Alladi Ramakrishnan Hall

Topology in a quantum magnet

R. Ganesh

IMSc

SrCu$_2$(BO$_3$)$_2$ is the archetypal quantum magnet, with Cu atoms forming a lattice of spin-1/2 moments. It serves as a realization of the Shastry-Sutherland model with a dimerized ground state -- pairs of spin-1/2 moments form singlets. Its low-lying excitations are 'triplons' involving breaking a singlet to form a triplet. When compared to the Shastry Sutherland model, there are small anisotropic corrections which turn out to be very interesting. Surprisingly, with a small magnetic field, we can give a topological character to the energy band structure of the triplons. We see a new type of Dirac cone which is a spin-1 generalization of graphene. We are left with a spin-analogue of the integer quantum Hall effect seen in electrons. We predict a thermal Hall signal, with a large measurable transverse heat current.