Quantum Spin Hall Effect Achieves Nonzero Spin Chern Number in Altermagnets

Researchers have demonstrated a novel pathway towards harnessing topological effects in altermagnetic materials, potentially revolutionising spintronics. Bo Yuan, Yingxi Bai and Ying Dai, all from Shandong University, alongside Baibiao Huang and Chengwang Niu et al., reveal a strategy for realising the magnonic quantum spin Hall effect within bilayer altermagnets. This work establishes a crucial link between topology and altermagnetism, moving beyond conventional electronic spintronics and showcasing protected helical edge states alongside an anisotropic thermal Hall response. Their findings, evidenced through both first-principles calculations on VWS bilayers and Heisenberg-DM model analysis, promise enhanced control and open exciting possibilities for developing dissipationless magnonic devices.

This breakthrough, detailed in new research, unveils a pathway to manipulate spin information with unprecedented control and efficiency. The team achieved this by focusing on the symmetry properties of altermagnets, a recently recognised class of magnetic materials exhibiting unique spin arrangements. Through a combination of theoretical modelling, first-principles calculations, and Heisenberg-DM model analysis, researchers uncovered that specific bilayer structures, notably V2WS4, exhibit d-wave altermagnetism alongside an integer spin Chern number and protected helical edge states.

The study reveals that chiral magnon splitting within altermagnets leads to an anisotropic, momentum-resolved thermal Hall response, a characteristic markedly different from that observed in conventional ferromagnets and antiferromagnets. This distinct behaviour offers enhanced flexibility for selectively controlling spin currents. Experiments show that the V2WS4 bilayer possesses the necessary properties to support these topological magnons, including a nonzero momentum-locked thermal Hall conductivity. This discovery establishes a direct link between topological magnons and altermagnetism, opening up possibilities for designing dissipationless magnonic devices.

Furthermore, the research establishes a rational design principle for topological magnon insulators with the magnonic quantum spin Hall effect in square bilayer altermagnets. By incorporating spin-group symmetries, the team demonstrated that helical edge states remain protected even when pseudo, time-reversal symmetry is broken, due to the combined effects of interlayer pseudospin flipping and the intrinsic rotational symmetry of the altermagnetic order. Analysis of site-symmetry groups predicted specific Wyckoff positions within square layer groups that simultaneously support altermagnetism and helical edge states, paving the way for material discovery. This work not only identifies candidate materials for realising the magnonic quantum spin Hall effect in altermagnets, but also establishes universal design principles. The findings offer a versatile platform for discovering and engineering topological magnons, potentially revolutionising the field of magnonics and enabling the creation of advanced, low-dissipation spin transport technologies for future information processing applications. The ability to manipulate magnons with such precision promises significant advancements in areas like data storage and quantum computing.

V2WS4 Bilayer Magnon Properties via Four-State Methodology

Scientists have uncovered a universal strategy for realising topological altermagnets exhibiting the magnonic quantum spin Hall effect, evidenced by a nonzero spin Chern number and protected helical edge states. This work demonstrates that chiral magnon splitting in altermagnets leads to an intrinsically anisotropic, momentum-resolved thermal Hall response, offering enhanced flexibility for selective manipulation. Researchers focused on the V2WS4 bilayer as a concrete material realisation, revealing its altermagnetic properties, integer spin Chern number, helical magnon edge states, and nonzero momentum-locked thermal Hall conductivity. To elucidate the magnon excitation properties of the V2WS4 bilayer, the team extracted magnetic exchange interactions using a four-state methodology (4SM), presenting the results in supplementary Table S1.

The magnonic band structure was then calculated, revealing two branches with opposite chirality, left-handed E(L,k) and right-handed E(R,k) , corresponding to Sz = ±1. This chiral band splitting exhibited a d-wave, type distribution, arising from the interplay between S4z symmetry and interlayer spin-flip mechanisms, where E(R,k) = E(L,S4zk). The antisymmetric DMI opened a magnonic band gap in both chiral branches, enabling the calculation of the spin Chern number to confirm topological nontriviality. Researchers calculated the distribution of magnonic Berry curvatures ΩL(R)(k) for bands with opposite chirality, finding dominant contributions near the M point and opposite signs for each chirality, resulting in Chern numbers CL = 1 and CR = -1.

