  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.

Strongly Correlated Electrons