Abstract
The exchange coupling of the helical edge state of a quantum spin-Hall insulator with an easy-plane magnet has no effect on its DC electrical conductance if the magnet's anisotropy axis is aligned with the spin quantization axis of the helical edge state [Meng et al., Phys. Rev. B 90, 205403 (2014)]. We here calculate the AC conductance G_V(ω) and the noise power S_V(ω) in the presence of a DC bias V. While both take the universal values G_V(ω = 0) = e²/h and S_V(ω = 0) = 4 e² k_ B T/h in the zero-frequency limit, G_V(ω) and S_V(ω) are quickly suppressed for finite ω, so that low-frequency transport is effectively noiseless.