**Seed 2
PI:** Luqiao

**Liu**

**INTELLECTUAL MERIT**

Topological Dirac semimetals are materials with a gapless Dirac-like band structure inside the bulk, which represent an unusual state of quantum matter that can be viewed as “3D graphene”. In contrast to 2D Dirac fermions in graphene or on the surface of 3D topological insulators, topological Dirac semimetals possess 3D Dirac fermions in the bulk. This new class of materials are expected to have rich applications in spin-current generation, spin-orbit torque, and spin-to-charge conversion, etc., spintronic devices.

MIT MRSEC researchers have achieved effective spin-pumping and spin-to-charge conversion in the topological Dirac semimetal/ ferromagnet bilayer structure (Cd_{3}As_{2}/Permalloy), which is promising for building quantum spintronic devices or spin-to-charge convertors that operate with exotic topological quantum states.

Figure. 1. (a) Schematic of the spin-pumping experiment in the Cd3As2/Py bilayer structure. (b) Typical spin-pumping signal measured at 3, 4, and 5GHz, when sweeping the in-plane magnetic field. (c) RF power dependence of the spin-pumping voltage Vsp at 5GHz. (d) Angle dependence of the spin-pumping voltage at 5GHz 17dBm. θ is the angle between the H field and the x-axis in the xz-plane.

**BROADER IMPACTS**

The ability to convert spin signals to charge signals indicate that the topological Dirac semimetals can be potentially used for energy harvesting purposes, where the spin signals are generated initially through spin-pumping by microwave, or through spin-Seebeck effect by heating. The spin signals are converted to charge current, and then stored as traditional electricity.

Another more useful application of topological Dirac semimetals is the spin-orbit torque switching device. Using the unique topological Dirac fermions in this material, a significant surface spin accumulation is expected when passing a charge current through it, which can be used to switch a top magnet (for example, CoTb) with perpendicular anisotropy. The switching is expected to be ultra-efficient, as has been calculated theoretically, and yet to be realized in experiment.

Figure 2: Schematic of the potential device applications based on topological Dirac semimetal. Left: spin-orbit torque switching device enabled by a Cd_{2}As_{3}/magnet structure, where the magnet shows perpendicular anisotropy. A current flow in Cd_{3}As_{2} can generate spin accumulation at the interface which can switch the top perpendicular magnet. Right: spin-to-charge convertor device based on topological Dirac semimetal. The pumped spins from the top magnet (Py) can be effectively converted to charge signals in the Cd_{3}As_{2} layer.

*Also supported by: NSF Grant 1639921, SRC-NRI 2700.001*