Results for "Kenji Watanabe"

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Loop series expansion with propagation diagramsAug 08 2008Nov 13 2008The Bethe approximation is a successful method for approximating partition functions of probabilistic models associated with a graph. Recently, Chertkov and Chernyak derived an interesting formula called Loop Series Expansion, which is an expansion of ... More
Anisotropic Dielectric Breakdown Strength of Single Crystal Hexagonal Boron NitrideOct 31 2016Dielectric breakdown has historically been of great interest from the perspectives of fundamental physics and electrical reliability. However, to date, the anisotropy in the dielectric breakdown has not been discussed. Here, we report an anisotropic dielectric ... More
Phase diagram of nuclear "pasta" and its uncertainties in supernova coresDec 01 2007Mar 11 2010We examine the model dependence of the phase diagram of inhomogeneous nulcear matter in supernova cores using the quantum molecular dynamics (QMD). Inhomogeneous matter includes crystallized matter with nonspherical nuclei - "pasta" phases - and the liquid-gas ... More
Simulation of Transitions between "Pasta" Phases in Dense MatterAug 24 2004Jan 28 2005Calculations of equilibrium properties of dense matter predict that at subnuclear densities nuclei can be rodlike or slablike. To investigate whether transitions between phases with non-spherical nuclei can occur during the collapse of a star, we perform ... More
Dry transfer of CVD graphene using MoS$_2$-based stampsJun 01 2017Recently, a contamination-free dry transfer method for graphene grown by chemical vapor deposition (CVD) has been reported that allows to directly pick-up graphene from the copper growth substrate using a flake of hexagonal boron nitride (hBN), resulting ... More
A Stochastic Temporal Model of Polyphonic MIDI Performance with OrnamentsApr 08 2014Aug 03 2016We study indeterminacies in realization of ornaments and how they can be incorporated in a stochastic performance model applicable for music information processing such as score-performance matching. We point out the importance of temporal information, ... More
Graph polynomials and approximation of partition functions with Loopy Belief PropagationMar 26 2009Nov 14 2009The Bethe approximation, or loopy belief propagation algorithm is a successful method for approximating partition functions of probabilistic models associated with a graph. Chertkov and Chernyak derived an interesting formula called Loop Series Expansion, ... More
Tunneling transport in a few monolayer-thick WS2/graphene heterojunctionNov 18 2014Dec 09 2014This paper demonstrates the high-quality tunnel barrier characteristics and layer number controlled tunnel resistance of a transition metal dichalcogenide (TMD) measuring just a few monolayers in thickness. Investigation of vertical transport in WS2 and ... More
BN/Graphene/BN Transistors for RF ApplicationsAug 09 2011In this letter, we demonstrate the first BN/Graphene/BN field effect transistor for RF applications. The BN/Graphene/BN structure can preserve the high mobility of graphene, even when it is sandwiched between a substrate and a gate dielectric. Field effect ... More
Ballistic transport exceeding 28 μm in CVD grown grapheneNov 27 2015Jan 12 2016We report on ballistic transport over more than 28 \mu m in graphene grown by chemical vapor deposition (CVD) that is fully encapsulated in hexagonal boron nitride. The structures are fabricated by an advanced dry van-der-Waals transfer method and exhibit ... More
Robust nonabelian ground states and continuous quantum phase transitions in a half filled bilayer graphene Landau levelNov 22 2016In a topological quantum bit, quantum information is protected from decoherence by the energy gap separating the nonabelian vacuum from higher energy modes. Experimental systems predicted to harbor nonabelions range from p-wave superfluids to superconducting ... More
Electroluminescence from multi-particle exciton complexes in monolayer WSe2Dec 10 2018Light emission from higher-order correlated excitonic states has been recently reported in hBN-encapsulated monolayer WSe2 upon optical excitation. These exciton complexes are found to be bound states of excitons residing in opposite valleys in momentum ... More
