Tunable spin-charge conversion in topological Dirac semimetals*

We theoretically demonstrate that topological Dirac semimetals (TDSMs) can provide a platform for realizing both electrically and magnetically tunable spin-charge conversion. With time-reversal symmetry, the spin component along the rotation axis (z-axis) is approximately conserved, which leads to an anisotropic spin Hall effect—the spin Hall current relies on the relative orientation between the external electric field and the z-axis. The application of a magnetic field, on the other hand, breaks time-reversal symmetry, driving the TDSM into a Weyl semimetal phase and thus, partially converting the spin current to a charge Hall current. Using the Kubo formulas, we numerically evaluate the spin and charge Hall conductivities based on a low-energy TDSM Hamiltonian together with the Zeeman coupling. Besides the conventional spin Hall conductivity element σ_xy^z, we find that unconventional components, such as σ_xy^x and σ_xy^y, also exist and vary as the magnetic field rotates. The charge Hall conductivity also exhibits appreciable tunability that is distinct from the spin Hall conductivities. We show that such tunability originates from the interplay of symmetry and band topology of the TDSMs.