A Schwinger Boson Theory for the Magnetic Excitations of Ba₃CoSb₂O₉

The spin 1/2 triangular lattice Heisenberg Hamiltonian is a prototypical model for studying the enhancement of quantum fluctuations induced by geometric frustration. P. W. Anderson proposed this model to introduce the resonance valence bond ground state that led to the concept of quantum spin liquids. While later theoretical studies demonstrated that the ground state exhibits 120º long range magnetic ordering, recent inelastic neutron scattering experiments on the triangular antiferromagnet Ba₃CoSb₂O₉ have revealed strong deviations from the dynamical spin structure factor predicted by a semi-classical theory, such as a strong downward renormalization of the low-energy dispersion, and a strong high-energy dispersive continuum. In this talk, I will show that the low-energy part of the spectrum (including the low-energy continuum) is reproduced to a very good approximation by a Schwinger-boson theory that includes corrections beyond the saddle-point approximation.