New graph polynomials from the Bethe approximation of the Ising partition functionAug 26 2009Jun 04 2010We introduce two graph polynomials and discuss their properties. One is a polynomial of two variables whose investigation is motivated by the performance analysis of the Bethe approximation of the Ising partition function. The other is a polynomial of ... More
Impact of nuclear "pasta" on neutrino transport in collapsing stellar coresJan 12 2007Apr 18 2007Nuclear ``pasta'', nonspherical nuclei in dense matter, is predicted to occur in collapsing supernova cores. We show how pasta phases affect the neutrino transport cross section via weak neutral current using several nuclear models. This is the first ... More
Nanosecond spin lifetimes in bottom-up fabricated bilayer graphene spin-valves with atomic layer deposited Al$_2$O$_3$ spin injection and detection barriersJul 09 2015We present spin transport studies on bi- and trilayer graphene non-local spin-valves which have been fabricated by a bottom-up fabrication method. By this technique, spin injection electrodes are first deposited onto Si$^{++}$/SiO$_2$ substrates with ... More
From diffusive to ballistic transport in etched graphene constrictions and nanoribbonsJun 29 2017Graphene nanoribbons and constrictions are envisaged as fundamental components of future carbon-based nanoelectronic and spintronic devices. At nanoscale, electronic effects in these devices depend heavily on the dimensions of the active channel and the ... More
Control of interlayer excitons in two-dimensional van der Waals heterostructuresMar 16 2018Long-lived interlayer excitons with distinct spin-valley physics in van der Waals heterostructures based on transition metal dichalcogenides make them promising for information processing in next-generation devices. While the emission characteristics ... More
Absorptive pinhole collimators for ballistic Dirac fermions in grapheneNov 16 2016Nov 17 2016Ballistic electrons in solids can have mean free paths far larger than the smallest features patterned by lithography. This has allowed development and study of solid-state electron-optical devices such as beam splitters and quantum point contacts, which ... More
Possibility of Deconfined Criticality in SU(N) Heisenberg Models at Small NJul 01 2013Dec 15 2013To examine the validity of the scenario of the deconfined critical phenomena, we carry out a quantum Monte Carlo simulation for the SU($N$) generalization of the Heisenberg model with four-body and six-body interactions. The quantum phase transition between ... More
High quality electrostatically defined hall bars in monolayer grapheneJan 04 2019Jan 22 2019Realizing graphene's promise as an atomically thin and tunable platform for fundamental studies and future applications in quantum transport requires the ability to electrostatically define the geometry of the structure and control the carrier concentration, ... More
Visualizing Strain-induced Pseudo magnetic Fields in Graphene through an hBN Magnifying GlassMar 01 2017The remarkable properties of graphene are inherent to its 2D honeycomb lattice structure. Its low dimensionality, which makes it possible to rearrange the atoms by applying an external force, offers the intriguing prospect of mechanically controlling ... More
Low energy band structure in Bernal stacked hexlayer graphene: Landau fan diagram and resistance ridgeOct 04 2018Jan 23 2019The low-energy band structure of Bernal-stacked hexlayer graphene was investigated by the measuring transport properties of high-mobility graphene samples equipped with a top and a bottom gate electrode at low temperature and in a magnetic field. By analyzing ... More
Aharonov-Bohm oscillations and magnetic focusing in ballistic graphene ringsSep 02 2017We present low-temperature magnetotransport measurements on graphene rings encapsulated in hexagonal boron nitride. We investigate phase-coherent transport and show Aharonov-Bohm (AB) oscillations in quasi-ballistic graphene rings with hard confinement. ... More
Graph Zeta Function in the Bethe Free Energy and Loopy Belief PropagationFeb 17 2010We propose a new approach to the analysis of Loopy Belief Propagation (LBP) by establishing a formula that connects the Hessian of the Bethe free energy with the edge zeta function. The formula has a number of theoretical implications on LBP. It is applied ... More
