Magnetic anisotropy reversal driven by structural symmetry-breaking in monolayer α-RuCl3.

Layered α-RuCl3 is a promising material to potentially realize the long-sought Kitaev quantum spin liquid with fractionalized excitations. While evidence of this state has been reported under a modest in-plane magnetic field, such behaviour is largely inconsistent with theoretical expectations of spin liquid phases emerging only in out-of-plane fields. These predicted field-induced states have been largely out of reach due to the strong easy-plane anisotropy of bulk crystals, however. We use a combination of tunnelling spectroscopy, magnetotransport, electron diffraction and ab initio calculations to study the layer-dependent magnons, magnetic anisotropy, structure and exchange coupling in atomically thin samples. Due to picoscale distortions, the sign of the average off-diagonal exchange changes in monolayer α-RuCl3, leading to a reversal of spin anisotropy to easy-axis anisotropy, while the Kitaev interaction is concomitantly enhanced. Our work opens the door to the possible exploration of Kitaev physics in the true two-dimensional limit.

Tunable layered-magnetism-assisted magneto-Raman effect in a two-dimensional magnet CrI3.

We used a combination of polarized Raman spectroscopy experiment and model magnetism-phonon coupling calculations to study the rich magneto-Raman effect in the two-dimensional (2D) magnet CrI3 We reveal a layered-magnetism-assisted phonon scattering mechanism below the magnetic onset temperature, whose Raman excitation breaks time-reversal symmetry, has an antisymmetric Raman tensor, and follows the magnetic phase transitions across critical magnetic fields, on top of the presence of the conventional phonon scattering with symmetric Raman tensors in N-layer CrI3 We resolve in data and by calculations that the first-order A g phonon of the monolayer splits into an N-fold multiplet in N-layer CrI3 due to the interlayer coupling [Formula: see text] and that the phonons within the multiplet show distinct magnetic field dependence because of their different layered-magnetism-phonon coupling. We further find that such a layered-magnetism-phonon coupled Raman scattering mechanism extends beyond first-order to higher-order multiphonon scattering processes. Our results on the magneto-Raman effect of the first-order phonons in the multiplet and the higher-order multiphonons in N-layer CrI3 demonstrate the rich and strong behavior of emergent magneto-optical effects in 2D magnets and underline the unique opportunities of spin-phonon physics in van der Waals layered magnets.

Observation and Manipulation of a Phase Separated State in a Charge Density Wave Material.

The 1T polytype of TaS2 has been studied extensively as a strongly correlated system. As 1T-TaS2 is thinned toward the 2D limit, its phase diagram shows significant deviations from that of the bulk material. Optoelectronic maps of ultrathin 1T-TaS2 have indicated the presence of nonequilibrium charge density wave phases within the hysteresis region of the nearly commensurate (NC) to commensurate (C) transition. We perform scanning tunneling microscopy on exfoliated ultrathin flakes of 1T-TaS2 within the NC-C hysteresis window, finding evidence that the observed nonequilibrium phases consist of intertwined, irregularly shaped NC-like and C-like domains. After applying lateral electrical signals to the sample, we image changes in the geometric arrangement of the different regions. We use a phase separation model to explore the relationship between electronic inhomogeneity present in ultrathin 1T-TaS2 and its bulk resistivity. These results demonstrate the role of phase competition morphologies in determining the properties of 2D materials.

High-Performance Mid-IR to Deep-UV van der Waals Photodetectors Capable of Local Spectroscopy at Room Temperature.

The ability to perform broadband optical spectroscopy with subdiffraction-limit resolution is highly sought-after for a wide range of critical applications. However, sophisticated near-field techniques are currently required to achieve this goal. We bypass this challenge by demonstrating an extremely broadband photodetector based on a two-dimensional (2D) van der Waals heterostructure that is sensitive to light across over a decade in energy from the mid-infrared (MIR) to deep-ultraviolet (DUV) at room temperature. The devices feature high detectivity (>109 cm Hz1/2 W-1) together with high bandwidth (2.1 MHz). The active area can be further miniaturized to submicron dimensions, far below the diffraction limit for the longest detectable wavelength of 4.1 µm, enabling such devices for facile measurements of local optical properties on atomic-layer-thickness samples placed in close proximity. This work can lead to the development of low-cost and high-throughput photosensors for hyperspectral imaging at the nanoscale.

Evolution of interlayer and intralayer magnetism in three atomically thin chromium trihalides.

