Layer-dependent antiferromagnetic Chern and axion insulating states in UOTe

TL;DR

The paper identifies layer-dependent topological phases in the van der Waals antiferromagnet UOTe, demonstrating that a 2-layer film realizes an antiferromagnetic Chern insulator with $C=1$ and a high Néel temperature around $T_N \approx 150\,\mathrm{K}$. It shows that external perturbations, such as in-plane strain and perpendicular electric fields, can drive a transition to a trivial insulator by inducing double-band inversions, with layer-resolved Chern densities quantitatively describing the redistribution of topological charge. For odd layers, PT symmetry leads to axion-insulator–like behavior with a finite, thickness-dependent magnetoelectric coupling; the 3-layer film additionally exhibits a quantized spin Hall response with $\sigma^z_{xy}=2e^2/h$ inside the gap, despite zero net charge conductance. Overall, UOTe provides a new intrinsic AFM platform for exploring correlated topological phases and potential spintronics applications, with a Dirac semimetal bulk and tunable topology in thin films.

Abstract

Magnetic topological insulators have received significant interest due to their dissipationless edge states, which promise advances in energy-efficient electronic transport. However, the magnetic topological insulator state has typically been found in ferromagnets (FMs) that suffer from low magnetic ordering temperatures and stray fields. Identifying an antiferromagnetic topological insulator that exhibits the quantum anomalous Hall effect (QAHE) with a relatively high Néel temperature has been a longstanding challenge. Here, we focus on the recently discovered van der Waals (vdW) antiferromagnet (AFM) UOTe, which not only features a high Néel temperature (\(\sim\)150K) but also exhibits intriguing Kondo interaction and topological characteristics. Our systematic analysis of the layer-dependent topological phases based on \textit{ab} initio computations predicts the two-layer UOTe film to be an ideal 2D AFM Chern insulator in which the Hall conductivity is quantized with a fully compensated spin magnetization. By applying an in-plane strain or electric field, we show how the itinerancy of U-5f electrons can be manipulated to trigger a transition between the nontrivial ($C = 1$) and trivial ($C = 0$) phases. Interestingly, the 3-layer UOTe film is found to have zero charge conductance but it hosts a quantized spin Hall conductivity (SHC) with finite magneto-electric coupling, suggesting the presence of an axion insulator-like state. The unique magnetic structure of UOTe supports a layer-tunable topology in which films with an odd number of layers are axion-like insulators, while films with an even number of layers are Chern insulators, and the bulk material is a Dirac semimetal. Our study offers a new intrinsic AFM materials platform for realizing correlated topological phases for next-generation spintronics applications and fundamental science studies.

Figures (6)

• Figure 1: Crystal structure and layer-dependent topology in UOTe.(a) Crystal structure and the experimental magnetic configuration of bulk UOTe. Two Uranium atoms in a quintuple layer are connected via two Oxygen atoms, resulting in antiferromagnetic coupling through a superexchange mechanism. (b) Square anti-prismic crystal field splitting of $U-5f$ electrons. (c) Orbital-resolved band structure of bulk UOTe with SOC. The color and size of the markers on the band structure represent different orbitals and their weights, respectively. Band inversion between the $Te-p$ and $U-f$ bands is prominent around the Fermi level. (d) Summary of the layer-dependent topological phases and their symmetries.
• Figure 2: (caption next page)
• Figure 2: AFM Chern insulating phase in 2-layer UOTe. (a) Crystal structure with open boundaries along the c-direction. (b) Orbital-resolved band structure, including the effect of SOC using HSE06 hybrid functional. A clear gap is seen around the Fermi energy, along with a band inversion between the conduction and valence bands. (c) The edge spectrum projected along the [100] direction shows the chiral edge state within the 2D bulk gap. (d) Variation of Hall conductivity (red curve) and orbital magnetization (blue curve) as a function of energy. The circulating edge current results in quantized Hall conductivity and generates a small orbital magnetization within the 2D bulk gap. (e) Temperature-dependent in-plane resistivity for a 25 nm flake shows the discontinuity at 140.46K, indicating the Neel temperature for the AFM transition. (f) Magnetization vs transition temperature phase diagram for comparing the predicted behavior of the 2-layer UOTe film with that of other reported Chern insulators Tcakaev_2020_magphaseTschirhart_2021_magphaseXu_2023_magphaseMei_2024_magphase, highlighting the uniqueness of the predicted AFM Chern insulating phase in the 2-layer UOTe film.
• Figure 3: Effects of strains and electric fields. (a) A schematic of the double-band- inversion mechanism under external effects such as strains and electric fields to show the transition from the nontrivial (topological) to the trivial phase. (b) $U_{eff}$ vs. bi-axial strain phase diagram, which shows two topologically distinct phases with Chern numbers C=1 and C=0. Intensity of the color represents the size of the bandgap between the highest occupied and lowest unoccupied band at the $\Gamma$ point. Negative values denote inverted band gaps and the presence of a double band inversion. Red line is fitted to the critical points to identify the phase boundary. (c) Layer-dependent Chern numbers capture the effect of the electric field, which changes the topological phase from C=1 to C=0 in the 2-layer UOTe film. (d) $\langle \sigma_z \rangle$-spin-resolved band structure for the 2-layer UOTe film with 0.005 $\mathrm{V/\AA}$ electric field, where a second band inversion can be seen. (e) The edge spectrum projected along the [1-10] direction shows the presence of two oppositely moving chiral states at Fermi energy, which add up to give a topologically trivial phase with C=0.
• Figure 4: (caption next page)