Spin Hall and spin Nernst effects in Th-based compounds: anab initiostudy of electron-phonon topology

The efficient generation and manipulation of spin currents via the spin Hall effect (SHE) has motivated extensive efforts to discover materials with enhanced SHE. Meanwhile, its thermoelectric counterpart, the spin Nernst effect (SNE), has attracted growing interest for spin-caloritronic applications. In this work, we provide key insights from a systematicab initioinvestigation of the electronic and phononic topology, alongside the SHE and SNE, in the Th-based compounds ThAsS, ThAsSe, and ThSbSe. With the exclusion of spin-orbit coupling (SOC), the electronic band structures of these compounds exhibit a nodal line in thekz=πplane. Upon introducing SOC, fourfold degeneracies emerge at severalkpoints across all three compounds. The examinedZ2indices suggest that these compounds are nontrivial topological materials. Using the Wannier tight-binding approach, we identify significant SHE and SNE in these materials. The magnitude and sign of the spin Hall conductivity and spin Nernst conductivity can be tuned by altering the direction of the applied electric field or spin current, and further amplified by adjusting the Fermi level. Detailed examination of the band- andk-resolved spin Berry curvatures suggests that the strong SHE and SNE stem from several nodal points and lines characterized by small SOC gaps in proximity to the Fermi level. Finally, the phonon dispersion analysis of these compounds reveals quadratic nodal lines and nodal surface states. In particular, the phonon spectrum of ThAsS exhibits aZ-centered hourglass nodal loop. This study underscores Th-based compounds as promising materials for exploring topological electronic and phononic structures with significant SHE and SNE.