Chemical design of monolayer altermagnets

TL;DR

This work tackles the scarcity of monolayer altermagnets by introducing symmetry-guided chemical-design principles rooted in the layered V2(Se,Te)2O archetype. It builds four structural frameworks and a 2600-candidate library via valence-adaptive substitutions and symmetry-preserving modifications, followed by high-throughput DFT screening that identifies 670 Néel-AFM altermagnets and 91 with crystal-symmetry-paired spin-momentum locking Dirac cones. The results reveal clear element-dependent trends governing altermagnetism and show that Dirac cones frequently co-occur with AM order, especially under Janus and B-site modification. The findings offer a rational route to discover and engineer monolayer altermagnets for atomically thin spintronic devices with potential for ultra-fast, spin-polarized transport.

Abstract

The crystal-symmetry-paired spin-momentum locking (CSML) arisen from the intrinsic crystal symmetry connecting different magnetic sublattices in altermagnets enables many exotic spintronics properties such as unconventional piezomagnetism and noncollinear spin current. However, the shortage of monolayer altermagnets restricts further exploration of dimensionally confined phenomena and applications of nanostructured devices. Here, we propose general chemical design principles inspired by sublattice symmetry of layered altermagnet V$_2$(Se,Te)$_2$O through symmetry-preserving structural modification and valence-adaptive chemical substitutions. In total, we construct 2600 candidates across four structural frameworks, M$_2$A$_2$B$_{1,0}$ and their Janus derivatives. High-throughput calculations identify 670 potential altermagnets with Néel-ordered ground states, among which 91 ones exhibiting CSML Dirac cones that enable spin-polarized ultra-fast transport. These materials also feature different ground-state magnetic orderings and demonstrate diverse electronic behaviors, ranging from semiconductors, metals, half-metals, to Dirac semimetals. This work not only reveals abundant monolayer altermagnets, but also establishes a rational principle for their design, opening gates for exploration of confined magnetism and spintronics in atomically thin systems.

Figures (5)

• Figure 1: (a) Schematic crystal structure of V$_2$Se$_2$O monolayers. Blue, green, and red spheres stand for V, Se, and O. Its mirror and rotation symmetries are presented by transparent colored planes and combinations of black dashed and gray rotating arrows, with respective notations shown beside. The Néel AFM-ordered spins on V are denoted by yellow arrows on blue spheres. (b) Structural framework M$_2$A$_2$B as design basis. Atomic sites M, A, and B resemble the in-plane locations of metal V, vertical-dimmer Se-Se, and single-ion O in V$_2$Se$_2$O, respectively; the symmetry requirements of altermagnetism is also marked inset. (c) Density of states of V$_2$Se$_2$O (gray) and its projected ones onto V, Se, and O ions (blue, green, and red, respectively). (d-h) Top view of crystal structures (top panel) of Janus V$_2$SSeO, V$_2$Se$_2$O, V$_2$Se$_3$, V$_2$Se$_2$, and Janus V$_2$SSe, as well as their band structures (bottom panel) under Néel-AFM order. The in-between arrows denote the symmetry-preserving structural modification by site-B engineering (blue) and Janus structuring (gray) in our design principles. Valence of each constituent elements is explicitly marked in crystal structures, and top S and bottom Se ions in c and g are distinguished by small yellow and large green spheres.
• Figure 2: Ground-state magnetic orderings and electronic band structures of designed candidates from framework (a) M$_2$A$_2$B, (b) M$_2$A$_2$, (c) Janus M$_2$AA$'$B, and (d) Janus M$_2$AA$'$. The vertical and horizontal axes denote the metal and non-metal parts of the chemical formula. The horizontal altermagnetism-absent metal belts, the vertical dependence of AM order on non-metal elements, and the strong relation between Dirac cones and AM order are all clearly demonstrated. (e) Schematic periodic table showing the transition-metal and non-metal elements considered in our design.
• Figure 3: Statistical count of candidates with four different ground-state magnetic orderings and four distinct types of band structures for material design framework (a) M$_2$A$_2$B, (b) M$_2$A$_2$, (c) Janus M$_2$AA$'$B, and (d) Janus M$_2$AA$'$. The specific number for each pair of magnetic orderings and band-structure types is explicitly marked on top.
• Figure 4: DFT-calculated spin-polarized band structures of selected AM candidates of material design frameworks (from top to bottom) Janus M$_2$AA$'$B, M$_2$A$_2$B, M$_2$A$_2$, and Janus M$_2$AA$'$, hosting (from left to right) semiconducting, metallic, Dirac-cone semimetallic, and half-metallic band features. The four types of band structures are marked at the top, while the top-view structures of four material frameworks are at the left, with the colored letters in chemical formula matching the respective atomic sites. AM ordering is the ground-state for all semiconducting, metallic, and Dirac-cone semimetallic candidates, while the FM ordering is for half-metallic ones.
• Figure 5: DFT-calculated phonon band structures (left) and AIMD simulations (right) for the same selected AM candidates of four material frameworks (from top to bottom) Janus M$_2$AA$'$B, M$_2$A$_2$B, M$_2$A$_2$, and Janus M$_2$AA$'$ with (from left to right) semiconducting, metallic, and Dirac-cone semimetallic band features. The top-view crystal structures after AIMD simulations at 300 K (except for 50 K in Cr$_2$OTe) after 10 ps are shown inset for different candidates, respectively, with colored letters on the right marking the names of ions. Almost all candidates presented exhibit both kinetic and thermodynamic stability.