid Mosallanejad
- CAS Key Laboratory of Quantum Information, University of Science and Technology of China, Hefei, Anhui 230026, China
- Synergetic Innovation Center of Quantum Information and Quantum Physics, University of Science and Technology of China, Hefei, Anhui 230026, China
| | - Takashi Taniguchi
- National Institute for Materials Science, Namiki 1-1, Ibaraki 305-0044, Japan
| | - Kenji Watanabe
- National Institute for Materials Science, Namiki 1-1, Ibaraki 305-0044, Japan
| | - Hai-Ou Li
- CAS Key Laboratory of Quantum Information, University of Science and Technology of China, Hefei, Anhui 230026, China
- Synergetic Innovation Center of Quantum Information and Quantum Physics, University of Science and Technology of China, Hefei, Anhui 230026, China
| | - Gang Cao
- CAS Key Laboratory of Quantum Information, University of Science and Technology of China, Hefei, Anhui 230026, China
- Synergetic Innovation Center of Quantum Information and Quantum Physics, University of Science and Technology of China, Hefei, Anhui 230026, China
| | - Guang-Can Guo
- CAS Key Laboratory of Quantum Information, University of Science and Technology of China, Hefei, Anhui 230026, China
- Synergetic Innovation Center of Quantum Information and Quantum Physics, University of Science and Technology of China, Hefei, Anhui 230026, China
| | - Franco Nori
- CEMS, RIKEN, Wako-shi, Saitama 351-0198, Japan
- Department of Physics, University of Michigan, Ann Arbor, MI 48109–1040, USA
| | - Guo-Ping Guo
- CAS Key Laboratory of Quantum Information, University of Science and Technology of China, Hefei, Anhui 230026, China
- Synergetic Innovation Center of Quantum Information and Quantum Physics, University of Science and Technology of China, Hefei, Anhui 230026, China
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30
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Downing CA, Portnoi ME. Localization of massless Dirac particles via spatial modulations of the Fermi velocity. JOURNAL OF PHYSICS. CONDENSED MATTER : AN INSTITUTE OF PHYSICS JOURNAL 2017; 29:315301. [PMID: 28685706 DOI: 10.1088/1361-648x/aa7884] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/07/2023]
Abstract
The electrons found in Dirac materials are notorious for being difficult to manipulate due to the Klein phenomenon and absence of backscattering. Here we investigate how spatial modulations of the Fermi velocity in two-dimensional Dirac materials can give rise to localization effects, with either full (zero-dimensional) confinement or partial (one-dimensional) confinement possible depending on the geometry of the velocity modulation. We present several exactly solvable models illustrating the nature of the bound states which arise, revealing how the gradient of the Fermi velocity is crucial for determining fundamental properties of the bound states such as the zero-point energy. We discuss the implications for guiding electronic waves in few-mode waveguides formed by Fermi velocity modulation.
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Affiliation(s)
- C A Downing
- Université de Strasbourg, CNRS, Institut de Physique et Chimie des Matériaux de Strasbourg, UMR 7504, F-67000 Strasbourg, France
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31
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Wang S, Kharche N, Costa Girão E, Feng X, Müllen K, Meunier V, Fasel R, Ruffieux P. Quantum Dots in Graphene Nanoribbons. NANO LETTERS 2017; 17:4277-4283. [PMID: 28603996 DOI: 10.1021/acs.nanolett.7b01244] [Citation(s) in RCA: 53] [Impact Index Per Article: 7.6] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/22/2023]
Abstract
Graphene quantum dots (GQDs) hold great promise for applications in electronics, optoelectronics, and bioelectronics, but the fabrication of widely tunable GQDs has remained elusive. Here, we report the fabrication of atomically precise GQDs consisting of low-bandgap N = 14 armchair graphene nanoribbon (AGNR) segments that are achieved through edge fusion of N = 7 AGNRs. The so-formed intraribbon GQDs reveal deterministically defined, atomically sharp interfaces between wide and narrow AGNR segments and host a pair of low-lying interface states. Scanning tunneling microscopy/spectroscopy measurements complemented by extensive simulations reveal that their energy splitting depends exponentially on the length of the central narrow bandgap segment. This allows tuning of the fundamental gap of the GQDs over 1 order of magnitude within a few nanometers length range. These results are expected to pave the way for the development of widely tunable intraribbon GQD-based devices.