We conduct a comprehensive study of three different magnetic semiconductors, CrI3, CrBr3, and CrCl3, by incorporating both few-layer and bilayer samples in van der Waals tunnel junctions. We find that the interlayer magnetic ordering, exchange gap, magnetic anisotropy, and magnon excitations evolve systematically with changing halogen atom. By fitting to a spin wave theory that accounts for nearest-neighbor exchange interactions, we are able to further determine a simple spin Hamiltonian describing all three systems. These results extend the 2D magnetism platform to Ising, Heisenberg, and XY spin classes in a single material family. Using magneto-optical measurements, we additionally demonstrate that ferromagnetism can be stabilized down to monolayer in more isotropic CrBr3, with transition temperature still close to that of the bulk.

Roadmap on quantum nanotechnologies.

Quantum phenomena are typically observable at length and time scales smaller than those of our everyday experience, often involving individual particles or excitations. The past few decades have seen a revolution in the ability to structure matter at the nanoscale, and experiments at the single particle level have become commonplace. This has opened wide new avenues for exploring and harnessing quantum mechanical effects in condensed matter. These quantum phenomena, in turn, have the potential to revolutionize the way we communicate, compute and probe the nanoscale world. Here, we review developments in key areas of quantum research in light of the nanotechnologies that enable them, with a view to what the future holds. Materials and devices with nanoscale features are used for quantum metrology and sensing, as building blocks for quantum computing, and as sources and detectors for quantum communication. They enable explorations of quantum behaviour and unconventional states in nano- and opto-mechanical systems, low-dimensional systems, molecular devices, nano-plasmonics, quantum electrodynamics, scanning tunnelling microscopy, and more. This rapidly expanding intersection of nanotechnology and quantum science/technology is mutually beneficial to both fields, laying claim to some of the most exciting scientific leaps of the last decade, with more on the horizon.

Photocurrent Imaging of Multi-Memristive Charge Density Wave Switching in Two-Dimensional 1T-TaS2.

Transport studies of atomically thin 1T-TaS2 have demonstrated the presence of intermediate resistance states across the nearly commensurate (NC) to commensurate (C) charge density wave (CDW) transition, which can be further switched electrically. While this presents exciting opportunities for memristor applications, the switching mechanism could be potentially attributed to the formation of inhomogeneous C and NC domains. Here, we present combined electrical driving and photocurrent imaging of ultrathin 1T-TaS2 in a heterostructure geometry. While micron-sized CDW domains are seen upon cooling, electrically driven transitions are largely uniform, indicating that the latter likely induces true metastable CDW states, which we then explain by a free energy analysis. Additionally, we are able to perform repeatable and bidirectional switching across the intermediate states without changing sample temperature, demonstrating that atomically thin 1T-TaS2 can be further used as a robust and reversible multimemristor material for the first time.

Atomic and electronic reconstruction at the van der Waals interface in twisted bilayer graphene.

Control of the interlayer twist angle in two-dimensional van der Waals (vdW) heterostructures enables one to engineer a quasiperiodic moiré superlattice of tunable length scale1-8. In twisted bilayer graphene, the simple moiré superlattice band description suggests that the electronic bandwidth can be tuned to be comparable to the vdW interlayer interaction at a 'magic angle'9, exhibiting strongly correlated behaviour. However, the vdW interlayer interaction can also cause significant structural reconstruction at the interface by favouring interlayer commensurability, which competes with the intralayer lattice distortion10-16. Here we report atomic-scale reconstruction in twisted bilayer graphene and its effect on the electronic structure. We find a gradual transition from an incommensurate moiré structure to an array of commensurate domains with soliton boundaries as we decrease the twist angle across the characteristic crossover angle, θc ≈ 1°. In the solitonic regime (θ < θc) where the atomic and electronic reconstruction become significant, a simple moiré band description breaks down and the secondary Dirac bands appear. On applying a transverse electric field, we observe electronic transport along the network of one-dimensional topological channels that surround the alternating triangular gapped domains. Atomic and electronic reconstruction at the vdW interface provide a new pathway to engineer the system with continuous tunability.

Tailored Tunnel Magnetoresistance Response in Three Ultrathin Chromium Trihalides.

