Valley-dependent electronic properties in two-dimensional altermagnetic iron-based transition metal chalcogenides

Altermagnets represent a newly identified third class of collinear magnets and have recently emerged as a focal point in condensed matter physics. In this work, through first-principles calculations and theoretical analysis, we identify monolayer Fe$_2$MoX$_4$ (X = S, Se, Te) and Fe$_2$WTe$_4$, a class of iron-based transition metal chalcogenides, as promising altermagnetic materials. These systems are found to be semiconductors exhibiting spin splitting in their nonrelativistic band structures, indicative of intrinsic altermagnetic ordering. Remarkably, their valence bands feature a pair of valleys at the time-reversal-invariant momenta X and Y points. Unlike conventional valley systems, these valleys are related by crystal symmetries rather than time-reversal symmetry. We investigate valley-dependent physical phenomena in these materials, including Berry curvature and optical circular dichroism, revealing strong valley-contrasting behavior. Furthermore, we investigate the effect of uniaxial strain and show that it effectively lifts the valley degeneracy, resulting in pronounced valley polarization. Under hole doping, this strain-induced asymmetry gives rise to a piezomagnetic response. We also explore the generation of anisotropic noncollinear spin currents in these systems, expanding the scope of their spin-related functionalities. Our findings unveil rich valley physics in monolayer Fe$_2$MoX$_4$ (X = S, Se, Te) and Fe$_2$WTe$_4$, highlighting their significant potential for applications in valleytronics, spintronics, and multifunctional nanoelectronic devices.

💡 Research Summary

In this work the authors identify a new family of two‑dimensional altermagnetic semiconductors, namely monolayer Fe₂MoX₄ (X = S, Se, Te) and Fe₂WTe₄, through extensive first‑principles calculations. Structural stability is confirmed by phonon spectra (no imaginary modes) and ab‑initio molecular dynamics at 300 K, indicating both dynamical and thermal robustness. Projected crystal‑orbital Hamilton population (pCOHP) analysis shows strong Fe–X bonding, supporting the structural integrity. Magnetic ground‑state calculations compare ferromagnetic (FM) and altermagnetic (AM) configurations; the AM state is energetically favored by 10–60 meV per primitive cell, with each Fe atom carrying ~3 μB. The AM order belongs to a d‑wave altermagnet characterized by spin‑space group operations {C₂‖M_xy} and {C₂‖S+4z}, which enforce a nonrelativistic spin splitting even without spin–orbit coupling (SOC).

Band‑structure analysis without SOC reveals indirect band gaps (0.66 eV for Fe₂MoTe₄, 0.91 eV for Fe₂WTe₄) and pronounced spin splitting of ~0.70 eV at the X and Y time‑reversal‑invariant momenta (TRIM). These two valleys are energetically degenerate but possess opposite spin polarization, a direct consequence of the crystal symmetry {C₂‖S+4z}. Inclusion of SOC slightly reduces the gaps (to 0.65 eV and 0.87 eV) and identifies the out‑of‑plane