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Imaging of pure spin-valley diffusion current in WS2-WSe2 heterostructures

2018-05-31T08:58:47+00:00 May 31st, 2018|Categories: Publications|Tags: , , |

Transition metal dichalcogenide (TMDC) materials are promising for spintronic and valleytronic applications because valley-polarized excitations can be generated and manipulated with circularly polarized photons and the valley and spin degrees of freedom are locked by strong spin-orbital interactions. In this study we demonstrate efficient generation of a pure and locked spin-valley diffusion current in tungsten disulfide (WS2)–tungsten diselenide (WSe2) heterostructures without any driving electric field. We imaged the propagation of valley current in real time and space by pump-probe spectroscopy. The valley current in the heterostructures can live for more than 20 microseconds and propagate over 20 micrometers; both the lifetime and the diffusion length can be controlled through electrostatic gating. The high-efficiency and electric-field–free generation of a locked spin-valley current in TMDC heterostructures holds promise for applications in spin and valley devices.

Published in: "Science".

Giant tunneling magnetoresistance in spin-filter van der Waals heterostructures

2018-05-04T02:20:05+00:00 May 4th, 2018|Categories: Publications|Tags: , |

Magnetic multilayer devices that exploit magnetoresistance are the backbone of magnetic sensing and data storage technologies. Here, we report multiple-spin-filter magnetic tunnel junctions (sf-MTJs) based on van der Waals (vdW) heterostructures in which atomically thin chromium triiodide (CrI3) acts as a spin-filter tunnel barrier sandwiched between graphene contacts. We demonstrate tunneling magnetoresistance which is drastically enhanced with increasing CrI3 layer thickness, reaching a record 19,000% for magnetic multilayer structures using four-layer sf-MTJs at low temperatures. Using magnetic circular dichroism measurements, we attribute these effects to the intrinsic layer-by-layer antiferromagnetic ordering of the atomically thin CrI3. Our work reveals the possibility to push magnetic information storage to the atomically thin limit and highlights CrI3 as a superlative magnetic tunnel barrier for vdW heterostructure spintronic devices.

Published in: "Science (Advanced Online Publication)".

Probing the ultimate plasmon confinement limits with a van der Waals heterostructure

2018-04-20T02:27:39+00:00 April 20th, 2018|Categories: Publications|Tags: , , |

The ability to confine light into tiny spatial dimensions is important for applications such as microscopy, sensing, and nanoscale lasers. Although plasmons offer an appealing avenue to confine light, Landau damping in metals imposes a trade-off between optical field confinement and losses. We show that a graphene-insulator-metal heterostructure can overcome that trade-off, and demonstrate plasmon confinement down to the ultimate limit of the length scale of one atom. This is achieved through far-field excitation of plasmon modes squeezed into an atomically thin hexagonal boron nitride dielectric spacer between graphene and metal rods. A theoretical model that takes into account the nonlocal optical response of both graphene and metal is used to describe the results. These ultraconfined plasmonic modes, addressed with far-field light excitation, enable a route to new regimes of ultrastrong light-matter interactions.

Published in: "Science".

Bottom-up synthesis of multifunctional nanoporous graphene

2018-04-13T02:27:46+00:00 April 13th, 2018|Categories: Publications|Tags: , |

Nanosize pores can turn semimetallic graphene into a semiconductor and, from being impermeable, into the most efficient molecular-sieve membrane. However, scaling the pores down to the nanometer, while fulfilling the tight structural constraints imposed by applications, represents an enormous challenge for present top-down strategies. Here we report a bottom-up method to synthesize nanoporous graphene comprising an ordered array of pores separated by ribbons, which can be tuned down to the 1-nanometer range. The size, density, morphology, and chemical composition of the pores are defined with atomic precision by the design of the molecular precursors. Our electronic characterization further reveals a highly anisotropic electronic structure, where orthogonal one-dimensional electronic bands with an energy gap of ~1 electron volt coexist with confined pore states, making the nanoporous graphene a highly versatile semiconductor for simultaneous sieving and electrical sensing of molecular species.

Published in: "Science".

Observation of fractional Chern insulators in a van der Waals heterostructure

2018-04-06T02:27:39+00:00 April 6th, 2018|Categories: Publications|Tags: , , |

Topologically ordered phases are characterized by long-range quantum entanglement and fractional statistics rather than by symmetry breaking. First observed in a fractionally filled continuum Landau level, topological order has since been proposed to arise more generally at fractional fillings of topologically nontrivial Chern bands. Here we report the observation of gapped states at fractional fillings of Harper-Hofstadter bands arising from the interplay of a magnetic field and a superlattice potential in a bilayer graphene–hexagonal boron nitride heterostructure. We observed phases at fractional filling of bands with Chern indices C=–1, ±2, and ±3. Some of these phases, in C=–1 and C=2 bands, are characterized by fractional Hall conductance—that is, they are known as fractional Chern insulators and constitute an example of topological order beyond Landau levels.

