SU(N) spin-wave theory: Application to spin-orbital Mott insulators

摘要

We present the application of the SU(N) spin-wave theory to spin-orbital Mott insulators whose ground states exhibit magnetic orders. When taking both spin and orbital degrees of freedom into account rather than projecting Hilbert space onto the Kramers doublet, which is the lowest spin-orbital locked energy levels, the SV(N) spin-wave theory should take the place of the SU(2) one due to the inevitable spin-orbital multipole exchange interactions. To implement the application, we introduce an efficient general local mean-field method, which involves all local fluctuations, and develop the SU(N) linear spin-wave theory. Our approach is tested firstly by calculating the multipolar spin-wave spectra of the SU(4) antiferromagnetic model. Then, we apply it to spin-orbital Mott insulators. It is revealed that the Hund's coupling would influence the effectiveness of the isospin-1/2 picture when the spin-orbital coupling is not large enough. We further carry out the SU(N) spin-wave calculations of two materials. a-RuCl_3 and Sr_2IrO_4. and find that the magnonic and spin-orbital excitations are consistent with experiments.