Loopy Belief Propagation, Bethe Free Energy and Graph Zeta FunctionMar 03 2011We propose a new approach to the theoretical analysis of Loopy Belief Propagation (LBP) and the Bethe free energy (BFE) by establishing a formula to connect LBP and BFE with a graph zeta function. The proposed approach is applicable to a wide class of ... More
Generation and detection of pure valley current by electrically induced Berry curvature in bilayer grapheneJan 20 2015Valley is a useful degree of freedom for non-dissipative electronics since valley current that can flow even in an insulating material does not accompany electronic current. We use dual-gated bilayer graphene in the Hall bar geometry to electrically control ... More
High-quality dry transferred CVD bilayer grapheneApr 14 2017We report on the fabrication and characterization of high-quality chemical vapor-deposited (CVD) bilayer graphene (BLG). In particular, we demonstrate that CVD-grown BLG can mechanically be detached from the copper foil by an hexagonal boron nitride (hBN) ... More
Photo-induced anomalous Hall effect in the type-II Weyl-semimetal WTe2 at room-temperatureOct 02 2018Using Hall photovoltage measurements, we demonstrate that an anomalous Hall-voltage can be induced in few layer WTe2 under circularly polarized light illumination. By applying a bias voltage along different crystal axes, we find that the photo-induced ... More
Superior valley polarization and coherence of 2s excitons in monolayer WSe2Dec 08 2017We report experimental observation of 2s exciton radiative emission from monolayer tungsten diselenide, enabled by hexagonal boron nitride protected high-quality samples. The 2s luminescence is highly robust and persists up to 150K, offering a new quantum ... More
One-dimensional edge contacts to monolayer semiconductorsFeb 14 2019Feb 17 2019Integration of Ohmic contacts into van der Waals (vdW) heterostructures is critical for realizing electronic and optoelectronic functionalities. However, to date no scalable methodology for gaining electrical access to buried monolayer two-dimensional ... More
Angular Power Spectrum in Modular Invariant Inflation ModelSep 25 2006Nov 28 2006We propose a scalar potential of inflation, motivated by modular invariant supergravity, and compute the angular power spectra of the adiabatic density perturbations that result from this model. The potential consists of three scalar fields, S, Y and ... More
Quantum Hall Effect, Screening and Layer-Polarized Insulating States in Twisted Bilayer GrapheneOct 20 2011We investigate electronic transport in dual-gated twisted bilayer graphene. Despite the sub-nanometer proximity between the layers, we identify independent contributions to the magnetoresistance from the graphene Landau level spectrum of each layer. We ... More
Imaging Electron Motion in a Few Layer MoS2 DeviceAug 07 2016Ultrathin sheets of MoS2 are a newly discovered 2D semiconductor that holds great promise for nanoelectronics. Understanding the pattern of current flow will be crucial for developing devices. In this talk, we present images of current flow in MoS2 obtained ... More
Coherent Control of Two-Dimensional ExcitonsFeb 13 2019Electric dipole radiation can be controlled by coherent optical feedback, as has previously been studied by modulating the photonic environment for point dipoles placed both in optical cavities and near metal mirrors. In experiments involving fluorescent ... More
Observation of superconductivity in bilayer graphene/hexagonal boron nitride superlatticesJan 27 2019Feb 21 2019A class of low-dimensional superconductivity (SC), such as most of "atomic-layer" SCs, has only survived under certain circumstances, implying a role of the substrate. Moreover, in some recent discoveries of SC at heterogeneous interfaces, SC was buried ... More
Electrical spin injection into graphene through monolayer hexagonal boron nitrideMay 30 2013We demonstrate electrical spin injection from a ferromagnet to a bilayer graphene (BLG) through a monolayer (ML) of single-crystal hexagonal boron nitride (h-BN). A Ni81Fe19/ML h-BN/BLG/h-BN structure is fabricated using a micromechanical cleavage and ... More
Exciton energy-momentum map of hexagonal boron nitrideAug 07 2015Understanding and controlling the way excitons propagate in solids is a key for tailoring materials with improved optoelectronic properties. A fundamental step in this direction is the determination of the exciton energy-momentum dispersion. Here, thanks ... More
