Chirality-selective topological magnon phase transition induced by interplay of anisotropic exchange interactions in honeycomb ferromagnet

TL;DR

The study demonstrates that competing anisotropic exchanges—out-of-plane DMI $D_z$ and PDI $F$—in a honeycomb ferromagnet can drive chirality-selective topological magnon phases. A bulk gap closure and reopening at the high-symmetry points $K$ and $K'$ induces band inversion (pseudo-orbital reversal) and a magnon valley degree of freedom, flipping the Chern number between TP-I ($C=+1$) and TP-II ($C=-1$). The phase boundary scales with the ratio $D_z/F^2$ and yields distinct Berry-curvature, edge-state, and magnon-thermal-Hall signatures, including a sign change of $\kappa^{xy}$. Realizable in 4d/5d materials, these results open routes to tunable topological magnonics and valley-like control in spin systems.

Abstract

A variety of distinct anisotropic exchange interactions commonly exist in one magnetic material due to complex crystal, magnetic and orbital symmetries. Here we investigate the effects of multiple anisotropic exchange interactions on topological magnon in a honeycomb ferromagnet, and find a chirality-selective topological magnon phase transition induced by a complicated interplay of Dzyaloshinsky-Moriya interaction (DMI) and pseudo-dipolar interaction (PDI), accompanied by the bulk gap close and reopen with chiral inversion. Moreover, this novel topological phase transition involves band inversion at high symmetry points $K$ and $K'$, which can be regarded as a pseudo-orbital reversal, i.e. magnon valley degree of freedom, implying a new manipulation corresponding to a sign change of the magnon thermal Hall conductivity. Indeed, it can be realized in 4$d$ or 5$d$ correlated materials with both spin-orbit coupling and orbital localized states, such as iridates and ruthenates, etc. This novel regulation may have potential applications on magnon devices and topological magnonics.

Figures (8)

• Figure 1: (Color online) (a) The honeycomb lattice with the NN vector $\mathbf{\delta_{1,2,3}}$ and the NNN vector $\mathbf{a_{1,2,3}}$ in the $x$-$y$ plane. Red and black dots indicate the sublattices A and B, respectively. (b) The corresponding Brillouin zone (BZ), and its high-symmetry $k$-points and $k$-paths.
• Figure 2: (Color online) (a)$D_{z}$-$F$ phase diagram for magnons in honeycomb ferromagnet. The symbols ($+/-$1, $-/+$1) denote the Chern number of conduction and valence bands, respectively. The dot line on the boundary is our analytical result. The directions of DMI for $D_{z}$$>$ 0 (b) and $D_{z}$$<$ 0 (c). The crossed and dotted circles denote the alternating DMI along the NNN bonds.
• Figure 3: (Color online) The evolution of magnon band structures depends on different DMI and PDI parameters: (a) $D_{z}$=$-$0.1 is fixed, and $F$=0, 4, 8, respectively; (b) $F$=7 is fixed, and $D_{z}$=$-$0.5, $-$0.3, 0, 0.3, respectively. The symbols ($+/-$1, $-/+$1) denote the Chern number of conduction and valence bands, respectively. The dash line represents the $D_{z}$=$F$=0 case.
• Figure 4: (Color online) Pseudo-orbital (sublattice A, i.e.$\alpha$-orbital) projection at different parameters: (a) $D_{z}$=0, $F$=0; (b) $D_{z}$=0.1, $F$=0; (c) $D_{z}$=0, $F$=7; (d) $D_{z}$=$-$0.5, $F$=7. The red, blue and green colors denote $\alpha$, $\beta$ and proportionally mixed orbital occupations, respectively. The symbols $+/-$1 denote the Chern number of the corresponding bands.
• Figure 5: (Color online) Spin structure factor with parameters $D_{z}$=$-$0.5 and $F$=7 for different cut frequencies: (a) ${\omega}$=10$JS$, (b) ${\omega}$=15$JS$, (c) ${\omega}$=20$JS$, (d) ${\omega}$=30$JS$.