Exceptional-Point Phase Transition in Coupled Magnonic Waveguides

Spontaneous symmetry breaking is a source of the phase transition in a variety of physical systems including such as non-Hermitian. It has recently been demonstrated that so-called exceptional points, which are singularities in non-Hermitian systems, separate the areas with symmetrical and asymmetrical eigenstates. Such separation is evidence of phase transitions in non-Hermitian systems taking place at exceptional points. In this paper, we experimentally demonstrate that exceptional-point phase transition in a system of two coupled magnonic waveguides coupled by dipole-dipole interaction occurs. Using infrared laser irradiation to control the spin-wave losses in one of the waveguides covered by a semiconductor $\mathrm{GaAs}$ layer, we observe a change in the wave numbers of the propagating spin waves and a change in the distribution of spin-wave intensity between the waveguides. Retrieving the experimental data, we find the spin-wave propagation parameters and, based on these and the use of coupled mode theory, we argue that in the system of magnonic waveguides the exceptional-point phase transition is observed.