Quantum confined Stark effect in a MoS$_2$ monolayer van der Waals heterostructureOct 26 2017The optics of dangling-bond-free van der Waals heterostructures containing transition metal dichalcogenides are dominated by excitons. A crucial property of a confined exciton is the quantum confined Stark effect (QCSE). Here, such a heterostructure is ... More
Giant valley-isospin conductance oscillations in ballistic grapheneAug 31 2017At high magnetic fields the conductance of graphene is governed by the half-integer quantum Hall effect. By local electrostatic gating a \textit{p-n} junction perpendicular to the graphene edges can be formed, along which quantum Hall channels co-propagate. ... More
Weak Localization and Electron-electron Interactions in Few Layer Black Phosphorus DevicesAug 01 2016Few layer phosphorene(FLP) devices are extensively studied due to its unique electronic properties and potential applications on nano-electronics . Here we present magnetotransport studies which reveal electron-electron interactions as the dominant scattering ... More
Ballistic miniband conduction in a graphene superlatticeMar 03 2016Rational design of artificial lattices yields effects unavailable in simple solids, and vertical superlattices of multilayer semiconductors are already used in optical sensors and emitters. Manufacturing lateral superlattices remains a much bigger challenge, ... More
A high-mobility electronic system at an electrolyte-gated oxide surfaceOct 11 2014Electrolyte gating is a powerful technique for accumulating large carrier densities in surface two-dimensional electron systems (2DES). Yet this approach suffers from significant sources of disorder: electrochemical reactions can damage or alter the surface ... More
Non-trivial quantum oscillation geometric phase shift in a trivial bandFeb 12 2019Feb 13 2019The accumulation of non-trivial geometric phases in a material's response is often a tell-tale sign of a rich underlying internal structure. Studying quantum oscillations provides one of the ways to determine these geometrical phases, such as Berry's ... More
Nanosecond spin lifetimes in single- and few-layer graphene-hBN heterostructures at room temperatureJun 10 2014We present a new fabrication method of graphene spin-valve devices which yields enhanced spin and charge transport properties by improving both the electrode-to-graphene and graphene-to-substrate interface. First, we prepare Co/MgO spin injection electrodes ... More
Electrical generation and detection of spin waves in a quantum Hall ferromagnetJan 25 2018Spin waves are essential to understanding the intrinsic ordering and thermodynamic properties of magnetic systems. An attractive candidate for studying long-lived spin-wave physics is the quantum Hall (QH) ferromagnet, which forms spontaneously in clean ... More
Low B Field Magneto-Phonon Resonances in Single-Layer and Bilayer GrapheneApr 14 2015Apr 16 2015Many-body effects resulting from strong electron-electron and electron-phonon interactions play a significant role in graphene physics. We report on their manifestation in low B field magneto-phonon resonances in high quality exfoliated single-layer and ... More
Large spin relaxation anisotropy and valley-Zeeman spin-orbit coupling in WSe2/Gr/hBN heterostructuresDec 15 2017Dec 18 2017Large spin-orbital proximity effects have been predicted in graphene interfaced with a transition metal dichalcogenide layer. Whereas clear evidence for an enhanced spin-orbit coupling has been found at large carrier densities, the type of spin-orbit ... More
New generation of moiré superlattices in doubly aligned hBN/graphene/hBN heterostructuresDec 25 2018The specific rotational alignment of two-dimensional lattices results in a moir\'e superlattice with a larger period than the original lattices and allows one to engineer the electronic band structure of such materials. So far, transport signatures of ... More
Gate-Defined One-Dimensional Channel and Broken Symmetry States in MoS$_2$ van der Waals HeterostructuresJan 30 2017Jul 21 2017We have realized encapsulated trilayer MoS$_2$ devices with gated graphene contacts. In the bulk, we observe an electron mobility as high as 7000~cm$^{2}$/(V s) at a density of 3 $\times$ 10$^{12}$~cm$^{-2}$ at a temperature of 1.9~K. Shubnikov--de Haas ... More
Probing electronic lifetimes and phonon anharmonicities in high-quality chemical vapor deposited graphene by magneto-Raman spectroscopyNov 29 2015We present a magneto-Raman study on high-quality single-layer graphene grown by chemical vapor deposition (CVD) that is fully encapsulated in hexagonal boron nitride by a dry transfer technique. By analyzing the Raman D, G, and 2D peaks, we find that ... More
