Chiral Spin Spirals at the Surface of the van der Waals Ferromagnet Fe3GeTe2.

Topologically protected magnetic structures provide a robust platform for low power consumption devices for computation and data storage. Examples of these structures are skyrmions, chiral domain walls, and spin spirals. Here, we use scanning electron microscopy with polarization analysis to unveil the presence of chiral counterclockwise Néel spin spirals at the surface of a bulk van der Waals ferromagnet Fe3GeTe2 (FGT) at zero magnetic field. These Néel spin spirals survive up to FGT's Curie temperature of TC = 220 K, with little change in the periodicity p = 300 nm of the spin spiral throughout the studied temperature range. The formation of a spin spiral showing counterclockwise rotation strongly suggests the presence of a positive Dzyaloshinskii-Moriya interaction in FGT, which provides the first steps towards the understanding of the magnetic structure of FGT. Our results additionally pave the way for chiral magnetism in van der Waals materials and their heterostructures.

Correlated Exciton Fluctuations in a Two-Dimensional Semiconductor on a Metal.

Excitons in nanoscale materials can exhibit fluorescence fluctuations. Intermittency is pervasive in zero-dimensional emitters such as single molecules and quantum dots. In contrast, two-dimensional semiconductors are generally regarded as stable light sources. Noise contains, however, valuable information about a material. Here, we demonstrate fluorescence fluctuations in a monolayer semiconductor due to sensitivity to its nanoscopic environment focusing on the case of a metal film. The fluctuations are spatially correlated over tens of micrometers and follow power-law statistics, with simultaneous changes in emission intensity and lifetime. At low temperatures, an additional spectral contribution from interface trap states emerges with fluctuations that are correlated with neutral excitons and anticorrelated with trions. Mastering exciton fluctuations has implications for light-emitting devices such as single-photon sources and could lead to novel excitonic sensors. The quantification of fluorescence fluctuations, including imaging, unlocks a set of promising tools to characterize and exploit two-dimensional semiconductors and their interfaces.

Spin-Orbit Torques in NbSe2/Permalloy Bilayers.

We present measurements of current-induced spin-orbit torques generated by NbSe2, a fully metallic transition-metal dichalcogenide material, made using the spin-torque ferromagnetic resonance (ST-FMR) technique with NbSe2/Permalloy bilayers. In addition to the out-of-plane Oersted torque expected from current flow in the metallic NbSe2 layer, we also observe an in-plane antidamping torque with torque conductivity σS ≈ 103 (ℏ/2e)(Ωm)-1 and indications of a weak field-like contribution to the out-of-plane torque oriented opposite to the Oersted torque. Furthermore, in some samples we also measure an in-plane field-like torque with the form m̂ × z, where m̂ is the Permalloy magnetization direction and z is perpendicular to the sample plane. The size of this component varies strongly between samples and is not correlated with the NbSe2 thickness. A torque of this form is not allowed by the bulk symmetries of NbSe2 but is consistent with symmetry breaking by a uniaxial strain that might result during device fabrication.

Electric control of optically-induced magnetization dynamics in a van der Waals ferromagnetic semiconductor.

Electric control of magnetization dynamics in two-dimensional (2D) magnetic materials is an essential step for the development of novel spintronic nanodevices. Electrostatic gating has been shown to greatly affect the static magnetic properties of some van der Waals magnets, but the control over their magnetization dynamics is still largely unexplored. Here we show that the optically-induced magnetization dynamics in the van der Waals ferromagnet Cr2Ge2Te6 can be effectively controlled by electrostatic gates, with a one order of magnitude change in the precession amplitude and over 10% change in the internal effective field. In contrast to the purely thermally-induced mechanisms previously reported for 2D magnets, we find that coherent opto-magnetic phenomena play a major role in the excitation of magnetization dynamics in Cr2Ge2Te6. Our work sets the first steps towards electric control over the magnetization dynamics in 2D ferromagnetic semiconductors, demonstrating their potential for applications in ultrafast opto-magnonic devices.

Locally Phase-Engineered MoTe2 for Near-Infrared Photodetectors.

Transition-metal dichalcogenides (TMDs) are ideal systems for two-dimensional (2D) optoelectronic applications owing to their strong light-matter interaction and various band gap energies. New techniques to modify the crystallographic phase of TMDs have recently been discovered, allowing the creation of lateral heterostructures and the design of all-2D circuitry. Thus, far, the potential benefits of phase-engineered TMD devices for optoelectronic applications are still largely unexplored. The dominant mechanisms involved in photocurrent generation in these systems remain unclear, hindering further development of new all-2D optoelectronic devices. Here, we fabricate locally phase-engineered MoTe2 optoelectronic devices, creating a metal (1T') semiconductor (2H) lateral junction and unveil the main mechanisms at play for photocurrent generation. We find that the photocurrent originates from the 1T'-2H junction, with a maximum at the 2H MoTe2 side of the junction. This observation, together with the nonlinear IV-curve, indicates that the photovoltaic effect plays a major role in the photon-to-charge current conversion in these systems. Additionally, the 1T'-2H MoTe2 heterojunction device exhibits a fast optoelectronic response over a wavelength range of 700-1100 nm, with a rise and fall times of 113 and 110 µs, respectively, 2 orders of magnitude faster when compared to a directly contacted 2H MoTe2 device. These results show the potential of local phase-engineering for all-2D optoelectronic circuitry.

Spin transport in high-quality suspended graphene devices.

We measure spin transport in high mobility suspended graphene (µ ≈ 10(5)cm(2)/(V s)), obtaining a (spin) diffusion coefficient of 0.1 m(2)/s and giving a lower bound on the spin relaxation time (τ(s) ≈ 150 ps) and spin relaxation length (λ(s) = 4.7 µm) for intrinsic graphene. We develop a theoretical model considering the different graphene regions of our devices that explains our experimental data.

Current-Induced Torques with Dresselhaus Symmetry Due to Resistance Anisotropy in 2D Materials.

We report measurements of current-induced torques in heterostructures of Permalloy (Py) with TaTe2, a transition-metal dichalcogenide (TMD) material possessing low crystal symmetry, and observe a torque component with Dresselhaus symmetry. We suggest that the dominant mechanism for this Dresselhaus component is not a spin-orbit torque but rather the Oersted field arising from a component of current that flows perpendicular to the applied voltage due to resistance anisotropy within the TaTe2. This type of transverse current is not present in wires made from a single uniform layer of a material with resistance anisotropy but will result whenever a material with resistance anisotropy is integrated into a heterostructure with materials having different resistivities, thereby producing a spatially nonuniform pattern of current flow. This effect will therefore influence measurements in a wide variety of heterostructures incorporating 2D TMD materials and other materials with low crystal symmetries.

Atomically Thin Ohmic Edge Contacts Between Two-Dimensional Materials.

With the decrease of the dimensions of electronic devices, the role played by electrical contacts is ever increasing, eventually coming to dominate the overall device volume and total resistance. This is especially problematic for monolayers of semiconducting transition-metal dichalcogenides (TMDs), which are promising candidates for atomically thin electronics. Ideal electrical contacts to them would require the use of similarly thin electrode materials while maintaining low contact resistances. Here we report a scalable method to fabricate ohmic graphene edge contacts to two representative monolayer TMDs, MoS2 and WS2. The graphene and TMD layer are laterally connected with wafer-scale homogeneity, no observable overlap or gap, and a low average contact resistance of 30 kΩ·µm. The resulting graphene edge contacts show linear current-voltage (I-V) characteristics at room temperature, with ohmic behavior maintained down to liquid helium temperatures.