Nonlocal drag by topological surface magnons in a pyrochlore ferromagnet

TL;DR

This paper demonstrates the existence of topologically protected surface magnon states in a pyrochlore ferromagnet and their measurable effects on conduction electrons, revealing new quantum phenomena in magnetic materials.

Contribution

It provides experimental evidence of topological surface magnon states and their nonlocal drag effects in a pyrochlore ferromagnet, expanding topological physics to magnetic quasiparticles.

Findings

Observation of magnon Hall effect via thermoelectric measurements

Detection of interfacial drag from surface magnon states on conduction electrons

Confirmation of topologically protected surface magnon states

Abstract

The nontrivial topology of quasiparticle wavefunctions can manifest themselves in the form of observable surface states. This is now well established in electronic systems, with Dirac and Weyl semimetals bringing to fore the exotic nature of these topologically protected entities. Magnons - which refer to collective excitations of localized spins - offer another sector where many of these concepts could be realized. Here, we report magneto-thermal measurements on a pyrochlore ferromagnet which is theoretically predicted to host such topological magnons. It is demonstrated that the thermoelectric potential across a metal layer deposited on single crystalline specimens of Y$_2$V$_2$O$_7$ can be used to measure the magnon Hall effect. Moreover, a direct manifestation of topologically protected magnon surface states is observed - via the interfacial drag which these surface spin currents impose on the conduction electrons of the adjacent metallic layer.