Gate-Tunable Quantum Dot in a High Quality Single Layer MoS$_{\mathrm{2}}$ Van der Waals HeterostructureJan 01 2018Mar 21 2018We have fabricated an encapsulated monolayer MoS$_{\mathrm{2}}$ device with metallic ohmic contacts through a pre-patterned hBN layer. In the bulk, we observe an electron mobility as high as 3000 cm$^{\mathrm{2}}$/Vs at a density of 7 $\times$ 10$^{\mathrm{12}}$ ... More
Strange metal in magic-angle graphene with near Planckian dissipationJan 11 2019Recent experiments on magic-angle twisted bilayer graphene have discovered correlated insulating behavior and superconductivity at a fractional filling of an isolated narrow band. In this paper we show that magic-angle bilayer graphene exhibits another ... More
Gate tunable dark trions in monolayer WSe$_2$Jan 30 2019We report the observation and gate manipulation of intrinsic dark trions in monolayer WSe$_2$. By using ultraclean WSe$_2$ devices encapsulated by boron nitride, we directly resolve the weak photoluminescence of dark trions. The dark trions can be tuned ... More
Photo-thermionic effect in vertical graphene heterostructuresJan 16 2016Jun 10 2016Finding alternative optoelectronic mechanisms that overcome the limitations of conventional semiconductor devices is paramount for detecting and harvesting low-energy photons. A highly promising approach is to drive a current from the thermal energy added ... More
Evidence for a Spin Phase Transition at ν=0 in Bilayer GrapheneDec 16 2012Feb 16 2013The most celebrated property of the quantum spin Hall effect is the presence of spin-polarized counter-propagating edge states. This novel edge state configuration has also been predicted to occur in graphene when spin-split electron- and hole-like Landau ... More
Electrical characterization of fully encapsulated ultra thin black phosphorus-based heterostructures with graphene contactsDec 03 2014The presence of finite bandgap and high mobility in semiconductor few-layer black phosphorus offers an attractive prospect for using this material in future two-dimensional electronic devices. Here we demonstrate for the first time fully encapsulated ... More
LHC constraints and prospects for $S_1$ scalar leptoquark explaining the $\bar B \to D^{(*)} τ\barν$ anomalyMar 16 2016Recently, deviations in flavor observables of B -> D(*) tau nu have been shown between the predictions in the Standard Model and the experimental results reported by BaBar, Belle, and LHCb collaborations. One of the solutions to this anomaly is obtained ... More
Cubic BN optical gap and intragap optically active defectsJun 29 2018We report a comprehensive study on the optical properties of cubic boron nitride (c-BN) and its optically active defects. Using electron energy-loss spectroscopy (EELS) within a monochromated scanning transmission electron microscope (STEM) on the highest-quality ... More
High-Mobility Holes in Dual-Gated WSe$_2$ Field-Effect TransistorsSep 13 2015We demonstrate dual-gated $p$-type field-effect transistors (FETs) based on few-layer tungsten diselenide (WSe$_2$) using high work-function platinum source/drain contacts, and a hexagonal boron nitride top-gate dielectric. A device topology with contacts ... More
Correlated Insulating and Superconducting States in Twisted Bilayer Graphene Below the Magic AngleFeb 13 2019The emergence of flat bands and correlated behaviors in 'magic angle' twisted bilayer graphene (tBLG) has sparked tremendous interest, though many aspects of the system are under intense debate. Here we report observation of both superconductivity and ... More
Integration of atomically thin layers of transition metal dichalcogenides into high-Q, monolithic Bragg-cavities - an experimental platform for the enhancement of optical interaction in 2D-materialsDec 03 2018Dec 05 2018We demonstrate a new approach to integrate single layer MoSeR2R and WSeR2R flakes into monolithic all-dielectric planar high-quality micro-cavities. These distributed-Bragg-reflector (DBR) cavities may e.g. be tuned to match the exciton resonance of the ... More
Switchable friction enabled by nanoscale self-assembly on grapheneApr 20 2015Sep 01 2015Graphene monolayers are known to display domains of anisotropic friction with twofold symmetry and anisotropy exceeding 200 percent. This anisotropy has been thought to originate from periodic nanoscale ripples in the graphene sheet, which enhance puckering ... More