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Affiliation(s)
- Shiyong Wang
- Empa, Swiss Federal Laboratories for Materials Science and Technology , Überlandstrasse 129, CH-8600 Dübendorf, Switzerland
| | - Neerav Kharche
- Department of Physics, Applied Physics and Astronomy, Rensselaer Polytechnic Institute , Troy, 12180 New York, United States
| | - Eduardo Costa Girão
- Departamento de Física, Universidade Federal do Piauí , CEP 64049-550, Teresina, Piauí Brazil
| | - Xinliang Feng
- Department of Chemistry and Food Chemistry, Technische Universität Dresden , Mommsenstrasse 4, 01062 Dresden, Germany
| | - Klaus Müllen
- Max Planck Institute for Polymer Research , Ackermannweg 10, 55128 Mainz, Germany
| | - Vincent Meunier
- Department of Physics, Applied Physics and Astronomy, Rensselaer Polytechnic Institute , Troy, 12180 New York, United States
| | - Roman Fasel
- Empa, Swiss Federal Laboratories for Materials Science and Technology , Überlandstrasse 129, CH-8600 Dübendorf, Switzerland
- Department of Chemistry and Biochemistry, University of Bern , Freiestrasse 3, CH-3012 Bern, Switzerland
| | - Pascal Ruffieux
- Empa, Swiss Federal Laboratories for Materials Science and Technology , Überlandstrasse 129, CH-8600 Dübendorf, Switzerland
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32
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Liu J, Hou WJ, Cheng C, Fu HX, Sun JT, Meng S. Intrinsic valley polarization of magnetic VSe 2 monolayers. JOURNAL OF PHYSICS. CONDENSED MATTER : AN INSTITUTE OF PHYSICS JOURNAL 2017; 29:255501. [PMID: 28516897 DOI: 10.1088/1361-648x/aa6e6e] [Citation(s) in RCA: 22] [Impact Index Per Article: 3.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/22/2023]
Abstract
Intrinsic valley polarization can be obtained in VSe2 monolayers with broken inversion symmetry and time reversal symmetry. First-principles investigations reveal that the magnitude of the valley splitting in magnetic VSe2 induced by spin-orbit coupling reaches as high as 78.2 meV and can be linearly tuned by biaxial strain. Besides conventional polarized light, hole doping or illumination with light of proper frequency can offer effective routes to realize valley polarization. Moreover, spin-orbit coupling in monolayer VSe2 breaks not only the valley degeneracy but also the three-fold rotational symmetry in band structure. The intrinsic and tunable valley splitting and the breaking of optical isotropy bring additional benefits to valleytronic and optoelectronic applications.
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Affiliation(s)
- Jian Liu
- Beijing National Laboratory for Condensed Matter Physics and Institute of Physics, Chinese Academy of Sciences, Beijing 100190, People's Republic of China
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33
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Xiong H, Jiang W, Song Y, Duan L. Bound state properties of ABC-stacked trilayer graphene quantum dots. JOURNAL OF PHYSICS. CONDENSED MATTER : AN INSTITUTE OF PHYSICS JOURNAL 2017; 29:215002. [PMID: 28367830 DOI: 10.1088/1361-648x/aa6aac] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/07/2023]
Abstract
The few-layer graphene quantum dot provides a promising platform for quantum computing with both spin and valley degrees of freedom. Gate-defined quantum dots in particular can avoid noise from edge disorders. In connection with the recent experimental efforts (Song et al 2016 Nano Lett. 16 6245), we investigate the bound state properties of trilayer graphene (TLG) quantum dots (QDs) through numerical simulations. We show that the valley degeneracy can be lifted by breaking the time reversal symmetry through the application of a perpendicular magnetic field. The spectrum under such a potential exhibits a transition from one group of Landau levels to another group, which can be understood analytically through perturbation theory. Our results provide insight into the transport property of TLG QDs, with possible applications to study of spin qubits and valleytronics in TLG QDs.
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Affiliation(s)
- Haonan Xiong
- Department of Physics, Tsinghua University, Beijing 100084, People's Republic of China. Center for Quantum Information, IIIS, Tsinghua University, Beijing 100084, People's Republic of China
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34
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Libisch F, Hisch T, Glattauer R, Chizhova LA, Burgdörfer J. Veselago lens and Klein collimator in disordered graphene. JOURNAL OF PHYSICS. CONDENSED MATTER : AN INSTITUTE OF PHYSICS JOURNAL 2017; 29:114002. [PMID: 28045377 DOI: 10.1088/1361-648x/aa565e] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/06/2023]
Abstract
We simulate electron transport through graphene nanoribbons of realistic size containing a p-n junction patterned by electrostatic gates. For a sharp p-n interface, Klein tunneling leads to refocusing of a divergent beam forming a Veselago lens. Wider transition regions allow only electrons with near-perpendicular incidence to pass the junction, forming a Klein collimator. Using a third nearest neighbor tight binding description we explore the influence of interface roughness and bulk disorder on guiding properties. We provide bounds on disorder amplitudes and p-n junction properties to be satisfied in order to experimentally observe the focusing effect and compare our predictions to very recent realizations.
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35
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Walkup D, Stroscio JA. Helical Level Structure of Dirac Potential Wells. PHYSICAL REVIEW. B 2017; 96:10.1103/PhysRevB.96.201409. [PMID: 31276077 PMCID: PMC6604641 DOI: 10.1103/physrevb.96.201409] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/09/2023]
Abstract
In graphene and other massless two-dimensional Dirac materials, Klein tunneling compromises electron confinement, and momentum-space contours can be assigned a Berry phase which is either zero or π. Consequently, in such systems the energy spectrum of circular potential wells exhibits an interesting discontinuity as a function of magnetic field B: for a given angular momentum the ladder of eigen-resonances is split at an energy-dependent critical field B c. Here we show that introducing a mass term Δ in the Hamiltonian bridges this discontinuity in such a way that states below B c are adiabatically connected to states above B c whose principal quantum number differs by unity depending on the sign of Δ. In the B-Δ plane, the spectrum of these circular resonators resembles a spiral staircase, in which a particle prepared in the ∣n, m⟩ resonance state can be promoted to the ∣n± 1, m⟩ state by an adiabatic circuit of the Hamiltonian about B c, the sign depending on the direction of the circuit. We explain the phenomenon in terms of the evolving Berry phase of the orbit, which in such a circuit changes adiabatically by 2π.
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Affiliation(s)
- Daniel Walkup
- Center for Nanoscale Science and Technology, National Institute of Standards and Technology, Gaithersburg, MD 20899, USA
- Maryland NanoCenter, University of Maryland, College Park, MD 20742, USA
| | - Joseph A. Stroscio
- Center for Nanoscale Science and Technology, National Institute of Standards and Technology, Gaithersburg, MD 20899, USA
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