Materials that demonstrate large magnetoresistance have attracted significant interest for many decades. Extremely large tunnel magnetoresistance (TMR) has been reported by several groups across ultrathin CrI3 by exploiting the weak antiferromagnetic coupling between adjacent layers. Here, we report a comparative study of TMR in all three chromium trihalides (CrX3, X = Cl, Br, or I) in the two-dimensional limit. As the materials exhibit different transition temperatures and interlayer magnetic ordering in the ground state, tunneling measurements allow for an easy determination of the field-temperature phase diagram for the three systems. By changing sample thickness and biasing conditions, we then demonstrate how to maximize and further tailor the TMR response at different temperatures for each material. In particular, near the magnetic transition temperature, TMR is nonsaturating up to the highest fields measured for all three compounds owing to the large, field-induced exchange coupling.

Atomic lattice disorder in charge-density-wave phases of exfoliated dichalcogenides (1T-TaS2).

Charge-density waves (CDWs) and their concomitant periodic lattice distortions (PLDs) govern the electronic properties in several layered transition-metal dichalcogenides. In particular, 1T-TaS2 undergoes a metal-to-insulator phase transition as the PLD becomes commensurate with the crystal lattice. Here we directly image PLDs of the nearly commensurate (NC) and commensurate (C) phases in thin, exfoliated 1T-TaS2 using atomic resolution scanning transmission electron microscopy at room and cryogenic temperature. At low temperatures, we observe commensurate PLD superstructures, suggesting ordering of the CDWs both in- and out-of-plane. In addition, we discover stacking transitions in the atomic lattice that occur via one-bond-length shifts. Interestingly, the NC PLDs exist inside both the stacking domains and their boundaries. Transitions in stacking order are expected to create fractional shifts in the CDW between layers and may be another route to manipulate electronic phases in layered dichalcogenides.

One Million Percent Tunnel Magnetoresistance in a Magnetic van der Waals Heterostructure.

We report the observation of a very large negative magnetoresistance effect in a van der Waals tunnel junction incorporating a thin magnetic semiconductor, CrI3, as the active layer. At constant voltage bias, current increases by nearly one million percent upon application of a 2 T field. The effect arises from a change between antiparallel to parallel alignment of spins across the different CrI3 layers. Our results elucidate the nature of the magnetic state in ultrathin CrI3 and present new opportunities for spintronics based on two-dimensional materials.

Structure and control of charge density waves in two-dimensional 1T-TaS2.

The layered transition metal dichalcogenides host a rich collection of charge density wave phases in which both the conduction electrons and the atomic structure display translational symmetry breaking. Manipulating these complex states by purely electronic methods has been a long-sought scientific and technological goal. Here, we show how this can be achieved in 1T-TaS2 in the 2D limit. We first demonstrate that the intrinsic properties of atomically thin flakes are preserved by encapsulation with hexagonal boron nitride in inert atmosphere. We use this facile assembly method together with transmission electron microscopy and transport measurements to probe the nature of the 2D state and show that its conductance is dominated by discommensurations. The discommensuration structure can be precisely tuned in few-layer samples by an in-plane electric current, allowing continuous electrical control over the discommensuration-melting transition in 2D.

Thickness and Stacking Sequence Determination of Exfoliated Dichalcogenides (1T-TaS2, 2H-MoS2) Using Scanning Transmission Electron Microscopy.

Layered transition metal dichalcogenides (TMDs) have attracted interest due to their promise for future electronic and optoelectronic technologies. As one approaches the two-dimensional (2D) limit, thickness and local topology can greatly influence the macroscopic properties of a material. To understand the unique behavior of TMDs it is therefore important to identify the number of atomic layers and their stacking in a sample. The goal of this work is to extract the thickness and stacking sequence of TMDs directly by matching experimentally recorded high-angle annular dark-field scanning transmission electron microscope images and convergent-beam electron diffraction (CBED) patterns to quantum mechanical, multislice scattering simulations. Advantageously, CBED approaches do not require a resolved lattice in real space and are capable of neglecting the thickness contribution of amorphous surface layers. Here we demonstrate the crystal thickness can be determined from CBED in exfoliated 1T-TaS2 and 2H-MoS2 to within a single layer for ultrathin ≲9 layers and ±1 atomic layer (or better) in thicker specimens while also revealing information about stacking order-even when the crystal structure is unresolved in real space.

Strain solitons and topological defects in bilayer graphene.

