Intrinsic suppression of the topological thermal Hall effect in an exactly solvable quantum magnet

In contrast to electron (fermion) systems, topological phases of charge neutral bosons have been poorly understood despite recent extensive research on insulating magnets. The most important unresolved issue is how the inevitable interbosonic interactions influence the topological properties. It has been proposed that the quantum magnet SrCu2(BO3)2 with an exact ground state serves as an ideal platform for this investigation, as the system is expected to be a magnetic analog of a Chern insulator with electrons replaced by bosonic magnetic excitations (triplons). Here, in order to examine topologically protected triplon chiral edge modes in SrCu2(BO3)2, we measured and calculated the thermal Hall conductivity κxy. Our calculations show that the sign of κxy is negative, which is opposite to the previous calculations, and its magnitude is 2π times smaller. No discernible κxy was observed, and its values are at most 20-30% of our calculations if present. This implies that even relatively weak interparticle interactions seriously influence the topological transport properties at finite temperatures. These findings demonstrate that, in contrast to fermionic cases, the picture of noninteracting topological quasiparticles cannot be naively applied to bosonic systems, calling special attention to the interpretation of the topological bosonic excitations reported for various insulating magnets.