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

Abstract

In contrast to electron (fermion) systems, topological phases of charge\nneutral bosons have been poorly understood despite recent extensive research on\ninsulating magnets. The most important unresolved issue is how the inevitable\ninter-bosonic interactions influence the topological properties. It has been\nproposed that the quantum magnet SrCu$_2$(BO$_3$)$_2$ with an exact ground\nstate serves as an ideal platform for this investigation, as the system is\nexpected to be a magnetic analogue of a Chern insulator with electrons replaced\nby bosonic magnetic excitations (triplons). Here, in order to examine\ntopologically protected triplon chiral edge modes in SrCu$_2$(BO$_3$)$_2$, we\nmeasured and calculated the thermal Hall conductivity $\\kappa_{xy}$. Our\ncalculations show that the sign of $\\kappa_{xy}$ is negative, which is opposite\nto the previous calculations, and its magnitude is 2$\\pi$ times smaller. No\ndiscernible $\\kappa_{xy}$ was observed, which is at most 20-30% of our\ncalculations if present. This implies that even relatively weak inter-particle\ninteractions seriously influence the topological transport properties at finite\ntemperatures. These demonstrate that, in contrast to fermionic cases, the\npicture of non-interacting topological quasi-particles cannot be naively\napplied to bosonic systems, calling special attention to the interpretation of\nthe topological bosonic excitations reported for various insulating magnets.\n