Nonlocal drag by topological surface magnons in a pyrochlore ferromagnet

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.