Van der Waals epitaxial growth of topological insulator Bi$_{2-x}$Sb$_x$Te$_{3-y}$Se$_y$ ultrathin nanoplate on electrically insulating fluorophlogopite micaJul 29 2014We report the growth of high quality Bi$_{2-x}$Sb$_x$Te$_{3-y}$Se$_y$ ultrathin nanoplates (BSTS-NPs) on an electrically insulating fluorophlogopite mica substrate using a catalyst-free vapor solid method. Under an optimized pressure and suitable Ar gas ... More
Unconventional Correlation between Quantum Hall Transport Quantization and Bulk State Filling in Gated Graphene DevicesNov 04 2015Oct 28 2016We report simultaneous transport and scanning microwave impedance microscopy to examine the correlation between transport quantization and filling of the bulk Landau levels in the quantum Hall regime in gated graphene devices. Surprisingly, a comparison ... More
Stacking transition in bilayer graphene caused by thermally activated rotationDec 07 2016Crystallographic alignment between two-dimensional crystals in van der Waals heterostructures brought a number of profound physical phenomena, including observation of Hofstadter butterfly and topological currents, and promising novel applications, such ... More
Emergence of Tertiary Dirac Points in Graphene Moiré SuperlatticesFeb 10 2017The electronic structure of a crystalline solid is largely determined by its lattice structure. Recent advances in van der Waals solids, artificial crystals with controlled stacking of two-dimensional (2D) atomic films, have enabled the creation of materials ... More
Spin and Valley States in Gate-defined Bilayer Graphene Quantum DotsMar 08 2018Aug 07 2018In bilayer graphene, electrostatic confinement can be realized by a suitable design of top and back gate electrodes. We measure electronic transport through a bilayer graphene quantum dot, which is laterally confined by gapped regions and connected to ... More
Strain-modulated Bandgap and Piezo-resistive Effect in Black Phosphorus Field-effect TransistorsJan 27 2017Energy bandgap largely determines the optical and electronic properties of a semiconductor. Variable bandgap therefore makes versatile functionality possible in a single material. In layered material black phosphorus, the bandgap can be modulated by the ... More
Thermoelectric detection of propagating plasmons in grapheneJan 08 2016Controlling, detecting and generating propagating plasmons by all-electrical means is at the heart of on-chip nano-optical processing. Graphene carries long-lived plasmons that are extremely confined and controllable by electrostatic fields, however electrical ... More
2D Semiconductor Nonlinear Plasmonic ModulatorsFeb 12 2019A plasmonic modulator is a device that controls the amplitude or phase of propagating plasmons. In a pure plasmonic modulator, the presence or absence of a pump plasmonic wave controls the amplitude of a probe plasmonic wave through a channel. This control ... More
Unconventional Quantum Hall Edge-Bulk Correlation in Gated Graphene DevicesNov 04 2015The quantum Hall effect in a two dimensional electron system has been understood as chiral edge states circulating a highly insulating bulk when the bulk Landau levels are completely filled. We report simultaneous transport and scanning Microwave Impedance ... More
Order parameters with higher dimensionful composite fieldsDec 23 2003We discuss the possibility of the spontaneous symmetry breaking characterized by order parameters with higher dimensionful composite fields. By analyzing general Ginzburg-Landau potential for a complex scalar field \phi=\phi_1 + i \phi_2 with O(2) symmetry, ... More
Tunable $Γ- K$ Valley Populations in Hole-Doped Trilayer WSe$_2$Jan 10 2018We present a combined experimental and theoretical study of valley populations in the valence bands of trilayer WSe$_2$. Shubnikov$-$de Haas oscillations show that trilayer holes populate two distinct subbands associated with the $K$ and $\Gamma$ valleys, ... More
Very Large Tunneling Magnetoresistance in Layered Magnetic Semiconductor CrI$_3$Jan 24 2018Jun 28 2018Magnetic layered van der Waals crystals are an emerging class of materials giving access to new physical phenomena, as illustrated by the recent observation of 2D ferromagnetism in Cr2Ge2Te6 and CrI3. Of particular interest in semiconductors is the interplay ... More