Magnonic Wannier centers were then plotted, further demonstrating a spin Chern number Cs = 1, confirming the V2WS4 bilayer as a topological magnetic insulator. To illustrate this, the team evaluated magnonic edge states of a nanoribbon, clearly showing the emergence of one pair of helical edge states, consistent with the nonzero spin Chern number. Furthermore, the emergence of nonzero magnonic Berry curvature, coupled with d-wave altermagnetism, implied the existence of momentum-locked κM xy. The distribution of κM xy in the Brillouin zone exhibited a pronounced d-wave, symmetric pattern, directly reflecting chiral magnon splitting and highlighting the connection between altermagnetic symmetry and directional thermal Hall transport. Calculations revealed an integral of κM xy around the M point on the order of 10−14W/K, suggesting a detectable thermal Hall conductivity can be realised in altermagnets by selectively exciting magnons at specific momenta. First-principles calculations were performed using VASP, employing the PBE GGA for the exchange-correlation potential, a 30Å vacuum layer, and convergence thresholds of 0.001 eV/Å and 10−6 for ionic relaxation and electronic self-consistency, respectively, with a 500 eV plane-wave cutoff.

V2WS4 bilayer exhibits topological altermagnetism and Hall effect

Scientists have uncovered a universal strategy for realizing topological altermagnets exhibiting the magnonic quantum spin Hall effect, as evidenced by a nonzero spin Chern number and protected helical edge states. Experiments revealed that chiral magnon splitting within altermagnets leads to an intrinsically anisotropic, momentum-resolved thermal Hall response, a characteristic sharply contrasting with those observed in ferromagnets and antiferromagnets. This discovery offers enhanced flexibility for selective manipulation of spin currents. First-principles calculations and Heisenberg-DM model analysis demonstrate that the V2WS4 bilayer exhibits d-wave altermagnetism, an integer spin Chern number accompanied by helical magnon edge states, and a nonzero momentum-locked thermal Hall conductivity.

The team measured a spin Chern number of Cs = ±1, confirming the presence of topologically protected edge states within the altermagnetic structure. These results establish a direct link between topological magnons and altermagnetism, potentially opening new avenues for dissipationless magnonic devices. Magnons, the quanta of spin waves, are fundamental quasiparticles in magnetically ordered materials and support almost dissipationless information transfer. Researchers focused on square bilayer altermagnets possessing chiral magnon band splitting to propose a rational design principle for a topological magnon insulator with the magnonic quantum spin Hall effect.

By incorporating spin-group symmetries, the study demonstrates that helical edge states in altermagnets remain protected even when pseudo, time-reversal symmetry is broken. Furthermore, analysis of site-symmetry groups predicts specific Wyckoff positions within square layer groups that simultaneously support altermagnetism and helical edge states. Remarkably, the V2WS4 bilayer was found to host exotic magnon transport characteristics distinct from both ferromagnetic and antiferromagnetic configurations, resulting in a nonzero momentum-resolved thermal Hall conductivity exhibiting momentum- and chirality-locked directional behaviour. This work not only identifies candidate materials supporting the magnonic quantum spin Hall effect in altermagnets but also establishes universal design principles for discovering and engineering topological magnons.

V2WS4 bilayer exhibits novel thermal transport properties

Scientists have demonstrated a universal strategy for creating topological altermagnets capable of exhibiting the magnonic quantum spin Hall effect. This was evidenced by a non-zero spin Chern number and the presence of protected helical edge states within the material. Researchers further showed that chiral magnon splitting in these altermagnets leads to an anisotropic, momentum-resolved thermal Hall response, a characteristic markedly different from that observed in conventional ferromagnets and antiferromagnets, and potentially enabling more precise control over thermal transport. Specifically, first-principles calculations and Heisenberg-DM model analysis identified the V2WS4 bilayer as a material exhibiting altermagnetism, an integer spin Chern number, helical magnon edge states, and a non-zero momentum-locked thermal Hall conductivity.

The observed 5d-wave, symmetric distribution in momentum space of the thermal Hall conductivity directly correlates with chiral magnon splitting, reinforcing the link between altermagnetic symmetry and directional thermal transport. The integral of the thermal Hall conductivity around a specific momentum point suggests a detectable signal could be achieved by selectively exciting magnons at particular momenta. The authors established definitive symmetry and structural criteria for realizing square altermagnets with chiral magnon band splitting and topologically nontrivial helical edge states, requiring specific layer group symmetries and inequivalent magnetic atom arrangements. They acknowledge limitations inherent in the computational methods used, such as the approximations within density functional theory and the chosen Hubbard U parameters. Future research could focus on exploring other material candidates predicted by these symmetry criteria and experimentally verifying the predicted magnonic and thermal transport properties, potentially leading to the development of dissipationless magnonic devices and advancements in transport engineering mediated by altermagnetism.