Published in: "Science".

Real-time imaging of adatom-promoted graphene growth on nickel

2018-03-16T02:27:57+00:00 March 16th, 2018|Categories: Publications|Tags: |

Single adatoms are expected to participate in many processes occurring at solid surfaces, such as the growth of graphene on metals. We demonstrate, both experimentally and theoretically, the catalytic role played by single metal adatoms during the technologically relevant process of graphene growth on nickel (Ni). The catalytic action of individual Ni atoms at the edges of a growing graphene flake was directly captured by scanning tunneling microscopy imaging at the millisecond time scale, while force field molecular dynamics and density functional theory calculations rationalize the experimental observations. Our results unveil the mechanism governing the activity of a single-atom catalyst at work.

Published in: "Science".

Coherent, atomically thin transition-metal dichalcogenide superlattices with engineered strain

2018-03-09T02:27:46+00:00 March 9th, 2018|Categories: Publications|

Epitaxy forms the basis of modern electronics and optoelectronics. We report coherent atomically thin superlattices in which different transition metal dichalcogenide monolayers—despite large lattice mismatches—are repeated and laterally integrated without dislocations within the monolayer plane. Grown by an omnidirectional epitaxy, these superlattices display fully matched lattice constants across heterointerfaces while maintaining an isotropic lattice structure and triangular symmetry. This strong epitaxial strain is precisely engineered via the nanoscale supercell dimensions, thereby enabling broad tuning of the optical properties and producing photoluminescence peak shifts as large as 250 millielectron volts. We present theoretical models to explain this coherent growth and the energetic interplay governing the ripple formation in these strained monolayers. Such coherent superlattices provide building blocks with targeted functionalities at the atomically thin limit.

Published in: "Science".

Observation of fractional Chern insulators in a van der Waals heterostructure

2018-03-02T02:20:04+00:00 March 2nd, 2018|Categories: Publications|Tags: , , |

Topologically ordered phases are characterized by long-range quantum entanglement and fractional statistics rather than by symmetry breaking. First observed in a fractionally filled continuum Landau level, topological order has since been proposed to arise more generally at fractional fillings of topologically non-trivial “Chern” bands. Here, we report the observation of gapped states at fractional fillings of Harper-Hofstadter bands arising from the interplay of a magnetic field and a superlattice potential in a bilayer graphene/hexagonal boron nitride heterostructure. We observe phases at fractional filling of bands with Chern indices C=–1, ±2, and ±3. Some of these, in C=–1 and C=2 bands, are characterized by fractional Hall conductance—they are “fractional Chern insulators” and constitute an example of topological order beyond Landau levels.

Published in: "Science (Advanced Online Publication)".

Infrared hyperbolic metasurface based on nanostructured van der Waals materials

2018-02-23T02:27:49+00:00 February 23rd, 2018|Categories: Publications|Tags: |

Metasurfaces with strongly anisotropic optical properties can support deep subwavelength-scale confined electromagnetic waves (polaritons), which promise opportunities for controlling light in photonic and optoelectronic applications. We developed a mid-infrared hyperbolic metasurface by nanostructuring a thin layer of hexagonal boron nitride that supports deep subwavelength-scale phonon polaritons that propagate with in-plane hyperbolic dispersion. By applying an infrared nanoimaging technique, we visualize the concave (anomalous) wavefronts of a diverging polariton beam, which represent a landmark feature of hyperbolic polaritons. The results illustrate how near-field microscopy can be applied to reveal the exotic wavefronts of polaritons in anisotropic materials and demonstrate that nanostructured van der Waals materials can form a highly variable and compact platform for hyperbolic infrared metasurface devices and circuits.

Published in: "Science".

Observation of chiral phonons

2018-02-02T14:27:43+00:00 February 2nd, 2018|Categories: Publications|Tags: , |

Chirality reveals symmetry breaking of the fundamental interaction of elementary particles. In condensed matter, for example, the chirality of electrons governs many unconventional transport phenomena such as the quantum Hall effect. Here we show that phonons can exhibit intrinsic chirality in monolayer tungsten diselenide. The broken inversion symmetry of the lattice lifts the degeneracy of clockwise and counterclockwise phonon modes at the corners of the Brillouin zone. We identified the phonons by the intervalley transfer of holes through hole-phonon interactions during the indirect infrared absorption, and we confirmed their chirality by the infrared circular dichroism arising from pseudoangular momentum conservation. The chiral phonons are important for electron-phonon coupling in solids, phonon-driven topological states, and energy-efficient information processing.

