Spin Generation and High-Frequency Detection via the Quantum Nonlinear Anomalous Hall Effect in Weyl Semimetals

In magnetic conductors, the passage of current yields an electric field in the transverse direction even without an external magnetic field – this is known as the anomalous Hall effect (AHE). This effect can act as a convenient probe of spin ordering, magnetic textures, spin-orbit coupling, and band topology in solids, and can be further exploited for developing spintronic devices. Recently, it has been shown that low-symmetry materials can exhibit a nonlinear version of the AHE, called the NLAHE, which allows for additional material functionalities with potential practical applications. Three common low-symmetry semimetals, WTe2, MoTe2, and TaIrTe4, have exhibited the NLAHE. The first goal of this project is to measure the spin accumulation and polarization direction on the surfaces and edges of these three materials using a unique approach combining spin transport techniques with van der Waals engineering in an inert atmosphere. With an applied current, there should be a net spin accumulation on the sample boundaries and by changing the direction of current injection or tuning with gates, the spin degrees of freedom can be manipulated. The spin polarization can also be different on the surfaces and the edges due to the thickness of the layers in the semimetals. By investigating the current- and gate-dependent accumulation and polarization in the sample, we may determine a new route for the electrical control of magnetism. Further, NLAHE in the semimetals can potentially be used as novel quantum sensors to detect radiofrequency, terahertz and infrared waves. For example, the Hall response of a NLAHE material coupled to a coplanar waveguide can be measured as broadband radiofrequency signal propagates in the waveguide. This would demonstrate the capability of NLAHE to detect broadband waves at radiofrequency. Success in these experiments will allow for exotic spintronic devices and sensors to be developed with functionalities unavailable with traditional materials, which with potential benefits to applications in the defense and security sectors.

Figure 1. A coplanar waveguide coupling with an RF source delivers microwaves across a broadband (Mo/W)Te2 device, with contacts to measure the response.