Abstract
We theoretically investigate dynamics of classical spins exchange-coupled through an isotropic medium. The coupling is treated at the adiabatic level of the medium's response, which mediates a first-order in frequency dissipative interaction along with an instantaneous Heisenberg exchange. The resultant damped spin precession yields exceptional points (EPs) in the coupled spin dynamics, which should be experimentally accessible with the existing magnetic heterostructures. In particular, we show that an EP is naturally approached in an antiferromagnetic dimer by controlling local damping, while the same is achieved by tuning the dissipative coupling between spins in the ferromagnetic case. Extending our treatment to one-dimensional spin chains, we show how EPs can emerge within the magnonic Brillouin zone by tuning the dissipative properties. The critical point, at which an EP pair emerges out of the Brillouin zone center, realizes a gapless Weyl point in the magnon spectrum. Tuning damping beyond this critical point produces synchronization (level attraction) of magnon modes over a finite range of momenta, both in ferro- and antiferromagnetic cases. We thus establish that damped magnons can generically yield singular points in their band structure, close to which their kinematic properties, such as group velocity, become extremely sensitive to the control parameters.
Highlights
Over the past several decades, it became clear that a class of unconventional degeneracies are abundant in diverse physical systems undergoing non-Hermitian dynamics [1]
We argue that exceptional points (EPs) are commonplace in pure spin dynamics, based on several generic examples, even in the absence of external driving, such as spin-transfer torque [13]
An isotropic antiferromagnetic spin pair harbors an EP already in its singletlike ground state. We show how this EP gets inherited by extended antiferromagnetic dynamics, manifesting in Weyl singularities and synchronization within magnonic band structure, which are tunable by the dissipative properties of the environment
Summary
Over the past several decades, it became clear that a class of unconventional degeneracies are abundant in diverse physical systems undergoing non-Hermitian dynamics [1]. Varying one of the complex-valued parameters w, the eigenenergies can be represented as single-valued functions on a multisheet Riemann surface, with branch points marking coalescence of two or more energy levels These EPs provide genuine singularities, which can manifest prominently in microwave [3] and optical [4] response properties and hybrid dynamical systems [5,6,7,8], scattering problems and sensing [9], open and quasi-Hermitian quantum systems [10], etc. An isotropic antiferromagnetic spin pair harbors an EP already in its singletlike ground state We show how this EP gets inherited by extended antiferromagnetic dynamics, manifesting in Weyl singularities and synchronization (level attraction) within magnonic band structure, which are tunable by the dissipative properties of the environment.
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