Tunable spin–charge conversion in class-I topological Dirac semimetals

Abstract

We theoretically demonstrate that class-I 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 uniaxial rotation axis (z axis) is approximately conserved, which leads to an anisotropic spin Hall effect; the resulting 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, consequently, 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 tensor element of the spin Hall conductivity σxyz, we find that unconventional components, such as σxyx and σxyy, also exist and vary as the magnetic field is rotated. Likewise, the charge Hall conductivity also exhibits appreciable tunability upon variation of the magnetic field. We show that such tunability—as well as large spin–charge conversion efficiency—arises from the interplay of symmetry and band topology of the TDSMs.