Published in: "Science".

Nanoscale chiral valley-photon interface through optical spin-orbit coupling

2018-01-26T02:27:42+00:00 January 26th, 2018|Categories: Publications|Tags: |

The emergence of two-dimensional transition metal dichalcogenide materials has sparked intense activity in valleytronics, as their valley information can be encoded and detected with the spin angular momentum of light. We demonstrate the valley-dependent directional coupling of light using a plasmonic nanowire–tungsten disulfide (WS2) layers system. We show that the valley pseudospin in WS2 couples to transverse optical spin of the same handedness with a directional coupling efficiency of 90 ± 1%. Our results provide a platform for controlling, detecting, and processing valley and spin information with precise optical control at the nanoscale.

Published in: "Science".

Synthesis of partially and fully fused polyaromatics by annulative chlorophenylene dimerization

2018-01-26T02:27:41+00:00 January 26th, 2018|Categories: Publications|Tags: |

Since the discovery by Ullmann and Bielecki in 1901, reductive dimerization (or homocoupling) of aryl halides has been extensively exploited for the generation of a range of biaryl-based functional molecules. In contrast to the single-point connection in these products, edge-sharing fused aromatic systems have not generally been accessible from simple aryl halides via annulation cascades. Here we report a single-step synthesis of fused aromatics with a triphenylene core by the palladium-catalyzed annulative dimerization of structurally and functionally diverse chlorophenylenes through double carbon-hydrogen bond activation. The partially fused polyaromatics can be transformed into fully fused, small graphene nanoribbons, which are otherwise difficult to synthesize. This simple, yet powerful, method allows access to functional -systems of interest in optoelectronics research.

Published in: "Science".

Observation of the quantum spin Hall effect up to 100 kelvin in a monolayer crystal

2018-01-05T02:27:43+00:00 January 5th, 2018|Categories: Publications|Tags: , |

A variety of monolayer crystals have been proposed to be two-dimensional topological insulators exhibiting the quantum spin Hall effect (QSHE), possibly even at high temperatures. Here we report the observation of the QSHE in monolayer tungsten ditelluride (WTe2) at temperatures up to 100 kelvin. In the short-edge limit, the monolayer exhibits the hallmark transport conductance, ~e2/h per edge, where e is the electron charge and h is Planck’s constant. Moreover, a magnetic field suppresses the conductance, and the observed Zeeman-type gap indicates the existence of a Kramers degenerate point and the importance of time-reversal symmetry for protection from elastic backscattering. Our results establish the QSHE at temperatures much higher than in semiconductor heterostructures and allow for exploring topological phases in atomically thin crystals.

Published in: "Science".

Imaging resonant dissipation from individual atomic defects in graphene

2017-12-08T02:27:40+00:00 December 8th, 2017|Categories: Publications|Tags: , |

Conversion of electric current into heat involves microscopic processes that operate on nanometer length scales and release minute amounts of power. Although central to our understanding of the electrical properties of materials, individual mediators of energy dissipation have so far eluded direct observation. Using scanning nanothermometry with submicrokelvin sensitivity, we visualized and controlled phonon emission from individual atomic-scale defects in graphene. The inferred electron-phonon “cooling power spectrum” exhibits sharp peaks when the Fermi level comes into resonance with electronic quasi-bound states at such defects. Rare in the bulk but abundant at graphene’s edges, switchable atomic-scale phonon emitters provide the dominant dissipation mechanism. Our work offers insights for addressing key materials challenges in modern electronics and enables control of dissipation at the nanoscale.

Published in: "Science".

Compacted dimensions and singular plasmonic surfaces

2017-11-17T02:27:54+00:00 November 17th, 2017|Categories: Publications|Tags: |

In advanced field theories, there can be more than four dimensions to space, the excess dimensions described as compacted and unobservable on everyday length scales. We report a simple model, unconnected to field theory, for a compacted dimension realized in a metallic metasurface periodically structured in the form of a grating comprising a series of singularities. An extra dimension of the grating is hidden, and the surface plasmon excitations, though localized at the surface, are characterized by three wave vectors rather than the two of typical two-dimensional metal grating. We propose an experimental realization in a doped graphene layer.

Published in: "Science".

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