Bilayer graphene has been a subject of intense study in recent years. The interlayer registry between the layers can have dramatic effects on the electronic properties: for example, in the presence of a perpendicular electric field, a band gap appears in the electronic spectrum of so-called Bernal-stacked graphene [Oostinga JB, et al. (2007) Nature Materials 7:151-157]. This band gap is intimately tied to a structural spontaneous symmetry breaking in bilayer graphene, where one of the graphene layers shifts by an atomic spacing with respect to the other. This shift can happen in multiple directions, resulting in multiple stacking domains with soliton-like structural boundaries between them. Theorists have recently proposed that novel electronic states exist at these boundaries [Vaezi A, et al. (2013) arXiv:1301.1690; Zhang F, et al. (2013) arXiv:1301.4205], but very little is known about their structural properties. Here we use electron microscopy to measure with nanoscale and atomic resolution the widths, motion, and topological structure of soliton boundaries and related topological defects in bilayer graphene. We find that each soliton consists of an atomic-scale registry shift between the two graphene layers occurring over 6-11 nm. We infer the minimal energy barrier to interlayer translation and observe soliton motion during in situ heating above 1,000 °C. The abundance of these structures across a variety of samples, as well as their unusual properties, suggests that they will have substantial effects on the electronic and mechanical properties of bilayer graphene.

Polycrystallinity and stacking in CVD graphene.

Graphene, a truly two-dimensional hexagonal lattice of carbon atoms, possesses remarkable properties not seen in any other material, including ultrahigh electron mobility, high tensile strength, and uniform broadband optical absorption. While scientists initially studied its intrinsic properties with small, mechanically exfoliated graphene crystals found randomly, applying this knowledge would require growing large-area films with uniform structural and physical properties. The science of graphene has recently experienced revolutionary change, mainly due to the development of several large-scale growth methods. In particular, graphene synthesis by chemical vapor deposition (CVD) on copper is a reliable method to obtain films with mostly monolayer coverage. These films are also polycrystalline, consisting of multiple graphene crystals joined by grain boundaries. In addition, portions of these graphene films contain more than one layer, and each layer can possess a different crystal orientation and stacking order. In this Account, we review the structural and physical properties that originate from polycrystallinity and stacking in CVD graphene. To begin, we introduce dark-field transmission electron microscopy (DF-TEM), a technique which allows rapid and accurate imaging of key structural properties, including the orientation of individual domains and relative stacking configurations. Using DF-TEM, one can easily identify "lateral junctions," or grain boundaries between adjacent domains, as well as "vertical junctions" from the stacking of graphene multilayers. With this technique, we can distinguish between oriented (Bernal or rhombohedral) and misoriented (twisted) configurations. The structure of lateral junctions in CVD graphene is sensitive to growth conditions and is reflected in the material's electrical and mechanical properties. In particular, grain boundaries in graphene grown under faster reactant flow conditions have no gaps or overlaps, unlike more slowly grown films. These structural differences can affect the material's electrical properties: for example, better-connected grain boundaries are more electrically conductive. However, grain boundaries in general are mechanically weaker than pristine graphene, which is an order of magnitude stronger than CVD graphene based on indentation measurements performed with an atomic force microscope. Vertical junctions in multilayer CVD graphene have two key structural features. First, bilayer graphene (BLG) with Bernal stacking exists in two mirrored configurations (AB or AC) that also form isolated domains. Similarly, oriented trilayer graphene also has alternating ABA and ABC stacked layers. Second, in twisted multilayer graphene, stacked layers lack long-range atomic registry and can move freely relative to each other, which generates unique optical properties. In particular, an interlayer optical excitation produces strong Raman and absorption peaks, dependent on the twist angle. A better understanding of the structural and physical properties of grain boundaries and multilayers in CVD graphene is central to realizing the full potential of graphene in large-scale applications. In addition, these studies provide a model for characterizing other layered materials, such as hexagonal boron nitride and MoS2, where similar polycrystallinity and stacking are expected when grown in large areas.

Gate-Tunable Multiband van der Waals Photodetector and Polarization Sensor.

A single photodetector with tunable detection wavelengths and polarization sensitivity can potentially be harnessed for diverse optical applications ranging from imaging and sensing to telecommunications. Such a device will require the combination of multiple material systems with different structures, band gaps, and photoelectrical responses, which is extremely difficult to engineer using traditional epitaxial films. Here, we develop a multifunctional and high-performance photosensor using all van der Waals materials. The device features a gate-tunable spectral response that is switchable between near-infrared/visible and short-/midwave infrared, as well as broad-band operation, at room temperature. The linear polarization sensitivity in the telecommunication O-band can also be directly modulated between horizontal, vertical, and nonpolarizing modes. These effects originate from the balance of photocurrent generation in two of the active layers that can be manipulated by an electric field. The photodetector features high detectivity (>109 cmHz1/2W-1) together with fast operation speed (∼1 MHz) and can be further exploited for dual visible and infrared imaging.

