Flux-induced strengthening of the magnetic couplings in a flat-band diamond chain

Abstract

The physics in flat bands has emerged as an essential field in condensed matter physics where a plethora of phenomena can be unveiled, such as anomalous transport properties, superconductivity dominated by quantum geometry or exotic topological phases. Our goal here is to show that even in magnetic systems, the presence of flat bands can give rise to unexpected features. More precisely, we address the impact of an Aharonov-Bohm (AB) flux on the exchange couplings in magnetic diamond chains. The most remarkable result is the significant amplification of magnetic couplings at short distances induced by the AB flux, leading to a considerable increase in the thermal conductivity of the magnons. We have also shown that the flux-dependent decaying length of the couplings is connected to the quantum metric of the flat bands. Our results could be of interest for the control of magnetic properties in spintronic devices and relevant for the heat transport by magnons at the nanoscale in quantum technologies.

Figures (12)

• Figure 1: (a) Representation of the magnetic diamond chain threaded by an external magnetic flux $\Phi$. The red arrows correspond to the localized classical spins located on $B$ and $C$ sites. The boxed area indicates the unit cell. The flux-dependent compact localized state (CLS) is depicted in panel (b). The CLS spreads over two unit cells, and its amplitudes in each site are shown.
• Figure 2: Energy dispersions in both $\uparrow$ and $\downarrow$ sectors as a function of the momentum ($k$), for different values of the flux $\Phi$. The labels $+$, $-$, and $0$ correspond respectively to the two dispersive bands and to the flat band of each spin sector (see main text). The green area indicates the occupied states. Here, we have chosen $JS=1$.
• Figure 3: Amplitude of the nearest neighbor coupling $J_{BB}(R=a)$ as a function of $JS$ for different values of the magnetic flux $\Phi$, as depicted in the figure. The inset shows $J_{BB}(R=a)/JS$ as a function of $JS$ in the weak coupling regime.
• Figure 4: Amplitude of the nearest neighbor coupling between $B$ sites $J_{BB}(R=a)$ as a function of $JS$ (black line). The other lines correspond to the different contributions as discussed in the main text. '[F]' or '[AF]' means that the contribution is either ferromagnetic or antiferromagnetic. Here we have chosen $\Phi=\pi$.
• Figure 5: Coupling $J_{BB}(R)$ as a function of $R$ for (a) $JS=0.1$ and (b) $JS=1$. The values of the flux $\Phi$ associated to each color are depicted in both the panels.