New Material Offers Stable, Energy-Efficient Electronics at 150 Kelvin

Researchers are actively pursuing antiferromagnetic topological insulators to overcome limitations found in their ferromagnetic counterparts, specifically low magnetic ordering temperatures and unwanted stray fields. Sougata Mardanya from Howard University, Barun Ghosh of the S. N. Bose National Centre for Basic Sciences, and Mengke Liu working with colleagues at Harvard University, Washington University in St. Louis, The University of Texas at Austin, University of Michigan, and Northeastern University, have investigated the van der Waals antiferromagnet UOTe to address this challenge. Their comprehensive ab initio computations reveal a layer-dependent topological behaviour, predicting that two-layer UOTe films function as ideal two-dimensional antiferromagnetic Chern insulators with quantized Hall conductivity and fully compensated spin magnetization. Furthermore, the team demonstrate how strain or electric fields can manipulate the material’s electronic structure, transitioning between nontrivial and trivial states, while three-layer films exhibit characteristics of an axion insulator with quantized spin Hall conductivity and magneto-electric coupling. This work establishes UOTe as a promising intrinsic antiferromagnetic platform for developing next-generation spintronics and furthering fundamental understanding of correlated topological states.

Scientists are unveiling a new platform for next-generation spintronics identifying an antiferromagnetic material that exhibits the quantum anomalous Hall effect (QAHE) at a relatively high Néel temperature. Unlike conventional materials, UOTe avoids issues with stray magnetic fields and low operating temperatures, offering a more robust and efficient solution for energy-efficient electronic transport. Detailed computational analysis reveals that the topological properties of UOTe are remarkably sensitive to its layered structure, dictating the material’s behaviour. A two-layer film of UOTe functions as an ideal two-dimensional antiferromagnetic Chern insulator, characterised by quantized Hall conductivity achieved with fully compensated spin magnetization, a crucial feature for low-power devices. Researchers discovered that manipulating the behaviour of uranium-5f electrons through the application of in-plane strain or an electric field can induce a transition between topologically distinct phases, switching the material between a nontrivial state (Chern number of 1) and a trivial insulator (Chern number of 0). Interestingly, a three-layer UOTe film exhibits different behaviour, displaying zero charge conductance but hosting a quantized spin Hall conductivity with significant magneto-electric coupling, indicative of an axion insulator-like state, a phase of matter where spin and electricity are intricately linked. This layer-dependent topology extends throughout the material, with odd-numbered layers favouring axion-like insulating behaviour, even-numbered layers forming Chern insulators, and the bulk material behaving as a Dirac semimetal. This discovery establishes UOTe as a versatile material for exploring correlated topological phases and advancing both fundamental scientific understanding and practical spintronic applications. Two-layer UOTe exhibits an antiferromagnetic Chern insulating phase characterised by a quantized Hall conductivity of e²/h within the bandgap. This quantization arises not from spin magnetization, but from an orbital magnetization calculated to be 0.08 μB at the Fermi energy, confirming a fully compensated spin configuration. Calculations reveal a linear relationship between the orbital magnetization and chemical potential, a hallmark of Chern insulators. Temperature-dependent in-plane resistance measurements demonstrate a Néel temperature of 140 K, remarkably consistent with the bulk value of approximately 150 K. This high Néel temperature distinguishes UOTe from previously investigated materials and suggests its potential for practical applications. The study further demonstrates the tunability of the topological state in two-layer UOTe through external stimul.
At lower Ueff values of 3.5 eV, the system remains a Chern insulator, but increasing Ueff to 4.5 eV promotes a transition to the trivial phase accompanied by unit cell expansion. Critical strain values defining the phase boundary were determined for each Ueff value, providing a detailed map of topological control. The three-layer UOTe film, conversely, exhibits zero charge conductance alongside a quantized spin Hall conductivity, indicative of an axion insulator-like state with finite magneto-electric coupling. Density functional theory (DFT) calculations form the basis of this work, systematically investigating the topological properties of UOTe, a recently discovered van der Waals antiferromagnet. Initial analysis focused on determining the magnetic exchange interactions between uranium atoms within the material, revealing an antiferromagnetic coupling of J1 = -1.72 meV mediated by oxygen atoms, alongside a ferromagnetic interaction of J2 = 1.23 meV between nearest-neighbour intralayer uranium atoms. To establish the ground state magnetic order, the total energy difference method was used to compare udud, uddu, and uudd configurations, finding udud to be the lowest in energy, though inconsistent with neutron scattering data which indicated uddu as the more likely arrangement. Consequently, the study models UOTe as consisting of three quintuple layers with reversed magnetic orderings (uddu), separated by a van der Waals gap, leveraging the material’s strong easy-axis anisotropy of 2 meV per uranium atom along the c-axis. The bulk electronic structure was then examined, revealing a semimetallic character with band overlap occurring around the Γ point, stemming from the interplay between tellurium-p orbitals in the valence bands and uranium-f and -d orbitals in the conduction bands. A key finding was the strong spin-orbit interaction of uranium-5f electrons, driving band inversion and forming a Dirac-like crossing along the Γ, Z direction, protected by rotational symmetry. To explore layer-dependent topological phases, calculations were performed on few-layer UOTe films, focusing on even and odd layer numbers. The breaking of PT symmetry, crucial for realising the anomalous Hall effect, was investigated in the two-layer system, utilising the HSE06 hybrid functional to calculate the electronic structure and confirm a Chern number of C = 1, indicative of a Chern insulator phase. The presence of chiral edge states and a quantized Hall conductivity of e²/h further substantiated this finding, while the orbital magnetization was calculated to confirm the topological origin of the effect. Scientists have long sought materials where electrons can flow without losing energy, a quest central to more efficient electronics and potentially revolutionary computing paradigms. This work on UOTe represents a significant step forward, not because it immediately delivers a practical solution, but because it fundamentally alters the landscape of materials considered for topological quantum phenomena. For years, the field has been dominated by ferromagnetic materials, which, while exhibiting these desirable properties, suffer from limitations like low operating temperatures and unwanted magnetic interference. The beauty of this research lies in the identification of an antiferromagnetic material, UOTe, that displays these topological characteristics at a comparatively high temperature. This is crucial because antiferromagnets offer inherent advantages in terms of stability and reduced stray fields, paving the way for more robust and scalable devices. The layer-by-layer control demonstrated, switching between Chern and axion insulator states simply by altering the number of atomic layers, is particularly compelling. However, translating these findings into real-world applications is not straightforward. The material is still relatively complex to synthesise, and the effects observed are, at present, largely theoretical and require precise control over external conditions like strain and electric fields. Furthermore, the observed quantum Hall effects are, as yet, limited to very low temperatures. Looking ahead, this work will likely stimulate a broader search for other antiferromagnetic materials with similar properties. The ability to tune topological states through layer thickness and external stimuli opens up exciting possibilities for creating novel spintronic devices, and potentially even exploring exotic states of matter with implications for fundamental physics. The challenge now is to move beyond proof-of-principle and engineer materials that are both robust and readily manufacturable.