Emergent Topological Phenomena in Thin Films of Pyrochlore Iridates

Because of the recent development of thin film and artificial superstructure growth techniques, it is possible to control the dimensionality of the system, smoothly between two and three dimensions. In this Letter we unveil the dimensional crossover of emergent topological phenomena in correlated topological materials. In particular, by focusing on the thin film of pyrochlore iridate antiferromagnets grown along the [111] direction, we demonstrate that the thin film can have a giant anomalous Hall conductance, proportional to the thickness of the film, even though there is no Hall effect in 3D bulk material. Moreover, in the case of ultrathin films, a quantized anomalous Hall conductance can be observed, despite the fact that the system is an antiferromagnet. In addition, we uncover the emergence of a new topological phase, the nontrivial topological properties of which are hidden in the bulk insulator and manifest only in thin films. This shows that the thin film of correlated topological materials is a new platform to search for unexplored novel topological phenomena.

Figure 1

(a) Structure of the pyrochlore lattice. (b) Local spin structure of the all-in–all-out (AIAO) antiferromagnet (AFM). (c) Schematic 3D bulk phase diagram as a function of local Coulomb interaction $U$. (d) Distribution of eight Weyl points for the Weyl SM phase. All eight WPs are aligned along the [111] or its symmetry-equivalent directions. Here a red (blue) dot indicates a WP with the chiral charge $+1$ ($-1$).

Figure 2

(a) Lattice structure of [111] thin films. (b) The detailed changes of $G_{xy}$ (blue) and $G_{xy}^{\max }$ (red). For $N_b=1$, $G_{xy}$ and $G_{xy}^{\max }$ are the same. The vertical dotted line indicates the critical point $m_{c1}$. (c) Evolution of the phase diagram of [111] thin films as the number of bilayers $N_b$ increases. The top (bottom) surface of each film is terminated by a triangular (kagome) layer. The dotted lines schematically describe the change of maximum anomalous Hall conductance $G_{xy}^{\max }$ of the system.

Figure 3

Fermi surface shape due to localized in-gap states. Here the film is composed of 20 bilayers with an additional kagome layer on the top; hence, it has kagome layers on both the top and bottom surfaces. The red (blue) lines indicate the states at $E_F$ localized on the top (bottom) surfaces, and there is no surface state in the green region. For $0<m<m_{c2}$, the surface states maintain the chiral nature, suppressing backscatterings.

Figure 4

(a) A film terminated by kagome layers on both the top and bottom surfaces when $m_{c1}<m<m_{c2}$. Two bands touching the Fermi level (the dotted horizontal line) are SSs, localized on the top (left band) and bottom (right band) kagome layers, respectively. (b) A film terminated by a triangular (kagome) layer on the top (bottom) surface. Both top and bottom surfaces support SSs, which are crossing in the gap (marked by a dotted circle).