Macroscopic tunneling probe of Moiré spin textures in twisted CrI3.

Various noncollinear spin textures and magnetic phases have been predicted in twisted two-dimensional CrI3 due to competing ferromagnetic (FM) and antiferromagnetic (AFM) interlayer exchange from moiré stacking-with potential spintronic applications even when the underlying material possesses a negligible Dzyaloshinskii-Moriya or dipole-dipole interaction. Recent measurements have shown evidence of coexisting FM and AFM layer order in small-twist-angle CrI3 bilayers and double bilayers. Yet, the nature of the magnetic textures remains unresolved and possibilities for their manipulation and electrical readout are unexplored. Here, we use tunneling magnetoresistance to investigate the collective spin states of twisted double-bilayer CrI3 under both out-of-plane and in-plane magnetic fields together with detailed micromagnetic simulations of domain dynamics based on magnetic circular dichroism. Our results capture hysteretic and anisotropic field evolutions of the magnetic states and we further uncover two distinct non-volatile spin textures (out-of-plane and in-plane domains) at ≈1° twist angle, with a different global tunneling resistance that can be switched by magnetic field.

Semiconductor nanowire metamaterial for broadband near-unity absorption.

The realization of a semiconductor near-unity absorber in the infrared will provide new capabilities to transform applications in sensing, health, imaging, and quantum information science, especially where portability is required. Typically, commercially available portable single-photon detectors in the infrared are made from bulk semiconductors and have efficiencies well below unity. Here, we design a novel semiconductor nanowire metamaterial, and show that by carefully arranging an InGaAs nanowire array and by controlling their shape, we demonstrate near-unity absorption efficiency at room temperature. We experimentally show an average measured efficiency of 93% (simulated average efficiency of 97%) over an unprecedented wavelength range from 900 to 1500 nm. We further show that the near-unity absorption results from the collective response of the nanowire metamaterial, originating from both coupling into leaky resonant waveguide and transverse modes. These coupling mechanisms cause light to be absorbed directly from the top and indirectly as light scatters from one nanowire to neighbouring ones. This work leads to the possible development of a new generation of quantum detectors with unprecedented broadband near-unity absorption in the infrared, while operating near room temperature for a wider range of applications.

The Magnetic Genome of Two-Dimensional van der Waals Materials.

Magnetism in two-dimensional (2D) van der Waals (vdW) materials has recently emerged as one of the most promising areas in condensed matter research, with many exciting emerging properties and significant potential for applications ranging from topological magnonics to low-power spintronics, quantum computing, and optical communications. In the brief time after their discovery, 2D magnets have blossomed into a rich area for investigation, where fundamental concepts in magnetism are challenged by the behavior of spins that can develop at the single layer limit. However, much effort is still needed in multiple fronts before 2D magnets can be routinely used for practical implementations. In this comprehensive review, prominent authors with expertise in complementary fields of 2D magnetism (i.e., synthesis, device engineering, magneto-optics, imaging, transport, mechanics, spin excitations, and theory and simulations) have joined together to provide a genome of current knowledge and a guideline for future developments in 2D magnetic materials research.

Giant c-axis nonlinear anomalous Hall effect in Td-MoTe2 and WTe2.

While the anomalous Hall effect can manifest even without an external magnetic field, time reversal symmetry is nonetheless still broken by the internal magnetization of the sample. Recently, it has been shown that certain materials without an inversion center allow for a nonlinear type of anomalous Hall effect whilst retaining time reversal symmetry. The effect may arise from either Berry curvature or through various asymmetric scattering mechanisms. Here, we report the observation of an extremely large c-axis nonlinear anomalous Hall effect in the non-centrosymmetric Td phase of MoTe2 and WTe2 without intrinsic magnetic order. We find that the effect is dominated by skew-scattering at higher temperatures combined with another scattering process active at low temperatures. Application of higher bias yields an extremely large Hall ratio of E⊥/E|| = 2.47 and corresponding anomalous Hall conductivity of order 8 × 107 S/m.