Altermagnetism induced surface Chern insulator

Abstract

We propose a new pathway to the quantized anomalous Hall effect (QAHE) by coupling an altermagnet to a topological crystalline insulator (TCI). The former gaps the topological surface states of the TCI, thereby realizing the QAHE in a robust and switchable platform with near- vanishing magnetization. We demonstrate the feasibility of this approach by studying a slab of the TCI SnTe coupled to an altermagnetic RuO2 layer. Our first-principles calculations reveal that the d-wave altermagnetism in RuO2 induces a 7 meV gap to the Dirac surface states on the (110) surface of SnTe, producing a finite anomalous Hall effect. Our approach generalizes to broader classes of altermagnetic materials and TCIs, thereby providing a family of topological altermagnetic heterostructures with small or vanishing magnetization that support nontrivial Chern numbers. Our results highlight a promising new topological platform with great tunability and applications to spintronics.

Figures (4)

• Figure 1: An altermagnet on the surface of a topological crystalline insulator gaps the surface Dirac cones of the latter to realize the quantum anomalous Hall effect. The momentum dependent spin-splitting of the altermagnet is indicated by purple/blue regions with red/blue arrows; green arrows indicate chiral edge modes.
• Figure 2: (a) Face-center BZ of SnTe and the surface BZ of the (110) surface. (b) Tight-binding band structure of 40 layer SnTe slab. Yellow and green color indicate the projection on the surface and bulk states, respectively, with the Dirac surface state near $\bar{X}$ indicated by the black arrow. (c) Top view of the RuO$_2$/SnTe interface; black and green dashed rectangles indicate the RuO$_2$ and SnTe (110) unit cells. Side (d) and front (e) view of RuO$_2$/SnTe interface. (f) DFT band structure of RuO$_2$/SnTe near $\bar{X}$ along the $\bar{X}-\bar{\Gamma}$ line. Color indicates the projection onto the top (dark purple) and bottom (yellow) layers; blue and green indicate bulk states. (g) Zoom-in to the gray dashed box in (f). Dashed lines in (g) guide the eye to avoided crossings between top surface (purple) and bulk (green) bands. (h) The tight-binding band structure near $\bar{X}$ along the $\bar{X}-\bar{\Gamma}$ line; zoom-in in (i). In (f)--(i), the red arrow indicates the gapped Dirac cone on the top surface, while the black arrow indicates the gapless Dirac cone on the bottom surface.
• Figure 3: (a) The tight-binding band structure near $\bar{X}$ along the $\bar{X}-\bar{\Gamma}$ line. Projection of the z-component of spin; red (blue) indicates spin up (down). (b) The spin texture at energy level $E$ indicated in (a). The arrows indicate the in-plane spin polarization and the color indicates the out-of-plane spin polarization consistent with (a). (c), (d) Same as (a),(b) after a $90^\circ$ rotation of the RuO$_2$ slab. The flipped spin polarization in (c),(d) compared to (a),(b) indicates that the slab heterostructure inherits the $d$-wave altermagnetic order in bulk RuO$_2$, even though the magnetization of the layer closest to SnTe does not change.
• Figure 4: (a) Berry curvature of the TB bands near the Dirac gap; red (blue) indicates positive (negative). The dashed lines indicate a continuous gap between bands (the gap at $\bar{X}$ is small but finite). The bands below the dashed line are included in the layer-resolved Chern number calculation shown in (b); $C(l)$ (blue) indicates the partial Chern number as a function of layer $l$ from the interface, up to the middle layer, and $C_\text{int}(n) = \sum_{l<n}C(l)$ (orange) indicates its integral.