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

Enhancement of interlayer exchange in an ultrathin two-dimensional magnet

Following the recent isolation of monolayer CrI3 (ref. 1), many more two-dimensional van der Waals magnetic materials have been isolated2,3,4,5,6,7,8,9,10,11,12. Their incorporation in van der Waals heterostructures offers a new platform for spintronics5,6,7,8,9, proximity magnetism13 and quantum spin liquids14. A primary question in this field is how exfoliating crystals to the few-layer limit influences their magnetism. Studies of CrI3 have shown a different magnetic ground state for ultrathin exfoliated films1,5,6 compared with the bulk, but the origin is not yet understood. Here, we use electron tunnelling through few-layer crystals of the layered antiferromagnetic insulator CrCl3 to probe its magnetic order and find a tenfold enhancement of the interlayer exchange compared with bulk crystals. Moreover, temperature- and polarization-dependent Raman spectroscopy reveals that the crystallographic phase transition of bulk crystals does not occur in exfoliated films. This results in a different low-temperature stacking order and, we hypothesize, increased interlayer exchange. Our study provides insight into the connection between stacking order and interlayer interactions in two-dimensional magnets, which may be relevant for correlating stacking faults and mechanical deformations with the magnetic ground states of other more exotic layered magnets such as RuCl3 (ref. 14)

Deep-Learning-Enabled Fast Optical Identification and Characterization of Two-Dimensional Materials

Advanced microscopy and/or spectroscopy tools play indispensable role in nanoscience and nanotechnology research, as it provides rich information about the growth mechanism, chemical compositions, crystallography, and other important physical and chemical properties. However, the interpretation of imaging data heavily relies on the "intuition" of experienced researchers. As a result, many of the deep graphical features obtained through these tools are often unused because of difficulties in processing the data and finding the correlations. Such challenges can be well addressed by deep learning. In this work, we use the optical characterization of two-dimensional (2D) materials as a case study, and demonstrate a neural-network-based algorithm for the material and thickness identification of exfoliated 2D materials with high prediction accuracy and real-time processing capability. Further analysis shows that the trained network can extract deep graphical features such as contrast, color, edges, shapes, segment sizes and their distributions, based on which we develop an ensemble approach topredict the most relevant physical properties of 2D materials. Finally, a transfer learning technique is applied to adapt the pretrained network to other applications such as identifying layer numbers of a new 2D material, or materials produced by a different synthetic approach. Our artificial-intelligence-based material characterization approach is a powerful tool that would speed up the preparation, initial characterization of 2D materials and other nanomaterials and potentially accelerate new material discoveries

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

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Magnetism in Two-Dimensional van der Waals Materials

Layered van der Waals crystals are a rich proving ground for exploring electronic behavior confined to two dimensions. Since the discovery of graphene in 2004, a large family of crystals has been isolated in the ultrathin limit, hosting a range of different properties, including semiconductors, superconductors, and topological insulators. These few-atom-thick sheets can be restacked into endless combinations of artificial heterostructures with atomically sharp interfaces that can be thought of as fundamentally new quantum materials. For over a decade, however, magnetism was noticeably absent from van der Waals materials. In this thesis, I present experiments on one of the first families of 2D magnets, the insulating chromium trihalides (CrX3), including CrI3 and CrCl3. These results were enabled by techniques developed to manipulate these air-sensitive few-layer crystals in an inert glovebox environment. I discuss magneto-optical experiments to measure and electrically control the magnetic ordering of ultrathin CrI3. I also present a new approach to probe the layer-dependent magnetic ordering by electron tunneling through van der Waals spin-filter magnetic tunnel junctions, where a few-layer crystal of CrX3 serves as the insulating tunnel barrier. Surprisingly, these magneto-optical and tunneling experiments reveal magnetic properties in ultrathin CrX3 differing from those of the bulk crystals. Using Raman spectroscopy, I connect these differences to changes in the lateral stacking arrangements between individual crystalline layers. The techniques established to handle air-sensitive 2D magnets lay the groundwork for the discovery of novel magnetic phenomena in the many yet unexplored layered magnetic insulators. Moreover, the development of 2D magnetic tunnel junctions with large magnetoresistances and highly spin-polarized currents paves the way for integration in the spintronics community. Finally, the more complete understanding of the layer-dependent magnetism in ultrathin CrX3 unlocks the potential to carefully incorporate 2D magnets in a variety of van der Waals heterostructures for proximity magnetism effects and beyond.Ph.D

Replication Data for: Probing magnetism in 2D van der Waals crystalline insulators via electron tunneling

Data repository for: Probing magnetism in 2D van der Waals crystalline insulators via electron tunnelin