Steering magnonic dynamics and permeability at exceptional points in a parity-time symmetric waveguide

Exceptional-point-enhanced nanoparticle sensor utilizing a linewidth broadening mechanism

Exceptional points (EPs) in non-Hermitian systems, where eigenvalues and eigenvectors coalesce, offer unique advantages in state transitions, non-reciprocal devices, and sensing, owing to their distinctive and extraordinary properties. Most prior studies for sensing at EPs focused on mode splitting, with limited focus on leveraging the linewidth broadening mechanism. In this study, we construct an EP by embedding two nanoholes within a microdisk cavity. With nanoparticle adsorption at the edge of the microcavity at the EP, the linewidth of two split modes exceeds the frequency splitting, enabling the use of the linewidth broadening mechanism for nanoparticle detection. By calculating the linewidth of the transmission spectra with or without the adsorption of the nanoparticle, an enhanced linewidth broadening based on the EP is achieved compared to that based on the diabolic point (DP). We observe that the linewidth broadening based on the EP varies periodically with the azimuthal position of the nanoparticle along the edge of the cavity. Specifically, the maximum of the linewidth broadening based on the EP is several times larger than that based on the diabolic point. This paper not only deepens our understanding of non-Hermitian physics in microcavities but also lays the groundwork for future research and applications in high-sensitivity sensing.

Non-hermiticity in spintronics: oscillation death in coupled spintronic nano-oscillators through emerging exceptional points

The emergence of exceptional points (EPs) in the parameter space of a non-hermitian (2D) eigenvalue problem has long been interest in mathematical physics, however, only in the last decade entered the scope of experiments. In coupled systems, EPs give rise to unique physical phenomena, and enable the development of highly sensitive sensors. Here, we demonstrate at room temperature the emergence of EPs in coupled spintronic nanoscale oscillators and exploit the system's non-hermiticity. We observe amplitude death of self-oscillations and other complex dynamics, and develop a linearized non-hermitian model of the coupled spintronic system, which describes the main experimental features. The room temperature operation, and CMOS compatibility of our spintronic nanoscale oscillators means that they are ready to be employed in a variety of applications, such as field, current or rotation sensors, radiofrequeny and wireless devices, and in dedicated neuromorphic computing hardware. Furthermore, their unique and versatile properties, notably their large nonlinear behavior, open up unprecedented perspectives in experiments as well as in theory on the physics of exceptional points expanding to strongly nonlinear systems.

Floquet Engineering the Exceptional Points in Parity-Time-Symmetric Magnonics

Magnons serve as a testing ground for fundamental aspects of Hermitian and non-Hermitian wave mechanics and are of high relevance for information technology. This study presents setups for realizing spatiotemporally driven parity-time- (PT) symmetric magnonics based on coupled magnetic waveguides and magnonic crystals. A charge current in a metal layer with strong spin-orbit coupling sandwiched between two insulating magnetic waveguides leads to gain or loss in the magnon amplitude depending on the directions of the magnetization and the charge currents. When gain in one waveguide is balanced by loss in the other waveguide, a PT-symmetric system hosting non-Hermitian degeneracies [or exceptional points (EPs)] is realized. For ac current, multiple EPs appear for a certain gain-loss strength and mark the boundaries between the preserved PT-symmetry and the broken PT-symmetry phases. The number of islands of broken PT-symmetry phases and their extensions is tunable by the frequency and the strength of the spacer current. At EP and beyond, the induced and amplified magnetization oscillations are strong and self-sustained. In particular, these magnetization auto-oscillations in a broken PT-symmetry phase occur at low current densities and do not require further adjustments such as tilt angle between electric polarization and equilibrium magnetization direction in spin-torque oscillators, pointing to a new design of these oscillators and their utilization in computing and sensorics. It is also shown how the periodic gain-loss mechanism allows for the generation of high-frequency spin waves with low-frequency currents. For spatially periodic gain and loss acting on a magnonic crystal, magnon modes approaching each other at the Brillouin-zone boundaries are highly susceptible to PT symmetry, allowing for a wave-vector-resolved experimental realization at very low currents.