Quantum Emission from Defects in Single Crystal Hexagonal Boron NitrideMar 07 2016Bulk hexagonal boron nitride (hBN) is a highly nonlinear natural hyperbolic material that attracts major attention in modern nanophotonics applications. However, studies of its optical properties in the visible part of the spectrum and quantum emitters ... More
Quantum Critical Transition and Kondo Screening of Magnetic Moments in GrapheneNov 19 2017Jan 11 2018In normal metals, the magnetic moment of impurity spins disappears below a characteristic Kondo temperature, TK, where coupling with the conduction-band electrons produces an entangled state that screens the local moment. In contrast, moments embedded ... More
Observation of spin-orbit coupling induced Weyl points and topologically protected Kondo effect in a two-electron double quantum dotApr 17 2018Recent years have brought an explosion of activities in the research of topological aspects of condensed-matter systems. Topologically non-trivial phases of matter are typically accompanied by protected surface states or exotic degenerate excitations ... More
Imaging bulk and edge transport near the Dirac point in graphene moiré superlatticesNov 21 2017Van der Waals structures formed by aligning monolayer graphene with insulating layers of hexagonal boron nitride exhibit a moir\'e superlattice that is expected to break sublattice symmetry. Despite an energy gap of several tens of millielectron volts ... More
Interlayer Exciton Optoelectronics in a 2D Heterostructure p-n JunctionJan 03 2017Semiconductor heterostructures are backbones for solid state based optoelectronic devices. Recent advances in assembly techniques for van der Waals heterostructures has enabled the band engineering of semiconductor heterojunctions for atomically thin ... More
Simultaneous imaging of voltage and current density of flowing electrons in two dimensionsOct 25 2018Electron transport in nanoscale devices can often result in nontrivial spatial patterns of voltage and current that reflect a variety of physical phenomena, particularly in nonlocal transport regimes. While numerous techniques have been devised to image ... More
Supercurrent Flow in Multi-Terminal Graphene Josephson JunctionsOct 27 2018We investigate the electronic properties of ballistic planar Josephson junctions with multiple superconducting terminals. Our devices consist of monolayer graphene encapsulated in boron nitride with molybdenum-rhenium contacts. Resistance measurements ... More
Anisotropic Etching of Graphite and Graphene in a Remote Hydrogen PlasmaMar 14 2017We investigate the etching of a pure hydrogen plasma on graphite samples and graphene flakes on SiO$_2$ and hexagonal Boron-Nitride (hBN) substrates. The pressure and distance dependence of the graphite exposure experiments reveals the existence of two ... More
Emergent Dirac gullies and gully-symmetry breaking quantum Hall states in ABA trilayer grapheneMay 02 2018Jul 19 2018We report on quantum capacitance measurements of high quality, graphite- and hexagonal boron nitride encapsulated Bernal stacked trilayer graphene devices. At zero applied magnetic field, we observe a number of electron density- and electrical displacement-tuned ... More
Tuning a Circular p-n Junction in Graphene from Quantum Confinement to Optical GuidingMay 20 2017The motion of massless Dirac-electrons in graphene mimics the propagation of photons. This makes it possible to control the charge-carriers with components based on geometrical-optics and has led to proposals for an all-graphene electron-optics platform. ... More
Interaction Driven Quantum Hall Wedding cake-like Structures in Graphene Quantum DotsSep 29 2018Quantum-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 table-top prototype of strongly confined ... More
Quantum Hall Effect in Electron-Doped Black Phosphorus Field-Effect TransistorsSep 20 2018The advent of black phosphorus field-effect transistors (FETs) has brought new possibilities in the study of two-dimensional (2D) electron systems. In a black phosphorus FET, the gate induces highly anisotropic 2D electron and hole gases. Although the ... More
Visualization and Control of Single Electron Charging in Bilayer Graphene Quantum DotsSep 13 2018Graphene 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 ... More
