Electrical switching of a moir\'{e} ferroelectric superconductor

Electrical control of superconductivity is critical for nanoscale superconducting circuits including cryogenic memory elements, superconducting field-effect transistors (FETs), and gate-tunable qubits. Superconducting FETs operate through continuous tuning of carrier density, but there has not yet been a bistable superconducting FET, which could serve as a new type of cryogenic memory element. Recently, unusual ferroelectricity in Bernal-stacked bilayer graphene aligned to its insulating hexagonal boron nitride (BN) gate dielectrics was discovered. Here, we report the observation of ferroelectricity in magic-angle twisted bilayer graphene (MATBG) with aligned BN layers. This ferroelectric behavior coexists alongside the strongly correlated electron system of MATBG without disrupting its correlated insulator or superconducting states. This all-van der Waals platform enables configurable switching between different electronic states of this rich system. To illustrate this new approach, we demonstrate reproducible bistable switching between the superconducting, metallic, and correlated insulator states of MATBG using gate voltage or electric displacement field. These experiments unlock the potential to broadly incorporate this new moir\'{e} ferroelectric superconductor into highly tunable superconducting electronics

Electrical Control of 2D Magnetism in Bilayer CrI3

The challenge of controlling magnetism using electric fields raises fundamental questions and addresses technological needs such as low-dissipation magnetic memory. The recently reported two-dimensional (2D) magnets provide a new system for studying this problem owing to their unique magnetic properties. For instance, bilayer chromium triiodide (CrI3) behaves as a layered antiferromagnet with a magnetic field-driven metamagnetic transition. Here, we demonstrate electrostatic gate control of magnetism in CrI3 bilayers, probed by magneto-optical Kerr effect (MOKE) microscopy. At fixed magnetic fields near the metamagnetic transition, we realize voltage-controlled switching between antiferromagnetic and ferromagnetic states. At zero magnetic field, we demonstrate a time-reversal pair of layered antiferromagnetic states which exhibit spin-layer locking, leading to a remarkable linear dependence of their MOKE signals on gate voltage with opposite slopes. Our results pave the way for exploring new magnetoelectric phenomena and van der Waals spintronics based on 2D materials.Comment: To appear in Nature Nanotechnolog

Layer-dependent ferromagnetism in a van der Waals crystal down to the monolayer limit

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Gigahertz Frequency Antiferromagnetic Resonance and Strong Magnon-Magnon Coupling in the Layered Crystal CrCl 3

© 2019 American Physical Society. We report broadband microwave absorption spectroscopy of the layered antiferromagnet CrCl3. We observe a rich structure of resonances arising from quasi-two-dimensional antiferromagnetic dynamics. Because of the weak interlayer magnetic coupling in this material, we are able to observe both optical and acoustic branches of antiferromagnetic resonance in the GHz frequency range and a symmetry-protected crossing between them. By breaking rotational symmetry, we further show that strong magnon-magnon coupling with large tunable gaps can be induced between the two resonant modes

Shape-Dependent Plasmonic Response and Directed Self-Assembly in a New Semiconductor Building Block, Indium-Doped Cadmium Oxide (ICO)

The influence of particle shape on plasmonic response and local electric field strength is well-documented in metallic nanoparticles. Morphologies such as rods, plates, and octahedra are readily synthesized and exhibit drastically different extinction spectra than spherical particles. Despite this fact, the influence of composition and shape on the optical properties of plasmonic semiconductor nanocrystals, in which free electrons result from heavy doping, has not been well-studied. Here, we report the first observation of plasmonic resonance in indium-doped cadmium oxide (ICO) nanocrystals, which exhibit the highest quality factors reported for semiconductor nanocrystals. Furthermore, we are able to independently control the shape and free electron concentration in ICO nanocrystals, allowing for the influence of shape on the optical response of a plasmonic semiconductor to be conclusively demonstrated. The highly uniform particles may be self-assembled into ordered single component and binary nanocrystal superlattices, and in thin films, exhibit negative permittivity in the near infrared (NIR) region, validating their use as a new class of tunable low-loss plasmonic building blocks for 3-D optical metamaterials

Uniform Bimetallic Nanocrystals by High-Temperature Seed-Mediated Colloidal Synthesis and Their Catalytic Properties for Semiconducting Nanowire Growth

A general procedure to prepare uniform gold-based bimetallic nanocrystals (NCs) is reported. The method relies on a seed-mediated approach in which deposition and <i>in-situ</i> alloying of a second metal (Ag, Pt, Hg, Sn, Cd) onto monodisperse Au seeds are performed at relatively high temperatures, giving access to bimetallic NCs of tunable compositions and properties. The position of the plasmon resonance in the original Au NCs is tunable over a wide range (∼300–520 nm) of the electromagnetic spectrum. We demonstrate the catalytic properties of these monodisperse NCs for growing single-crystalline semiconductor nanowires of uniform, small diameter (∼15–30 nm) via a vapor–liquid–solid (VLS) mechanism at low temperatures. This seeded-mediated approach is not restricted to Au but can be extended to several other combinations, making this procedure a straightforward method to prepare highly monodisperse and controllable multimetallic nanocrystals for optical and catalytic applications

Shape-Dependent Plasmonic Response and Directed Self-Assembly in a New Semiconductor Building Block, Indium-Doped Cadmium Oxide (ICO)

The influence of particle shape on plasmonic response and local electric field strength is well-documented in metallic nanoparticles. Morphologies such as rods, plates, and octahedra are readily synthesized and exhibit drastically different extinction spectra than spherical particles. Despite this fact, the influence of composition and shape on the optical properties of plasmonic semiconductor nanocrystals, in which free electrons result from heavy doping, has not been well-studied. Here, we report the first observation of plasmonic resonance in indium-doped cadmium oxide (ICO) nanocrystals, which exhibit the highest quality factors reported for semiconductor nanocrystals. Furthermore, we are able to independently control the shape and free electron concentration in ICO nanocrystals, allowing for the influence of shape on the optical response of a plasmonic semiconductor to be conclusively demonstrated. The highly uniform particles may be self-assembled into ordered single component and binary nanocrystal superlattices, and in thin films, exhibit negative permittivity in the near infrared (NIR) region, validating their use as a new class of tunable low-loss plasmonic building blocks for 3-D optical metamaterials