Persistent spin textures, altermagnetism and charge-to-spin conversion in metallic chiral crystals TM$_{3}$X$_{6}$

Chiral crystals, due to the lack of inversion and mirror symmetries, exhibit unique spin responses to external fields, enabling physical effects rarely observed in high-symmetry systems. Here, we show that materials from the chiral dichalcogenide family TM$3$X$6$ (T = 3d, M = 4d/5d, X = S) exhibit persistent spin texture (PST) - unidirectional spin polarization of states across large regions of the reciprocal space - in their nonmagnetic metallic phase. Using the example of NiTa${3}$S${6}$ and NiNb${3}$S${6}$, we show that PSTs cover the full Fermi surface, a rare and desirable feature that enables efficient charge-to-spin conversion and suggests long spin lifetimes and coherent spin transport above magnetic ordering temperatures. At low temperatures, the materials that order antiferromagnetically become chiral altermagnets, where spin textures originating from spin-orbit coupling and altermagnetism combine in a way that sensitively depends on the orientation of the Neel vector. Using symmetry analysis and first-principles calculations, we classify magnetic ground states across the family, identify cases with weak ferromagnetism, and track the evolution of spin textures and charge-to-spin conversion across magnetic phases and different Neel vector orientations, revealing spin transport signatures that allow one to distinguish Neel vector directions. These findings establish TM$_3$X$_6$ as a tunable platform for efficient charge-to-spin conversion and spin transport, combining structural chirality, persistent spin textures, and altermagnetism.

💡 Research Summary

The authors investigate the electronic structure, spin textures, and charge‑to‑spin conversion mechanisms of the chiral metallic dichalcogenide family TM₃X₆ (T = 3d transition metal, M = 4d/5d, X = S). Using NiTa₃S₆ and NiNb₃S₆ as representative compounds, they combine symmetry analysis, density‑functional theory (DFT), and linear‑response calculations to reveal a rich interplay between structural chirality, spin‑orbit coupling (SOC), and antiferromagnetic (AFM) order.

In the non‑magnetic metallic phase, the lack of inversion and mirror symmetries forces a global, unidirectional spin polarization— a persistent spin texture (PST)—that spans the entire Fermi surface. The PST follows a “g‑wave” momentum dependence (∝ k_y k_z(3k_x² − k_y²)), dictated by the point group D₆ of the P6₃22 space group. Because the spin orientation does not rotate with momentum, spin dephasing is strongly suppressed, leading to long spin lifetimes and diffusion lengths.

Charge‑to‑spin conversion is quantified via the Kubó linear‑response formalism. The authors separate the response into a T‑even term (χ_I), which corresponds to the conventional Rashba‑Edelstein effect (REE), and a T‑odd term (χ_II), which captures the intrinsic spin Hall conductivity (SHC). In the PST regime the intraband (Fermi‑surface) contribution dominates, dramatically enhancing both REE and SHC compared with typical non‑chiral metals—by roughly a factor of two to three. This demonstrates that a full‑Fermi‑surface PST can simultaneously provide high conversion efficiency and long spin coherence, a combination that is rarely achieved.

Upon cooling, NiTa₃S₆ becomes a collinear antiferromagnet while NiNb₃S₆ adopts a helical AFM order. The chiral lattice remains, so the systems become altermagnets: spin splitting arises from magnetic crystal symmetries rather than SOC alone. The authors employ Landau theory and point‑group irreducible representations to analyze the allowed couplings between the Néel vector N and any net magnetization M. For NiTa₃S₆ (N‖