Exceptional points in magnetic systems*

While several works have recently addressed the emergence of exceptional points (EPs), i.e., spectral singularities of non-Hermitian Hamiltonians, in the dynamics of coupled magnetic systems, their experimental signatures remain relatively unexplored. In the vicinity of an EP, magnon spectral features can be drastically modified, which, in turn, can affect the the properties routinely probed in spintronics setups, such as, e.g., spin conductivity and spin diffusion length. However, the systems discussed so far display EPs only at isolated momenta, which are not likely to influence system’s properties, such as, e.g., transport coefficients, that depend on integrals over a large number of momenta. 

In the first part of my talk I will show, by focusing on the driven magnetization dynamics of a van der Waals ferromagnetic bilayer, that EPs can appear over extended portions of the first Brillouin zone as well. Furthermore, I will discuss how the effective non-Hermitian magnon Hamiltonian, whose eigenvalues are purely real or come in complex-conjugate pairs, respects an unusual wavevector-dependent pseudo-Hermiticity. In the second part of my talk, I will show that, in the presence of gain, EPs are smoking guns of dynamical phase transitions of the non-linear spin dynamics.