## SCL Seminar by Slobodan Radosevic

SCL seminar of the Center for the Study of Complex Systems, will be held on Thursday, 13 July 2017 at 14:00 in the library reading room “Dr. Dragan Popović" of the Institute of Physics Belgrade. The talk entitled

will be given by Dr. Slobodan Radošević (Department of Physics, Faculty of Sciences, University of Novi Sad).

Most important methods for studying magnon-magnon interactions in localized magnets, including (non)linear spin-wave theory (SWT), linearizations of equations of motion or Monte Carlo simulations, are based on spin operators which define the Heisenberg model. Correspondingly, magnon-magnon interactions are viewed as a consequence of nontrivial algebra and the Hilbert space of spin operators coupled through the exchange integrals. Since su(2) algebra of spin operators and the dynamics of the spin system are fully satisfied only with exact boson/fermion Hamiltonian and corresponding Hilbert space, inevitable approximations in practical calculations often produce uncontrollable errors. On the other hand, from the effective field theory (EFT) perspective, magnons are interpreted as local coordinates on O(3)/O(2) = S

[1] S. Radošević, M. Pantić, M. Pavkov-Hrvojević, D. Kapor, Ann. Phys. 339, 382 (2013).

[2] S. Radošević, Ann. Phys. 362, 336 (2015).

**"****Magnon-magnon interactions in O(3) ferromagnets: spin operators vs. effective Lagrangian method"**will be given by Dr. Slobodan Radošević (Department of Physics, Faculty of Sciences, University of Novi Sad).

**Abstract of the talk:**Most important methods for studying magnon-magnon interactions in localized magnets, including (non)linear spin-wave theory (SWT), linearizations of equations of motion or Monte Carlo simulations, are based on spin operators which define the Heisenberg model. Correspondingly, magnon-magnon interactions are viewed as a consequence of nontrivial algebra and the Hilbert space of spin operators coupled through the exchange integrals. Since su(2) algebra of spin operators and the dynamics of the spin system are fully satisfied only with exact boson/fermion Hamiltonian and corresponding Hilbert space, inevitable approximations in practical calculations often produce uncontrollable errors. On the other hand, from the effective field theory (EFT) perspective, magnons are interpreted as local coordinates on O(3)/O(2) = S

^{2}and all information concerning magnon-magnon interactions is encoded in the nonlinear Lagrangian. Being based upon physical degrees of freedom (magnons), EFT approach has certain advantages over conventional methods such as SWT or linearizations of equations of motion. The perturbation theory for magnon fields in O(3) ferromagnets provides basis for detailed calculations and geometric classification of magnon-magnon interactions and can also increase understanding of some standard, spin-operator based, methods.[1] S. Radošević, M. Pantić, M. Pavkov-Hrvojević, D. Kapor, Ann. Phys. 339, 382 (2013).

[2] S. Radošević, Ann. Phys. 362, 336 (2015).