Full control of quantum Hall supercurrent in a side gated graphene Josephson junctionJan 17 2019We present a study of a graphene-based Josephson junction with dedicated side gates carved from the same sheet of graphene as the junction itself. These side gates are highly efficient, and allow us to modulate carrier density along either edge of the ... More
Electrical 2π phase control of infrared light in a 350nm footprint using graphene plasmonsMay 29 2017Modulating the amplitude and phase of light is at the heart of many applications such as wavefront shaping, transformation optics, phased arrays, modulators and sensors. Performing this task with high efficiency and small footprint is a formidable challenge. ... More
Inducing Superconducting Correlation in Quantum Hall Edge StatesSep 26 2016Sep 28 2016The quantum Hall (QH) effect supports a set of chiral edge states at the boundary of a 2-dimensional electron gas (2DEG) system. A superconductor (SC) contacting these states induces correlation of the quasi-particles in the dissipationless 1D chiral ... More
Hot Carrier-Assisted Intrinsic Photoresponse in GrapheneAug 18 2011Nov 17 2011Graphene is a new material showing high promise in optoelectronics, photonics, and energy-harvesting applications. However, the underlying physical mechanism of optoelectronic response has not been established. Here, we report on the intrinsic optoelectronic ... More
Emergent ferromagnetism near three-quarters filling in twisted bilayer grapheneJan 11 2019When two sheets of graphene are stacked at a small twist angle, the resulting flat superlattice minibands are expected to strongly enhance electron-electron interactions. Here we present evidence that near three-quarters ($3/4$) filling of the conduction ... More
X-ray pumping of the Th-229 nuclear clock isomerFeb 13 2019Thorium-229 is a unique case in nuclear physics: it presents a metastable first excited state Th-229m, just a few electronvolts above the nuclear ground state. This so-called isomer is accessible by VUV lasers, which allows transferring the amazing precision ... More
Large N Expansion for Strongly-coupled Boson-Fermion MixturesOct 20 2010We study a many-body mixture of an equal number of bosons and two-component fermions with a strong contact attraction. In this system bosons and fermions can be paired into composite fermions. We construct a large N extension where both bosons and fermions ... More
Noise Response Data Reveal Novel Controllability Gramian for Nonlinear Network DynamicsJun 03 2016Control of nonlinear large-scale dynamical networks, e.g., collective behavior of agents interacting via a scale-free connection topology, is a central problem in many scientific and engineering fields. For the linear version of this problem, the so-called ... More
Asymptotic States in Two Black Hole Moduli SpaceJul 27 2004Oct 14 2004We discuss the quantum states in the moduli space, which constructed with maximally charged dilaton black holes. Considering the quantum mechanics in the moduli space, we obtain the asymptotic states for the near-coincident black holes and the widely ... More
On the symmetry of commuting differential operators with singularities along hyperplanesSep 03 2003Feb 14 2004We study the commutants of a Schr\"{o}dinger operator whose potential function possesses inverse square singularities along some hyperplanes passing through the origin. It is shown that the Weyl group symmetry of the potential function and the commutants ... More
Floer homology of Lagrangian submanifoldsJun 24 2011This is a translation of an article appeared in Japanese in Suugaku 63 (2011), no. 1, 43-66 (MR2790665) and is a survey of Lagrangian Floer homology which the author studies jointly with Y.-G.Oh, H. Ohta, and K. Ono. It also contains some explanation ... More
Exceptional String: Instanton Expansions and Seiberg-Witten CurveOct 15 2001Feb 25 2002We investigate instanton expansions of partition functions of several toric E-string models using local mirror symmetry and elliptic modular forms. We also develop a method to obtain the Seiberg--Witten curve of E-string with arbitrary Wilson lines with ... More
Kähler Moduli Space of a D-Brane at Orbifold SingularitiesJun 08 1998Jun 10 1998We develop a method to analyze systematically the configuration space of a D-brane localized at the orbifold singular point of a Calabi--Yau $d$-fold of the form ${\Bbb C}^d/\Gamma$ using the theory of toric quotients. This approach elucidates the structure ... More